Nerve Physiology

NERVE PHYSIOLOGY
DR SARAN AJAY
DEPT OF PHYSIOLOGY, GMCM

DEPT OF PHYSIOLOGY, GMCM 10
What is an excitable tissue?

Excitable Tissue
• Property of responding to a stimulus is called
excitability or irritability
• A sudden and appreciable change in environment
of living matter, lasting for a minimum time
constitute a stimulus
• Capable of generation of electrical impulses at
their membranes

• These impulses are used to transmit signals along the
membranes
• Mainly 2 excitable tissues – Nerve & muscle

• Nerves - specialized for function of reception,
integration and transmission of information in the body
• Muscles - also excitable tissues but are characterized
by mechanical contraction

CNS : Central Nervous System
PNS : Peripheral Nervous System
NERVOUS SYSTEM

Cellular elements of CNS
• Neurons - basic building blocks
• 1011 neurons
• Also contain Glial cells
• 40% of human genes participate in the formation of CNS

STRUCTURE
OF NEURON

Cell body/ Soma/ Perikaryon
1. Round , stellate, pyramidal, fusiform
2. Nucleus with nucleoli
3. No centrosome
4. Cytoplasm – mitochondria, ER, Golgi apparatus, Nissl
granules

5. Nissl granules
• Basophilic granules
• Contains rough ER
• Involved in protein synthesis
• Absent from axon & axon hillock
• Chromatolysis - disintegrate and finally disappear

7. Neurofibrils
• Microfilaments and Microtubules
• Alzheimer's disease: microfilament proteins →
neurofibrillary tangles

8. Pigment granules
• Neuromelanin- in substantia nigra
• Lipofuscin- aging neurons

Site of cell bodies
• Gray matter of CNS
• Autonomic and posterior nerve root ganglia
Group of cell bodies
• Inside the CNS- Nucleus
• Outside CNS- Ganglia

Dendrites
• Small multiple branched processes
• Short course, irregular number
• Contains Nissl bodies
• Thorny projections – Dendritic spines
• Impulses travel towards the cell body

Axon
• Single long process
• Arise from Axon hillock
• Initial segment – first part of axon
• Axolemma and axoplasm

• Axon divides into presynaptic terminals, which end in
• Synaptic knobs/ terminal buttons/ axon telodendrion
• Carries impulses away from cell body
• Myelinated and unmyelinated nerve fibers

MYELINATION

Myelination in PNS

• Schwann cell - wraps its membrane around an axon up to
100 times
• Axons invaginate into cytoplasm of Schwann cell
• So becomes suspended in a fold of Schwann cell
membrane - mesaxon

• Lipids are deposited between the layers of Schwann cell
• Compacted to form myelin sheath – by Protein zero( P0)
• Extracellular portions of this membrane proteins on the
apposing membranes lock to each other
• Mutations in the gene for P0 → Peripheral neuropathy
• Outermost cell membrane → Neurilemma

• Each Schwann cell provide myelin sheath for a short
segment of axon
• Between 2 segments - 1µm constrictions – Nodes of
Ranvier
• Myelin is absent at axonal endings

Functions of Myelin
1. Increases velocity of conduction: Saltatory conduction
2. Reduces energy expenditure for conduction
3. Acts as an insulator – prevents cross stimulation

In unmyelinated nerves,
• Axon is surrounded by Schwann cell without wrapping of
membrane

Myelination in CNS
• Oligodendrocytes form myelin
• They extend multiple processes and form myelin on many
axons

Myelinogenesis
• Begins at 4th month of intra uterine life
• Sensory fibers – 4th-5th month
• Pyramidal tract (motor fibers) – 2 years

1. Babinski Test in children – Extensor Response
2. Mutations in gene for Protein0 → Peripheral
neuropathies
Applied Aspects

2. Multiple sclerosis - demyelinating disease
• Autoimmune
• Antibodies and WBCs attack myelin →
inflammation & injury to the sheath → eventually
the nerves are affected

• Loss of myelin → leakage of K+ through voltage-gated
channels → hyperpolarization → failure to conduct
impulses
• Nerve conduction tests show slowed conduction in motor
& sensory pathways
• Numbness and weakness
• Optic neuritis

NERVE PHYSIOLOGY- 2
DR SARAN AJAY

Specific Learning Objectives
• Functional Zones of a Neuron
• Classification of Neuron
• Transport and Metabolism
• Peripheral Nerve
• Nerve Injury and Degeneration
• Neuroglia
• Neurotropins

