VPB 121: Veterinary Physiology - II (2+1)

PROPERTIES OF NERVE FIBRE

• Ability of nerve fibre to respond to stimulus.

Stimulus

• It is an external agent that brings about visual response. It can be also be from within and can elicit response which can not be visualized.
• Types of stimulus
  • Electrical: this is the most common stimulus to bring about neuronal response.
  • Chemical: osmotic also come under this. Hypo, hyperosmotic, acid or alkali.
  • Mechanical:  touch, thermal, light, sound, pressure changes are included in this.

All or none law

• In a nerve fibre, action potential are equal in amplitude and velocity. Because amplitude and velocity are independent of intensity of the stimulus. This is the basis of all or none law. Nerve fibres obey all or none law.
• When adequate strength of stimulus is applied to a nerve, the action potential propagates in the same magnitude all along the nerve. If the stimulus is low in threshold, there is no action potential and if the stimulus is above the threshold the action potential is uniform, magnitude of action potential does not increase with the stronger stimuli. This is known as all or none principle. As the action potential travels along it triggers the subsequent axon to fibre.

Refractory period

• However, for a period following the passage of an impulse, there is un responsiveness , though a second stimulus is strong axons are incapable of generating new impulses. This interval known as absolute refractory period. This period varies wit the type of nerve and with species.
• In mammalian nerve it is about 1.04m sec. absolute refractory period is due to complete closure of sodium gates which resists the membrane to produce an action potential most of the axons exhibit 1 ms as their absolute refractory period.

Relative refractory period

• It follows the absolute refractory period. In this, the response to second stimuli of threshold strength is absent but if the second stimuli is of higher strength than threshold response is appreciated.
• The excitability gradually rises to normal. Because of this the nerve fibre can be stimulated for hours without excitability fatigue.
• Relative refractory period is due to the opening ok K⁺ gates with Na⁺ gates resting their usual state. Due to free flow of K⁺, stronger than usual stimulus is necessary to initiate action potential.
• Relative refractory period in axons experimented was 2-4 m s. The refractory period governs the rate at which a membrane can fire.

Accomodation

• Strength of the stimulus applied bears relationship with length of the time of its application to produce a response.
• Short duration, strong stimuli and long duration weak stimuli fail to produce a response.
• Rate of rise of stimuli when applied rapidly produces better response. But slow rise in stimuli for longer duration fail to provoke a response due to adaptation by nerves to weaker stimuli which is known as accommodation. This phenomenon is due to partial inactivation of sodium carrier by nerve cell.