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CHAPTER 13The Spinal Cord &

Spinal Nerves

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INTRODUCTION• Mediate reactions to environmental changes.

• Process reflexes

• Site for integration of EPSPs and IPSPs that arise locally or are triggered by nerve impulses from periphery and brain

• Conduction pathway for sensory & motor nerve impulses

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I. SPINAL CORD ANATOMY• Protective structures

– vertebral column provides a bony covering of spinal cord• vertebral foramen form vertebral cavity• added protection from vertebral ligaments & CSF

– meninges• CT layers surrounding brain & spinal cord• 3 layers

– dura mater = outermost layer» epidural space

– arachnoid mater = middle layer» subdural space contains ISF

– pia mater = innermost layer» vascularized» subarachnoid space contains CSF

– denticulate ligaments prevent displacement of cord

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Applications• Subarachnoid space is between the

arachnoid mater & pia mater and contains cerebrospinal fluid (CSF).

• Inflammation of the meninges is known as meningitis.

• Removal of CSF from the subarachnoid space is called a spinal tap (lumbar puncture).– used to diagnose pathologies and to

introduce antibiotics, contrast media, anesthetics, and chemotherapeutic drugs.

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I-B. Ext. Anat. of Spinal Cord

• Begins as a continuation of medulla oblongata and terminates at about L2 vertebra in an adult

• Spinal nerves– 31 pair emerge @ regular intervals from vertebral cav.– roots = bundles of axons that connect nerves to cord

• posterior (dorsal) root contains only sensory neurons

– sensory info from skin, muscles, organs to CNS– post. (dorsal) root ganglion = swelling of root

that contains cell bodies of sensory neurons• anterior (ventral) root contains axons of motor

neurons– CNS to effector organs– no ventral ganglion

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I-C. Internal Anat. of Spinal Cord

• Gray matter surrounded by white matter• Gray matter

– cell bodies, unmyelinated axons and dendrites – central canal = center of gray matter

• filled with CSF• extends entire length of spinal cord

– nuclei = functional groups of neuronal cell bodies• sensory• motor

– horns• anterior gray horns contain somatic motor

neurons• posterior gray horns contain autonomic/somatic

sensory neurons• lateral gray horns contain autonomic motor

neurons

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I-C. Internal Anat. of Spinal Cord

• White matter

– bundles of myelinated axons of w/ common origin/destination form tracts in CNS

• sensory (ascending) tracts carry sensory info to brain

• motor (descending) tracts carry impulses from brain to effector

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Sensory and Motor Tracts• Figure 13.4 shows the principal sensory and

motor tracts in the spinal cord. (detailed in Chapter 16)

• Sensory (ascending) tracts conduct nerve impulses toward the brain.– the lateral and anterior spinothalamic tracts and

the posterior column tract.

• Motor (descending) tracts conduct impulses down the cord.– Direct pathways include lateral and anterior

corticospinal and corticobulbar tracts.– Indirect pathways include rubrospinal, tectospinal,

and vestibulospinal tracts.

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II. SPINAL NERVES• Spinal nerves are part of the PNS!!!

• Connective tissue coverings– endoneurium covers individual axons (superficial

to myelin sheath)– perineurium surrounds groups of axons (fascicles)– epineurium is outer covering of entire nerve– blood vessels associated w/ peri-/epineuria

• Distribution– branches arise just outside vertebral column

• posterior (dorsal) ramus• anterior (ventral) ramus

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II. SPINAL NERVES• Distribution

– plexuses• network of interlacing, adjacent axons• do not directly connect to structures they innervate• 5 plexuses

– cervical: skin/muscles of head/neck/upper chest– brachial: shoulders/upper limbs– lumbar: lower limbs/abdominal wall– sacral: buttocks/perineum/lower limbs– coccygeal: small area of skin in coccygeal region

– intercostal nerves• T2-T12 rami do not form plexuses• directly connected to structures they supply

• Dermatomes provide sensory input from regions of skin

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III. SPINAL CORD PHYSIOLOGY

• Sensory & motor tracts– naming of tracts indicates direction of information

flow• example = anterior spinothalamic tract• impulses travel from spinal cord toward brain

