Peripheral Nervous System Part 2

 

Plexuses

In the cervical, lumber, and sacral regions, anterior rami unite near their origins and form large masses of nerves, called as plexuses. Nerve fibers are regrouped and rearranged before proceeding to supply skin, bones, muscles and joints of a particular area. These structures have a nerve supply from more than one spinal nerve. Therefore, damage to one spinal nerve does not cause loss of function of a region. They lie deep within the body, often under large muscles. Therefore, well protected from injury. In the thoracic region, the anterior rami do not form plexuses. Five large plexuses of mixed nerves formed on each side of the vertebral column. They are, cervical plexuses, brachial plexuses, lumber plexuses, sacral plexuses, coccygeal plexuses.

 

Cervical plexuses

            Formed by anterior rami of the first 4 cervical nerves. Lies deep within the neck opposite the 1^st, 2^nd, 3^rd, and 4^th cervical vertebrae under the protection of the sternocleidomastoid muscle. It serves the head, neck and shoulders. The superficial branches supply the structures at the back and side of the head and the skin of the front of the neck to the level of the sternum. The deep branches supply muscles of the neck. Example, the sternocleidomastoid and the trapezius.

            The phrenic nerve originates from cervical nerve roots 3, 4 and 5. It passes downward through the thoracic cavity in front of the root of the lung to supply the diaphragm, initiating inspiration. Disease or spinal cord injury at this level will result in death (due to apnoea without assisted ventilation as spontaneous respiration is not possible.)

 

 

Brachial plexus

Anterior rami of the lower 4 cervical nerves (C5 – C8) and a large part of the 1^st thoracic nerve form the brachial plexus. This plexus is situated deeply within the neck and shoulder above and behind the subclavian vessels and in the axilla. Serves the chest shoulders, arms and hands. The branches of the brachial plexus supply the skin and muscles of the upper limbs and some of the chest muscles. Five large nerves and a number of smaller ones emerge from this plexus, each with a contribution from more than 1 nerve root, containing sensory motor and autonomic fibers,

                        Axillary (circumflex) nerve: C5, 6

                        Radial nerve: C5, 6,7, 8, T1

                        Musculocutaneous nerve: C5, 6, 7

                        Median nerve: C5, 6, 7, 8, T1

                        Ulnar nerve: C7, 8, T1

                        Medial cutaneous nerve: C8, T1

 

Axillary nerve

            Supply the deltoid muscle, shoulder joint and overlying skin

 

Radial nerve

            Supply the triceps muscle behind the humerus, crosses in front of the elbow joint winds round to the back of the forearm to supply extensor muscles of the wrist and finger joints. Continuous into the back of the hand to supply the skin of the posterior aspect of the thumb, 1^st two fingers and the lateral half of the 3^rd finger.

 

Musculocutaneous nerve

            Supply the muscles of the upper arm and the skin of the forearm

 

Median nerve

            Supply the muscles of the front of the forearm. Supplies small muscles and the skin of the front (palmer aspect) of the thumb, 1^st two fingers and the lateral half of the 3^rd finger

 

Ulnar nerve

            Supply the muscles on the ulnar aspect of the forearm. Supply the muscles in the palm of the hand and the skin of the whole of the little finger and the medial half of the 3^rd finger.

 

Medial cutaneous nerve

            Supply the dermatomes