The medulla oblongata contains vital centers as those of respiration, swallowing, vomiting, cough, sneezing, and blood vessels.

B) The spinal cord:

The spinal cord exists inside a canal in the vertebral column called the neural canal. It extends from the medulla oblongata in the form of a cylindrical cord about 45 cm long. The spinal cord is hollow containing a central canal and covered by meningies as those surrounding the brain (dura mater, pia mater, and arachnoid). Along the midline there are two fissures (dorsal, and ventral) which divide the spinal cord incompletely into two halves. The spinal cord consists of 2 layers; outer white matter formed of nerve fibers and inner grey matter formed of nerve cells with their dendrites and neuroglia. Gray matter is H-shaped with two dorsal horns and two ventral horns.

Functions of the spinal cord:

The grey matter is the main center of reflex action as it contains thousands of reflex arcs. The white matter transmits impulses from different parts to the brain and vice versa.

Spinal nerves:

There are 31 pairs of spinal nerves that originate as successive pairs from both sides of the spinal cord as follows:

Eight pairs of cervical nerves.

Twelve pairs of thoracic nerves.

Five pairs of lumbar nerves.

Five pairs of sacral nerves.

One pair of coccigeal nerves.

Each spinal nerve originates from the spinal cord by two roots (dorsal, and ventral). The dorsal root carries sensory nerve fibers that transmit impulses from the receptors to the spinal cord and then to the brain. The ventral root carries motor nerve fibers that transmit impulses to the responding organs (effectors) as muscles and glands.

Peripheral Nervous System

This system consists of a network of nerves distributed all over the body connecting the central nervous system to all parts of the body. These nerves are of two types:

1) Cranial nerves:

12 pairs connected to the brain:

A. Some cranial nerves are purely sensory containing sensory fibres only and carry impulses from receptors to the brain.

B. Others are purely motor containing motor fibres only and carry impulses from the brain to the effecter organs.

C. Some cranial nerves are mixed with both motor and sensory fibres.

2) Spinal nerves:

Pairs connected to the spinal cord and these are mixed nerves with both sensory and motor fibres.

The reflex action ( reflex arc):

The reflex action is the unit of nervous activity. The majority of the nervous functions can be analyzed to a group of reflex actions at different levels.

The reflex action consists of at least two nerve cells (neurons), one sensory (afferent) and the other is motor (efferent).

The majority of reflex actions consist of 5 elements:

Receptor (sense organ)

Afferent (sensory) neuron.

Connector (intermediate) neuron.

Efferent (motor) neuron.

5. Effector (responding) organ.

Reflex actions are of two types:

a. Voluntary (somatic) reflex:

In which effector organ is a voluntary (skeletal) muscle.

b. Involuntary (autonomic) reflex:

In which the effector organ is an involuntary muscle, a gland or the heart muscle.

Autonomic Nervous System

Date: 2015-12-17; view: 902