FUNCTIONAL
ZONES OF
A NEURON

Functional Zones of a Neuron
1. Receptor zone- dendritic zone- local potentials
2. Site of origin of conducted impulses
• Motor neuron – initial segment
• Sensory – 1st node of Ranvier
3. Zone of all or none transmission- axon
4. Zone of secretion of neurotransmitters

Granules or vesicles – neurotransmitter
Termination
• Synapse
• Effecter organ (muscle or gland)
• Peripheral ganglion

Classification of neuron
1. Depending upon number of poles/ structural classification
2. Depending upon the function
3. Depending upon length of axon

A. Depending upon number of poles
1. Unipolar neuron
• Single pole
• Both dendrites and axon arise from that

2. Bipolar neuron
2 poles- one for axon, one for dendrite

3. Pseudo unipolar neuron
• A subclass of bipolar cells
• A single process splits into two, both of which function as axons
• Peripheral one going to skin or muscle and central one to spinal
cord
• Eg: Dorsal root ganglion (DRG)

4. Multi polar neuron
• One pole- axon
• Other poles- dendrites
Q. KUHS AUGUST 2015

B. Depending on function
1. Sensory /afferent neuron
2. Motor / efferent neuron
3. Interneuron

C. Depending upon length of axon
Golgi type 1- also known as projection neurons
• Have long axons
• Neurons forming peripheral nerves & long tracts of brain
Golgi type 2- short circuit neuron
• Short axons
• Numerous in cerebral cortex, cerebellar cortex, retina

Biological activities in a neuron
• Protein synthesis & transport
• Metabolism
• Excitation and conduction of impulses

Protein synthesis & Transport
• Soma is the machinery for protein synthesis
• Neurotransmitters and other proteins that are needed at the
axon terminals
• Transport of proteins and polypeptides to axonal ending
occurs by axoplasmic flow

Axoplasmic flow/ transport
1. Anterograde transport: from cell body → axon terminal
• Fast axonal transport- 400 mm/day
• Slow axonal transport- 0.5-10 mm/day

• Occurs along microtubules
• Requires 2 molecular motors
Dynein & Kinesin

Substances transported are
• Fast Axonal transport
1. Mitochondria and other membrane bound organelles
2. Small Vesicles
3. Actin and Myosin
• Slow Axonal Transport
1. Tubulins of microtubules
2. Protein subunits of neurofilaments

2. Retrograde transport
• From axon terminal → cell body
• 200mm/ day
• Occurs along microtubules

Substances transported are
1. Used synaptic vesicles,
2. Nerve growth factors (NGF)
3. Viruses like polio, herpes simplex and rabies
4. Toxins e.g.: tetanus toxin

Metabolism
• Low level
• 70% total energy is used to maintain the polarity of membrane
• By action of Na+- K+ ATPase
• At maximum activity– metabolic rate doubles

Peripheral Nerve
• Compact bundle of axons located outside CNS – Nerve
• Axons are arranged in different bundles – Fasciculi
• Each axon is covered by Endoneurium
• Each fasciculus is covered by Perineurium
• Whole nerve is covered by Epineurium

NERVE INJURY -
DEGENERATION
REGENERATION

Response to injury
Types of injury
• Cut injury
• Crush injury
• Ischemia
• Injection of toxic substance
• Diseases like DM, syphilis
• Pressure on the nerve

A. Physical and chemical degenerative changes
1. Axon distal to the injury (anterograde degeneration)
2. Proximal to the injury
3. Cell body
(retrograde degeneration)

1. Anterograde degeneration (Wallerian)
• Axon distal to the injury
• Augustus Waller
Q. KUHS 2022 JULY

a) Changes in impulse conduction
b) Changes in the axon
c) Changes in the myelin sheath
d) Changes in the Neurilemma – Ghost tube

a. Changes in impulse conduction
1. If severed - stops within hours
2. If crushed
• Up to 3 days normal conduction
• After 3 days deteriorates seriously
• After 5 days no impulse evoked

b. Changes in Axon
1. Swells up
2. Irregular in shape
3. Breaks into small fragments
4. Axon terminals retract from post synaptic target
5. Within several weeks axon and its terminals degenerate

c. Myelin Sheath
1. Small fragments
2. Hydrolysis of myelin to fatty droplets
3. Begins by 8th day and completed by 32 days

d. Neurilemma
1. Intact
2. Schwann cells – increase size and multiply
3. Macrophages remove degenerating axons, myelin
and cellular debris
4. Neurilemmal tube becomes empty – Ghost tube

