1. The document describes the structure and properties of excitable tissues like nerves and muscles. It discusses the anatomy of neurons including the cell body, dendrites, axon, and synaptic terminals.
2. Key properties of nerves are described, including excitability, conductivity, the all-or-none principle, accommodation, and infatiguability. The mechanisms of the resting membrane potential and action potential are summarized.
3. The stages of an action potential are outlined as depolarization, repolarization, after-depolarization, and after-hyperpolarization. Factors influencing stimulus effectiveness are also noted.
It is over 60 years since Hodgkin and
Huxley1 made the first direct recording of
the electrical changes across the neuronal
membrane that mediate the action
potential. Using an electrode placed inside a
squid giant axon they were able to measure a
transmembrane potential of around 260 mV
inside relative to outside, under resting
conditions (this is called the resting membrane
potential). The action potential is a
transient (,1 millisecond) reversal in the
polarity of this transmembrane potential
which then moves from its point of initiation,
down the axon, to the axon terminals. In a
subsequent series of elegant experiments
Hodgkin and Huxley, along with Bernard
Katz, discovered that the action potential
results from transient changes in the permeability
of the axon membrane to sodium (Na+)
and potassium (K+) ions. Importantly, Na+ and
K+ cross the membrane through independent
pathways that open in response to a change
in membrane potential.
As testimony to their pioneering work, the
fundamental mechanisms described by
Hodgkin, Huxley and Katz remain applicable
to all excitable cells today. Indeed, the
predictions they made about the molecular
mechanisms that might underlie the changes
in membrane permeability showed remarkable
foresight. The molecular basis of the action
potential lies in the presence of proteins
called ion channels that form the permeation
pathways across the neuronal membrane.
Although the first electrophysiological
recordings from individual ion channels were
not made until the mid 1970s,2 Hodgkin and
Huxley predicted many of the properties now
known to be key components of their
function: ion selectivity, the electrical basis
of voltage-sensitivity and, importantly, a
mechanism for quickly closing down the
permeability pathways to ensure that the
action potential only moves along the axon in
one direction.
It is over 60 years since Hodgkin and
Huxley1 made the first direct recording of
the electrical changes across the neuronal
membrane that mediate the action
potential. Using an electrode placed inside a
squid giant axon they were able to measure a
transmembrane potential of around 260 mV
inside relative to outside, under resting
conditions (this is called the resting membrane
potential). The action potential is a
transient (,1 millisecond) reversal in the
polarity of this transmembrane potential
which then moves from its point of initiation,
down the axon, to the axon terminals. In a
subsequent series of elegant experiments
Hodgkin and Huxley, along with Bernard
Katz, discovered that the action potential
results from transient changes in the permeability
of the axon membrane to sodium (Na+)
and potassium (K+) ions. Importantly, Na+ and
K+ cross the membrane through independent
pathways that open in response to a change
in membrane potential.
As testimony to their pioneering work, the
fundamental mechanisms described by
Hodgkin, Huxley and Katz remain applicable
to all excitable cells today. Indeed, the
predictions they made about the molecular
mechanisms that might underlie the changes
in membrane permeability showed remarkable
foresight. The molecular basis of the action
potential lies in the presence of proteins
called ion channels that form the permeation
pathways across the neuronal membrane.
Although the first electrophysiological
recordings from individual ion channels were
not made until the mid 1970s,2 Hodgkin and
Huxley predicted many of the properties now
known to be key components of their
function: ion selectivity, the electrical basis
of voltage-sensitivity and, importantly, a
mechanism for quickly closing down the
permeability pathways to ensure that the
action potential only moves along the axon in
one direction.
Muscle spindles are proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath (spindle). They run parallel to the extrafusal muscle fibers and act as receptors that provide information on muscle length and the rate of change in muscle length. The spindles are stretched when the muscle lengthens. This stretch causes the sensory neuron in the spindle to transmit an impulse to the spinal cord, where it synapses with alpha motor neurons. This causes activation of motor neurons that innervate the muscle. The muscle spindles determine the amount of contraction necessary to overcome a given resistance. When the resistance increases, the muscle is stretched further, and this causes spindle fibers to activate a greater muscle contraction.
these slides contain a brief introduction of neurons and its classification as well as details of generation of action potential, resting potential and eletrotonic potential.
https://nabeelbeeran.blogspot.com/
https://youtu.be/ur3LZGVuLI0
CLASSIFICATION & PROPERTIES OF NERVE FIBERS-
CLASSIFICATION OF NERVE FIBERS
PROPERTIES OF NERVE FIBERS :
1. EXCITABILITY
2. CONDUCTIVITY
3. ALL OR NONE LAW
4. REFRACTORY PERIOD
Stimulus – A change in environment which brings about a change in potential across a membrane in an excitable tissue
Electrical Chemical Thermal Mechanical 14
STRENGTH-DURATION CURVE TIME
UTILISATION TIME
STRENGTH RHEOBASE 2 X RHEOBASE
CHRONAXIE
Muscle spindles are proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath (spindle). They run parallel to the extrafusal muscle fibers and act as receptors that provide information on muscle length and the rate of change in muscle length. The spindles are stretched when the muscle lengthens. This stretch causes the sensory neuron in the spindle to transmit an impulse to the spinal cord, where it synapses with alpha motor neurons. This causes activation of motor neurons that innervate the muscle. The muscle spindles determine the amount of contraction necessary to overcome a given resistance. When the resistance increases, the muscle is stretched further, and this causes spindle fibers to activate a greater muscle contraction.
these slides contain a brief introduction of neurons and its classification as well as details of generation of action potential, resting potential and eletrotonic potential.
https://nabeelbeeran.blogspot.com/
https://youtu.be/ur3LZGVuLI0
CLASSIFICATION & PROPERTIES OF NERVE FIBERS-
CLASSIFICATION OF NERVE FIBERS
PROPERTIES OF NERVE FIBERS :
1. EXCITABILITY
2. CONDUCTIVITY
3. ALL OR NONE LAW
4. REFRACTORY PERIOD
Stimulus – A change in environment which brings about a change in potential across a membrane in an excitable tissue
Electrical Chemical Thermal Mechanical 14
STRENGTH-DURATION CURVE TIME
UTILISATION TIME
STRENGTH RHEOBASE 2 X RHEOBASE
CHRONAXIE
The nervous system includes the brain, spinal cord, and a complex network of nerves. This system sends messages back and forth between the brain and the body.
