A&P 1 Ch. 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes

The Spinal Cord: Conduction

nerve fibers that conduct information up and down the cord, connecting different levels of the trunk with each other and the brain

Neural integration

input from multiple sources, integrated, and executed output

Locomotion

repetitive, coordinated contractions of several muscle groups in the limbs

Reflexes

involuntary stereotyped responses to stimuli

Surface Anatomy: Spinal cord

cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull

Passes through the vertebral canal

Inferior margin ends at L1 or a little beyond

Averages 1.8 cm thick and 45 cm long

Occupies the upper two-thirds of the vertebral canal

Gives rise to 31 pairs of spinal nerves

First pair passes between the skull and C1

Rest pass through intervertebral foramina

Segment

part of the spinal cord supplied by each pair of

spinal nerves

Two areas of the cord are thicker than elsewhere

Cervical enlargement-nerves to upper limb

Lumbar enlargement-nerves to pelvic region and lower limbs

Medullary cone (conus medullaris)

cord tapers to a point inferior to lumbar enlargement

Cauda equina

bundle of nerve roots that occupy the vertebral canal from L2 to S5

Meninges

three fibrous connective tissue membranes that enclose the brain and spinal cord

Separate soft tissue of central nervous system from

bones of cranium and vertebral canal

From superficial to deep: dura mater, arachnoid

mater, and pia mater

Spina Bifida

congenital defect in which one or more vertebrae fail

to form a complete vertebral arch for enclosure of the spinal cord

In 1 baby out of 1,000

Common in lumbosacral region

Spina bifida occulta and spina bifida cystica

Cross-Sectional Anatomy

Gray matter-neuron cell bodies with little myelin

Site of information processing, synaptic integration

White matter-abundantly myelinated axons

Carry signals from one part of the CNS to another

Gray Matter

Spinal cord has a central core of gray matter that looks butterfly- or

H-shaped in cross section

Pair of posterior (dorsal) horns

Posterior (dorsal) root of spinal nerve carries only sensory fibers

Pair of thicker anterior (ventral) horns

Anterior (ventral) root of spinal nerve carries only motor fibers

Gray commissure connects right and left sides

Punctured by central canal lined with ependymal cells and filled

with CSF

Lateral horn: visible from T2 through L1

Contains neurons of sympathetic nervous system

White Matter

White matter of the spinal cord surrounds the gray matter

Consists of bundles of axons that course up and down the cord

Provide avenues of communication between different levels of the CNS

Columns or funiculi?three pairs of these white matter bundles

Posterior (dorsal), lateral, and anterior (ventral) columns on each side

Tracts or fasciculi?subdivisions of each column

Spinal Tracts

Ascending tracts-carry sensory information up the spinal cord

Descending tracts-carry motor information down the spinal cord

All nerve fibers in a given tract have a similar origin, destination, and

function

Decussation-as the fibers pass up or down the brainstem and spinal

cord they cross over from the left to the right side and vice versa

Contralateral-when the origin and destination of a tract are on

opposite sides of the body

Ipsilateral-when the origin and destination of a tract are on the same

side of the body; does not decussate

Ascending Tracts

Ascending tracts carry sensory signals up the spinal cord

Sensory signals travel across three neurons from origin

in receptors to the destination in the sensory areas of the

brain

First-order neurons: detect stimulus and transmit signal to spinal

cord or brainstem

Second-order neurons: continues to the thalamus at the upper

end of the brainstem

Third-order neurons: carries the signal the rest of the way to the

sensory region of the cerebral cortex

Ascending Tracts: parts

Gracile fasciculus

Cuneate fasciculus

Spinothalamic tract

Spinoreticular tract

Posterior (dorsal) and

anterior (ventral)