– two routes for sensory input from spinal cord to brain• spinothalamic tracts• posterior columns

– sensory systems keep CNS apprised of Δ in environ.– motor pathways

•direct: info from cortex; results in controlled voluntary movements (corticospinal/corticobulbar)

• indirect: info from brain for autonomic movements– Fig 13.12 summarizes functions of cord & nerves

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III-B. Spinal Reflexes & Reflex Arcs• Spinal cord = integrating center for spinal reflexes

• Terms– reflex =fast, automatic response occurring in response to

particular stimulus– Reflexes may be spinal, cranial, somatic, or autonomic.– reflex arc = pathway followed by nerve impulses in

generation of reflex• sensory receptor produces graded potential in

response to stimulus• sensory neuron sends info to gray matter of cord• integrating ctr = synapse btwn sensory/motor neurons• motor neuron carries impulses from integrating ctr to

effector organ• effector = body part that responds to motor impulse

– somatic effectors = skeletal muscles– autonomic effectors = visceral organs

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III-B. Spinal Reflexes & Reflex Arcs

Somatic spinal reflexes (4)1) Stretch reflex

– causes contraction of skeletal muscle in response to stretching

– prevents overstretch of muscle (feedback mechanism)– sensory receptors = muscle spindles– spindles detect Δ in length sends info to CNS if

strong enough, motor neuron generates a.p. results in ACh release muscle contracts to prevent overstretch

– antagonistic muscles relax (via inhibitory potentials) as part of reflex (reciprocal innervation)

– brain sets overall level of muscle tone by adjusting sensitivity of spindles• tone = degree of contraction present in resting muscle

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2) Tendon reflex– controls muscle tension by causing muscle

relaxation when muscle force becomes too extreme

– sensory receptors = Golgi tendon organs• detect change in tension caused by

stretch/contract.

– activation of inhibitory interneuron results in release of inhibitory NT @ NMJ (hyperpolarization of postsynaptic neuron) muscle relaxation results

– in this case reciprocal innervation causes contraction of antagonist muscle

III-B. Spinal Reflexes & Reflex Arcs

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3) Withdrawal (flexor) reflex– protective reflex that moves a limb to avoid pain – results in contraction of flexor muscles to

move a limb to avoid injury or pain• EX: stepping on tack, touching hot burner• ACh release causes contraction of muscle to

w/draw from stimulus

4) Crossed extensor reflex helps maintain balance during withdrawal reflex– causes synchronized extension of the joints of

one limb and flexion of the joints in the opposite limb (Figure 13.9)• EX: step on tack, one leg flexes, other leg

extends to maintain balance• ACh release @ NMJ results in contraction of

extensors

III-B. Spinal Reflexes & Reflex Arcs

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Clinically Useful ReflexesPresence or absence of certain reflexes is useful in

diagnosing disorders/injuries in nervous tissue• Patellar reflex: extension of knee results from tapping

patellar ligament– blocked by damage to L2-L4 segment of spinal cord– absent in some cases of diabetes mellitus– exaggerated in certain brain injuries

• Achilles reflex: plantar flexion of foot in response to tapping of Achilles tendon– absence indicates damage to nerves supplying leg muscles– may be absent in diabetics, alcoholics & as result of

subarachnoid hemorrhage– exaggerated reflex indicates spinal cord compression in

cervical region or lesion in sacral region

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Clinically Useful Reflexes

• Babinski sign: pressure on sole of foot results in dorsiflexion of big toe and fanning of others– normal under age 1½ incomplete myelination– if present after age 1½ damage to corticospinal tract– (--) Babinski results in curling under of all toes

• Abdominal reflex: contraction of muscles in abdominal wall causes umbilicus to move in direction of stimulus– absence indicates lesions in corticospinal tract

• Pupillary light reflex (autonomic reflex)– pupils of both eyes ↓ in diameter when either eye is

exposed to light– absence indicates brain damage or injury