The brain is what controls all the body's functions. The spinal cord runs from the brain down through the back. It contains threadlike nerves that branch out to every organ and body part. This network of nerves relays messages back and forth from the brain to different parts of the body.What Are the Parts of the Nervous System?
The nervous system is made up of the central nervous system and the peripheral nervous system:
The central nervous system includes the brain and spinal cord.
The peripheral nervous system includes the nerves that run throughout the whole body.How Does the Nervous System Work?
The nervous system uses tiny cells called neurons (NEW-ronz) to send messages back and forth from the brain, through the spinal cord, to the nerves throughout the body.
Billions of neurons work together to create a communication network. Different neurons have different jobs. For example, sensory neurons send information from the eyes, ears, nose, tongue, and skin to the brain. Motor neurons carry messages away from the brain to the rest of the body to allow muscles to move. These connections make up the way we think, learn, move, and feel. They control how our bodies work — regulating breathing, digestion, and the beating of our hearts.
The following power point presentation talks about neural control and coordination in humans. In this, we study about neurons, the conduction of nerve impulse, about Central Nervous System and also about sense organs
New Directions in Targeted Therapeutic Approaches for Older Adults With Mantl...i3 Health
i3 Health is pleased to make the speaker slides from this activity available for use as a non-accredited self-study or teaching resource.
This slide deck presented by Dr. Kami Maddocks, Professor-Clinical in the Division of Hematology and
Associate Division Director for Ambulatory Operations
The Ohio State University Comprehensive Cancer Center, will provide insight into new directions in targeted therapeutic approaches for older adults with mantle cell lymphoma.
STATEMENT OF NEED
Mantle cell lymphoma (MCL) is a rare, aggressive B-cell non-Hodgkin lymphoma (NHL) accounting for 5% to 7% of all lymphomas. Its prognosis ranges from indolent disease that does not require treatment for years to very aggressive disease, which is associated with poor survival (Silkenstedt et al, 2021). Typically, MCL is diagnosed at advanced stage and in older patients who cannot tolerate intensive therapy (NCCN, 2022). Although recent advances have slightly increased remission rates, recurrence and relapse remain very common, leading to a median overall survival between 3 and 6 years (LLS, 2021). Though there are several effective options, progress is still needed towards establishing an accepted frontline approach for MCL (Castellino et al, 2022). Treatment selection and management of MCL are complicated by the heterogeneity of prognosis, advanced age and comorbidities of patients, and lack of an established standard approach for treatment, making it vital that clinicians be familiar with the latest research and advances in this area. In this activity chaired by Michael Wang, MD, Professor in the Department of Lymphoma & Myeloma at MD Anderson Cancer Center, expert faculty will discuss prognostic factors informing treatment, the promising results of recent trials in new therapeutic approaches, and the implications of treatment resistance in therapeutic selection for MCL.
Target Audience
Hematology/oncology fellows, attending faculty, and other health care professionals involved in the treatment of patients with mantle cell lymphoma (MCL).
Learning Objectives
1.) Identify clinical and biological prognostic factors that can guide treatment decision making for older adults with MCL
2.) Evaluate emerging data on targeted therapeutic approaches for treatment-naive and relapsed/refractory MCL and their applicability to older adults
3.) Assess mechanisms of resistance to targeted therapies for MCL and their implications for treatment selection
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
Title: Sense of Taste
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the structure and function of taste buds.
Describe the relationship between the taste threshold and taste index of common substances.
Explain the chemical basis and signal transduction of taste perception for each type of primary taste sensation.
Recognize different abnormalities of taste perception and their causes.
Key Topics:
Significance of Taste Sensation:
Differentiation between pleasant and harmful food
Influence on behavior
Selection of food based on metabolic needs
Receptors of Taste:
Taste buds on the tongue
Influence of sense of smell, texture of food, and pain stimulation (e.g., by pepper)
Primary and Secondary Taste Sensations:
Primary taste sensations: Sweet, Sour, Salty, Bitter, Umami
Chemical basis and signal transduction mechanisms for each taste
Taste Threshold and Index:
Taste threshold values for Sweet (sucrose), Salty (NaCl), Sour (HCl), and Bitter (Quinine)
Taste index relationship: Inversely proportional to taste threshold
Taste Blindness:
Inability to taste certain substances, particularly thiourea compounds
Example: Phenylthiocarbamide
Structure and Function of Taste Buds:
Composition: Epithelial cells, Sustentacular/Supporting cells, Taste cells, Basal cells
Features: Taste pores, Taste hairs/microvilli, and Taste nerve fibers
Location of Taste Buds:
Found in papillae of the tongue (Fungiform, Circumvallate, Foliate)
Also present on the palate, tonsillar pillars, epiglottis, and proximal esophagus
Mechanism of Taste Stimulation:
Interaction of taste substances with receptors on microvilli
Signal transduction pathways for Umami, Sweet, Bitter, Sour, and Salty tastes
Taste Sensitivity and Adaptation:
Decrease in sensitivity with age
Rapid adaptation of taste sensation
Role of Saliva in Taste:
Dissolution of tastants to reach receptors
Washing away the stimulus
Taste Preferences and Aversions:
Mechanisms behind taste preference and aversion
Influence of receptors and neural pathways
Impact of Sensory Nerve Damage:
Degeneration of taste buds if the sensory nerve fiber is cut
Abnormalities of Taste Detection:
Conditions: Ageusia, Hypogeusia, Dysgeusia (parageusia)
Causes: Nerve damage, neurological disorders, infections, poor oral hygiene, adverse drug effects, deficiencies, aging, tobacco use, altered neurotransmitter levels
Neurotransmitters and Taste Threshold:
Effects of serotonin (5-HT) and norepinephrine (NE) on taste sensitivity
Supertasters:
25% of the population with heightened sensitivity to taste, especially bitterness
Increased number of fungiform papillae
Flu Vaccine Alert in Bangalore Karnatakaaddon Scans
As flu season approaches, health officials in Bangalore, Karnataka, are urging residents to get their flu vaccinations. The seasonal flu, while common, can lead to severe health complications, particularly for vulnerable populations such as young children, the elderly, and those with underlying health conditions.
