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Chapter 13:

The Spinal Cord, Spinal Nerves,

and Spinal Reflexes

2Figure 13–1

General Organization of the Nervous System

• Highly organized, very efficient

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Nervous System• Organization:

– CNS = brain and spinal cord– PNS = all other neural tissue

• Structures in the PNS1. Ganglia = collection of somas together in one

place

2. Nerves = bundles of axons

Nervous System• Structures in the CNS:

1. Center = collection of somas with a common function

2. Nucleus = a center with a visible boundary

3. Neural cortex = gray matter (somas) covering the brain

4. Tracts = bundles of axons with common origins, destinations and function

5. Columns/funiculi = large tracts in the spinal cord

6. Pathways = centers and tracts that link brain w/ body• Sensory pathways: receptor CNS• Motor pathways: CNS effector

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5Figure 13-2

Gross Anatomy of the Spinal Cord

• 45cm (18”) from brain to L2

• Inside vertebral canal– Stacked vertebral

foramen• Surrounded by

CT:– Spinal Meninges

• Support spinal cord• Protect spinal cord

6Figure 13–3

Spinal MeningesThree Layers:

1. Dura Mater2. Arachnoid3. Pia Mater

Functions:– Protect/support spinal cord– carry blood supply

All three layers are continuous with cranial meninges of the same name and the connective tissue around the spinal nerves.

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Spinal Meninges1. Dura Mater: Outermost

- Dense collagen fibers- Attaches to periosteum of occipital bone and coccyx

by coccygeal ligament

2. Arachnoid: Middle- Two layers

1. Arachnoid membrane: - Simple squamous epithelium, contacts dura

mater2. Arachnoid trabeculae:

- Collagen and elastin fibers that bind to pia mater- Fibers pass through the subarachoid space

which contains cerebrospinal fluid (CSF)

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Spinal Meninges3. Pia Mater: Innermost

- Fine mesh of collagen and elastin fibers bound to neural tissue

- Attached to arachnoid trabeculae- Dentriculate ligaments

- extend through arachnoid to dura mater to prevent lateral movement of the cord

9Figure 13–5a

Anatomy of the Spinal Cord

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Anatomy of the Spinal Cord• Posterior median sulcus

– Posterior surface bears a shallow longitudinal groove• Anterior median fissure

– Deeper groove along the anterior surface• Central canal: contains CSF for diffusion• Gray matter: somas, neuroglia, unmyelinated

axons– Posterior gray horn:

• somatic and visceral sensory nuclei– Anterior gray horn:

• somatic motor nuclei– Lateral gray horn:

• throacic and lumbar only, visceral motor nuclei (ANS)– Gray commissure:

• axons cross from on side of the cord to the opposite side

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Anatomy of the Spinal Cord• White Matter: myelinated axons

– Posterior white column/funiculus– Anterior white column/funiculus– Lateral white column/funiculus– All 6 column contains tracts:

• Ascending tracts: sensory to brain• Descending tracts: motor from brain• Transverse tracts: decussation

• Axons exit as spinal roots (dorsal +ventral = spinal nerve)– Dorsal root: sensory axons from receptor to CNS– Dorsal root ganglion: somas of the sensory neurons– Ventral root: motor axons from CNS to effectors

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KEY CONCEPT• Spinal cord has a narrow central canal

– surrounded by gray matter that contains sensory and motor nuclei

• Sensory nuclei are dorsal• Motor nuclei are ventral • Gray matter:

– is covered by a thick layer of white matter• White matter:

– consists of ascending and descending axons – organized in columns – containing axon bundles with specific functions

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Anatomy of the Spinal Cord• Spinal roots exit vertebral canal

through intervertebral foramen• Dorsal and ventral roots combine

to form spinal nerve

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Damage to which root of a spinal nerve would interfere with motor

function?

1. posterior root2. anterior root3. dorsal root4. ventral root

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Where is the cerebrospinal fluid that surrounds the spinal cord located?