2. Retrograde degeneration
• Proximal to injury
• Similar to changes seen in distal part of axon
• Occurs only up to first or second node of Ranvier
• Changes seen in proximal part of axon along with those
in cell body constitute retrograde degeneration

3. Changes in cell body
• Starts within 48hr → continues up to 15-20 days
• Cell organelles are fragmented and eventually disappear
• Cell body draws in more fluid, swells up
• Nissl substances undergo Chromatolysis
• Nucleus is displaced to periphery and extruded out of cell

B. Regeneration
Under favorable conditions
• Neurilemma present
• Cell body & nucleus are intact
• Gap less than 3mm
• Sliced cut, cut ends in straight line

• Starts within 4 days of injury
• More active after 30 days
• Complete recovery- several months to one year
• 1 – 4 mm per day

Changes in the axon
• From proximal stump axon elongates and give out fibrils
• Schwann cells of distal end guide the fibril to enter the endo
neural tube
• Fibril makes contact with the peripheral end organ
• Myelination – completed in 1 year
• 80% of original diameter is regained

Changes in the cell body
• 20-80d
• Nissl granules followed by Golgi apparatus appear in
cell body
• Cell loses excess water, regains its normal size
• Nucleus occupies the central position

Complication following nerve injury
1. Complete atrophy
2. Functional complications- misinterpretation of
sensations
3. Neuroma formation- if fibrils could not find distal cut
end, it will form a mass

4. Phantom Limb
• Feeling perceived by a patient after amputation of limb
• Neuroma when stimulated, patient feel the lost limb

NERVE PHYSIOLOGY- 2b
DR SARAN AJAY

Glial cells/ Neuroglia
• Supporting cells in nervous system
• 2 types: Microglia & Macroglia
• Microglia arise from macrophages
• Act as scavenger cells – remove debris resulting from
injury, infection, and diseases

• Macroglia: Oligodendrocytes, Schwann cells, & Astrocytes
• Oligodendrocytes
• In white matter provide myelin
• In gray matter support neurons
• Schwann cells provide myelin to PNS
• Astrocytes most common glia in the CNS, star like shape

Astrocytes
Astrocytes help to regulate the microenvironment in CNS
under normal conditions and also in response to damage.

• Fibrous
• contain intermediate filaments
• found in white matter
• Protoplasmic – found in gray matter

Functions
• Astrocytes send processes to blood vessels → induce
capillaries to form tight junctions → Blood–brain barrier
• Also send processes that envelop synapses and surface
of nerve cells
• Produce neurotropins

• Help to maintain concentration of ions and neurotransmitters
• Regulate microenvironment of neuron
• Membrane potential of protoplasmic astrocyte varies with
external K+

Neurotrophins
• Proteins necessary for survival and growth of neurons
• Site of production- muscles, structures innervated by
neuron, astrocytes, neurons
• Retrograde transport and Anterograde transport

Nerve growth factor (NGF)
• First neurotrophin identified
• Sympathetic nerves, sensory nerves
• Trk A Receptor
Brain derived neurotrophic factor (BDNF)
• Trk B
• Peripheral sensory nerves

Neurotrophin -3
• Trk B and Trk C
• Cutaneous mechanoreceptors
• Proprioceptor neurons that innervate muscle spindle

NERVE PHYSIOLOGY- 3
DR SARAN AJAY
DEPT. OF PHYSIOLOGY, GMCM

Hodgkin and Huxley
Nobel Prize in Physiology
and Medicine 1963

• Membrane Potential
• Resting Membrane Potential
• Genesis of RMP
• Electrotonic Potentials
DEPT. OF PHYSIOLOGY, GMCM 7

MEMBRANE
POTENTIAL

• The potential difference across the membrane of all living
cells
• Results from the separation of charges across the cell
membrane

+

• Note that the membrane itself is not charged.
• The term membrane potential refers to the difference in charge
between the wafer-thin regions of ICF and ECF lying next to the
inside and outside of the membrane, respectively.

1. Transport processes
2. Signaling purposes

• The energy stored in this miniature battery can drive a variety of
transmembrane transport processes.
• Electrically excitable cells such as brain neurons and heart
myocytes also use this energy for signalling purposes. The brief
electrical impulses produced by such cells are called action
potentials.