Dr. Vidisha Kumari, a leading epidemiologist in Bangalore, emphasizes the importance of getting vaccinated. "The flu vaccine is our best defense against the influenza virus. It not only protects individuals but also helps prevent the spread of the virus in our communities," he says.
This year, the flu season is expected to coincide with a potential increase in other respiratory illnesses. The Karnataka Health Department has launched an awareness campaign highlighting the significance of flu vaccinations. They have set up multiple vaccination centers across Bangalore, making it convenient for residents to receive their shots.
To encourage widespread vaccination, the government is also collaborating with local schools, workplaces, and community centers to facilitate vaccination drives. Special attention is being given to ensuring that the vaccine is accessible to all, including marginalized communities who may have limited access to healthcare.
Residents are reminded that the flu vaccine is safe and effective. Common side effects are mild and may include soreness at the injection site, mild fever, or muscle aches. These side effects are generally short-lived and far less severe than the flu itself.
Healthcare providers are also stressing the importance of continuing COVID-19 precautions. Wearing masks, practicing good hand hygiene, and maintaining social distancing are still crucial, especially in crowded places.
Protect yourself and your loved ones by getting vaccinated. Together, we can help keep Bangalore healthy and safe this flu season. For more information on vaccination centers and schedules, residents can visit the Karnataka Health Department’s official website or follow their social media pages.
Stay informed, stay safe, and get your flu shot today!
Pulmonary Thromboembolism - etilogy, types, medical- Surgical and nursing man...VarunMahajani
Disruption of blood supply to lung alveoli due to blockage of one or more pulmonary blood vessels is called as Pulmonary thromboembolism. In this presentation we will discuss its causes, types and its management in depth.
These lecture slides, by Dr Sidra Arshad, offer a quick overview of physiological basis of a normal electrocardiogram.
Learning objectives:
1. Define an electrocardiogram (ECG) and electrocardiography
2. Describe how dipoles generated by the heart produce the waveforms of the ECG
3. Describe the components of a normal electrocardiogram of a typical bipolar leads (limb II)
4. Differentiate between intervals and segments
5. Enlist some common indications for obtaining an ECG
Study Resources:
1. Chapter 11, Guyton and Hall Textbook of Medical Physiology, 14th edition
2. Chapter 9, Human Physiology - From Cells to Systems, Lauralee Sherwood, 9th edition
3. Chapter 29, Ganong’s Review of Medical Physiology, 26th edition
4. Electrocardiogram, StatPearls - https://www.ncbi.nlm.nih.gov/books/NBK549803/
5. ECG in Medical Practice by ABM Abdullah, 4th edition
6. ECG Basics, http://www.nataliescasebook.com/tag/e-c-g-basics
ARTIFICIAL INTELLIGENCE IN HEALTHCARE.pdfAnujkumaranit
Artificial intelligence (AI) refers to the simulation of human intelligence processes by machines, especially computer systems. It encompasses tasks such as learning, reasoning, problem-solving, perception, and language understanding. AI technologies are revolutionizing various fields, from healthcare to finance, by enabling machines to perform tasks that typically require human intelligence.
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Ve...kevinkariuki227
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Verified Chapters 1 - 19, Complete Newest Version.pdf
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Verified Chapters 1 - 19, Complete Newest Version.pdf
- Video recording of this lecture in English language: https://youtu.be/lK81BzxMqdo
- Video recording of this lecture in Arabic language: https://youtu.be/Ve4P0COk9OI
- Link to download the book free: https://nephrotube.blogspot.com/p/nephrotube-nephrology-books.html
- Link to NephroTube website: www.NephroTube.com
- Link to NephroTube social media accounts: https://nephrotube.blogspot.com/p/join-nephrotube-on-social-media.html
Ozempic: Preoperative Management of Patients on GLP-1 Receptor Agonists Saeid Safari
Preoperative Management of Patients on GLP-1 Receptor Agonists like Ozempic and Semiglutide
ASA GUIDELINE
NYSORA Guideline
2 Case Reports of Gastric Ultrasound
Ocular injury ppt Upendra pal optometrist upums saifai etawah
Excitable tissues nerve
1. 1
EXCITABLE TISSUES
Cells and tissues in which excitation is accompanied by
action potential, distributed along the cellular membrane.
This is a property of the bodies of nerve cells and their processes—
nerve fibers, muscle fibers or cells, and some elongated plant cells
Nerves and muscles are studied together because both arc excitable
structures that conduct impulses. but the muscle contract
2. 2
NERVES
Neurons in the mammalian central nervous system come in
many different shapes and sizes.
3. 3
NERVES
Neurons in the mammalian central nervous system come in
many different shapes and sizes.