1. epidural space2. subarachnoid space3. above the dura mater4. between the pia mater and

the nervous tissue of the brain

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Spinal Nerves• 31 pairs• Exit via intervertebral

or sacral foramen• Name for location of

exit on spin beginning between skull and C1

• Nerves:– C1-C8– T1-T12– L1-L5– S1-S5– Co1

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Spinal Nerves• Cord and column grow

together until age 4• Column continues to grow

but cord does not• Adult: cord ends at L1-L2• “stretched” spinal roots

after L2 = cauda equina• Lumbar puncture:

– “spinal tap” at L3-L4 draw CSF from subarachnoid space

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Intervertebral foramen Maintained by intervertebralDiscs between vertebrae

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• Herniated disc:– Nucleus pulposus ruptures through

anulus fibrosis– Compresses nerves in intervertebral

foramen and/or spinal cord in vertebral canal

• Slipped disc:– Intervertebral disc distorted or

displaced– Causes pressure

20Figure 13–6

Nerve Structure• Dorsal root + ventral root +

blood vessels + connective tissue

1. Epineurium- Outermost layer- Dense collagen fibers

2. Perineurium- Partitions that extend inward

from epineurium and divide axons into bundles called fascicles

3. Endoneurium- Innermost layer- Thin collagen fibers that

surround each axon

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Nerve Structure• Axons repair if cut, it will follow original path• Severed nerves do not usually repair

– Axons do not line up correctly• Spinal nerves branch off cord near to what

they innervate• Cervical and lumbar enlargements of cord

house cell bodies of motor neurons for muscles of appendages

• Dermatome: Region of skin surface– Innervate by one pair spinal nerves

22Figure 13–8

Dermatomes

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Nerve Plexus• Most spinal nerves do not go

directly to target:– Axons from multiple nerves

intermingle in a nerve plexus

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Spinal Nerve Plexus• Nerve plexus:

– Interwoven network of nerves

1. Cervical plexus:– Nerves C1-C5– Innervate muscles of

neck and diaphragm2. Brachial plexus:

– Nerve C5-T1– Innervate pectoral

girdle and upper limbs3. Lumbar plexus:

– Nerves T12-L4– Innervate pelvic girdle

and lower limbs4. Sacral plexus:

– Nerves L4-S4– Innervate lower limbs

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Trauma and Disorders• Often result form damage or pressure1. Paralysis:

– Loss of motor function– Disorder of ventral root or anterior gray horn

2. Paresthesia:– Sensory loss– Disorder of dorsal root or posterior gray horn– Complete cut results in loss of both motor and sensory below

injury3. Paraplegia:

– Sever between T1 and L4– Loss of lower limb function

4. Quadriplegia:– Sever in cervical – Loss of all limb function (above C5 can kill)

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Organization of Neural Pathways

• 10 million sensory neurons – receptor to CNS

• 500 thousand motor neurons – CNS to effector

• 20 billion interneurons – coordinate sensory and motor, they are between

sensory and motor neurons located in the CNS• Interneurons organized into neuronal pools

– functional groups with limited input sources (sensory) and output locations (motor)

• Spread of info organized into neural circuits

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5 Neural Circuits

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Reflexes

• Rapid, automatic nerve responses triggered by specific stimuli

• Used to maintain homeostasis• Simple reflex:

– Sensory perception in, motor response out

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Reflex Arcs = Single Reflex• Negative Feedback

– Action opposes stimulus as form of defense

– Fast response, but not always coordinated

30Figure 13–14

5 Steps in a Neural Reflex

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5 Steps in a Neural Reflex • Step 1: Stimulus Activates Receptor

– Receptor = specialized cell or dendrite of sensory neuron

– Receptor responds to a particular type of stimuli• physical or chemical changes

• Step 2: Sensory Neuron is Activated– Stimulation causes action potential on axon of

sensory neuron– Nerve impulse travels into spinal cord via dorsal

root

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5 Steps in a Neural Reflex • Step 3: Information Processing

