12-1

Structure of Nerves (including roots and rami and plexuses)

• Consist of– Axon bundles/nerve fibers– Schwann cells– Connective tissue– Blood vessels

• Endoneurium: surrounds individual neurons

• loose CT with capillaries (for neurons)

• Perineurium: Surrounds fascicles

• blood vessels Epineurium: surrounds the entire nerve

• Dense CT

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Spinal Cord and Spinal NervesChapter 12

Spinal Cord Functions• carry/transmit sensory and

motor impulses between spinal nerves and the brain– in columns/white matter

• integration center for spinal reflexes– in gray matter

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Spinal Cord • Extends from foramen magnum to ~L1-L2

• Vertebral canal continues length of sacrum– a portion of the vertebral canal is not

occupied by the actual spinal cord– creates the opportunity for spinal

tap/lumbar puncture (see next side)

• Gives rise to 31 pairs of spinal nerves– Exit through intervertebral and sacral foramina

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Lumbar puncture

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Spinal Cord • Not uniform in diameter – Cervical enlargement:

• supplies upper limbs

– Lumbar enlargement: • supplies lower limbs

• Conus medullaris: – Tapered/pointed inferior end of

cord.

• Cauda equina: – Roots and nerves extending down

vertebral canal below L2 that exit intervertebral and sacral foramina

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12-8

Spinal Nerves• Thirty-one pairs of spinal

nerves• First pair exit vertebral

column between skull and atlas

• Exit vertebral canal through intervertebral and sacral foramina

• 8 pair cervical, 12 pair thoracic, 5 pair lumbar, 5 pair sacral, & coccygeal

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Rami of spinal nerve (thoracic region): rami branch off the spinal nerve

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Cervicalnerves

Thoracicnerves

Lumbarnerves

Sacralnerves

Coccygealnerves

(a) Posterior view

C1

T1

2345678

2345

6

78

9

10

11

12

L1

2

3

45

T2

S1

L5L5

S1

L4 L4

L3L3

L2L2

S3

S3

C8C8

T1T1

C7C7

T3T4T5T6

T7

T8

T9

T10

T11

T12

L1

C6C6

C5

C4

C4C3

C2

S1

C5

T1T1

S4

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Dermatomal Map• Dermatomal map: skin area supplied with somatic

sensory innervation by spinal nerves—general trends

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Plexuses: intermingling nerves arising from multiple anterior rami

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Branches of

Spinal Nerves

Dorsal/posterior Ramus: • innervate deep muscles of the trunk

responsible for movements of the vertebral column and skin near dorsal midline

Ventral/anterior Ramus: • innervates structures anterior and lateral to

spinal cord.– Thoracic region: form intercostal nerves

that innervate the intercostal muscles and the skin over the thorax

– Other regions they form plexuses (intermingling of nerves).

• Ventral rami C1-C4= cervical plexus• Ventral rami C5-T1= brachial plexus• Ventral rami of L1-L4= lumbar plexus• Ventral rami of L4-S4= sacral plexus• Ventral rami S4 & S5= coccygeal

plexus

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Plexus

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Cervical Plexus

• C1-C4 • Innervates superficial

neck structures, skin of neck, posterior portion of head

Selected nerve of cervical plexus--Phrenic nerve

– Innervate diaphragm• Sole motor innervation of

diaphragm

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Brachial Plexus• C5-T1 and some from C4 • Nerves arising from:

– Axillary– Radial– Musculocutaneous– Ulnar– Median

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Lumbar Plexus• Lumbar plexus: ventral rami

of L1-L4• Selected Nerves:

– Femoral– Obturator

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sacral Plexus• Sacral plexus: ventral rami

of L4-S4– sometimes considered

together because of their close relationship

• major nerves exit and enter lower limb

• Selected Nerves:– Sciatic

• Tibial• Common fibular (peroneal)

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Cross Section of Spinal Cord

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Cross Section of Spinal Cord• Gray matter: mostly glial cell, cell bodies, dendrites

Horns• Posterior (dorsal)

– sensory neurons enter the cord

• Anterior (ventral)– cell bodies of somatic motor neurons

• Lateral – associated with ANS—cell bodies of visceral motor neurons– distinct lateral horn not present in all regions of cord

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• Commissures: connections between left and right halves (of CNS)– Gray & White Commisures

• Roots: nerves connecting to the cord– Dorsal (posterior) root

• Sensory Nerve• contains sensory neurons (unipolar)

– Dorsal Root Ganglion: cell bodies of sensory neurons

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• Ventral (anterior) Root– motor nerve

• axons of motor neurons (multipolar)

• Spinal Nerve: Two roots merge to form a spinal nerve– then passes through intervertebral foramen– mixed nerves

• axons of both motor and sensory neurons

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Cross Section of Spinal CordWhite matter: myelinated axons forming nerve tracts

– columns (funiculi): • divided into tracts (fasciculi; pathways)• Carry information:

– to and from the brain (ascending and descending)– to and from other regions of the spinal cord

Figure 16.4 locations of various neurons within spinal cord (e.g., somatic motor, visceral motor [autonomic], somatic sensory)