Perikayon (soma) - nerve cell body, contains nucleus and typical cell
organelles. It is the metabolic center of the neuron.
a. Nucleus - large, central in most, large amount of euchromatin (intense
synthetic activity), Barr body (Dormant X chromosome of females).
b. Rough endoplasmic reticulum (RER) - lots for synthesis of structural
and transport proteins, Nissl bodies are condensations of the RER and
free ribosomes.
c. Golgi apparatus - only found near nucleus in perikaryon. Expected,
since intense synthetic activity of neurotransmitters and/or
neurohormones.
4. 4
NERVES
Dendrite - cell process, may be branched, forms receptive area for
synaptic contacts from other neurons,
Has tiny rough projections or spines called gemmules that may be
points of synaptic contact,
Dendrites from larger neurons may be lightly myelinated by
oligodendroglia.
Neurons may have more than one dendrite.
Cytoplasm in these processes similar to that of perikaryon, but no
golgi bodies.
5. 5
NERVES
Axon – a single, long, cell process extending away from perikaryon,
may be branched,
Ends of branches form synapses with other neurons or muscle cells
May be myelinated by either oligodendroglia in CNS or Schwann cells
in PNS..
6. 6
NERVES
Each neuron has only one axon..
Axon hillock (pyramid shaped region where axon originates from the
perikaryon)
Initial segment (unmyelinated intitial portion of axon)
remainder of axon (may be myelinated or unmyelinated, may be
branched)
7. 7
NERVES
Axons carry electrical impulses (action potentials) to synapses at end
of axon.
Except for the axon hillock and the synaptic bouton, the axon
cytoplasm (axoplasm) has few organelles, microtubules, or
microfilaments.
Not much synthetic activity in this part of neuron. The synaptic button
granules or vesicles in which the synaptic transmitters secreted by the
nerves are stored.
The axon divides into presynaptic terminals, each ending in a number
of synaptic knobs which are also called terminal buttons or boutons
8. 8
NERVES
Based on the number of processes that emanate from their cell body,
neurons can be classified as unipolar, bipolar, and multipolar
Multipolar - more than two processes (one axon plus multiple
dendrites), most of neurons in brain and spinal cord are of this type
Bipolar - two major processes (axon and dendrite), but may be
branched at ends, sensory neurons in retina, cochlea, and olfactory
epithelium are of this type
9. 9
NERVES
Pseudounipolar - two major processes that are fused along portions
closet to perikaryon - found in spinal ganglia and some cranial ganglia.
Unipolar have one process, with different
segments serving as receptive surfaces and
releasing terminals.
10. 10
NEURON CLASSIFICATION BASED ON FUNCTION
Motor neurons - efferent, action potential moves from CNS to
effector organ (e.g. muscle)
Sensory neurons - afferent, action potential moves from sensory
organ to CNS (e.g. neuron processes associated with pacinian corpuscles,
touch, pressure)
Interneurons - form connections between neurons
11. 11
GLIAL CELLS
There are many more glial cells in the nervous system than there are
neurons.
These cells are situated among the neurons and are generally smaller.
In sections stained with hematoxylin - eosin, only the glial cell nuclei
show up.
Special staining techniques are necessary if their cell bodies are to be
easily differentiated from surrounding cells.
12. 12
GLIAL CELLS - Astrocytes
Two types:
1. Protoplasmic astrocytes:
a. Granular cytoplasm, many branches on short processes
b. Some of processes are closely applied to neurons, while others form
intimate contacts with blood vessels.
c. Thought to form a conduit for nutrients from blood vessels to neurons.
d. Found in gray matter.
e. Protoplasmic astrocytes have a membrane potential that varies with the
external K+ concentration but do not generate propagated potentials
f. Produce substances that are tropic to neurons, and they help maintain
the appropriate concentration of ions and neurotransmitters by taking up
K+ and the neurotransmitters glutamate and γ-aminobutyrate (GABA)
13. 13
GLIAL CELLS - Astrocytes
2. Fibrous astrocytes:
a. Contain many intermediate filaments, are found primarily in white
matter
b. Function not well understood
Both types send processes to blood vessels, where they induce
capillaries to form the tight junctions making up the blood–brain barrier.
14. 14
GLIAL CELLS - oligodendroglia
Smaller than astrocytes, fewer processes
In white matter, these cells form the myelin sheaths that are around
many axons, in gray mater they may lightly myelinate some dendrites
Also called oligodendrocytes
Found in both gray and white matter
Anaologous to Schwann cells of peripheral nervous system
These cells must be cultured with neurons in order to get neurons to
grow in tissue culture. Suggests intimate interactive association.
15. 15
GLIAL CELLS - microglia
Elongate nucleus with mostly heterochromatin
Small cell body that is elongated
Can be differentiated from other glia by elongate nucleus. Other glia
have a spherical nucleus
16. 16
GLIAL CELLS - ependymal cells
Ciliary action acts to circulate cerebral spinal fluid.
Ciliated cells forming single layer of cuboidal epithelium that lines the
entire neurocoel
Neurocoel is the cavity of the chordate cerebrospinal system,
consisting of the ventricles of the brain and the central canal of the spinal
cord, regarded as a unit.
17. 17
PROPERTIES OF NERVES
This is the ability of living tissues to respond to various stimuli.
It is an electric phenomenon, and the electric changes that accompany
nerve excitation are called the action potential.
Such changes are very small and very rapid. so their magnitudes are
measured in millivolts (mV) while their durations are measured in
milliseconds (msec).
They are recorded by microelectrodes connected to either a
galvanometer or a cathode ray oscilloscope (CRO ).
1. Excitability
18. 18
A stimulus is a change in the environment around the nerve (or
muscle)which may be either chemical, thermal, mechanical or electrical.
