NERVE ROOTS

Nerve roots

The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina. Each nerve is formed by the union of a motor (anterior) and a sensory (posterior) nerve root and is, therefore, a mixed nerve. Thoracic and upper lumb ar (L1 and L2) spinal nerves have a contribution from the sympathetic part of the autonomic nervous system in the form of a preganglionic fibre.

Chapter 16 describes the bones and muscles mentioned in the following section. Bones and joints are supplied by adjacent nerves.

The anterior nerve root consists of motor nerve fibres, which are the axons of the lower motor neurons from the anterior column of grey matter in the spinal cord and, in the thoracic and lumbar regions, sympathetic nerve fibres, which are the axons of cells in the lateral columns of grey matter.

The posterior nerve root consists of sensory nerve fibres. Just outside the spinal cord there is a spinal ganglion (posterior root ganglion), consisting of a little cluster of cell bodies. Sensory nerve fibres pass through these ganglia before entering the spinal cord. The area of skin whose sensory receptors contribute to each nerve is called a dermatome.

For a very short distance after leaving the spinal cord the nerve roots have a covering of dura and arachnoid maters. These terminate before the two roots join to form the mixed spinal nerve. The nerve roots have no covering of pia matter.

Branches

Immediately after emerging from the intervertebral foraminen, spinal nerves divide into brances, or rami: a ramus communicans, a posterior ramus and an anterior ramus.

The rami communicante are part of preganglionic sympathetic neurons of the autonomic nervous system.

The posterior rami pass backwards and divide into medial and lateral branches to supply skin and muscles of relatively small area of the posterior aspect of the head, neck and trunk.

The anterior rami supply the anterior and lateral aspects of the neck, trunk, and upper and lower limbs.

Plexuses

In the cervical, lumbar and sacral regions the anterior rami unite near their origins to form large masses of nerves, or plexuses, where nerve fibres are regrouped and rearranged before proceeding to supply skin, bones, muscles and joints of a particular area. This means that these structures have a nerve supply from more than one spinal nerve and therefore damage to one spinal nerve does not cause loss of function of a region.

In the thoracic region the anterior rami do not form plexuses.

There are five large plexuses of mixed nerves formed on each side of the vertebral column. They are the:

1. cervical plexuses

2. brachial plexuses

3. lumbar plexuses

4. sacral plexuses

5. coccygeal plexuses.

Cervical plexus  

This is formed by the anterior rami of the first four cervical nerves. It lies opposite the 1^st, 2^nd, 3^rd and 4^th cervical vertebrae under the protection of the sternocleidomastoid muscle.

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, e.g. the sternocleidomastoid and the trapezius.

The phrenic nerve originates from cervical roots 3, 4 and 5 and passes downwards through the thoracic cavity in front of the root of the lung to supply the diaphragm with impulses that stimulate contraction, initiating inspiration.

Brachial plexus

The anterior rami of the lower four cervical nerves and a large part of the first thoracic nerve from the brachial plexus shows its formation and the nerves that emerge from it. The plexus is situated in the neck and shoulder above and the behind the subclavian vessels and in the axilla.

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 one nerve root, containing sensory, motor and autonomic fibres.

. 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

. meidal cutaneous nerve: C8, T1.

The axillary (circumflex) nerve winds round the humerus at the level of the surgical neck. It then breaks up into minute branches to supply the deltoid muscle, shoulder joint and overlying skin.

The radial nerve is the largest branch of the brachial plexus. It supplies the triceps muscle behind the humerus, crosses in front of the elbow joint then winds round to the back of the forearm to supply extensors of the wrist and finger joints. It continues into the back of the hand to supply the skin of the thumb, the first two fingers and the lateral half of the third finger.

The musculocutaneous nerve passes downwards to the lateral aspect of the forearm. It supplies the muscles of the upper arm and the skin of the forearm.

The median nerve passes down the midline of the arm in close association with the brachial artery. It passes in front of the elbow joint then down to supply the muscles of the front of the forearm. It continues into the hand where it supplies small muscles and the skin of the front of the thumb, the first two fingers and the lateral half of the third finger. It gives off no branches above the elbow.

The ulnar nerve descends through the upper arm lying medial to the brachial artery. It passes behind the medial epicondyle of the humerus to supply the muscles on the ulnar aspect of the forearm. It continues downwards to 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 third finger. It gives off no branches above the elbow.

The main nerves of the arm are presented. The distribution and origins of the cutaneous sensory nerves of the arm, i.e. the dermatomes, are shown.

Lumbar plexus

The lumbar plexus is formed by the anterior rami of the first three and part of the fourth lumbar nerves. The plexus is situated in front of the transverse processes of the lumbar vertebrae and behind the psoas muscle. The main branches, and their nerve roots are:

. iliohypogastric nerve: L1

. ilioinguinal nerve: L1

. genitofemoral: L1, 2

. lateral cutaneous nerve of thigh: L2, 3

. femoral nerve: L2, 3, 4

. obturator nerve: L2, 3, 4

. lumbosacral trunk: L4, (5).

The iliohypogastric, ilioinguinal and genitofemoral nerves supply muscles and the skin in the area of the lower abdomen, upper and medial aspects of the thigh and the inguinal region.

The lateral cutaneous nerve of the thigh supplies the skin of the lateral aspect of the thigh including part of the anterior and posterior surfaces.

The femoral nerve is one of the larger branches. It passes behind the inguinal ligament to enter the thigh in close association with the femoral artery.