spinocerebellar tracts

Gracile Fasciculus

Carries signals from midthoracic and lower parts of

the body

Below T6, it composes the entire posterior column

At T6, it joins cuneate fasciculus

Consists of first-order nerve fibers that travel up the

ipsilateral side of the spinal cord

Terminates at the gracile nucleus of the medulla

oblongata

Carries signals for vibration, visceral pain, deep and

discriminative touch, and proprioception from lower

limbs and lower trunk

Proprioception?nonvisual sense of the position and

movements of the body

Cuneate Fasciculus

Joins gracile fasciculus at T6

Occupies lateral portion of the posterior column

Forces gracile fasciculus medially

Carries the same type of sensory signals

Originates from the level of T6 and up

Upper limb and chest

Fibers end in the cuneate nucleus on the ipsilateral side of the

medulla oblongata

Medial lemniscus: formed from the second-order neurons of

gracile and cuneate systems that decussate in the medulla

Tracts of these nerve fibers lead the rest of the way to the

thalamus

Third-order neurons go from thalamus to cerebral cortex

Carry signals to contralateral cerebral hemisphere

Spinothalamic Tract

One of the smaller tracts of the anterolateral system

Passes up the anterior and lateral columns of the spinal cord

Carry signals for pain, pressure, temperature, light touch,

tickle, and itch

First-order neurons end in posterior horn of spinal cord

Synapse with second-order neurons which decussate to

other side of spinal cord and form the ascending

spinothalamic tract

Third-order neurons continue from there to cerebral cortex

Sends signals to the contralateral cerebral hemisphere

Spinoreticular Tract

Travels up the anterolateral system

Carries pain signals resulting from tissue injury

First-order neurons enter posterior horn and immediately

synapse with second-order neurons

Decussate to opposite anterolateral system

Ascend the cord

End in reticular formation: loosely organized core of gray

matter in the medulla and pons

Third-order neurons continue from the pons to the

thalamus

Fourth-order neurons complete the path to the cerebral

cortex

Spinocerebellar Tracts

Travel through lateral column

Carry proprioceptive signals from limbs and trunk up

to the cerebellum

First-order neurons originate in the muscles and

tendons

End in posterior horn of the spinal cord

Second-order nerves ascend spinocerebellar tracts

and end in cerebellum

Spinocerebellar Tracts 2

Fibers of the posterior tract travel up the ipsilateral

side of the spinal cord

Fibers of the anterior tract cross over and travel up

the contralateral side

Cross back in the brainstem to enter the ipsilateral

side of the cerebellum

Provide cerebellum with feedback needed to

coordinate muscle actions

Descending Tracts

Descending tracts-carry motor

signals down the brainstem and

spinal cord

Involve two neurons

Upper motor neuron originates

in cerebral cortex or brainstem

and terminates on a lower motor

neuron

Lower motor neuron in

brainstem or spinal cord

Axon of lower motor neuron

leads the rest of the way to the

muscle or other target organ

Corticospinal Tracts

From cerebral cortex for precise, finely coordinated limb

movements

Pyramids?ridges on anterior surface of the medulla

oblongata formed from fibers of this system

Decussate in lower medulla

Lateral corticospinal tract on contralateral side of spinal

cord

Anterior (ventral) corticospinal tract on ipsilateral side of

spinal cord

Two neuron pathways

Upper motor neuron in cerebral cortex

Lower motor neuron in spinal cord

Descending Tracts 2

Tectospinal tract-begins in midbrain region (tectum)

Crosses to contralateral side of midbrain

Reflex turning of head in response to sights and sounds

Lateral and medial reticulospinal tract

Originate in the reticular formation of brainstem

Control muscles of upper and lower limbs

Especially those for posture and balance

Contain descending analgesic pathways

Reduce the transmission of pain signals to brain

Lateral and medial vestibulospinal tract

Begins in brainstem vestibular nuclei

Receives impulses for balance from inner ear

Controls extensor muscles of limbs for balance control

The Spinal Nerves

Spinal cord communicates with the rest of the

body by way of spinal nerves

Nerve-a cordlike organ composed of numerous

nerve fibers (axons) bound together by

connective tissue

Mixed nerves contain both afferent (sensory) and

efferent (motor) fibers

The Spinal Nerves 2

Nerves of peripheral nervous system are ensheathed in

Schwann cells

Forms neurilemma

Myelin sheath around the axon

Endoneurium

Fascicles: nerve fibers gathered in bundles

Perineurium

Epineurium

Blood vessels penetrate connective tissue coverings

Nerves have high metabolic rate and need plentiful blood

supply

General Anatomy of Nerves

and Ganglia

Sensory (afferent) nerves:

