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Nervous System
By
Mrs. B. Srividya (Asst. Prof)
RCPHS, Berhampur
Contents
Introduction
Organization of nervous system
Neuron and its classification
Neuroglia
Properties of nerve fibre
Electrophysiology, nerve impulse, action
potential
Receptors
Synapse
Neurotransmitters
Nervous System
(Introduction)
• Nervous system is the most important
organization which control and integrates the
different body functions and maintains the
consistency of internal environment.
• A network of billions of nerve cells linked
together in a highly organized fashion to form
the rapid control center of the body
Functions of Nervous System
Depending on the type of nerve impulse and its interpretation,
functions of the nervous system may be of three types:
1. Sensory functions: These may be either conscious or
unconscious. When conscious they are called „sensations‟. In
the autonomic nervous system, they are usually unconscious.
2. Motor functions: These may be of two types:
▫ Reflex or involuntary and
▫ Voluntary: In the autonomic nervous system, all motor
effects are reflex. In the central nervous system, motor
effects are both reflex and voluntary.
3. Associated functions: For instance, idea, memory,
intelligence, etc. These are carried out mainly by the
cerebrum.
NERVES
• A nerve consists of numerous neurones collected
into bundles (bundles of nerve fibers in the CNS
known as tract). Each bundle has several coverings
of connective tissue:
1. Endonerium- surrounding the individual fiber.
2. Perineurium- smooth connective tissue
surrounding each bundle of fibers.
3. Epineurium- is the fibrous tissue which
surrounds and encloses a number of bundles of
nerve fibers .
Sensory or afferent nerves
• Sensory nerves carry information from the body to
the spinal cord.
• And then information pass to brain or to connector
neuron of reflex arc in the spinal cord.
Sensory receptors
• Specialized endings respond to different stimuli inside
and outside the body
1. Somatic, cutaneous or common senses- these
originate in the skin, they are pain, touch, heat and
cold , sensory nerve ending in the skin are fine
branching filaments without myelin sheaths. When
stimulated an impulse is generated and transmitted by
the sensory nerves to the brain where sensation is
perceived.
Sensory receptors
2. Proprioceptor senses- these originates in the
muscles and joints and contribute to the maintenance
of balance and posture.
3. Special senses- These are sight, hearing, balance,
smell and taste.
4. Autonomic afferent nerves- These originates in
the internal organs, glands, and tissues e.g.-
baroreceptors involved in control of blood pressure.,
chemoreceptors involved in control of respiration .
Motor or efferent nerves
• Motor nerves originates in the brain, spinal cord and
autonomic ganglia.
• They transmit impulses to the effector organs ; muscles
and glands there are two types
1. Somatic nerve- Involved in involuntary and reflex
skeletal muscle contraction.
2. Autonomic nerves-(Sympathetic or
parasympathetic) involved in cardiac and smooth
muscle contraction and glandular secretions.
MIXED NERVES
• In the spinal cord, sensory and motor nerves are
arranged in separate groups or tracts.
• Outside the spinal cord, when sensory and motor nerves
are enclosed within same sheath of connective tissue
they are called mixed nerves.
Organization of nervous system
Nerve Tissue
Nerve Tissue Consists of two types of cells:
1. Neurons/Nerve cells: Structural and functional
unit of nervous system that generates and
transmit nerve impulse.
2. Neuroglia/Glial cells: Supporting cells
Neuron
Cell body/soma:
• The following structures are found in the cell body:
a) nucleus: large, spherical
b) neuroplasm: a cytoplasmic matrix containing nissl
bodies(participate in conduction of nerve impulses)
and neurofibrils (fine filaments).
c) mitochondria, golgi apparatus, ribosome,
endoplasmic reticulum, etc.
• Cell body forms the grey matter, in periphery of
brain, in centre of spinal cord.
Neuron
Axon:
• It is a process arises from cell body that carries impulse
away from it.
• The term nerve fiber usually refers to the axons.
• It arises from axon hillock of the cell body.
• It is generally long with few branches (axon terminals).
• Axon is single but constant. If a neuron has only one
process, it will be the axon.
