Transmission of Nerve Pulses

The nervous system of all the animals around the world is very complex. This system helps us react to stimuli and respond in the right manner to the respective situation. There is a constant exchange of information between the various parts of the body through the nerves and nerve cells.

The neurons are the very functional unit of the neural system; they have this amazing ability to detect, receive, and transmit various kinds of stimuli. In scientific terms, the way these neurons communicate is a nerve impulse. These signals are generally in the form of electrical signals that travel along the neuron’s axon.

The neurons communicate at specific points, or junctions called synapses. These synapses can be chemical, communicating through chemical messengers or electrical, where ions flow between the cells.

Generation and Conduction of Nerve Impulse

Nerve impulse

A wave of reversed polarity or depolarization (action potential) moving down an axon is called a nerve impulse.

The most accepted mechanism of nerve impulse conduction is an ionic theory proposed by Hodgkin and Huxley. This theory states that nerve impulse is an electro-chemical process governed by differential permeability of neurilemma to Na+ and K+, which is regulated by the electric field.

Neurons are excitable cells because their membranes are in a polarised state. Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e. the axonal membrane is comparatively more permeable to potassium ions (K) and nearly impermeable to sodium ions (Na+). 

Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of Nat. In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na** and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the sodium-potassium pump’s active transport of ions, which transports 3 Na+ outwards for 2 K into the cell. 

As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised. The membrane is called the resting potential (-70 mV). When a stimulus is applied at a site (point A) on an I polarised membrane, the membrane at site A becomes freely permeable to Na*. This leads to a rapid influx of Na followed by the reversal of the polarity at that site, i.e. The outer surface of the membrane becomes negatively charged, and the inner side becomes charged. 

The polarity of the membrane at site A is thus reversed and hence depolarised. The electrical potential difference across the plasma membrane at site A is called the action potential (+20 to +40 mV), which is termed a nerve impulse.

The axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface at sites immediately ahead. As a result, a current flows on the inner surface from site A to B. On the outer surface, current flows from site B to site A to complete the current flow circuit. Hence, the polarity at the site is reversed, and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B. The sequence is repeated along the length of the axon, and consequently, the impulse is conducted. 

The rise in the stimulus-induced permeability to Na is extremely short-lived. It is quickly followed by a rise in permeability to K. Within a fraction of a second, K diffuses outside the membrane and restores the membrane’s resting potential at the site of excitation, and the fibre becomes once more responsive to further stimulation.

When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma membrane. Arrival of the disturbance at the neuron’s endings, or output zone, triggers events in adjacent neurons and other cells. The new cells developed may be either excitatory or inhibitory.

Synapse:

Synapses are usually found between the fine terminal branches of the axon of one neuron and the dendrites or cell bodies of another. This type of neuron is called an axon-dendrite synapse.

A nerve impulse is transmitted from one neuron through junctions called synapses. A synapse is formed by membranes of a presynaptic neuron and a postsynaptic neuron, which may or may not be separated by a synaptic cleft gap.

Structure of chemical synapse:

A typical (generalised) synapse consists of a bulbous expansion of a nerve terminal called a presynaptic knob lying close to the membrane of a The cytoplasm of the synaptic knob contains mitochondria, smooth endoplasmic reticulum, microfilaments and numerous synaptic vesicles.

Transmission of impulse across synapse:

• When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft.
• The released neurotransmitters bind to their specific receptors, present on the postsynaptic membrane.
• This binding opens ion channels, allowing ions to enter, which can generate a new potential in the postsynaptic neuron.
• The new potential developed may be either excitatory or inhibitory.

After generating a change in the permeability of the postsynaptic membrane, the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine is hydrolyzed by the enzyme acetylcholinesterase.

(AChE) which is present in high conc. at the synapse. The product of hydrolysis, acetate & choline are reabsorbed into the synaptic knob, where they are resynthesized into acetylcholine by using ATP.

Conclusion:

The nervous system of all the animals around the world is very complex. This system helps us react to stimuli and respond in the right manner to the respective situation. There is a constant exchange of information between the various parts of the body through the nerves and nerve cells. This article covers concepts regarding nerve impulses, transmission, a synapse and structure of chemical synapses.