Nerve impulse| classification| structure| function| nervous system|

    TRANSMISSION OF NERVE IMPULSE  

In a neuron, nerve impulse is conducted from axon terminal of one neuron to dendrites of next neuron, so it a unidirectional process. The nerve impulse travels along axon in the form of a self-propagative wave of certain fixed electrochemical changes, The conduction of nerve impulse depends upon following facts like: Permeability of axolemma., Osmotic equilibrium between axoplasm and extracellular fluid, Electrical equivalence between axoplasm and extracellular fluid. In thin nerves the impulses travel at less than 1m/sec, but in large nerves they travel much faster at 100m/sec.

✅Resting Membrane Potential (Polarization) 

Membrane of nerve cells have electrical potential difference (voltage), known as membrane potential. Resting nerve cell has -70mv (ranging from – 40 to – 90mv) electrical potential on the inner side of membrane. It is called resting membrane potential. This state of nerve cell is, called polarized state. Resting membrane potential is the unequal distribution of ions on both sides of the membrane of neuron determined by the concentration of ions. During polarized state, membrane is negatively charged from inner side and positively charged on outer side. The resting membrane potential is determined primarily by three factors: -

1) Concentration of ions on the inside and outside of the cell. 

2) Permeability of membrane to the ions through specific ion channels 

3) By the activity of the electrogenic sodium potassium pump. Thus, polarization is established by maintaining excess sodium ions (Na+) on the outside and an excess of potassium ions K+ on the inside. A certain amount of Na+ and K+ ions is always leaking across the membrane through leakage channels but the sodium –potassium pumps in the membrane actively restore the ions to the appropriate side. The main factor that determines the resting membrane potential is the difference in permeability of K+ and Na+. The resting membrane is more permeable to K+ than to Na+ resulting in slightly more net K+ diffusion (from inside of the neuron to the inside) causing a slight difference in polarity along the membrane. 

Graded potential: -

It is a change in the resting potential of the plasma membrane in response to a stimulus. A graded potential occurs when the stimulus causes Na+ or K+ gated channels to open. If Na+ channels open Na+ enters inside and the membrane depolarizes (becomes more positive). If K+ Channels open K+ exit across the membrane and the membrane hyperpolarizes (becomes more negative). A grade potential is a local event that does not travel far from its origin. It occurs in cell bodies and dendrites. Light, heat, mechanical. Pressure and chemicals may generate potential depending upon the neuron.

When a nerve fiber is stimulated it propagates a nerve impulse and the conduction of such an impulse along the axon is associated with an action potential. The factors which can elicit an action potential of a nerve fiber are: -

✅Chemical stimulation: Certain chemicals such as acids, bases, salt solutions of strong concentrations and some hormones stimulate nerve fires by disturbing resting potential.

✅ Mechanical stimulation: - Crushing, pinching or pricking a nerve fiber can cause a sudden surge of Na+ influx and cause stimulation of the nerves. There are numerous mechanoreceptors found distributed throughout the body which pick up even slightest sensation of pressure, pain, or vibrations.

✅ Electrical stimulation: - Electrical charge can also initiate an action potential because it causes an excess flow of ions across the membrane. 

 Action Potential: -

Any factor that suddenly increases the permeability of the nerve membrane to sodium ions is likely to elicit a sequence of changes in the membrane potential lasting a fraction of a minute. followed immediately by the return of the membrane potential to its resting value. This sequence of potential changes by a factor or stimulus is called action potential. Following sequence of events occur during an action potential. 

Depolarization: When the stimulus picked up by a nerve is strong enough the sodium channels in the trigger zone open increasing the flow of Na+. The permeability of the nerve membrane to Na+ increases and the ions rush to the inside of the membrane. This is known as activation of the membrane at the onset of action potential. As sodium diffuse into the interior the internal negativity becomes less and there is reversal potential. Thus, the membrane become depolarized.

✅Repolarization: -

Almost immediately after depolarization the sodium channels close and the nerve membrane again become impermeable to Na+. As soon as sodium channels close potassium channels open, thus allowing K+ from inside to rush out of the cell. This causes repolarization by restoring g the original membrane polarization. Unlike the resting potential, in repolarization the K+ are on the outside and Na+ are on the inside. 

✅Hyper polarization: -

By the time potassium channels close more K+ have moved out of the cell than is actually necessary to establish original polarized potential. Thus, the membrane is said to be hyperpolarized(-80mV). Na+ and K+ diffuse through sodium and potassium channels present in the membrane. The sodium channels are believing d to be oval in shape and having a diameter of 3x5ang while potassium channels are rounded, with a diameter of 3x3Aº.Each channel is believed to be guarded by a gate which can open and close the channel. Under resting condition both sodium and potassium channels are completely closed. The sodium and potassium gates are positively charged. The positive charge creates a positive electric field that spread far into the channels and thus blocks ion permeability. The opening and closing of the gate is caused by electrical potential called gating potential.