Axolemma Axon hillock
Myelin
sheath
Nodes of
Ranvier
Schwann
cells
Myelinated neuron
• Large axon & those of peripheral nerves are surrounded
by a myelin sheath.
• This consists of series of Schwann cells arranged along
with the length of axon.
• Each one is wrapped around the axon so that it is
covered by number of concentric layers of Schwann cells
plasma membrane.
• Between these layers of plasma membrane there is a
small amount of fatty substance called myelin.
Myelinated neuron
Myelinated neuron
• The outermost layer of Schwann cell plasma
membrane is the neurilemma.
• There are tiny areas of exposed axolemma between
adjacent Schwann cells, nodes of Ranvier, which
assist the rapid transmission of nerve impulses in
Myelinated neurons.
Non- Myelinated neurons
• Postganglionic fibers and some small fibers in the
CNS are non-Myelinated.
• In this type a number of axons are embedded in
Schwann cells are in close association and there is no
exposed axolemma.
• The speed of transmission of nerve impulses is slower.
Neuron
Dendrites:
• It is the process that carries impulse towards the
cell body.
• It collects impulses from other neurons and
carries them towards the cell body.
• It is generally short with many branches.
• Number varies from nil to numerous.
• They are the part of receptor membrane of the
neuron.
A)
Neuroglia/Glial
cells
• These are the non-excitable supporting
connective tissues present in nervous system,
called neuroglia or glial cells.
• There are two major types of glial cells in the
vertebrate nervous system: -
▫ Microglia
▫ Macroglia
Neuroglia/Glial cells
• Macroglia: There are 3 types of macroglia:
1. Astrocytes
2. Oligodendrocytes
3. Schwann cells
• Microglia: Specialized immune cells that act as
the macrophages of the CNS
Macroglia:
1. Astrocytes
• Main supporting tissue of CNS.
• Star shaped with fine branching .
• At the free end of some of the processes are small swellings
called foot processes.
• Found in large number adjacent to blood vessels with their
foot processes forming a sleeve around them.
• This means blood is separated from the neurones by the
capillary wall and a layer of foot processes together
constitute the blood brain barrier.
• Fibrous astrocytes, which contain many intermediate
filaments, are found primarily in white matter.
• Protoplasmic astrocytes are found in gray matter and have
a granular cytoplasm.
Astrocytes in cerebral cortex
Functions
• Supporting network in brain.
• Form the blood brain barrier (BBB).
• Maintain chemical environment of extra cellular fluid.
• Recycles neurotransmitters.
Blood brain barrier
A. Longitudinal section
B. Transverse section
Blood brain barrier
• That protect the brain from potentially harmful
toxic substances and chemical variations in the
blood e.g. after a meal.
• These cells are smaller than astrocytes and
• Found in clusters round nerve cell bodies in grey matter,
where they are thought to have a supportive function.
• They are found adjacent to, and along the length of,
myelinated nerve fibres.
• They form and maintain myelin.
Macroglia:
2. Oligodendrocytes
• Schwann cells are involved in myelin formation
around axons in the peripheral nervous system.
Macroglia:
3. Schwann cells
Microglia
• The smallest and least numerous glial cells.
• Derived from monocytes that migrate from the blood
into the nervous system before birth.
• They are found mainly in the area of blood vessels.
• They enlarge and become phagocytic, removing
microbes and damaged tissue, in areas of inflammation
and cell destruction.
Ependymal cells
• These cells form the epithelial lining of the ventricles of
the brain and the central canal of the spinal cord.
• Those cells that form the choroid plexusess (a network
of blood vessels that secret CSF in ventricles) of the
ventricles secrete cerebrospinal fluid.
Satellite Cells
• Satellite cells are flat cells that surround the cell bodies of
neurons in the PNS.
• They appear to enclose and support the cell bodies, and
have intertwined processes that link them with other
parts of the neuron, other satellite cells, and also
neighboring Schwann cells.
Nerve fibers
• A nerve fiber is a threadlike extension of a
nerve cell and consists of an axon
(microfilament + microtubule) and myelin
sheath (if present) in the nervous system.