More the separation of charges, more is the membrane potential.

• The inside is negative with respect to the outside
• Varies from cell to cell
• Varies with functional status

• The cells of excitable tissues—namely, nerve cells and muscle
cells—have the ability to produce rapid, transient changes in their
membrane potential when excited.
• These brief fluctuations in potential serve as electrical signals or
action potentials.

• Genesis of RMP

• It is the potential difference across the membrane while the
cell is at rest.
Neuron : - 70mV
Skeletal & cardiac muscle : - 90mV
RBC : - 10mV
• Transmembrane / Steady state potential / Diffusion potential
Resting Membrane Potential

• The constant membrane potential present in the cells of non excitable
tissues and those of excitable tissues when they are at rest—that is,
when they are not producing electrical signals—is known as the
resting membrane potential.

Recording of RMP

• Microelectrodes
• Amplifier
• Voltmeter
• Cathode Ray Oscilloscope (CRO) for rapid changes
Equipment

• Genesis of RMP

Genesis of RMP
• Due to the unequal distribution of ions across the cell
membrane (concentration gradient)
• Their selective movement through the cell membrane

The ions primarily responsible for RMP
1. Na+
2. K+
3. A-
A- = Large, negatively charged (anionic) intracellular proteins

FACTORS involved
in Genesis of RMP
5

“The phenomenon of predictable and unequal
distribution of permeant charged ions on either
side of a semipermeable membrane, in the presence
of impermeant charged ions”
1. The Gibbs-Donnan effect

Non
permeable
anion

“The phenomenon of predictable and unequal
distribution of permeant charged ions on either
side of a semipermeable membrane, in the presence
of impermeant charged ions”
The Gibbs-Donnan effect

When 2 ionized solutions are separated by a semipermeable
membrane, at equilibrium
1. Each solution will be electrically neutral
2. (Diffusible cations)A X (Diffusible anions)A =
(Diffusible cations)B X (Diffusible anions)B

• Diffusible cations (K+) more on the side of non-diffusible
anions (proteins) : ICF
• Less diffusible cations (Na+) more on the side of diffusible
anions (Cl-) : ECF

Na+, Cl-
K+
K+ , anions
Na+ ,Cl-
ECF
ICF

Ions ECF mmol/L ICF mmol/L
Na+ 150 15
K+ 5.5 150
Cl- 125 9
Proteins 0 65
Distribution of ions

2. Selective Permeability of the cell membrane
• Separates ECF & ICF

• Cytoplasm has diffusible & non-diffusible ions
• Membrane is freely permeable to K+ and Cl-
• Moderately permeable to Na+
• Impermeable to intracellular proteins and organic
phosphates (negatively charged ions)

Permeability to K+
• Potassium sodium leak channels
• About 50-100 times than that for
Na+
• Hydrated K+ ion is smaller

• K+ efflux - not compensated by Na influx
• K+ efflux is not accompanied by negatively charged
anions
• More negative ions remain inside the cell → creates
negativity inside the cell

Asymmetric distribution of ions
↓
Concentration & Electrical gradients
↓
Diffusion

K+ as the only diffusible ion

ECF 5.5 mmol/L of H2O
ICF 150 mmol/L of H2O
C
G
E
G

• Equilibrium is reached
• Driving forces down conc. & electrical gradients are
equal and opposite
• No net movement

3. Nernst Equation

Nernst potential/ Equilibrium potential for K+
Ek = -61.5 log [Ki
+ ] = - 90mV (37 degC)
[Ko
+]
[Ko
+] = K+ concentration outside the cell
[Ki
+] = K+ concentration inside the cell

K+ as the only diffusible ion

1. Polarity/ electrical charge
2. Permeability of membrane to each
3. Concentration inside & outside
When multiple ions are involved,
Membrane potential depends on

4. Goldman Hodgkin – Katz equation
• With this equation RMP calculated is –67 mV

• Greater the permeability of an ion, greater is the tendency
for that ion to drive the membrane potential towards its own
equilibrium potential

5. Na+ - K+ ATPase
• Steady ion leaks cannot continue forever without
eventually dissipating the ion gradients
• This is prevented by the Na+-K+ ATPase
• It actively moves 3 Na+ out and 2 K+ in against their
electrochemical gradients

Jens Skou
Nobel Prize in Physiology and Medicine 1997

• Results in continuous loss of positive charges from inside
the membrane
• Creates an additional degree of negativity
• So it is electrogenic and contributes to RMP about – 4mV
• Helps to maintain osmotic balance.