– Simple case: sensory neuron synapses on motor neuron– More complex: sensory neuron synapses on an

interneuron

• Step 4: Motor Neuron is Activated – Motor neuron is stimulated to threshold– Action potential travels down motor neuron axon to the

effector tissue

• Step 5: Peripheral effector responds– Neurotransmitters released by motor neuron trigger

events in effector– E.g. muscle contraction

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Reflex Classification: 4 Ways to Classify

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4 Classifications of Reflexes1. By early development2. By type of motor response3. By complexity of neural circuit4. By site of information processing

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Response• Nature of resulting motor response:

– somatic reflexes:• involuntary control of nervous system

– superficial somatic reflexes »Stimuli originate at skin or mucous

membranes– stretch reflexes (deep tendon reflexes)

»Stimuli from overstretched tendon»e.g., patellar reflex

– visceral reflexes (autonomic reflexes):• control systems other than muscular system

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Complexity• Complexity of neural circuit:

– monosynaptic reflex:• sensory neuron synapses directly onto motor

neuron– polysynaptic reflex:

• at least 1 interneuron between sensory neuron and motor neuron

• Response delayed by each synapse but capable of more complex output

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Examples of Common Spinal Reflexes

1. Patellar Reflex2. Withdrawal Reflex

38Figure 13–15

1. Patellar Reflex• Monosynaptic stretch reflex• Carried on type A fibers: largest myelinated axons

– Carries action potential at the fastest rate• Sudden stretch of patellar ligament activates

muscle spindle signal quadriceps group to contract

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Stretch Reflex• Muscle spindles = sensory receptors

involved in the stretch reflex– Consist of a bundle of small, specialized

skeletal muscle fibers called intrafusal muscle fibers• Innervated by sensory and motor neurons

– Surrounded by a larger extrafusal muscle fiber responsible for• The resting muscle tone• The contraction of the entire muscle, when

greater levels of stimulation are present

40Figure 13–16

A Muscle Spindle

Axons of motor neurons innervating intrafusal fibers

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Muscle Spindle:Specialized Muscle Fiber

• Constantly signal CNS• Relaxed = signal less• Stretched = signal more

– threshold, trigger reflex arc• Prevent overstretching of muscle

and tendons• Aid in maintaining upright position

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2. Withdrawal Reflexes

• Complex polysynaptic spinal reflex• Consists of three parts:

1. Flexor reflex: flex to withdraw2. Reciprocal Inhibition: inhibit extensors3. Crossed extensor reflex: maintain

balance

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Withdrawal Reflexes• Pain flexor muscles pull limb away

extensors same limb inhibited to prevent opposition to flexion

limbs on opposite side extend to provide balance for sudden flexion

44Figure 13–17

A Flexor Reflex

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Reflexes• Reflexes automatic but can be

impacted by higher brain centers:– Take cues from reflex for coordinated

voluntary movements– Facilitate or inhibit reflexes

• Reflexes serve as diagnostic tool to assess health and function of spinal cord and brain

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Integration and Control of Spinal Reflexes

• Though reflex behaviors are automatic:– processing centers in brain can

facilitate or inhibit reflex motor patterns based in spinal cord

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KEY CONCEPT• Reflexes:

– rapid, automatic responses to stimuli– “buy time” to plan and execute complex,

conscious responses• Somatic motor reflexes:

– fastest reflexes– involve myelinated axons– involve only 1 spinal cord segment or brain

nucleus – are monosynaptic

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SUMMARY • General organization of nervous system:

– CNS, PNS• Afferent (sensory) and efferent (motor)

fibers• Structures and functions of spinal meninges• Gray matter and horns of spinal cord• 4 major nerve plexuses:

– cervical, brachial, lumbar, sacral• Neuronal pools and neural circuit patterns:

– divergence, convergence, serial, parallel, reverberation

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SUMMARY• Reflexes and reflex arcs• Classifications of reflexes:

– innate vs. acquired– somatic vs. visceral– cranial vs. spinal– monosynaptic, polysynaptic, or intersegmental

• Characteristics of monosynaptic reflexes:– stretch reflex, postural reflex, muscle spindles

• Characteristics of polysynaptic reflexes:– tendon, withdrawal, flexor, and crossed

extensor reflexes