Properties of nerve fibers
1. Excitability: the nerve can be stimulated by a
suitable stimulus, which may be: -
 mechanical
 thermal
 chemical
 electrical
• After excitation, nerve impulse is generated through
depolarisation, repolarisation and hyperpolarisation.
• Excitability depends upon the following factors:
a) Strength of stimulus
b) Duration of stimulus
c) Direction of the current
d) Frequency of stimulus Injury
Properties of nerve fibers
2. Conductivity:
 Impulse is propagated along a nerve in both directions [but under
normal conditions the nerve impulse travels in one direction only;
in the motor nerve towards the responding organ, in sensory nerve
toward the center].
 The nerve impulse is propagated with a definite speed. The
conduction velocity depends upon the diameter of the nerve fibers,
the thicker fibers showing higher velocity. The velocity also depends
on the myelination and on temperature.
3. All-or-none law:
▫ If the stimulus be adequate, a single nerve will always give a
maximum response.
▫ If the strength or duration of the stimulus be further increased,
no alteration in the response will take place.
Properties of nerve fibers
4. Refractory period:
 When the nerve fiber is once excited, it will not respond to
a second stimulus for a brief period. This period is called
refractory period.
4. Summation:
 In a nerve fiber summation of two submaximal stimuli is
possible.
4. Adaptation:
 The nerve fiber quickly adapts itself. Due to this adaptation
there is no excitation during the passage of a constant
current.
 Only when the strength of the current is suddenly altered
or the current is made or broken excitation takes place.
Properties of nerve fibers
7. Accommodation: -
 If a stimulus even in stronger strength is applied very
slowly to a nerve, then there may have no response
only due to lack of attaining the threshold strength.
This phenomenon is called accommodation.
8. Indefatigability: -
 In the nerve muscle preparation, if the nerve is
stimulated repeatedly, then after a certain period the
muscle fails to give any response but nerve is not
fatigued.
NERVE PHYSIOLOGY AND THE NERVE
IMPULSES (ACTION POTENTIAL)
The nerve cells are excitable cells.
There is a different electrical charge on each side of the
membrane, which is known as membrane potential.
Any stimulus will change the membrane potential and
cause an action potential to generate.
The synchronized opening and closing of Na+ and K+
gates result in the movement of electrical charges that
generates a nerve impulse or action potential.
Action potentials reach the end of each neuron where
these electrical signals are either transmitted directly to
the next cell in the sequence via gap junctions, or are
responsible for activating the release of specialized
neurotransmitter chemicals.
THE NERVE IMPULSES (ACTION
POTENTIAL)
• Impulses is initiated by stimulation of sensory nerve
ending or by passage of an impulse from another nerve.
• Transmission of impulse or action potential is due to
movement of ion across the nerve cell membrane.
• In resulting state nerve cell membrane is polarised due
to difference in concentration of ions across plasma
membrane.
• This is called RESTING MEMBRANE POTENTIAL.
• Na+ main extracellular cation.
• K+ main intracellular cation.
THE NERVE IMPULSE (ACTION
POTENTIAL)
When stimulated
the permeability of
nerve cell
membrane
Na+ floods into
neurone from
extracellular fluid
causing
depolarisation
Creation of nerve
impulse or action
potential
Conduction of
nerve impulse in
one direction only
K+ flood out of
neurone
Return of
membrane
potential to
resting state
Action of sodium
potassium pump
expels Na+ from
cell in exchange
for k+
Steps involved
1. Closed Sodium and Potassium channels (Resting
potential)
2. Membrane depolarization and sodium channel
activation (Depolarization)
3. Sodium channel inactivation (Repolarization)
4. Potassium channel activation (Hyperpolarization)
5. Return to normal permeability (Resting potential)
2. DEPOLARIZATION
• Na+ floods into neuron from the extracellular fluid to
intracellular fluids causing depolarization, creating a
nerve impulse or action potential.
• It is very rapid enabling the conduction of a nerve
impulse along the entire length of a neuron in a few
milliseconds.
• It passes from area of stimulation to resting potential, it
takes time for repolarization to occur.
