Nerve fibre types, functions and properties

1. PY 3.2 Describe the types,
function & properties of nerve
fibre
Dr Shikha Saxena

2. TYPES OF NEURONS
• I. Depending upon the number of poles
• Depending upon the number of poles from which processes arise,
neurons are divided into

3. • 1. Unipolar neurons:
• Have a single pole, from which both the processes—axon and
dendrite arise.
• True unipolar cells are present only in embryonic stage in human
being.
• The primary sensory neurons (neurons conveying impulses from a
sensory receptor to spinal cord) are pseudounipolar

4. • 2. Bipolar neurons:
• Two poles, one for axon and other for dendrite.
• Bipolar neurons are found in the
• vestibular and cochlear ganglia,
• nasal olfactory epithelium and
• bipolar cells in the retina

5. • 3. Multipolar neurons:
• Many poles. One of the poles gives rise to axon and all others to
dendrites.
• Most vertebrate neurons, especially in the central nervous system
(CNS) are multipolar.
• The dendrites branch profusely to form the dendritic tree.

6. • II. Depending upon the function
• Two types—motor and sensory.
• 1. Motor neurons: also known as efferent nerve cells, carry the motor
impulses from the CNS to the peripheral effector organs like muscles,
glands and blood vessels.
• These neurons have very long axon and short dendrites.
• 2. Sensory neurons: also known as afferent nerve cells, carry the sensory
impulses from the periphery to the CNS.
• These neurons have short axon and long dendrites.

7. According to the Length of Axon
• Golgi Type 1:
• These are the neurons with short axons.
• Dendrites of these neurons terminate near the soma.
• The example is cortical inhibitory neurons.
• Golgi Type 2:
• Axons are long.
• Cortical motor neurons (neurons that give rise to corticospinal tract) are the
examples.

8. According to Dendritic Pattern
• Pyramidal Cells:
• Dendrites of these cells spread like pyramids.
• The example is hippocampal pyramidal neurons.
• Stellate Cells:
• Radial shaped spread of dendrites occurs in these cells.
• The examples are cortical stellate cells.

9. ZONES OF THE NEURON
• 1. Receptor zone (dendritic zone) is the region where local potential changes are
generated by integration of the synaptic connections.
• 2. Site of origin of conducted impulse is the site, where propagated action
potentials are generated. In case of spinal motor neuron, initial segment and in
cutaneous sensory neurons first node of Ranvier is the site of origin of conducted
impulses.
• 3. Zone of all or none transmission in the neuron is the axon.
• 4. Zone of secretion of transmitter (nerve endings). The propagated impulses
(action potential) to nerve endings cause the release of neurotransmitter.

11. CLASSIFICATION OF NERVE FIBRES
• 1. Letter classification of Erlanger and Gasser:
• This is the best known classification based on the diameter and conduction velocity
of the nerve fibres.
• The nerve fibres have been classified as follows:
• ‘Type A’ nerve fibres:
• The fastest conducting fibres
• Their diameter varies from 12–20 μm and conduction velocity from 70–120 m/s.
• They are myelinated fibres.
• Further subdivided into α, β, γ and δ.
• Subserve both motor and sensory functions.

12. • ‘Type B’ nerve fibres:
• These fibres are myelinated have a diameter of less than 3 μm and their
conduction velocity varies from 4–30 m/s.
• They form preganglionic autonomic efferent fibres, afferent fibres from skin and
viscera, and free nerve ending in connective tissue of muscle.
• ‘Type C’ nerve fibres:
• These are unmyelinated, have a diameter of 0.4–1.2 μm and their conduction
velocity varies from 0.5–4 m/s.
• These form the postganglionic autonomic fibres, some sensory fibres carrying
pain sensations, some fibres from thermoreceptors and some from viscera.

14. 2. Numerical classification
• Some physiologists have classified sensory nerve fibres by a
numerical system into type
• Ia,
• Ib,
• II,
• III and
• IV

16. 3. Susceptibility of nerve fibres
• Hypoxia:
• the type B fibres are most susceptible to hypoxia.
• the preganglionic autonomic fibres are of type B, therefore, hypoxia is
associated with alteration of the autonomic functions in the body such
as rise in heart rate, blood pressure and respiration.

17. • Pressure. Type A fibres are most susceptible to pressure and type C least.
• pressure on a nerve can produce temporary paralysis due to loss of
conduction in motor, touch and pressure fibres (type A),
• while pain sensation (carried by type C fibres) remain relatively intact.
• This is common observation after sitting cross-legged for long periods and
after sleeping with arms under the head.
• Local anaesthetics. Type C fibres (conducting pain, touch and temperature
sensations generated by cutaneous receptors) are most susceptible to local
anaesthetics.

19. PROPERTIES OF NERVE
FIBERS

20. Excitability
• Excitability is the property by virtue of which cells or tissues respond to
changes in the external or internal environments.
• It is due to the disturbances in the ionic equilibrium across the receptive
zone of cell membrane.
• The nerve fibers are highly excitable tissues.
• They respond to various forms of stimuli—mechanical, thermal, chemical
or electrical. In experiment set-up, ‘electrical’ stimulus is usually
employed, because its strength and frequency can be accurately
controlled, nerves respond well to chemical and thermal stimuli.
• The production of a wave of depolarization, and (excitation or activation)
impulse demonstrates that a nerve has been excited.

