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Chapter 12

The Central Nervous System

Spinal CordReflexesPlexuses

Marieb’s HumanAnatomy and

PhysiologyNinth Edition

Marieb Hoehn

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Lecture Overview• The spinal cord

– Spinal cord structure– Spinal meninges– Cross-sectional anatomy of the spinal cord– Ascending and descending spinal tracts– Reflexes– Spinal Nerves– Nerve plexuses

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Spinal Cord Structure• extends from the foramen magnum to 2nd lumbar vertebra

• cervical and lumbar enlargements

• cauda equina (horse’s tail) – thin nerve fibers that exit at different level than they arise (note that spinal cord does not extend into this area of the lumbar spine). Begins around L2 and extends to S5. Good area for lumbar puncture and collection of CSF.

Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Overview of the Spinal Cord

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What type of vertebra is shown above?

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Meninges of the Spinal Cord

- dura mater – outer, tough (anchoring dural folds)

- arachnoid mater – web-like

- pia mater – inner, delicate

- Subdural space – like interstitial fluid

- Subarachnoid space – CSF

Figure from: Marieb Human Anatomy & Physiology, Pearson 2013

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Organization of Spinal Cord

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

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Organization of Spinal Gray MatterFigure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Posterior gray horn = sensory

Lateral gray horn = visceral motor

Anterior gray horn = somatic motor

Anterior root

Posterior root

Gray matter = dendrites and unmyelinated axons

Spinal Cord and Nerve Roots

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Ventral root - axons of motor neurons whose cell bodies are in spinal cord

Dorsal root - axons of sensory neurons in the dorsal root ganglion

Dorsal root ganglion - cell bodies of sensory neurons

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Organization of Spinal White Matter

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

White matter = Myelinated axons

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Functions of the Spinal Cord

a. is a conduit for nerve impulses to and from the brain (nerve tracts) b. is a center for spinal reflexes

The spinal cord…

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Tracts of the Spinal Cord• Ascending tracts conduct sensory impulses to the brain• Descending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glands

All the axons in a tract share a common origin and destination

Tracts are usually named for their place of origin (1st) and termination (2nd)

Most axons cross over during their travel. What will this mean clinically?

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Ascending Tracts• fasciculus cuneatus/gracilis - fine touch, pressure, body movement - cross (decussate) in medulla

• spinothalamic - crude pain, temperature, pressure, and touch - cross in spinal cord

• spinocerebellar - subconscious coordination of muscle movements (1st and 2nd order neurons) - ipsilateral

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2

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Decussation

(crossing over)

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1st, 2nd, and 3rd Order Sensory Neurons

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2

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1st order neuron – from receptor to the spinal cord (cell bodies are located in the dorsal root ganglion)

2nd order neuron – from spinal cord to thalamus

3rd order neuron – from thalamus to sensory cerebral cortex - terminate in the cerebral cortex

Decussation

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Descending Tracts• corticospinal (direct, pyramidal) - voluntary movement of skeletal muscles - lateral cross in medulla - contralateral

• reticulospinal (indirect, extrapyramidal) - subconscious muscle tone, sweat glands - some lateral cross, anterior do not cross

• rubrospinal (indirect, extrapyramidal) - subconscious regulation of upper limb tone/movement - cross in brain (less important in humans)

Upper motor – begin in precentral gyrus of cortex

Lower

Upper MN – Cerebral cortex to spinal cord

Lower MN – Spinal cord to effector

Decussation

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Somatic Reflex ArcsReflexes – automatic, subconscious, quick, stereotyped responses to stimuli either within or outside the body

They occur in both the somatic and autonomic divisions

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Knee-jerk (Stretch) Reflex

• helps maintain posture

Monosynaptic, Ipsilateral

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Withdrawal Reflex

• protective

Polysynaptic, Ipsilateral, Intersegmental

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Crossed-Extensor Reflex

• flexor muscles contract• flexor muscles on opposite side inhibited• extensor muscles on opposite side contract for balance

Polysynaptic, Contralaterial, Intersegmental

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Spinal Nerves• mixed nerves

• 31 pairs• 8 cervical (C1 to C8)• 12 thoracic (T1 to T12)• 5 lumbar (L1 to L5)• 5 sacral (S1 to S5)• 1 coccygeal (Co)

THIRTY ONEderful flavors of spinal nerves!

Below cervical spine, each spinal nerve leaves inferior to the same numbered vertebra Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

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Spinal Nerves

These are ‘mixed’ nerves (sensory and motor nerve fibers)

Spinal nerves are named according to the level of the spinal cord from which they exit.

Ventral (anterior) ramus leads to formation of plexuses

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Cervical PlexusNerve plexus – complex network formed by anterior (ventral) branches of spinal nerves; fibers of various spinal nerves are sorted and recombined

Cervical Plexus• C1-C4• lies deep in the neck• supplies muscles and skin of the neck• contributes to phrenic nerve (diaphragm)

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Contains both sensory and motor fibers

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Brachial Plexus• C5-T1• lies deep within shoulders• supplies shoulder and upper limbs• musculocutaneous nerves

• supply muscles of anterior arms and skin of forearms

• ulnar nerves• supply muscles of forearms and hands• supply skin of hands

• radial nerves• supply posterior muscles of arms and skin of forearms and hands

• axillary nerves• supply muscles and skin of superior, lateral, and posterior arms

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Lumbosacral Plexus

• T12 – S5• supplies pelvis and lower limbs• extend from lumbar region into pelvic cavity• obturator nerves

• supply adductors of thighs

• femoral nerves• supply muscles and skin of thighs and legs

• sciatic nerves• supply muscles and skin of thighs, legs, and feet

May be separated into lumbar, sacral, pudendal, and coccygeal plexuses

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Classification of Nerve Fibers

Table from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

SOMAtic

- Skin- BOnes- Muscles- Articulations

SAME Sensory = Afferent Motor = Efferent