Carry signals from sensory

receptors to the CNS

Motor (efferent) nerves:

Carry signals from CNS to

muscles and glands

Mixed nerves:

Consists of both afferent and

efferent fibers

Conduct signals in two directions

Both sensory and motor fibers can

also be described as:

Somatic or visceral

General or special

Ganglion-cluster of neurosomas outside the CNS

Enveloped in an endoneurium continuous with that of the nerve

Among neurosomas are bundles of nerve fibers leading into and

out of the ganglion

Posterior root ganglion associated with spinal nerves

pairs of spinal nerves

31 pairs of spinal nerves (mixed nerves)

8 cervical (C1?C8); C1 between skull and atlas

Others exiting at intervertebral foramen

12 thoracic (T1?T12)

5 lumbar (L1?L5)

5 sacral (S1?S5)

1 coccygeal (Co)

Proximal branches

forming spinal nerves

Each spinal nerve has two points of attachment to the

Posterior (dorsal) root is sensory input to spinal cord

Posterior (dorsal) root ganglion-contains the somas of

sensory neurons carrying signals to the spinal cord

Six to eight rootlets that enter posterior horn of cord

Anterior (ventral) root is motor output out of spinal cord

Six to eight rootlets that leave spinal cord and converge to

These merge to form spinal nerve proper that enters

intervertebral foramen

Cauda equina: formed from roots arising from L2 to Co

Distal branches of spinal nerves

Distal to vertebral foramen, the nerve divides into:

