That includes the cranial nerves and the spinal nerves.

3. Autonomic nervous system:

That controls the involuntary muscles and the glands. This system is sub-divided into:

a. Sympathetic nervous system: The nerve fibers of this system originate from the thoracic and lumbar region (segments) of the spinal cord.

b. Parasympathetic nervous system: The nerve fibers of this system originate from the brain and the sacral region of the spinal cord.

The nerve cell (neuron):

The nerve cell is the unit of structure of the nervous system. Its small in size and cannot be recognized by the naked eye. It consists of:

1. The cell body:

It contains rounded nucleus surrounded by cytoplasm (neuroplasm). The neuroplasm contains neurofilaments and Nissl granules (which are unique for nerve cells). Nissl granules are considered as a stored food for the cell. The neuroplasm contains all other cell’s organelles as mitochondria, and Golgi bodies, except the centrioles (That is why neurons cannot divide)

2. Cell processes:

a. Dendrites:

Dendrites are short processes which increase the surface area available to receive nerve impulses and through which most nerve impulses enter to the cell, while some of which enter to the cell through the cell body.

b. Axon:

It is a long cytoplasmic extension of the cell (may reach more than a meter in length) and usually known as the nerve fiber. In some nerve cells, the axon is surrounded by a sheath of lipid called myelin sheath secreted by special cells called Schawnn’s cells. The outer cover of the axon (nerve fiber) is the neurolemma. The myelin sheath is not continuous around the axon but interrupted at certain points called nodes of Raniver.

The conduction rate of verve impulses in myelinated axons (covered with myelin sheath) is much more rapid than in non-myelinated nerve fibers (axons) because the myelin sheath is an insulator.

Normally, the nerve impulse is propagated and conducted through the nerve cell in one direction only, from the dendrite to the nerve cell body to the axon, then to another next neuron (nerve cell) through a synapse. The axon ends in a group of branches called terminal arborizations.

Types of nerve cells:

According to the function, nerve cells are classified into three types:

1. Sensory neurons: Convey (transmit) impulses from receptors to the central nervous system.

2. Motor neurons: Convey impulses from the central nervous system to the effectors organs as muscles and glands.

3. Connector (intermediate) neurons: Relay impulses from sensory to motor neurons.

Neuroglia:

Another type of cells in the nervous system, have the ability to divide and perform the following functions:

They act as a connective tissue to support neurons.

They act as insulators between neurons.

Nutrition of the neurons.

Have a role in repair of injured parts of some neurons.

The structure of the nerve:

The nerve consists of a group of nerve bundles, each of which is surrounded by a connective tissue sheath. The whole nerve is surrounded by another connective tissue called epineurim which contains blood vessels. Each nerve bundle is formed of a group of nerve fibres (axons) and connected by supporting neuroglia cells (glial cells)

The nerve impulse:

The nerve impulse is the message transmitted through the nerves from sense oragns (receptors) to the central nervous system, and from the latter to the effectors (responding) organs. It is an electrical phenomenon with a chemical nature (electro-chemical phenomenon). To understand the nature of the nerve impulse, we should study the nerve cells (neurons) during 4 different conditions:

1. The nerve cell at rest:

At rest, there is a difference in distribution and concentration of some ions outside and inside the nerve cell, as follows:

a. The concentration of positive Sodium ions (Na+) outside the cell is 10 up to 15 times higher than inside.

b. The concentration of positive Potassium ions (K+) inside the cell is 30 times higher than outside.

Date: 2015-12-17; view: 672