3. Repolarization
• As the membrane potential approaches +50 mV,
voltage gated potassium channels open and positively
charged potassium ions begin to flow out of the cell.
• This begins to repolarise, the cell by reducing the excess
internal positive charge and moving the membrane
potential closer to the resting potential.
• At this point the cell is basically impermeable to sodium
and very permeable to potassium which rapidly flows
out of the cell.
4. Hyperpolarization
• Potassium efflux (exiting) continues past the resting
potential of -70 mV due to the slow closing voltage gated
potassium channels.
• This causes a hyperpolarisation known as undershoot
which takes the membrane potential to around -75mV.
5. Return to normal permeability/resting
state (Resting potential)
• Soon afterward the cell returns to resting potential via
the standard membrane proteins.
• Refractory periods prevent the backward movement of
action potentials and limit the rate of firing.
Action potential conduction
Continuous conduction
Saltatory conduction
• An action potential at one node of Ranvier causes
inwards currents that move down the action,
depolarizing the membrane and stimulating a new action
potential at the next node of Ranvier.
Saltatory conduction
• Myelin sheath reduces membrane capacitance and
increases membrane resistance in the inter-node
intervals, thus allowing a fast, saltatory movement of
action potentials from node to node.
• Myelin prevents ions from entering or leaving the axon
along myelinated segments.
• As a general rule, myelination increases the conduction
velocity of action potentials and makes them more
energy-efficient.
Receptors (According to the type of
binding with neurotransmitter)
• Nearly all neurotransmitters induce postsynaptic
potentials by binding to their receptors in the
postsynaptic membrane.
• The type of receptor to which a neurotransmitter binds
determines the postsynaptic response.
• Two types of neurotransmitter receptors have been
identified:
1. Ionotropic receptors
2. Metabotropic receptors
1. Ionotropic receptors
• Are simply receptors that are part of ligand-gated ion
channels.
• They are called ionotropic because they directly
control the movement of ions into or out of the neuron
when bound by a neurotransmitter.
• Neurotransmitters that bind ionotropic receptors have
very rapid but short-lived effects on the membrane
potential of the postsynaptic neuron.
2. Metabotropic receptors
• Are receptors within the plasma membrane that are
connected to a separate ion channel.
• They are called metabotropic because they are directly
connected to metabolic processes that begin when they
are bound by neurotransmitters.
• Most are connected through a group of intracellular
enzymes called G-proteins.
2. Metabotropic receptors
When the
neurotransmitter
molecule (“first
messenger”) binds
to the receptor
activates one or
more G-proteins
followed by
enzyme-
catalyzed
reactions
formation of a
second
messenger
inside the
postsynaptic
neuron
The second messenger
(Eg; cyclic adenosine
monophosphate (or
cAMP)opens or closes
an ion channel
Causes slow changes in
the membrane potential
of the postsynaptic
neuron, but are
typically longer-lasting.
THE SYNAPSE
• There is always more than one neuron is involved in the
transmission of a nerve impulses from its origin to its
destination, whether its sensory or motor.
• No physical contact between these neurones.
• Synapse is the region where communication occurs
between 2 neurons or between a neuron and a target
cell.
THE SYNAPSE
• At its free ends the axon of the presynaptic neurone breaks up
into minute branches that terminate into small swellings
called synaptic knobs or terminal buttons.
• These are in close proximity to the dendrites and the cell body
of postsynaptic neuron.
• These chemicals are synthesized by nerve cells, actively
transported along the axons and stored in vesicles.
• They are released by exocytosis in response to action potential
and diffuse across the synaptic cleft.
• They act on the specific receptor site on the postsynaptic
membrane. There action is short lived as they act on effector
organ or neurone, or taken up by synaptic knob.
THE SYNAPSE
• Synapse has the following elements:
• Presynaptic terminal: It contains synaptic vesicles encapsulated
around the neurotransmitter substance.
• Synaptic cleft: It refers to the 20 nm wide synaptic gap, which
separates the two adjacent neurons.
