Neuro-anatomy

Nerve Plexuses

By

Khaled Na3im

The C.N.S = Brain + Spinal Cord

• C.N.S tissue is enclosed within the skull and vertebral column

• The C.N.S.is protected by bones, meninges, and cerebrospinal fluid (CSF)

• Epidural space is the space between the bones and the Dural sheath (dura mater)

THE MENINGES

• The meninges are a set of 3 connective tissue layers that surround the brain and spinal cord.– The dura mater – toughest & outermost– The arachnoid mater – middle layer

containing most major blood vessels and cerebrospinal fluid.

– The pia mater – inner most layer with fine blood vessels.

Meninges

Spinal Cord

• Cervical and lumbosacral enlargements – sites where nerves serving the limbs emerge

• Conus medullaris – terminal portion of the spinal cord

• Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

• Spinal nerves – 31 pairs

Spinal Gray Matter

• Gray matter consists of neurons, unmyelinated processes, and neuroglia

• Gray commissure – connects masses of gray matter; encloses central canal

• Posterior (dorsal) horns – interneurons (sensory)• Anterior (ventral) horns – some interneurons but

mostly somatic motor neurons• Lateral horns – contain autonomic nerve fibers

Gray Matter and Spinal Roots

Cross Section of Spinal Cord

Reflex Arc• Basic functional unit of nervous system and simplest

portion capable of receiving a stimulus and producing a response

• Automatic response to stimulus, occurs without conscious thought.

• Components– Action potentials produced in sensory receptors transmitted to– Sensory neuron. To-– Interneurons. To-– Motor neuron. To-– Effector organ which responds with a reflex

Variety of Reflexes• Some integrated within spinal cord; some

within brain

• Some involve excitatory neurons yielding a response; some involve inhibitory neurons that prevent an action

• Higher brain centers can influence, suppress, or exaggerate reflex responses

Spinal Nerve Organization

• Dorsal half – sensory roots and ganglia

• Ventral half – motor roots

• Dorsal & ventral roots fuse laterally to form spinal nerves

Cross-Sectional Anatomy of Spinal Cord

Spinal Nerves• 31 pairs of mixed nerves supply all parts of

body except head• Named according to their point of issue

– 8 cervical (C1-C8)

– 12 thoracic (T1-T12)

– 5 Lumbar (L1-L5)

– 5 Sacral (S1-S5)

– 1 Coccygeal (C0)

Spinal Nerve• Each spinal nerve connects to the spinal

cord via two medial roots

• Each root forms a series of rootlets that attach to the spinal cord

• Ventral roots arise from the ventral horn and contain motor (efferent) fibers

• Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves: Roots

Nerve Plexuses• All ventral rami except T2-T12 form interlacing

networks called plexuses• Plexuses are found in the:

– cervical, – brachial, – lumbar, – and sacral regions

• 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

Cervical Plexus

• The 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 major motor and sensory nerve of the diaphragm

Cervical Plexus

Figure 13.8

Brachial Plexus

• Formed by ventral rami of C5-C8 and T1 (C4 and T2 may also contribute to this plexus)

• It gives rise to the nerves that innervate the upper limb ( except trapezes is supplied by accessory nerve and skin of tip of shoulder )

• The major ones :– Axillary n. - Nerve to serratus ant.– Radial n. - Nerve to L.D.– Musculocutaneous n.– Ulnar n. - Median n.

Brachial Plexus

• C4 from cervical plus C5-T1• Five ventral rami 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.

Brachial Plexus: Distribution of Nerves

Lumbosacral Plexus(Lumbar and Sacral Plexuses)

• Arises from L1-L4 & L4-S4 and innervates the thigh, abdominal wall, psoas muscle, the buttock, lower limb, pelvic structures, and the perineum

• The major nerves are the – Femoral (Lumbar)– Obturator (Lumbar)– Sciatic (Sacral)

• Tibial • Common fibular (peroneal)

QuestionsWhich of the following is NOT a nerve

plexus? 

A. The thoracic rami 

B. The lumbosacral plexus 

C. The cervical plexus  

D. The brachial plexus 

Questions

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

Questions

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

Questions

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

Questions

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

Questions

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

 True False

Questions

A sensory nerve transmits afferent impulses to the CNS. 

 True False