In laboratories, electrical stimuli are preferred because they can be
accurately controlled (both in strength and duration) and, in addition,
they leave the stimulated structures without damage.
2 types of electric currents can be used for stimulation of excitable
tissues :
a. The galvanic current: This is a constant (or direct) current (D.C.)
which is obtained from a battery.
b. The faradic current: This is an alternating current (A.C.) like the
induction currents used in laboratories for nerve stimulation.
The stimulus
19. 19
The physicochemical change produced by various stimuli in the nerve
is called the nerve impulse.
Such impulse is actively conducted along the nerve fibre and it can be
conducted in both directions.
In the body, each nerve conducts impulses in one direction only (motor
nerves toward the effector organs and sensory nerves toward the nervous'
system).
2. Conductivity
Conduction in the normal direction is called orthodromic conduction,
If it occurs in the opposite direction due to any cause, it will be called
antidromic cmuluction
Does antidromic conduction occur in the brain under normal conditions?
20. 20
States that "A threshold (minimal) stimulus produces a maximal
response " i.e. a maximal action potential in nerve and muscle fibres and
a maximal contraction in muscle fibres.
Therefore as long as other factors that affect excitability remain
constant:
Increasing the intensity of the stimulus above the threshold value
produces no further increase in the action potential or muscle contraction
(3) All or none law (or rule)
21. 21
The all or none law is obeyed in the following structures:
a. A single nerve fibre
- A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated
by that motor neuron's axonal terminals.
(3) All or none law (or rule)
c. The cardiac muscle and some smooth muscles which act as one unit
called syncytium
b. A single skeletal muscle fibre and the motor unit
- Groups of motor units often work together to coordinate the contractions of a single
muscle; all of the motor units within a muscle are considered a motor pool.
22. 22
However, nerve trunks and whole skeletal muscles (which contain
many fibres) do not obey the law.
(3) All or none law (or rule)
Why?
The threshold intensity for stimulation varies in the different types of
nerve and muscle fibres (i.e. it is not equal).
Therefore if the intensity of stimulation is increased in these structures,
the response will also increase till reaching a maximum.
23. 23
The nerve fibre adapts to stimulation by a constant current so no
response occurs during passage of the current.
(4) Accommodation (or adaptation)
(5) Infatiguability
Nerve fibres are not fatigued by continuous stimulation
24. 24
1. Intensity (strength) of the stimulus: Sub-threshold stimuli produce only
local responses that don't initiate action potentials .
Factors that determine the effectiveness of stimuli
If the intensity is increased slowly the nerve will not respond because of
the property of accommodation
2. Rate of increase in the intensity of stimuli; Sub-threshold stimuli
that are gradually increased produce a response only with a rapid
increase in the intensity of stimuli
25. 25
3. Duration of stimulus (duration of current): The relation between the
intensity of a stimulating current and the duration (time) of its flow
necessary to set up an impulse is shown in the strength-duration curve
Factors that determine the effectiveness of stimuli
26. 26
Within limits, there is a reciprocal relationship between the current
strength and duration of flow required to produce an impulse
There is a minimal duration needed for excitation below which no
excitation occurs whatever be the strength of the stimulus.
RHEOBASE: This is the minimal strength (or threshold intensity ) of a
galvanic current that can set up an impulse.
CHRONAXIE: This is the duration of current flow required for
excitation when using a strength equal to twice (or double) the rheobase.
The time required for excitation when using the rheobase is called the
utilization time
27. 27
What is the significance of the Chronaxie?
Its measurement can be used to compare the excitability of
different tissues, or that of the same tissue under different conditions.
The chronaxie is a good index for the degree of excitability (the shorter
the chronaxie the greater the excitability versa).
In the strength-duration curve to
the right, which nerve is more
excitable?
28. 28
The curve for the slower fibres would be shifted to the right, indicating
that for a given stimulus strength, a longer stimulus duration would be
needed to bring the slower fibres to threshold.
Q How would the strength-duration curve for a set of slow fibres (not
very excitable) compare to the strength-duration curve for a set of quick
fibres (very excitable)?
29. 29
RESTING MEMBRANE POTENTIAL REVIEW
Outside the cell membranes there are mainly Na+, Cl- and HCO3 while
inside the cell there are mainly K+ and organic protein anions
In neurons the potential difference about -70 mV
RM P is due to an unequal distribution of ions on both sides of the
membrane with relatively excess cations outside and excess anions inside
This is produced as a result or 2 main factors (a) Selective
permeability of cell membranes (b) The N a+/K+ ATPase
30. 30
RESTING MEMBRANE POTENTIAL REVIEW
(a) The electrical gradient is directed inwards
K+ ions tend to diffuse outside the cells. However, this is limited
because:
(b) The +ve charge on the outside of the membranes repels K+ ions
inwards.
(c) The sodium-potassium pump actively drives K+ ions inwards
The concentration gradient for Cl- and HCO3 is directed inwards, so
these anions tend to diffuse into the cells.
This is limited because the interior of the cells has a great -ve charge,
and accordingly, they are expelled out of the cell along this electrical
gradient
31. 31
CHANGES THAT ACCOMPANY PROPAGATION OF A NERVE
IMPULSE
The Action Potential (AP)
(1) ELECTRIC CHANGES
This refers to the changes in potential that occur in excitable tissues
when stimulated
It is transmitted as a self-propagated disturbance called impulse
Stimulating the nerve (by an electric stimulator) is marked by a
stimulus artifact, which is due to current leakage from the stimulating
electrode to the recording (external) electrode.
Latent period (which is an isopotcntial interval representing the time
taken by the impulse to reach the recording electrode) after which the AP
is recorded.
33. 33
The AP consists of 2 main stages (called depolarization and
repolarization)
This is followed by 2 other stages known as after-
depolarization and after-hyperpolarization.