• Postsynaptic terminal: It possesses receptor sites for the
binding of neurotransmitters, which can either inhibit or promote
the passage of nerve signal from one cell to the next.
THANK YOU

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central nervous system

  • 1. Nervous System By Mrs. B. Srividya (Asst. Prof) RCPHS, Berhampur
  • 2. Contents Introduction Organization of nervous system Neuron and its classification Neuroglia Properties of nerve fibre Electrophysiology, nerve impulse, action potential Receptors Synapse Neurotransmitters
  • 3. Nervous System (Introduction) • Nervous system is the most important organization which control and integrates the different body functions and maintains the consistency of internal environment. • A network of billions of nerve cells linked together in a highly organized fashion to form the rapid control center of the body
  • 4. Functions of Nervous System Depending on the type of nerve impulse and its interpretation, functions of the nervous system may be of three types: 1. Sensory functions: These may be either conscious or unconscious. When conscious they are called „sensations‟. In the autonomic nervous system, they are usually unconscious. 2. Motor functions: These may be of two types: ▫ Reflex or involuntary and ▫ Voluntary: In the autonomic nervous system, all motor effects are reflex. In the central nervous system, motor effects are both reflex and voluntary. 3. Associated functions: For instance, idea, memory, intelligence, etc. These are carried out mainly by the cerebrum.
  • 5. NERVES • A nerve consists of numerous neurones collected into bundles (bundles of nerve fibers in the CNS known as tract). Each bundle has several coverings of connective tissue: 1. Endonerium- surrounding the individual fiber. 2. Perineurium- smooth connective tissue surrounding each bundle of fibers. 3. Epineurium- is the fibrous tissue which surrounds and encloses a number of bundles of nerve fibers .
  • 6.
  • 7. Sensory or afferent nerves • Sensory nerves carry information from the body to the spinal cord. • And then information pass to brain or to connector neuron of reflex arc in the spinal cord.
  • 8.
  • 9. Sensory receptors • Specialized endings respond to different stimuli inside and outside the body 1. Somatic, cutaneous or common senses- these originate in the skin, they are pain, touch, heat and cold , sensory nerve ending in the skin are fine branching filaments without myelin sheaths. When stimulated an impulse is generated and transmitted by the sensory nerves to the brain where sensation is perceived.
  • 10. Sensory receptors 2. Proprioceptor senses- these originates in the muscles and joints and contribute to the maintenance of balance and posture. 3. Special senses- These are sight, hearing, balance, smell and taste. 4. Autonomic afferent nerves- These originates in the internal organs, glands, and tissues e.g.- baroreceptors involved in control of blood pressure., chemoreceptors involved in control of respiration .
  • 11. Motor or efferent nerves • Motor nerves originates in the brain, spinal cord and autonomic ganglia. • They transmit impulses to the effector organs ; muscles and glands there are two types 1. Somatic nerve- Involved in involuntary and reflex skeletal muscle contraction. 2. Autonomic nerves-(Sympathetic or parasympathetic) involved in cardiac and smooth muscle contraction and glandular secretions.
  • 12. MIXED NERVES • In the spinal cord, sensory and motor nerves are arranged in separate groups or tracts. • Outside the spinal cord, when sensory and motor nerves are enclosed within same sheath of connective tissue they are called mixed nerves.
  • 14.
  • 15.
  • 16.
  • 17.
  • 18. Nerve Tissue Nerve Tissue Consists of two types of cells: 1. Neurons/Nerve cells: Structural and functional unit of nervous system that generates and transmit nerve impulse. 2. Neuroglia/Glial cells: Supporting cells
  • 19.
  • 20.
  • 21. Neuron Cell body/soma: • The following structures are found in the cell body: a) nucleus: large, spherical b) neuroplasm: a cytoplasmic matrix containing nissl bodies(participate in conduction of nerve impulses) and neurofibrils (fine filaments). c) mitochondria, golgi apparatus, ribosome, endoplasmic reticulum, etc. • Cell body forms the grey matter, in periphery of brain, in centre of spinal cord.
  • 22.
