ANATOMY & PHYSIOLOGY Ch 12 & 13 Spinal Cord and Spinal Nerves 1

ANATOMY & PHYSIOLOGY

Ch 12 & 13

Spinal Cord and Spinal Nerves

1

Spinal Cord• This part of the central nervous system

(CNS) is enclosed in the vertebral cavity• It s protected by:

– Vertebrae – Meninges –

• dura, arachnoid and pia • covered in previous lecture

– Cerebrospinal Fluid (CSF)• Epidural space – space between the

vertebrae and the dural sheath (dura mater)

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Spinal Cord• The spinal cord extends inferiorly

from the medulla oblongata to the lumbar region

• Along its length there are two notably thicker regions: The Cervical and Lumbosacral enlargements – These are sites where the

nerves serving the limbs emerge• Conus medullaris – the end of the

spinal cord in the lumbar region• Cauda equina – the tail-like

collection of spinal nerve roots inferior to the conus medullaris

• Spinal nerves (PNS) will be covered below.

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Spinal Cord Gray Matter• Gray matter consists of cell bodies, unmyelinated processes,

and neuroglia• Gray commissure – connects masses of gray matter on the left

and right• The central canal runs through the gray commissure• Posterior (dorsal) horns – interneurons (sensory)• Anterior (ventral) horns – some interneurons but mostly somatic

motor neurons• Lateral horns – contain autonomic nerve fibers

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Gray Matter and Spinal Roots

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Cross Section of Spinal Cord

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Reflex Arc• The reflex arc is the simplest network of cells capable of receiving a stimulus

and producing a response

• A reflex is an automatic response to a stimulus that occurs without conscious thought.

• Components of a reflex arc– Sensory receptor is stimulated and transmits an action potential to – Sensory neuron. (in the case of pain reception 2-in-1 cell) To-– Interneurons. To-– Motor neuron. To-– Effector organ which responds with a reflex

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Spinal Nerves• 31 pairs of spinal nerves supply all of the body except head• Each nerve is named according to its nearby vertebra

– 8 cervical (C1-C8)

– 12 thoracic (T1-T12)

– 5 Lumbar (L1-L5)

– 5 Sacral (S1-S5)

– 1 Coccygeal (C0)

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Spinal Nerves• Each spinal nerve is formed from two roots

– Dorsal root – sensory roots and ganglia– Ventral root – motor roots

• A dorsal root and a ventral root fuse laterally (on the side) to form spinal nerves

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Spinal Nerves: Roots• Each spinal nerve arises from the spinal cord via two roots• Each root is formed from a set of rootlets that attach to the

spinal cord • Ventral roots arise from the ventral horn of the spinal cord and

contain motor (efferent) fibers• Dorsal roots arise from sensory neurons in the dorsal root

ganglia and contain sensory (afferent) fibers

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Spinal Nerves: Roots

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Spinal Nerve Rami• Spinal nerves are actually quite short. • Two spinal nerve roots give rise to each spinal nerve.

– The dorsal root carries afferent (sensory) neurons.– The ventral root carries efferent (motor) neurons. – So each spinal nerve is a mixed nerve carrying both sensory

and motor neurons. • Each spinal nerve, in turn, gives rise to two spinal nerve rami

(singular: ramus). – Both the dorsal and ventral spinal nerve rami carry both

sensory and motor neurons.

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Spinal Nerve: Rami

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Ventral Rami Form Nerve Plexuses• All ventral rami except T2-T12 form interlacing networks called

plexuses (i.e. spaghetti bowls)• Plexuses are found in the:

– Cervical region– Brachial region– Lumbar region– Sacral region– The last two will be dealt with as the Lumbosacral plexus

• Each resulting branch of a plexus contains fibers from several spinal nerves

• Each muscle receives a nerve supply from more than one spinal nerve

• Damage to one spinal segment cannot completely paralyze a muscle

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Cervical PlexusThe cervical plexus is formed by

ventral rami of C1-C4

Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders

The most important nerve of this plexus is the phrenic nerve .

The phrenic nerve is the major motor and sensory nerve of the diaphragm.

As long as an injury to the spinal cord occurs inferior to the emergence of the phrenic nerve the ability to breathe independently may be spared.

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Brachial Plexus• Formed by C5-C8 and T1

– In some individuals C4 and T2 may also contribute to this plexus.

• The brachial plexus gives rise to nerves supplying the arm.• The five major branches of this plexus are:

– Axillary– Radial– Musculocutaneous– Ulnar– Median

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Brachial Plexus for PT and OT• Additional detail is provided for

those headed to PT and OT programs:

• Five ventral rami (C5-T1) form • three trunks that separate into

six divisions then form • three cords that give rise to:• Branches/nerves

– Axillary– Radial– Musculocutaneous– Ulnar– Median– Smaller nerves, too.

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Lumbosacral Plexus(Lumbar and Sacral Plexuses)

• Arises from L1-S4 and innervates the thigh, abdominal wall, psoas muscle, the buttocks, lower limb, pelvic structures, and the perineum

• The major nerves are the – Femoral (Lumbar)

– Obturator (Lumbar)

– Sciatic (Sacral)

• Tibial • Common fibular (peroneal)

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Practice Questions

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Which of the following is NOT a nerve plexus? 

A. The thoracic rami 

B. The lumbosacral plexus 

C. The cervical plexus  

D. The brachial plexus 

21

Which of the following nerves does NOT arise from the brachial plexus? 

A. The musculocutaneous nerve 

B. The radial nerve 

C. The axillary nerve 

D. The ulnar nerve 

E. The phrenic nerve

22

The muscle fiber or gland cell that responds to the efferent impulses is known as the  

A. Effector  

B. Motor neuron 

C. Integration center 

D. Sensory neuron 

E. Receptor

23

The basic pattern of the CNS in the spinal cord is a central cavity surrounded by a gray matter core and a white matter layer lying outside of that. 

 True False

24

Although the pairs of spinal nerves correspond to the number of vertebrae in the each area, there are eight pairs of cervical cranial nerves, but only seven cervical vertebrae. 

 True False

25

Each muscle in a limb receives its nerve supply from one spinal nerve. 

 True False

26

A sensory neuron transmits afferent impulses to the CNS. 

 True False

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