21. • Factors Affecting Excitability:
• 1. Strength and duration of the stimulus
• 2. Effect of extracellular Ca++
• i. Decrease in ECF Ca++ increases excitability of neuron by
decreasing the amount of depolariza- tion necessary to initiate the
changes in the Na+ and K+ permeability that produces the action
potential.
• ii. Increase in ECF Ca++ stabilizes the membrane by decreasing
excitability. Ca++ entry contributes to depolarization.

22. Conductivity
• On stimulation, action potential is generated in the nerve fiber, which is
propagated along its entire length to the axon terminal.
• Orthodromic and Antidromic Conduction
• An axon can conduct in either direction.
• If the stimulus is applied in the middle junction of axon, the action
potential initiated in the middle of it can travel in both directions, due to
set-up of electronic depolarization on either side of the initial current
sink.

23. • 1. Impulses normally pass from synaptic junction to the axon terminal,
which is called orthodromic conduction.
• 2.Conduction in the opposite direction is called antidromic conduction,
seen in sensory nerve supplying the blood vessels. Axon reflex is an
example of antidromic conduction.

24. Summation
• Application of a subthreshold stimulus does not evoke an action
potential.
• If subthreshold stimuli are applied in rapid succession, they are
summated and they produce an action potential.
• This property is called summation.

25. Accommodation
• Application of continuous stimuli may decrease the excit- ability of the
nerve fiber, a phenomenon called accommodation.
• Nerve endings that adapts cause decreases the transmission of impulse
across the neurons.

26. • 1. If a nerve is submitted to the passage of constant strength of current,
the site of stimulation shows decrease in excitability. The
accommodation consists of a rise in threshold of the membrane during
stimulation
• 2. A similar feature observed at at nerve endings is called adaptation.
• 3. Thus, nerve fiber accommodates while the nerve end- ings adapt.

27. Unfatigability
• Nerve fibers cannot be fatigued, even when they are stimulated
continuously.
• This is because the nerve fibers primarily conduct impulses
(propagation of action potential) that do not involve expenditure of
energy (ATP).

28. All or None response
• When a stimulus of subthreshold intensity is applied to the axon, then no
action potential is produced (none response).
• A response in the form of spike of action potential is observed when the
stimulus is of threshold intensity
• There occurs no increase in the magnitude of action potential when the
strength of stimulus is more than the threshold level (all response).
• This all or none relationship observed between strength of stimulus and
response achieved is known as All or None law

30. Refractory period
• During the action potential, the stimulated area of the membrane happens to
be unresponsive to a second stimulus in most part, and later it requires a
stronger stimulus to get excited again.
• The length of time during which the membrane is unresponsive to a second
stimulus no matter how strong is the stimulus, is known as refractory period.
• The periods of total and relative refractoriness are known as absolute and
relative refractory periods respectively

31. • Absolute Refractory Period:
• (ARP) is defined as the period in the action potential during which,
application of a second stimulus of any strength and duration does not
produce another action potential.
• The ARP corresponds to the period from the time the firing level is
reached until repolarization is about one-third complete

32. • Mechanism:
• At the peak of the action potential, the inactivation gates of the voltage-
gated sodium channels close and they remain in that inactivated state for
some time before returning to the resting state.
• These sodium channels can reopen in response to a second stimulus, only
after attaining the resting state.
• Hence, even if a stronger stimulus is applied during this interval, it will
not produce a second action potential, and the membrane is said to be in
its absolute refractory period.

33. • Physiological Importance
• 1. ARP determines the rate of discharge of nerve fiber.
• 2. The ARP is also responsible for the one-way conduction of action
potentials

34. • Relative Refractory Period
• Relative refractory period (RRP) is defined as the period following
ARP during which, application of a suprathreshold stimulus can elicit
a second action potential.
• The RRP starts from the end of ARP to the start of after-
depolarization.

35. • Mechanism :
• 1. All the sodium channels present at the site of stimulus do not achieve the open state or
inactivated state or resting state, exactly at the same time.
• Few of them open when the membrane potential is –63 mV, causing local response.
• By the time of relative refractory period, some of the channels have returned to their
initial resting state.
• These channels in resting state can open their activation gate and allow the influx of Na+
• 2. A suprathreshold stimulus can spread to larger area over the membrane and open extra
voltage-gated sodium channel

37. FUNCTIONS OF NEURONS
• The cell body and dendrites serve as the receptor zone to receive the information,
axon hillock and initial segment for generation of action potential, axon for
transmission of nerve impulse, axon terminal for discharge of neurotransmitters.
• Cell body:
• It maintains the functional and anatomical integrity of the axon.
• The proteins associated with synaptic transmitters are synthesized in Nissl granules
of the cell body and are transported to axon terminal by axoplasmic flow

38. • Dendrites:
• They form the receptor zone of the neuron, i.e. they receive impulses and
transmit the impulses toward the cell body.
• In this region, non-conducted local potential changes generated by synaptic
connections are integrated.
• Axon:
• The initial segment is the site where propagated action potentials are generated.
• The axonal process transmits propagated impulses from the cell body to the
axon terminal.

39. • Synaptic knobs:
• This is the nerve ending where arrival of action potentials results in
the release of synaptic transmitter