  • 23. Neuron Axon: • It is a process arises from cell body that carries impulse away from it. • The term nerve fiber usually refers to the axons. • It arises from axon hillock of the cell body. • It is generally long with few branches (axon terminals). • Axon is single but constant. If a neuron has only one process, it will be the axon. Axolemma Axon hillock Myelin sheath Nodes of Ranvier Schwann cells
  • 24.
  • 25. Myelinated neuron • Large axon & those of peripheral nerves are surrounded by a myelin sheath. • This consists of series of Schwann cells arranged along with the length of axon. • Each one is wrapped around the axon so that it is covered by number of concentric layers of Schwann cells plasma membrane. • Between these layers of plasma membrane there is a small amount of fatty substance called myelin.
  • 27. Myelinated neuron • The outermost layer of Schwann cell plasma membrane is the neurilemma. • There are tiny areas of exposed axolemma between adjacent Schwann cells, nodes of Ranvier, which assist the rapid transmission of nerve impulses in Myelinated neurons.
  • 28. Non- Myelinated neurons • Postganglionic fibers and some small fibers in the CNS are non-Myelinated. • In this type a number of axons are embedded in Schwann cells are in close association and there is no exposed axolemma. • The speed of transmission of nerve impulses is slower.
  • 29. Neuron Dendrites: • It is the process that carries impulse towards the cell body. • It collects impulses from other neurons and carries them towards the cell body. • It is generally short with many branches. • Number varies from nil to numerous. • They are the part of receptor membrane of the neuron.
  • 30. A)
  • 31.
  • 32.
  • 33. Neuroglia/Glial cells • These are the non-excitable supporting connective tissues present in nervous system, called neuroglia or glial cells. • There are two major types of glial cells in the vertebrate nervous system: - ▫ Microglia ▫ Macroglia
  • 34. Neuroglia/Glial cells • Macroglia: There are 3 types of macroglia: 1. Astrocytes 2. Oligodendrocytes 3. Schwann cells • Microglia: Specialized immune cells that act as the macrophages of the CNS
  • 35. Macroglia: 1. Astrocytes • Main supporting tissue of CNS. • Star shaped with fine branching . • At the free end of some of the processes are small swellings called foot processes. • Found in large number adjacent to blood vessels with their foot processes forming a sleeve around them. • This means blood is separated from the neurones by the capillary wall and a layer of foot processes together constitute the blood brain barrier. • Fibrous astrocytes, which contain many intermediate filaments, are found primarily in white matter. • Protoplasmic astrocytes are found in gray matter and have a granular cytoplasm. Astrocytes in cerebral cortex
  • 36. Functions • Supporting network in brain. • Form the blood brain barrier (BBB). • Maintain chemical environment of extra cellular fluid. • Recycles neurotransmitters. Blood brain barrier A. Longitudinal section B. Transverse section
  • 37. Blood brain barrier • That protect the brain from potentially harmful toxic substances and chemical variations in the blood e.g. after a meal.
  • 38. • These cells are smaller than astrocytes and • Found in clusters round nerve cell bodies in grey matter, where they are thought to have a supportive function. • They are found adjacent to, and along the length of, myelinated nerve fibres. • They form and maintain myelin. Macroglia: 2. Oligodendrocytes
  • 39. • Schwann cells are involved in myelin formation around axons in the peripheral nervous system. Macroglia: 3. Schwann cells
  • 40. Microglia • The smallest and least numerous glial cells. • Derived from monocytes that migrate from the blood into the nervous system before birth. • They are found mainly in the area of blood vessels. • They enlarge and become phagocytic, removing microbes and damaged tissue, in areas of inflammation and cell destruction.
  • 41. Ependymal cells • These cells form the epithelial lining of the ventricles of the brain and the central canal of the spinal cord. • Those cells that form the choroid plexusess (a network of blood vessels that secret CSF in ventricles) of the ventricles secrete cerebrospinal fluid.
  • 42. Satellite Cells • Satellite cells are flat cells that surround the cell bodies of neurons in the PNS. • They appear to enclose and support the cell bodies, and have intertwined processes that link them with other parts of the neuron, other satellite cells, and also neighboring Schwann cells.
