Spinal Cord, Nerves, and Reflexes

Length of spinal cord

18 inches (45 cm)

width of spinal cord

1/2 inch (14 mm)

Where does the spinal cord end?

between L1 and L2 vertebrae

symmetry of spinal cord

bilateral

Grooves divide the spinal cord into left and right

Posterior median sulcus - on posterior side and Anterior median fissure - deeper groove on anterior side

posterior median sulcus

posterior side of spinal cord

anterior median fissure

deeper groove on anterior side

enlargements of spinal cord are caused by

Amount of gray matter in segment. Involvement with sensory and motor nerves of limbs.

enlargements of the spinal cord

cervical and lumbar

cervical enlargement

nerves of shoulders and upper limbs

lumbar enlargement

nerves of pelvis and lower limbs

distal end of spinal cord

conus medullaris, filum terminale, cauda equina

conus medullaris

thin, conical spinal cord below lumbar enlargement

filum terminale

thin thread of fibrous tissue at end of conus medullaris, attaches to coccygeal ligament

cauda equina

nerve roots extending below conus medullaris

spinal cord segments

cervical, thoracic, lumbar, sacral

cervical nerves

named for inferior vertebra

all other nerves

named for superior vertebra

Two branches of spinal nerves

ventral root and dorsal root

ventral root

contains axons of motor neurons

dorsal root

contains axons of sensory neurons

dorsal root ganglion

contains cell bodies of sensory neurons

spinal nerve

each side of spine, dorsal and ventral roots join to form this

meningeal layers

dura mater, arachnoid mater, pia mater

dura mater

Outermost layer of the meninges

arachnoid mater

middle layer of the meninges

pia mater

Innermost layer of the meninges

dura mater

fibrous membrane forming the outermost covering of the brain and spinal cord

Dura matter caudally

Tapers to dense cord of collagen fibers, Joins filum terminale in coccygeal ligament

dura mater cranially

Fuses with periosteum of occipital bone, Is continuous with cranial dura mater

epidural space of dura mater

Between spinal dura mater and walls of vertebral canal. Contains loose connective and adipose tissue. Anesthetic injection site.

arachnoid mater

middle meningeal layer

arachnoid membrane

simple squamous epithelia, covers outer surface of arachnoid mater

Interlayer Spaces of Arachnoid Mater

subdural space; subarachnoid space

subdural space

space between dura mater and arachnoid mater

subarachnoid space

space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae

subarachnoid space is filled with

cerebrospinal fluid

cerebrospinal fluid (CSF)

Carries dissolved gases, nutrients, and wastes, Lumbar puncture or spinal tap withdraws CSF

what withdraws CSF?

lumbar puncture or spinal tap

pia mater

the innermost layer of the meninges bound to the underlying neural tissue

mesh of collagen and elastic fibers

pia mater

gray matter

Surrounds the central canal of spinal cord, Contains neuron cell bodies, neuroglia, unmyelinated axons, Has projections (gray horns)

gray horns

posterior, anterior, lateral

posterior gray horns

contain somatic and visceral sensory nuclei

anterior gray horns

contain somatic motor nuclei

lateral gray horns

are only in thoracic and lumbar segments; contain visceral motor nuclei

gray commissures

Axons that cross from one side of cord to the other before reaching gray matter

nuclei

functional groups of cell bodies

two types of nuclei

sensory and motor

sensory nuclei

Dorsal (posterior), Connect to peripheral receptors

motor nuclei

Ventral (anterior), Connect to peripheral effectors

Control and Location

Sensory or motor nucleus location within the gray matter determines which body part it controls

Anatomy of Spinal Nerves

Each spinal cord segment: Is connected to a pair of spinal nerves, Each spinal nerve: Is surrounded by three connective tissue layers, That support structures and contain blood vessels

each spinal cord segment

is connected to a pair of spinal nerves

each spinal nerve

is surrounded by 3 connective tissue layers that support structures and contain blood vessels

Three Connective Tissue Layers of Spinal Nerves

epineurium, perineurium, endoneurium

Epineurium

outer layer, dense network of collagen fibers

Perineurium

middle layer, divides nerve into fascicles (axon bundles)

Endoneurium

inner layer, surrounds individual axons

peripheral nerves

interconnecting branches of spinal nerves that are surrounded by connective tissue sheaths

Peripheral Distribution of Spinal Nerves

Form lateral to intervertebral foramen where dorsal and ventral roots unite then branch and form pathways to destination

peripheral neuropathy

regional loss of sensory or motor function due to trauma or compression

nerve plexuses

Complex, interwoven networks of nerve fibers. Formed from blended fibers of ventral rami of adjacent spinal nerves. Control skeletal muscles of the neck and limbs.

The Four Major Plexuses of Ventral Rami

cervical, brachial, lumbar, and sacral plexus

cervical plexus

C1-C5

brachial plexus

C5-T1

lumbar plexus

T12-L4

sacral plexus

L4-S4

cervical plexus

Includes ventral rami of spinal nerves C1-C5, Innervates neck, thoracic cavity, diaphragmatic muscles, Major nerve (phrenic nerve (controls diaphragm))

brachial plexus

Includes ventral rami of spinal nerves C5-T1, Innervates pectoral girdle and upper limbs

brachial plexus major nerves

Musculocutaneous nerve (lateral cord), Median nerve (lateral and medial cords), Ulnar nerve (medial cord), Axillary nerve (posterior cord), Radial nerve (posterior cord)

musculocutaneous nerve

lateral cord

median nerve

lateral cord & medial cord both turn into this

ulnar nerve

medial cord

axillary nerve

posterior cord

radial nerve

posterior cord

lumbar plexus

Includes ventral rami of spinal nerves T12-L4

major nerves of lumbar plexus

genitofemoral, lateral femoral cutaneous, femoral

sacral plexus

Includes ventral rami of spinal nerves L4-S4

major nerves of sacral plexus

sciatic nerve and pudendal nerve

Two branches of the sciatic nerve

fibular nerve and tibial nerve

reflexes

automatic responses to stimuli

Reflexes are controlled by

spinal cord

simple and complex reflexes

types of reflexes

autonomic responses coordinated within spinal cord

through interconnected sensory neurons, motor neurons, and interneurons

Two types of reflexes

monosynaptic and polysynaptic

monosynaptic reflex

stretch reflex

monosynaptic reflex

direct communication between sensory and motor neuron

The stretch reflex is a

monosynaptic reflex

patellar reflex is a

stretch reflex

20-40 msec

How long/quick does a monosynaptic reflex complete?

stretch reflex receptor

muscle spindle

muscle spindles

receptors sensitive to change in length of the muscle and the rate of that change

intrafusal muscle fibers

Specialized muscle fibers found in muscle spindles.

intrafusal muscle fibers

innervated by sensory and motor neurons

extrafusal muscle fibers

maintain tone and contract muscle

sensory region

Central region of intrafusal fibers wound with dendrites of sensory neurons

gamma efferents

axons of the gamma motor neurons, complete reflex arc

gamma efferents are important in

voluntary muscle contractions

polysynaptic reflexes

More complicated than monosynaptic reflexes. Interneurons control more than one muscle group. Produce either EPSPs or IPSPs.