The nerve impulse is the propagation of the action potential along the nerve cell (fiber)

3. Refractory period: from 0.001 up to 0.003 second following the stimulation, the nerve cell will not respond to any stimulus whatever its strength. This period is called the refractory period. During this period, the membrane of the nerve cell regains its physiological properties to be ready to respond to new stimulus and to transmit another nerve impulse.

Properties of the nerve impulse:

1. The speed of propagation of the nerve impulse along a nerve fibre depends on its diameter:

- It reaches 140 meters/second in thick (myelinated) nerve fibres.

- It reaches 12 meters/second in thin (non-myelinated) nerve fibres.

Stimulation of the nerve (and also muscles) obeys the all or none law, which means that the nerve responds maximumly or does not respond at all; the sufficient stimulus produces a maximum response (generation of a nerve impulse), after which, the response does not increase whatever the stimulus strength increases. Weak stimuli are insufficient to produce an action potential (nerve impulse).

The synapse:

The synapse is site between the terminal branches (arborizations) of the axon of one neuron and the dendrites of the next neuron.

Types of synapses:

Synapse between two neurons.

Synapse between a neuron and a muscle fibre.

3. Synapse between a neuron and gland cells.

The structure of the synapse:

The terminal branches of the axon end with swellings called buttons which are very close to the dendrites of the next neuron. In between, there is a very narrow space called the synaptic cleft. This cleft separates a presynaptic membrane (axon) from a postsynaptic membrane (dendrite). The synaptic button contains small vesicles (sacs) called synaptic vesicles, filled with chemical transmitters as Acetylcholine and Noradrenalin which play an important role in synaptic transmission of the nerve impulse from one neuron to the next.

Mechanism of transmitting a nerve impulse across a synapse:

Arrival of a nerve impulse to the buttons leads to entrance of Calcium ions by the action of a Calcium pump in the cell membrane. The inflow of Calcium ions leads to rupture of the synaptic vesicles and the release of the chemical transmitters.

The chemical transmitters cross the synaptic cleft and reach the membrane of the dendrites of the next neuron.

Binding of the chemical transmitters to special receptors on the membrane of the dendrites leads to stimulation of these points and changes the permeability of the membrane to Sodium and Potassium ions. These results in depolarization and the production of an action potential (nerve impulse) as mentioned before. This nerve impulse is propagated through the body, then the axon of the neuron, then to a next synapse, and so on.

Acetylcholine (chemical transmitter) is destroyed (after performing its function) under the effect of an enzyme called cholinesterase to terminate its action. Then, the postsynaptic membrane (dendrite) returns to the resting state again.

The Central Nervous System (C.N.S)

A) The brain:

The brain constitutes the major part of the central nervous system, with a weight that ranges from 350 grams at birth, and reaches 1400 grams in adults. The brain occupies a bony space called the brain case or the cranium (a part of the skull).

The brain is surrounded by three membranes called the meningies which are responsible for the protection and nutrition of the brain cells. These membranes are:

1. The dura mater: The membrane which lines the skull.

2. The pia mater: The membrane which is in direct contact and adheres to the brain.

3. The arachnoid: The membrane which is in between the other two membranes and contains a transparent fluid to protect the brain from mechanical trauma.

The brain consists of three main parts:

1. Forebrain: It includes the two cerebral hemispheres (the brain cortex), thalamus, and hypothalamus.

Midbrain.

3. Hindbrain: It includes the cerebellum, pons Varolii, and medulla oblongata.

Date: 2015-12-17; view: 756