1. Donnan effect & Gibbs Donnan Equilibrium
2. Selective permeability of cell membrane to ions
3. Nernst Equation
4. Goldman Hodgkin Katz Equation
5. Sodium- potassium ATPase pump
Factors involved in genesis of RMP

Factors affecting RMP
1. ECF K+ concentration
ECF K+ ↑: magnitude of RMP ↓ → neuron becomes more
excitable
2. Inhibition of Na+ K+ ATPase
3. ECF Na+: no effect

• If the extracellular level of K + is increased (hyperkalemia), the resting
potential moves closer to the threshold for eliciting an action
potential, thus the neuron becomes more excitable.
• If the extracellular level of K + is decreased (hypokalemia), the
membrane potential is reduced and the neuron is hyperpolarized.

NERVE PHYSIOLOGY- 4
DR SARAN AJAY

Electrical Properties of a Neuron
1. Excitability
2. Conductivity

Excitability
• Stimulus
• Types
• Strength/ intensity
Threshold stimulus
Subthreshold stimulus
Suprathreshold stimulus

Application of stimulus will produce
1. Local non-propagated potentials
Electrotonic potentials
2. Propagated potentials
Action potential/ nerve impulse

• Electrotonic Potential
• Action Potential
• Ionic Basis of Action Potential
• Graded Potential v/s Action Potential

Electrotonic Potentials
ELECTROTONIC
POTENTIALS

• Demonstrated by placing recording electrodes within a
few millimeters of a stimulating electrode
• Applying subthreshold stimuli of fixed duration
Recording of electrotonic potential

• Leads to a localized potential change
• Rises sharply and decays exponentially with time
• Graded response can be produced both at cathode and
anode
1. Catelectrotonic potential
2. Anelectrotonic potential

Graded Potential – Examples
1. End plate potential
2. Post synaptic potential
3. Generator or receptor potential

• Brief, rapid, large depolarizing changes in membrane
potential
• Transiently, resting negative potential → positive potential
• Conducted, propagated along the nerve fiber
• Non decremental fashion
• Nerve signals are transmitted by action potentials

How is an action potential recorded?
1. Stimulating Electrodes (min - threshold stimuli)
2. Recording Electrodes
3. Amplifier
4. Cathode Ray Oscilloscope

1. Resting stage
2. Depolarization
3. Repolarization
Stages of Action Potential

1. RMP –70mV (polarized state)
2. Stimulus artifact
3. Latent period (Isoelectric potential interval)
4. Depolarization (reverse polarization)
5. Firing level
6. Spike potential
7. After depolarization
8. After hyperpolarization

Marked changes in membrane permeability and
ion movement lead to action potential.

NERVE PHYSIOLOGY- 5
DR SARAN AJAY

• Properties of Action Potential
• Types of Action Potential

IONIC BASIS OF
ACTION POTENTIAL

1. Stimulus artifact – Current leakage from stimulating
to recording electrodes
2. Latent Period – Time taken by the impulse to travel
along the axon
• Distance between electrodes
• Velocity of conduction

3. Depolarization
Threshold stimulus
↓
Initial slow depolarization (a change of 7–15mV)
(opening of few voltage gated Na+ channels)

Voltage Gated Sodium Channel

Threshold stimulus
↓
Initial slow depolarization (a change of 7–15mV)
(opening of few voltage gated Na+ channels)
↓
Rapid depolarization (at –55mV) Firing Level
(opening of more and more voltage gated Na+ channels)

Feedback Control in Voltage Gated Ion Channels / Hodgkin Cycle

Firing Level
• At -55mV more Na channels opens → gush of Na+ into
cell (500 – 5000 fold)

• Membrane potential moves towards equilibrium potential
for Na+ : +61 mV
• But membrane potential does not reach to +61 mV
because Na+ conductance is short lived.