  • 43. Nerve fibers • A nerve fiber is a threadlike extension of a nerve cell and consists of an axon (microfilament + microtubule) and myelin sheath (if present) in the nervous system.
  • 44. Properties of nerve fibers 1. Excitability: the nerve can be stimulated by a suitable stimulus, which may be: -  mechanical  thermal  chemical  electrical • After excitation, nerve impulse is generated through depolarisation, repolarisation and hyperpolarisation. • Excitability depends upon the following factors: a) Strength of stimulus b) Duration of stimulus c) Direction of the current d) Frequency of stimulus Injury
  • 45. Properties of nerve fibers 2. Conductivity:  Impulse is propagated along a nerve in both directions [but under normal conditions the nerve impulse travels in one direction only; in the motor nerve towards the responding organ, in sensory nerve toward the center].  The nerve impulse is propagated with a definite speed. The conduction velocity depends upon the diameter of the nerve fibers, the thicker fibers showing higher velocity. The velocity also depends on the myelination and on temperature. 3. All-or-none law: ▫ If the stimulus be adequate, a single nerve will always give a maximum response. ▫ If the strength or duration of the stimulus be further increased, no alteration in the response will take place.
  • 46. Properties of nerve fibers 4. Refractory period:  When the nerve fiber is once excited, it will not respond to a second stimulus for a brief period. This period is called refractory period. 4. Summation:  In a nerve fiber summation of two submaximal stimuli is possible. 4. Adaptation:  The nerve fiber quickly adapts itself. Due to this adaptation there is no excitation during the passage of a constant current.  Only when the strength of the current is suddenly altered or the current is made or broken excitation takes place.
  • 47. Properties of nerve fibers 7. Accommodation: -  If a stimulus even in stronger strength is applied very slowly to a nerve, then there may have no response only due to lack of attaining the threshold strength. This phenomenon is called accommodation. 8. Indefatigability: -  In the nerve muscle preparation, if the nerve is stimulated repeatedly, then after a certain period the muscle fails to give any response but nerve is not fatigued.
  • 48. NERVE PHYSIOLOGY AND THE NERVE IMPULSES (ACTION POTENTIAL) The nerve cells are excitable cells. There is a different electrical charge on each side of the membrane, which is known as membrane potential. Any stimulus will change the membrane potential and cause an action potential to generate. The synchronized opening and closing of Na+ and K+ gates result in the movement of electrical charges that generates a nerve impulse or action potential. Action potentials reach the end of each neuron where these electrical signals are either transmitted directly to the next cell in the sequence via gap junctions, or are responsible for activating the release of specialized neurotransmitter chemicals.
  • 49. THE NERVE IMPULSES (ACTION POTENTIAL) • Impulses is initiated by stimulation of sensory nerve ending or by passage of an impulse from another nerve. • Transmission of impulse or action potential is due to movement of ion across the nerve cell membrane. • In resulting state nerve cell membrane is polarised due to difference in concentration of ions across plasma membrane. • This is called RESTING MEMBRANE POTENTIAL. • Na+ main extracellular cation. • K+ main intracellular cation.
  • 50. THE NERVE IMPULSE (ACTION POTENTIAL) When stimulated the permeability of nerve cell membrane Na+ floods into neurone from extracellular fluid causing depolarisation Creation of nerve impulse or action potential Conduction of nerve impulse in one direction only K+ flood out of neurone Return of membrane potential to resting state Action of sodium potassium pump expels Na+ from cell in exchange for k+
  • 51.
  • 52. Steps involved 1. Closed Sodium and Potassium channels (Resting potential) 2. Membrane depolarization and sodium channel activation (Depolarization) 3. Sodium channel inactivation (Repolarization) 4. Potassium channel activation (Hyperpolarization) 5. Return to normal permeability (Resting potential)
  • 53.
  • 54.
  • 55. 2. DEPOLARIZATION • Na+ floods into neuron from the extracellular fluid to intracellular fluids causing depolarization, creating a nerve impulse or action potential. • It is very rapid enabling the conduction of a nerve impulse along the entire length of a neuron in a few milliseconds. • It passes from area of stimulation to resting potential, it takes time for repolarization to occur.