34. 34
Depolarization (DP)
This is loss of the normal resting polarized state of the
membrane
It is recorded as a rise of the membrane potential in the
positive direction from -70 mV towards the isopotential line
(zero potential)
It produces the ascending limb of the A.P
DP develops slowly. but after an initial 15 mV of DP (i.e.
MP becomes about -55 mV). the rate of DP suddenly increases
(so this point is called the firing level)
35. 35
Depolarization (DP)
DP then proceeds rapidly till the resting membrane potential
is lost
The potential difference between both sides of the
membrane becomes zero
this change is called overshoot or reversal of polarity, and it
results in an A.P. having a magnitude of 105 mV (from -70 to
+35 mV)
The membrane potential the reaches +35m V (indicating
that the inner surface of the membrane becomes positive
relative to the outer surface.
36. 36
Repolarization (RP)
This is restoration or the normal resting polarized state of
the membrane
It is recorded as a fall of the membrane potential in the
negative direction from +35 mV to -70 mV.
RP proceeds immediately and rapidly after the overshoot is
reached
It produces the descending limb of the A.P.
When RP is 70-80% completed, its rate decreases for about
4 msec. This stage is called after-depolarization (or negative
after-potential)
37. 37
Repolarization (RP)
After RP is completed the membrane potential overshoots to
the negative side (by about 1-2 mV) leading to
hyperpolarization of the membrane
This stagc is called after-hyperpolarization (or positive
after-potential).
It lasts about 40 msec but its magnitude gradually declines
till the normal resting membrane potential is restored
38. 38
Ionic basis (or mechanisms) of DP and RP
The initial slow DP is produced by the stimulating current
itself . How?
Stimulating current arc cathodic in nature, which adds
negative charges outside the nerve membrane
Thus the potential difference between both sides of the
membrane is decreased
The membrane potential becomes less -vc than at the resting
state. This is called electrotonic DP.
Depolarization
39. 39
The rapid phase of DP and the overshoot are produced by an
increase in Na+ influx (= entrance) into the nerve fiber as a
result of increased Na+ conductance of the nerve membrane
Increased permeability occurs secondary to marked increase
in the Na+ permeability of the membrane through opening of
specific Na+ channels in the membrane.
Each Na+ channel has an activation gate at the outer surface
of the membrane and an inactivation gate at its inner surface.
The membrane Na+ conductance (and consequently the Na
influx ) is increased only when both gates are opened.
40. 40
In the resting state, only the inactivation gates are open so
the membrane permeability to Na+ is low
When the nerve is stimulated the Na+ activation gates also
open thus the membrane permeability and conductance to Na+
as well as Na+ influx are markedly increased
Opening of the Na+ activation gates is voltage-dependent.
They start to open when the initial electrotonic DP becomes
7 mV
This allows Na influx. which further decreases the
membrane polarity leading to opening of more gates.
41. 41
This results in more Na+ influx and more decrease of the
membrane polarity, which leads to opening of more and more
gates and more Na+ influx.
Such process continues in the form of a vicious circle
(i.e. by a positive feedback mechanism ) till all gates open
The rate of opening of the Na+ activation gates is slow
between 7 and I5 mV of DP (i.e. till a membrane potential of
about -55 mV) leading to slow DP.
After15 mV of DP, the rate of opening of these gates
suddenly increases leading to much acceleration of DP (the
firing level)
42. 42
(a) Closure of the Na+ inactivation gates
(b) Reversal ofthe direction of eleclrical gradient for Na+
(2) K+ efflux (exit) from the nerve fibre:
Repolarization (RP) and after-potentials
RP of the membrane takes place rapidly after DP as a result of:
(1) Stoppage of Na+ influx due to:
This occurs through specific K+ channels that contain a
single gate located toward the inside of the membrane.
The decrease in membrane polarity during DP leads to
opening of the K+ gates, thus the K+ conductance is markedly
increased and K+ ef1ux occurs.
43. 43
The negative after-potential stage is clue to slowing of the
rate of K+ efflux.
The positive after-potential stage is due to slow return of the
K+ channels to the closed state (which allows prolonged K+
efflux)
The process is however slower than opening of the Na+
channels so the increase in K+ conductance is slightly delayed
The slow opening and delayed closure of the K+ channels
may explain the phenomenon of accommodation that occurs in
nerves
Following the AP, the resting Na+ and K+ ionic gradients is
restored by the action of the Na+/K+ pump.
44. 44
Normally, both the concentration and electrical gradients for
Ca2+are directed inwards
The Ca2+ conductance increases during nerve excitation
leading to Ca2+ influx.
Role of Ca2+ in Nerve excitation
Ca2+ contributes to DP (and in some invertebrates. it is
primarily responsible for the AP)
Ca2+ influx occurs via voltage-gated Ca2+ channels which
are also slightly permeable to Na+ (so they arc called Ca2+-
Na+ channels)
45. 45
They are however very slow to become activated, so they
are also called slow channels (in contrast to the voltage-gated
Na+ channels which arc called fast channels)
Role of Ca2+ in Nerve excitation
The extracellular Ca2+ concentration also affects nerve
excitability
Its decrease increases the excitability while its increase
decreases the excitability and stabilizes the nerve membrane
46. 46
ETPs are localized potential changes that occur in nerves
when stimulated by sub-threshold constant currents
Electrotonic potentials (ETPs) and the local response
There are 2 types of the ETPs both of which are passive
changes in the membrane polarization
This is produced by addition of subtraction of charges
through the stimulating current) that decay (i.e. disappear)
gradually
47. 47
This is the potential change that occurs when using anodal
(+ve) currents for stimulation
(A) Anelectrotonic potential (or anelectrotonus)
It is a state of hyperpolarization caused by addition of +ve
charges at the outer surface of the nerve membrane.