✅All or none response: -

Once an action potential has been set up by a stimulus above the threshold potential at any point on the membrane of a resting nerve fiber, the process of nerve depolarization will travel over the entire membrane. This process of impulse formation and transmission is independent of the strength of stimulus. Had the stimulus been less strong than the threshold value (sub-threshold potential), the impulse would have not generated at all. This is because the conduction follows an all or none response.i.e.the stimulus either fails to set up an impulse or it sets up a full-sized impulse.

TRANSMISSION OF NERVE IMPULSE 

Nerve impulse is transmitted from axon of one neuron to dendrites of next neuron, though. synapse. If dendrites of more than one neuron are in contact of one axon, then nerve impulse will be transmitted to all neurons with same velocity. This transmission of nerve impulse from is a chemical process that is stored in synaptic vesicles.

At a synapse, telodendrion of one axon are not in direct contact of dendrites of next neuron but are separated by a space called synaptic cleft. Synaptic cleft is from 200 to 400 E wide. Tissue fluid is filled in the synaptic cleft. When nerve impulse reaches telodendrion Ca++ from tissue fluid diffuse into synaptic vesicles. The concentration of Ca++ is 10,000 times more outside the cells than in the axoplasm. After entering inside synaptic vesicles, Ca++ stimulates release of neurotransmitters at the synapse. Many synaptic vesicles fuse with the plasma membrane and release the neurotransmitters in the fluid of synaptic cleft. The molecules of neurotransmitters bind to some surface receptors of dendrites, which change the permeability of postsynaptic membranes and generate nerve impulse in the next neuron. In this manner neurotransmitters transmit nerve impulse to next neuron. 

Neurotransmitters are also called neurohumor. Adrenaline, dopamine, serotonin and sympathy are some other excitatory neuron transmitters that are secreted at some nerve endings. Glycine and GABA (gamma amino butyric acid) are impulse inhibitory substances. Nerve impulse is transmitted from one neuron to next neuron within milliseconds after which neurotransmitters are hydrolyzed. Enzyme acetylcholinesterase (AChE) breaks acetylcholine at synaptic cleft of cholinergic neurons. At synapse of adrenergic neurons, norepinephrine is released. Norepinephrine is disintegrated by catechol-O-methyltransferase enzyme (COMT). The time required for the impulse to cross at the synapse is, called synaptic delay. Synaptic delay is of about 0.8 milliseconds.

✅Propagation of the impulse: -

Once elicited at one spot on an excitable membrane, the action potential then excites adjacent portions of the membrane resulting in propagation of the action potential. The fig A shows a rating nerve and a nerve excited in its middle portion has been shown in fig B which has developed a local circuit of current flow between the polarized and the resting membrane, show that more areas on the membrane become depolarized and the process of depolarization occur in both directions along the nerve fiber. The transmission of this depolarization along a nerve fiber is called a nerve impulse. 

✅The Refractory Period: -

After a nerve impulse has passed, there is a short period of time during which the nerve is not able to respond to another stimulus as it is still depolarized from the previous action potential. This brief interval of in excitability is known as absolute refractory period and it is about 1/1000 second for a large, myelinated nerve. Thus, there is limit to the frequency of the impulses the nerve fiber can transmit and it is usually 500-1000 impulses per second. In the later part of the refractory period second stimulus of higher intensity can generate a fresh action potential by it, second stimulus of same intensity cannot. This is known as relative, refractory period. 

✅The velocity of conduction: -

The velocity of conduction through nerve fibers varies from as little as 0.5 meter per second in small unmyelinated fibers up to as high as 130metres per second in large, myelinated fibers. 

Saltatory conduction: - 

In 1925, Lillie observed that when the velocity of propagation of nerve impulse in an unmyelinated axon is compared with that in a myelinated axon of the same diameter, it is found that the impulse travels at a much greater rate in myelinated fiber. This suggests that the mechanism of propagation may be different and has led to the hypothesis of saltatory conduction. Saltatory means “Leaping” (saltire-jump) and the term is used to describe a process in which the active process of conduction ‘leaps ’from one node of Ranvier to another node of Ranvier where the membrane is some 500times more permeable. Saltatory conduction is of physiological importance for two reasons: First, by causing the depolarization process to jump long intervals along the axon, which greatly increases the velocity of conduction in myelinated fibers. Second, saltatory conduction conserves energy for the axon, for only the nodes depolarize and that metabolic energy is saved which would have been otherwise required to re transport the ions across the membrane.