1. After few 10,000th of a second inactivation gate closed
→ Na+ influx ↓
• Inactivation gate will remain closed till (or near) reaching
RMP again.
4. Repolarization

2. Direction of electrical gradient of Na+ is reversed during
overshoot
• Leads to decrease in Na+ influx
3. Opening of voltage gated K+ channel also contributes to
repolarization

Voltage Gated Potassium Channel

• Only 1 gate inside, resting state gate is closed
• As membrane potential rises -70 to 0 → slow
conformational change in the K+ gate occurs →
opening of gate
• So repolarization is due to both ↑ K+ efflux and ↓ Na+
influx

Feedback Control in Voltage Gated Ion Channels

5. After depolarization due to slowing of K+ efflux
6. After hyperpolarization due to slow return of K+
channel to closed state

7. Re-establishing Na+ and K+ ionic gradients
• Transmission of action potential slightly reduces
concentration differences of Na+ and K+ inside and
outside the membrane

• Achieved by action of the Na+ - K+ pump
• This pump requires energy derived from ATP
• So “recharging” of the nerve fiber is an active
metabolic process.

Jens Skou
Nobel Prize in Physiology and Medicine 1997

• Decrease in ECF calcium increase excitability
• By decreasing the amount of depolarization necessary
for opening of Na+ channels
• Increase in ECF calcium levels decreases excitability
and stabilizes the membrane
Effect of Ca2+ on excitability

PROPERTIES OF
ACTION POTENTIAL
30

Properties of Action Potential
1. Threshold stimulus
2. All or none response
3. Propagation of action potential
4. Refractory period

• The stimulus that brings the membrane potential to the
firing level (–55 mV) is known as threshold stimulus.
• Threshold stimulus is defined as the lowest strength of
stimulus that elicits an action potential.
1. Threshold Stimulus

Strength Duration Curve

• Rheobase : minimum intensity of stimulus which
if applied for adequate time (utilization time)
produces a response.
• Chronaxie: minimum duration for which stimulus
of double the rheobase intensity must be applied to
produce a response

• Within physiological limits, chronaxie of a given excitable tissue
is constant.
• Gives an idea about the excitability of a tissue. The lesser the
chronaxie, greater is the excitability.
• Nerves have a shorter chronaxie compared to muscles.

Slowly rising currents fail to induce an action potential in
the nerve because the nerve undergoes adaptation.

2. All or None Response
• Action potential fails to occur if the stimulus is
subthreshold in magnitude.
• It occurs with constant amplitude and shape
regardless of the strength of stimulus, if stimulus is at
or above threshold intensity.

2. All or None Response
• Action potential fails to occur if the stimulus is
subthreshold in magnitude
• It occurs with constant amplitude and shape
regardless of the strength of stimulus, if stimulus is
at or above threshold intensity
Why?

• After the threshold level is achieved, the amount of sodium
influx becomes independent of the stimulus factor.
• The activation gates of voltage-gated Na+ channels open as
soon as -55mV is reached.

3. Propagation of Action Potential (AP)
• AP elicited at any point on an excitable membrane
excites adjacent portions of the membrane.
• Results in propagation of AP along membrane.

1. Excited portion rapidly develops ↑ permeability to
Na+ and influx begins
In unmyelinated axon

2. Positive charges from membrane ahead of and behind
AP flow into area of negativity - Current Sink
3. Charges are carried by inward-diffusing Na+ ions
through depolarized membrane for several millimeters
in both directions.

4. These positive charges increase the voltage to levels
above the threshold voltage for initiating an action
potential
5. So Na+ channels in these new areas immediately
open and the explosive action potential spreads

Saltatory conduction (Myelinated Axon)
1. Myelin is an effective insulator, and current flow
through it is negligible
2. Depolarization travels from one node of Ranvier to
the next

3. Current sink at active node depolarize node ahead of
the action potential to firing level
4. This “jumping” of depolarization from node to node
is called saltatory conduction
5. So conduction up to 50 times faster than the fastest
unmyelinated fibers

Advantages
1.The velocity of conduction is faster
2.Helps to conserves energy
In unmyelinated axons, voltage-gated channels open throughout
the axonal length causing activation of larger number of Na+-K+
ATPase and higher expenditure of energy.

1. Orthodromic conduction
2. Antidromic conduction

• When AP is initiated in the middle of the axon, two
impulses traveling in opposite directions are set up.
• Impulses usually pass in one direction only, i.e., from
synaptic junctions or receptors along axons to their
termination – orthodromic conduction.

• Since the impulse conducted in opposite
direction (antidromic) fails to pass the first
synapse they encounter & die out.
• Because synapses permit conduction only in
one direction.