The magnitude of the potential change is proportionate to
the strength of the stimulus
It is associated with a decrease of excitability of the nerve
and with strong anodal currents, the nerve excitability may be
completely lost ( anodal block)
It takes the membrane potential away from firing level
(which inhibits discharge of impulses).
48. 48
This is the potential change that occurs when using cathodal
(-ve) currents for stimulation
(A) Catelectrotonic potential (or catelectrotonus)
It is a state of partial DP caused by addition of –ve charges
at the outer surface of the nerve membrane.
The magnitude of the potential change varies with the
strength of the stimulating current
It is associated with an increase of excitability of the nerve,
50. 50
(A) In unmyelinated nerve fibres
Nerve impulses are propagated along unmyelinated nerve
fibres in the form of a wave of APs
Local circular currents flow between the activated point and
the neighbouring inactive areas of the nerve membrane
The initial stimulus causes reversal of polarity and an AP at
the point of stimulation
+ve charges from the inactive areas flow into the initial area
of negativity produced by the AP (= area of current sink )
51. 51
This decreases the polarity at the inactive areas (
electrotonic depolarization) which produces an AP initiating
on reaching the firing level.
The latter areas, in turn, electrotonically depolarize the
membrane in front of it through local circular currents and this
sequence of events moves regularly along the nerve fibre to its
end.
the nerve impulse is self-propagated and once it leaves a
point this point will soon repolarize
thus a repolarization wave starts after the depolarization
wave and is propagated in the same direction
52. 52
(B) In myelinated nerve fibres
Nerve impulses are propagated along myelinated nerve
fibres by a mechanism called the saltatory conduction
Myelin surrounds the nerve axon and is interrupted at
regular intervals at the nodes of Ranvier
It is an insulator to current flow (in contrast to the nodes of
Ranvier which easily permit current flow because of their high
permeability to Na)
Circular currents also flow in myelinated nerve fibres but
the +ve charges jump from the inactive nodes to the area of
current sink at the active node
53. 53
This leads to electrotonic depolarization and production of
an AP at the inactive nodes, which in turn activates the
neighbouring nodes.
This jumping of DP from node to node is called saltatory
conduction and it results in:
(a) Increasing velocity of conduction
(b) Conservation of energy (because excitation occurs only in
the nodes and not allover the nerve membrane)
54. 54
(2) EXCITABILITY CHANGES
During propagation of a nerve impulse (i.e. during an AP).
The excitability of nerve fibres passes in the following phases
(a) Absolute refractory period (ARP):
During this period, the nerve is completely inexcitable.
No stimulus can excite it whatever its strength
It corresponds to the ascending limb of the AP from the time
the firing level is reached
That is during DP and overshoot and upper part of the
descending limb (until RP is about 1/3 complete)
55. 55
(b) Relative refractory period (RRP):
During this period, nerve excitability is partiality recovered
Stronger stimuli than normal are required for excitation
It corresponds to the remaining part of the descending limb
of the AP till the start of after-depolarization
That is during the later 2/3 of RP
56. 56
(c) Supernormal phase (or period):
During this period nerve excitability is increased.
Weaker stimuli than normal can excite the nerve
It corresponds to the period of after-depolarization
57. 57
(d) Subrnormal phase (or period):
During this period nerve excitability is decreased.
Stronger stimuli than normal are required for excitation
It corresponds to the period of afler-hyperpolarization.
59. 59
Factors that affect nerve excitability
(1) Temperature: Cooling decreases nerve excitability while
warming increases it
(2) Pressure: Mechanical pressure on a nerve reduces its
excitability.
(3) Blood supply: Nerve excitability is decreased in cases of
ischemia.
(4) Oxygen supply: O2 lack decreases nerve excitability.
(5) H+ concentration: Alkalinity increases while acidity
decreases excitability of nerves.
60. 60
(6) Chemicals: Nerve excitability is decreased by excess CO2
and alcohol as well as by anesthetic drugs e.g. ether,
chloroform and cocaine.
(8) Electrolytes: The concentrations of Na+ K+ and Ca2+ in
the extracellular fluid affect nerve excitability as follows
61. 61
Ionic changes that increase nerve excitability
1- Decreased Ca2+ concentration: This increases the
membrane permeability to Na+ and decreases the amount of
DP necessary to initiate the changes in Na+ and K+
conductances that produce the AP.
2- Increased Na concentration: This facilitates the process of
DP
3- Increased K+ concentration: This favours K influx which
leads to DP thus the nerve excitability will increase
62. 62
Ionic changes that decrease nerve excitability
1- Increased Ca2+ concentration : This decreases the
membrane permeability to Na+ and increases the amount of
DP necessary to initiate the changes in Na+ and K+
conductances that produce the AP
2-Decreased Na concentration : This decreases nerve
excitability by delaying the process of DP
3- Decreased K+ concentration : This favours K+ efflux
which leads to hyperpolarization, thus the nerve excitability
will decrease
63. 63
Methods of producing nerve block
Propagation of impulses by nerves can be blocked (i.e.
prevented) by one of the following methods :
(1) Physical methods e.g. application of cold or a strong
anelectrotonus
(2) Mechanical methods (e.g. application of pressure on the
nerve).
(3) Chemical methods: These include:
(a) The ionic changes that decrease nerve excitability i.e.
increased Ca2+ and decreased Na+ or K+ concentration in the
extracellular fluid
64. 64
(b) Use of certain chemical substances known as the
membrane stabilizers which include mainly the local
anesthetic drugs (e.g. cocaine and novocaine)
These drugs markedly decrease the membrane
permeability to Na+ (by preventing opening of the Na+
channel activation gales)
So the depolarization process is inhibited and nerve
impulses fail to be produced
65. 65
(3) METABOLIC CHANGES
The metabolic processes that occur in nerves are generally
similar to those occurring in muscles (O2 and ATP)
In resting nerves, they occur at a low rate (mainly to
maintain the polarized state) but they are much increased
during transmission of nerve impulses
The breakdown of ATP supplies the energy required for the
sodium potassium pump and nerve impulse propagation
It is resynthesized by energy derived from breakdown of
glycogen.