Anterior ramus: innervates the anterior and lateral

skin and muscles of the trunk

Gives rise to nerves of the limbs

Posterior ramus: innervates the muscles and joints in

that region of the spine and the skin of the back

Meningeal branch: reenters the vertebral canal and

innervates the meninges, vertebrae, and spinal

ligaments

Nerve Plexuses

Anterior rami branch and anastomose repeatedly to

form five nerve plexuses

Cervical plexus in the neck, C1 to C5

Supplies neck and phrenic nerve to the diaphragm

Brachial plexus near the shoulder, C5 to T1

Supplies upper limb and some of shoulder and neck

Median nerve?carpal tunnel syndrome

Lumbar plexus in the lower back, L1 to L4

Supplies abdominal wall, anterior thigh, and genitalia

Sacral plexus in the pelvis, L4, L5, and S1 to S4

Supplies remainder of lower trunk and lower limb

Coccygeal plexus, S4, S5, and Co

Nerve Plexuses 2

Somatosensory function-carry sensory signals

from bones, joints, muscles, and the skin

Proprioception: brain receives information about body

position and movements from nerve endings in

muscles, tendons, and joints

Motor function-primarily to stimulate muscle

contraction

Shingles

Chickenpox-common disease of early childhood

Caused by varicella-zoster virus

Produces itchy rash that clears up without complications

Virus remains for life in the posterior root ganglia

Kept in check by the immune system

Shingles (herpes zoster)-localized disease caused by

the virus traveling down the sensory nerves by fast axonal

transport when immune system is compromised

Common after age 50

Painful trail of skin discoloration and fluid-filled vesicles along path

of nerve

Usually in chest and waist on one side of the body

Pain and itching

Childhood chickenpox vaccinations reduce the risk of shingles later

in life

Cutaneous Innervation

and Dermatomes

Dermatome-a specific area of

the skin that receives sensory

input from a pair of spinal nerves

Dermatome map-a diagram of

the cutaneous regions innervated

by each spinal nerve

Dermatomes overlap their edges

as much as 50%

Necessary to sever or anesthetize

three successive spinal nerves to

produce a total loss of sensation in

one dermatome

The Nature of Reflexes

Reflexes-quick, involuntary, stereotyped

reactions of glands or muscle to stimulation

Automatic responses to sensory input that occur without

our intent or often even our awareness

The Nature of Reflexes 2

Four important properties of a reflex

Reflexes require stimulation

Not spontaneous actions, but responses to sensory input

Reflexes are quick

Involve few if any interneurons and minimum synaptic delay

Reflexes are involuntary

Occur without intent and difficult to suppress

Automatic response

Reflexes are stereotyped

Occur essentially the same way every time

The Nature of Reflexes 3

Reflexes include glandular secretion and

contraction of all three types of muscle

Include some learned responses: conditioned

reflexes

We are dealing with unlearned skeletal muscle

reflexes mediated by the brainstem and spinal

cord

Somatic reflexes-since they involve the

somatic nervous system

The Nature of Reflexes 4

Pathway of reflex arc

Somatic receptors

In skin, muscles, or tendons

Afferent nerve fibers

Carry information from receptors to posterior horn of spinal cord

or the brainstem

Integrating center

A point of synaptic contact between neurons in gray matter of

spinal cord or brainstem

Determines whether efferent neurons issue signal to muscles

Efferent nerve fibers

Carry motor impulses to skeletal muscle

Effectors

The somatic effectors carry out the response

The Muscle Spindle

Muscle spindle-stretch receptors embedded in skeletal

muscles

Proprioceptors-specialized sense organs to monitor

position and movement of body parts

Muscle spindles inform the brain of muscle length and

body movement

Enables brain to send motor commands back to the

muscles that control coordinated movement, corrective

reflexes, muscle tone, and posture

The Muscle Spindle 2

Intrafusal fibers-muscle fibers within spindle

Nuclear chain fibers

Nuclear bag fibers

Nerve fibers in muscle spindle

Primary afferent (group Ia) fiber

Secondary afferent (group II) fibers

Gamma motor neurons

The Stretch Reflex

Stretch (myotatic) reflex-when a muscle is

stretched, it "fights back" and contracts, maintaining

increased tonus and making it stiffer than unstretched

muscle

Helps maintain equilibrium and posture

Head starts to tip forward as you fall asleep

Muscles contract to raise the head

Stabilize joints by balancing tension in extensors and

flexors smoothing muscle actions

Stretch reflex is mediated primarily by the brain

Not strictly a spinal reflex

The Stretch Reflex 2

Tendon reflex-reflexive contraction of a muscle

when its tendon is tapped

Knee-jerk (patellar) reflex is a monosynaptic reflex

One synapse between the afferent and efferent

neurons

Testing somatic reflexes helps diagnose many

diseases

Reciprocal inhibition-reflex phenomenon that

prevents muscles from working against each other

by inhibiting the antagonist

The Flexor and Crossed Extension Reflexes

Flexor reflex-the quick

contraction of flexor

muscles resulting in the

withdrawal of a limb from an

injurious stimulus

Requires contraction of the

flexors and relaxation of the

extensors in that limb

Polysynaptic reflex arc-

pathway in which signals

travel over many synapses

on their way back to the

muscle

The Flexor and Crossed Extension Reflexes 2

Crossed extension reflex-the

contraction of extensor muscles in limb

opposite of the one that is withdrawn

Maintains balance by extending

other leg

Ipsilateral reflex arc-one in which the

sensory input and motor output are on

the same sides of the spinal cord

Flexor reflex

Contralateral reflex arc-one in which

the input and output are on opposite

sides

Crossed extension reflex

Intersegmental reflex-one in which

the input and output occur at different

levels (segments) of the spinal cord

The Tendon Reflex

Tendon organs-

proprioceptors in a tendon

near its junction with a muscle

Golgi tendon organ: 1 mm long,

nerve fibers entwined in collagen

fibers of the tendon

Tendon reflex-in response

to excessive tension on the

tendon

Inhibits muscle from contracting

strongly

Moderates muscle contraction

before it tears a tendon or pulls it

loose from the muscle or bone