Typical spinal nerve learning objectives at the end of class student should be able to

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At the end of class student should be able to:

1. Define a spinal nerve.

2. Recognize the spinal nerve as a part of PNS.

3. Enumerate the spinal nerves in different regions.

4. Identify their location and site of emergence.

5. Identify various components of a typical spinal nerve.

6. Recall the fate of rami.

7. Associate the rami communicans with typical spinal nerve.

8. Recall the distribution of gray rami.



What Is A Spinal Nerve?

  • The nerves that arise in pairs from the spinal cord (within the vertebral or spinal canal).

  • The spinal nerves are distinguished from the cranial nerves, which arise in pairs from the brainstem and (apart from the vagus nerve) are limited in their distribution to the head and neck.

  • Like the cranial nerves, the spinal nerves are part of the peripheral nervous system.

Cranial Nerves Attached toSurface Of Brainstem:

There are 31 pairs of spinal nerves in the human body:

8 cervical,

12 thoracic,

5 lumbar,

5 sacral, and

1 coccygeal.

Spinal Nerve:

Site of Emergence from Vertebral Canal

  • These emerge through the intervertebral foramina on the sides of adjacent vertebrae.

  • The uppermost pair passes between the occipital bone and the atlas.

  • The lowest pair passes through the intervertebral foramen in the coccyx.

Structure Of A Typical Spinal Nerve:

Common to spinal nerves at all levels : Roots

  • Each spinal nerve has two nerve roots (except the first, which has no sensory root).

  • The root in the front, called anterior or ventral root, transmits impulses from the spinal cord to the muscles.

  • The root in the back, known as the posterior or dorsal root, carries sensory information (about touch, position, pain, and temperature) from the body (receptors) to the spinal cord.

Structure of a typical spinal nerve

The Anterior Root Of Each Spinal Nerve :

  • Made up of nerve fibers which come from the anterior gray column of the spinal cord.

  • These nerve fibers are axons of neurons having cell bodies in anterior gray horn of spinal cord.

  • Consists solely of motor nerve fibers supplying ultimately a skeletal muscle fiber.

  • Also Called Efferent Root As Fibers Are Conveying Messages Away From CNS.

The Posterior Root Of Each Spinal Nerve:

  • Consists of fibers which lead to the posterior gray column of the cord.

  • These fibers are all sensory fibers bringing impulse from some peripheral receptors.

  • Also called afferent root as fibers are conveying messages towards CNS.

  • On the posterior root there is a small bulge called the posterior root ganglion. It is produced by the cell bodies of the pseudo unipolar neuron with the peripheral process innervating the receptor and central process entering the cord.

Parts Of A Typical Spinal Nerve

Trunks & Rami:

  • Within the spinal canal or vertebral canal the anterior and posterior roots unite to form the trunk of a spinal nerve which is a mixed nerve having both sensory and motor fibers.

  • Each spinal nerve leaves the vertebral canal through intervertebral foramen and divides into two branches called rami.

Branches Of The Spinal Nerves:

Posterior Primary Ramus:

  • Smaller of the two branches.

  • Contains both motor and sensory fibers.

  • Curves sharply backwards.

  • Divides into many small branches to supply the muscles and the skin of the back (dorsal body wall).

Branches Of The Spinal Nerves :

Anterior Primary Ramus:

  • The larger of the two branches.

  • Contains both motor and sensory nerve fibers.

  • Runs forward in the tissues, dividing into many small branches which serve the muscles and the skin of anterolateral body wall including limbs.

Fate of anterior primary ramus:

  • In the lower neck region and the lumbar region the anterior rami of the spinal nerves are very large indeed. In each of these regions they fuse together in groups in a complicated way to form nerve plexuses. From these plexuses spring the series of large nerves which supply motor nerve fibers to the muscles, and sensory nerve fibers to the skin of the arms and the legs.

  • C1 - C4: form cervical plexus, supply neck.

  • C5 - C8 & T1: form brachial plexus, supply upper limb.

  • TI - T12 & L1: supply anterolateral boy wall(forms no plexus).

  • L1 - L4: form lumbar plexus, supply lower limb.

  • L4, L5, S1 - S4: form sacral plexus, supply lower limb and perineum.

  • S4, S5 & all Co.

Rami Communicans:

  • Rami communicans (plural rami communicantes) is the term used for a nerve which connects two other nerves.

  • It almost always refers to a communicating branch between a spinal nerve and the sympathetic trunk.

  • More specifically, it usually refers to one of the following:

  • Gray ramus communicans

  • White ramus communicans

White Ramus Communicans:

  • The thoracic and the first and second lumbar nerves each contribute a branch, white ramus communicans to the adjoining sympathetic ganglion.

  • They contain myelinated preganglionic sympathetic fibers with cells of origin located in lateral gray column of spinal cord extending from T1-L2

  • Unlike the gray rami, white rami communicantes do not extend below L2.

Gray Ramus Communicans:

  • Each spinal nerve receives a branch, gray ramus communicans, from the adjacent ganglion of the sympathetic trunk.

  • They contain unmyelinated postganglionic sympathetic fibers that join the spinal nerve at the level of origin,

  • Or ascend or descend to join spinal nerve above the neck or belowL2


Above T1 and below L2, the sympathetic trunk is attached to the spinal nerve ONLY by gray rami communicantes

There are NO white rami in neck above T1 and below L2

The fibers in the grey ramus are distributed via the branches of spinal nerve to

  • Bloodvessels(Vasomotor),

  • Sweat Glands(Sudomotor) AndArrector Pili Muscles (Pilomotor )

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