Nerve fibres are also rich in vitamin B1 which is necessary
for their metabolic activities
66. 66
(4) THERMAL CHANGES
Resting nerves liberate little heat as a result of their low
metabolic rate.
During nerve impulse transmission the heat production by
nerves is markedly increased, and is liberated in the following
2 stages :
(1) Initial heat: This coincides with propagation of spike (i.e.
during ionic migration), and it is due to anaerobic breakdown
of ATP
(2) Recovery (delayed) heat: This follows propagation of the
spike, and it is due to aerobic reactions that liberate the energy
required for ATP resynthesis.
67. 67
Types of mammalian nerve fibres
The mammalian nerve fibres are classified according to their
diameters into the following types:
( 1) Group A nerve fibres :
These have the largest diameters (3-20 microns)
The have highest speeds of conduction ( 15-120 m/sec)
They are further subdivided into alpha, beta, gamma & delta
nerve fibres
E.g. the somatic nerve fibres that transmit motor impulses &
deep sensations
They are most sensitive to pressure (i.e. the conduction of
impulses in these nerves can be readily blocked by pressure)
68. 68
( 2) Group B nerve fibres :
These have the smaller diameters (1.3 - 3 microns)
They have moderate speeds of conduction (3-15 m/sec)
E.g. the myelinated preganglionic autonomic nerves
They arc most sensitive to O2 lack
69. 69
( 3) Group C nerve fibres :
These have the smallest diameters (0.3-1.3 microns)
The have slowest speeds of conduction (0.5-3 m/sec)
E.g. the unmyelinated postganglionic autonomic nerves
They arc most sensitive to local anaesthetics
71. 71
The area of contact between a nerve fibre and a muscle fibre
is called the motor end plate (M.E.P. ) or neuromuscular
junction
As the nerve approaches the muscle, it loses its myelin
sheath and its axon is branched.
The tip of each branch (sole foot ) is covered by the
neurolemma which continues with the sarcolemma ((the outer
membrane of the n1uscle fibre)
Each sole foot lies in a depression in the plasma membrane
(the inner membrane of the muscle fibre) called the synaptic
gutter
73. 73
Where the membrane is thrown into many folds called the
palisades
There is no protoplasmic continuity between the nerve and
muscle fibres.
They are separated by the synaptic cleft in which the
chemical transmitter acetylcholine is released from the
vesicles present in the sole feet.
The sarcoplasm at the motor end plate is granular and
contains the cholinesterase enzyme which hydrolyzes
acetylcholine.
74. 74
Miniature end plate potential ( M. E.P.P.)
This is a state of persistent localized subthreshold DP (0.5
mV in amplitude) of the muscle fibres at the M.E.P. during
rest.
It does not reach the firing level (so it does not lead to an
AP)
It is due to continuous release of small amounts of
acetylcholine
Arising from the continuous rupture of a few vesicles that
contain this chemical transmitter at the nerve terminals
75. 75
Mechanism of neuromuscular transmission
When a motor nerve is stimulated, the generated impulse is
propagated toward the M.E.P
it causes movement of'Ca2+ from the extracellular fluid into
the nerve terminals
Ca2+ cause rupture of acetylcholine vesicles leading to
release (exocytosis) of this transmitter into the synaptic cleft
Acetylcholine then combines with nicotinic receptors in the
muscle membrane
76. 76
This increases its permeability to Na+ through opening of
ligand-gated Na+ channel.
Na+ influx increases resulting in DP of the muscle at the
M.E.P.. which is called the end plate potential
This normally reaches the firing level which starts an A P
along the surface of the muscle
The released acetylcholine is rapidly hydrolyzed by the
cholinesterase enzyme so that the re-excitation of the muscle
wouldn’t occur.
78. 78
Properties of neuromuscular transmission
1. At the M.E.P. impulses are conducted in one direction only
2. Transmission of impulses at the M. E.P. is delayed 0.5- 0. 7
millisecond due to the time required for:
(a) Release of acetylcholine
(b) Passage of acetylcholine across the synaptic cleft and its
combination with the nicotinic receptors in the muscle
(c) Increase in the permeability of the muscle membrane to Na
(d) Increase in Na+ influx till the firing level
3. Transmission of impulses at the M.E.P. readily fatigues after
repeated stimulations (mostly due to exhaustion of Ach)
4. Transmission of impulses at the M.E.P. is readily affected
by drugs
79. 79
Drugs that affect neuromuscular transmission
(A) Drugs that block neuromuscular transmission
1. Curare (tubocurarine) competitively inhibits Ach interaction
at the Nm receptors. α- neurotoxins (α- bungarotoxin, α-
cobrotoxins) irreversibly blocks.
2. Botulinum toxin: From Clostridium botulinum. The toxin
inhibits the release of ACh
80. 80
(B) Drugs that stimulate neuromuscular transmission
1. Acetylcholine-like drugs that are not rapidly hydrolyzed by
the cholinesterase enzyme and act mainly at the M. E.P. e.g.
carbacol.
2. Drugs that preserve the liberated acetylcholine at the M.E.P.
by antagonizing the cholinesterase enzyme e.g. prostigmine
and eserine
Editor's Notes
+256786014374
+256786014374
Electrotonic potential (or graded potential) — a non-propagated local potential, resulting from a local change in ionic conductance (e.g. synaptic or sensory that engenders a local current). When it spreads along a stretch of membrane, it becomes exponentially smaller (decrement).