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Pathways through roots and cord

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Ascending and Descending Tracts/Pathways

= sensory

= motor

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Specific Ascending Pathways within spinal cord white matter

• Anteriolateral System — within anterior and lateral columns– Spinothalamic Tracts: somatic sensory information

from cutanous receptors to Thalamus• pain, crude touch, temperature

• Dorsal Column System — in dorsal columns– proprioception, fine touch, two point discrimination

• Spinocerebellar--periphery of lateral and dorsal columns– proprioception to cerebellum

• Anteriolateral System —Spinothalamic Tracts: somatic sensory information from cutanous receptors to Thalamus

• pain, pressure, crude touch, temperature

• Dorsal Column System – proprioception, fine-touch,

two point discrimination, pressure

• Spinocerebellar--periphery of lateral and dorsal columns– proprioception to cerebellum

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12-32

Descending Pathways through spinal cord white matter

• Corticospinal (pyramidal)—within dorsal and anterior columns– voluntary movements

• Indirect Pathways– involuntary movement, upright posture,

balance, walking, reflexive movements of head and neck in response to visual and auditory stimuli

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roots

Spinal nerve

ramusplexus“nerves”

e.g., femoral, median, phrenic, sciatic

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Spinal Meninges and Protection of cord

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Cross Section of Spinal Cord

Figure 16.2

12-39

Meninges & Associated Spaces • Connective tissue membranes

surrounding spinal cord and brainEpidural Space: – Contains blood vessels, areolar CT

and fat.Dura mater: – continuous with epineurium of the

spinal nervesArachnoid mater: thin and wispySubarachnoid space: – Contains CSF and blood vessels

within web-like strands of arachnoid tissue

Pia mater: – bound tightly to surface of brain

and spinal cord. • filum terminale: anchors spinal

cord to coccyx—longitudinal support

• denticulate ligaments: attach the spinal cord to the dura mater laterally—lateral support

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Protection of the Spinal Cord

Physical Protection• Vertebrae

– rigid protection• Epidural Space with adipose

– padding/cushioning• Meninges• CSF

– cushioning• Filum Terminale

– longitudinal support• Denticulate Ligaments

– lateral support

Chemical Protection• Blood Brain Barrier—blood CNS barrier

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Somatic Motor and Sensory:--Single sensory neurons carry sensory impulses from the receptor/site of stimulus all the way into the spinal cord.

--Single motor neurons carry motor impulses from the spinal cord all the way to the effector muscle

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Pathways through roots and cord

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ReflexesAutomatic responses to specific stimuli (do

not require conscious thought/processing)

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

Types:• Learned• Innate (typically homeostatic)

– Spinal, integrated in spinal cord– Cranial, integrated in brain

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Spinal Reflexes

• represent some of the most basic nerve pathways and CNS integration

• Brain not necessary for spinal reflexes to occur– Although brain can modify (suppress or enhance )

spinal reflexes.

• The nerve pathway is called a reflex arc

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Reflex Arc Components1. Electrical Impulse (Action potentials) produced in sensory receptors

transmitted to2. Sensory neuron. To-3. Interneurons—in most cases. To-4. Motor neuron. To-5. Effector organ which responds with a reflex

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Stretch/Extensor Reflexes• Monosynaptic:

– Two neurons– One synapse– Sensory neuron synapses directly with motor

neuron

1. Stretch Receptor (Muscle spindle):2. Sensory neuron3. synapse with motor neurons of the spinal cord4. Motor neuron innervates muscle that was

stretch causing contraction

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Stretch/Extensor Reflex

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Withdrawal/Flexor Reflex:Function is to remove a body limb or other part from a painful stimulus.

• Polysynaptic:– 2+ synapses– 3+ neurons– Interneuron(s) between sensory and motor neuron

Variations:• Reciprocal inhibition

– causes relaxation of antagonistic extensor muscle when flexor muscle contracts.

• Crossed extensor reflex: – when a withdrawal reflex is initiated in one lower limb, the crossed extensor reflex causes extension of

opposite lower limb.

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Withdrawal Reflex

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Withdrawal Reflex with Reciprocal Inhibition

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Relationship of Brain and Spinal Cord Reflexes

• Sensory information goes to brain; e.g., pain.

• Descending tracts from brain carry info to reflexes.

• Neurotransmitters produce inhibitory or excitatory effects modifying the reflex.

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PNS Disorders• General disorders

– Anesthesia: loss of sensation• Hyperesthesia: increased sensitivity to pain, pressure, light• Paresthesia: tingling, prickling, burning• Neuralgia: nerve inflammation causing stabbing pain• Sciatica: pain radiating down back of thigh and leg

• Infections– Herpes: skin lesions– Shingles or herpes zoster: adult disease of chickenpox, virus latent in

peripheral ganglia– Poliomyelitis: infantile paralysis– Anesthetic leprosy: bacterial infection of peripheral nerves– Diptheria: demylenation, motor/sensory decline, resp and heart failure

• Genetic and autoimmune disorders– Myasthenia gravis: results in fatigue and muscular weakness due to

inadequate ACh receptors