NERVE PHYSIOLOGY- 5b
DR SARAN AJAY

• Properties of AP Continued
• Velocity of Nerve Conduction
• Properties of Mixed Nerve

Monophasic action potential

4. Refractory period
• Absolute Refractory Period
• Relative Refractory Period

Absolute Refractory Period
• Period in which a stimulus, no matter how strong, will not
be able to excite the nerve
• Corresponds to the period from the firing level to the
point at which repolarization is about one-third
complete.

Basis
1. Shortly after action potential is initiated, Na+ channels
become inactivated.
2. No amount of excitatory signal will re-open inactivation
gates till membrane potential return to or near the RMP.

Relative Refractory period
• Period in which a stimulus, stronger than normal
stimuli can cause excitation
• Corresponds to the period from the one-third of
repolarization to the start of after-depolarization

Na+ channels reopen when membrane potential returns
to or near the original resting membrane potential.

Graded Potential v/s Action Potential

Graded potential Action potential
1. No threshold stimuli
needed
2. Amplitude of potential is
proportionate to the
intensity of stimulus
3. Can get summated
1. A threshold stimuli is
needed
2. Amplitude remains
constant even if intensity of
stimulus is increased above
threshold stimuli
3. Cannot be summated

4. So does not obey “All or
none” law
5. Can be either depolarizing
or hyperpolarizing
6. Change in membrane
potential that is confined to
relatively small region on
membrane
4. Obeys “All or none” law
5. It is always depolarizing
6. Spreads over a larger area
on the membrane

7. These potentials cannot be
conducted as impulses –
Non-propagating
potentials
8. No Refractory period
7. Can be conducted as
impulses – Propagating
potentials
8. Has a Refractory period

Types of Action Potentials

Types of action potential
1. Spike potential (Nerve & Skeletal muscle)
2. Plateau potential (Cardiac muscle)
3. Rhythmic type of action potential

Rhythmic type of action potential
• Seen in rhythmically discharging cells
• SA node, AV node & conducting system of heart
• Smooth muscles of intestine & some neurons in CNS
• Pacemaker potential or pre-potential

Velocity of Nerve Conduction

Velocity of nerve conduction
Factors that increase velocity Factors that decrease velocity
Diameter Pressure
Myelination Narcotic drugs
Increase in Temperature Decrease in temperature

“highly differentiated functions of single nerve fibers”
Erlanger and Gassser
Nobel Prize in Medicine
or Physiology, 1944

Erlanger Gasser classification of nerve fibers

Numerical classification of sensory nerve fibers

Susceptibility to Hypoxia, Pressure and LAs

Properties of a Mixed Nerve

1. Effect of stimuli with different intensities
• Sub threshold stimulus
• Threshold stimulus
• Maximal stimulus
• Supra maximal stimulus
2. Compound action potential

• Potential changes recorded extracellularly from mixed nerves
represent an algebraic summation of the all-or-none action
potentials of many axons.

• With subthreshold stimuli, no response occurs
• When the stimuli are of threshold intensity, axons
with low thresholds fire and a small potential change
is observed.
1. Effect of stimuli with different intensities

• As the intensity of the stimulating current is increased,
the axons with higher thresholds are also stimulated
• Electrical response increases proportionately until
stimulus is strong enough to excite all of the axons
in the nerve

• Stimulus that produces excitation of all the axons is the
maximal stimulus
• Application of greater, supramaximal stimuli produces
no further increase in the potential

• In mixed nerves, there is the appearance of multiple
peaks in the action potential.
• The multipeaked action potential is called a
compound action potential.
2. Compound action potential

• Asynchronous discharge of the various fibers
• Different conduction velocities

WBC and Nerve Lecture Feedback (google.com)

saran.adhoc@gmail.com

Short Essay (8 marks)
1. Explain chronaxie, rheobase, utilization time. Draw strength
duration curve.
Write briefly (4 marks)
2. Ionic Basis of Resting membrane potential.
3. Ionic Basis of Action Potential.
4. Absolute Refractory Period.
Q - KUHS Mar 2021, May 2022
Q - KUHS Feb 2022, May 2022
Q - KUHS July 2022
Q - KUHS Feb 2016

Physiological Basis (2 marks)
1. Wallerian Degeneration
2. Sodium Potassium pump helps to maintain RMP.
Draw and Label
1. Action Potential in a Nerve
2. Multipolar Neuron
Q - KUHS July 2022
Q - KUHS Jan 2019
Q - KUHS Nov 2020, Sep 2021
Q - KUHS Aug 2015

Nerve Physiology

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