  • 56. 3. Repolarization • As the membrane potential approaches +50 mV, voltage gated potassium channels open and positively charged potassium ions begin to flow out of the cell. • This begins to repolarise, the cell by reducing the excess internal positive charge and moving the membrane potential closer to the resting potential. • At this point the cell is basically impermeable to sodium and very permeable to potassium which rapidly flows out of the cell.
  • 57. 4. Hyperpolarization • Potassium efflux (exiting) continues past the resting potential of -70 mV due to the slow closing voltage gated potassium channels. • This causes a hyperpolarisation known as undershoot which takes the membrane potential to around -75mV. 5. Return to normal permeability/resting state (Resting potential) • Soon afterward the cell returns to resting potential via the standard membrane proteins.
  • 58.
  • 59.
  • 60. • Refractory periods prevent the backward movement of action potentials and limit the rate of firing.
  • 63. Saltatory conduction • An action potential at one node of Ranvier causes inwards currents that move down the action, depolarizing the membrane and stimulating a new action potential at the next node of Ranvier.
  • 64. Saltatory conduction • Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node. • Myelin prevents ions from entering or leaving the axon along myelinated segments. • As a general rule, myelination increases the conduction velocity of action potentials and makes them more energy-efficient.
  • 65.
  • 66.
  • 67.
  • 68.
  • 69.
  • 70. Receptors (According to the type of binding with neurotransmitter) • Nearly all neurotransmitters induce postsynaptic potentials by binding to their receptors in the postsynaptic membrane. • The type of receptor to which a neurotransmitter binds determines the postsynaptic response. • Two types of neurotransmitter receptors have been identified: 1. Ionotropic receptors 2. Metabotropic receptors
  • 71. 1. Ionotropic receptors • Are simply receptors that are part of ligand-gated ion channels. • They are called ionotropic because they directly control the movement of ions into or out of the neuron when bound by a neurotransmitter. • Neurotransmitters that bind ionotropic receptors have very rapid but short-lived effects on the membrane potential of the postsynaptic neuron.
  • 72. 2. Metabotropic receptors • Are receptors within the plasma membrane that are connected to a separate ion channel. • They are called metabotropic because they are directly connected to metabolic processes that begin when they are bound by neurotransmitters. • Most are connected through a group of intracellular enzymes called G-proteins.
  • 73. 2. Metabotropic receptors When the neurotransmitter molecule (“first messenger”) binds to the receptor activates one or more G-proteins followed by enzyme- catalyzed reactions formation of a second messenger inside the postsynaptic neuron The second messenger (Eg; cyclic adenosine monophosphate (or cAMP)opens or closes an ion channel Causes slow changes in the membrane potential of the postsynaptic neuron, but are typically longer-lasting.
  • 74. THE SYNAPSE • There is always more than one neuron is involved in the transmission of a nerve impulses from its origin to its destination, whether its sensory or motor. • No physical contact between these neurones. • Synapse is the region where communication occurs between 2 neurons or between a neuron and a target cell.
  • 75. THE SYNAPSE • At its free ends the axon of the presynaptic neurone breaks up into minute branches that terminate into small swellings called synaptic knobs or terminal buttons. • These are in close proximity to the dendrites and the cell body of postsynaptic neuron. • These chemicals are synthesized by nerve cells, actively transported along the axons and stored in vesicles. • They are released by exocytosis in response to action potential and diffuse across the synaptic cleft. • They act on the specific receptor site on the postsynaptic membrane. There action is short lived as they act on effector organ or neurone, or taken up by synaptic knob.
  • 76.
  • 77. THE SYNAPSE • Synapse has the following elements: • Presynaptic terminal: It contains synaptic vesicles encapsulated around the neurotransmitter substance. • Synaptic cleft: It refers to the 20 nm wide synaptic gap, which separates the two adjacent neurons. • Postsynaptic terminal: It possesses receptor sites for the binding of neurotransmitters, which can either inhibit or promote the passage of nerve signal from one cell to the next.
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