Chapter 14: Nervous System - Spinal Cord and Spinal Nerves

direct pathway (pyramidal pathway)

originates in the primary motor cortex in the frontal lobe; descend through the internal capsule, enter the cerebral peduncles, and ultimately synapse in either the brainstem or the spinal cord

denticulate ligaments

paired, lateral triangular extensions of the spinal pia mater that attach to the dura mater; help suspend and anchor the spinal cord laterally

anterolateral pathway

pathway in the anterior and lateral white funiculi of the spinal cord; axons within these pathways relay sensory input related to crude touch and pressure as well as pain and temperature

spinocerebellar pathway

pathway that extends through the anterior and posterior white funiculi of the spinal cord; axons within these pathways conduct nerve signals from proprioceptors related to postural input to the cerebellum

contralateral reflex

reflex that involves an effector on the opposite side of the body of the receptor that detected the stimulus

acquired reflex

reflex that is developed after birth

innate reflex

reflex that you are born with

lateral pathway

regulates and controls precise, discrete movements and tone in flexor muscles of the limbs

medial pathway

regulates reflexive muscle tone and gross movements of the muscles of the head, neck, proximal parts of the limbs, and trunk

subcostal nerve

the anterior rami of spinal nerve T12 because it arises inferior to the ribs, not between two ribs

intercostal nerves

the anterior rami of spinal nerves T1-T11

deep fibular nerve

travels in the anterior compartment of the leg and terminates between the first and second toes; innervates the anterior leg muscles and the muscles on the dorsum of the foot

superficial fibular nerve

travels in the lateral compartment of the leg; innervates the lateral compartment muscles of the leg

axillary nerve

traverses through the axilla and posterior to the surgical neck of the humerus; innervates both the deltoid and teres minor muscles

indirect pathway

upper motor neurons originate within brainstem nuclei and take complex, circuitous route through the brain to the spinal cord; modifies or helps control the pattern of somatic motor activity by exciting or inhibiting the lower motor neurons that innervate the muscles

middle trunk

brachial plexus division; nerve C7 remains as this

superior trunk

brachial plexus division; nerves C5 and C6 unite to form this

inferior trunk

brachial plexus division; nerves C8 and T1 unite to form this

posterior root ganglion

cell bodies of the sensory neurons in the posterior root are located here

gray matter

centrally located in the spinal cord and its shape resembles a letter H or a butterfly

anterior/posterior spinocerebellar tract

compose the spinocerebellar pathway; derives names from origin and destination of the secondary neurons of this pathway

anterior funiculus

composed of white matter that occupies the space on each anterior side of the cord between the anterior gray horns and the anterior median fissure

intrafusal muscle fibers

composes a muscle spindle; surrounded by a connective tissue capsule; lack myofilaments in their central regions and are contractile only at their distal regions

anterior spinothalamic tract

composes the anterolateral pathway; extends through the anterior funiculi

lateral spinothalamic tract

composes the anterolateral pathway; extends through the lateral funiculi

anterior root

contains motor axons only and is formed from the merging of multiple anterior rootlets

posterior root

contains sensory axons only and is formed from the merging of multiple posterior rootlets

corticospinal tracts

descend from the primary motor cortex of the cerebrum through the medulla oblongata and into the spinal cord where they synapse on lower motor neurons in the anterior horn of the spinal cord

ulnar nerve

descends along the medial side of the arm; travels posterior to the medial epicondyle of the humerus and then extends along the ulnar side of the forearm; innervates some of the anterior forearm muscles and most of the intrinsic hand muscles

pia mater

directly adheres to the spinal cord; delicate, most intermediate meningeal layer composed of elastic and collagen fibers that support some of the blood vessels supplying the spinal cord

tibial nerve

formed from the anterior divisions of the sciatic nerve; innervates the hamstrings and the hamstring part of the adductor magnus

lumbar plexuses

formed from the anterior rami of spinal nerves L1-L4 located lateral to the L1-L4 vertebrae and along the psoas major muscle in the posterior abdominal wall; innervates the inferior abdominal wall, anterior thigh medial thigh, and the skin of the medial leg

sacral plexuses

formed from the anterior rami of spinal nerves L4-S4 and are located immediately inferior to the lumbar plexuses; innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot

common fibular nerve

formed from the posterior division of the sciatic nerve; innervates the short head of the biceps femoris muscle

phrenic nerve

formed primarily from the C4 nerve; travels through the thoracic cavity to innervate the thoracic diaphragm

lateral horns

found in the T1-L2 parts of the spinal cord only; contain the the cell bodies of autonomic motor neurons, which innervate cardiac muscle

nuclei

functional groups of neuron cell bodies

sensory nuclei

in the posterior horns and contain interneuron cell bodies

monosynaptic reflex

reflex that has only a sensory neuron and motor neuron

lateral funiculus

the white matter on each lateral side of the spinal cord

five steps of a reflex

1. A stimulus activates a sensory receptor. 2. The sensory neuron transmits a nerve signal to the CNS. 3. Information from the nerve signal is processed in the integration center by interneurons. 4. The motor neuron transmits a nerve signal from the CNS to an effector. 5. The effector response to the nerve signal from the motor neuron.

properties of reflexes

1. A stimulus is required to initiate a reflex. 2. A rapid response requires that few neurons are involved and synaptic delay is minimal. 3. A preprogrammed response occurs the same way every time. 4. An involuntary response requires no conscious intent or preawareness of the reflex activity.

characteristics of pathways

1. Common location of neuron components 2. Composed of two or more neurons 3. Paired tracts 4. Decussation

classifications of reflexes

1. Spinal reflex or cranial reflex 2. Somatic reflex or visceral reflex 3. Monosynaptic reflex or polysynaptic reflex 4. Ipsilateral reflex or contralaateral reflex 5. Innate reflex or acquired reflex

gray commissure

a horizontal bar of gray matter that surrounds a central canal; primarily contains unmyelinated axons and serves as a communication route between the right and left sides of the gray matter

sulcus limitans

a horizontal groove developed by the sixth week of development that forms in the lateral walls of the central canal

stretch reflex

a muscle reflexively contracting in response to stretching of a muscle

posterior median sulcus

a narrow groove that dips internally on the posterior surface of the spinal cord

nerve plexus

a network of interweaving anterior rami of spinal nerves

referred visceral pain

a phenomenon in which pain or discomfort from one organ is mistakenly referred to a dermatome

Golgi tendon reflex

a polysynaptic reflex that results in muscle lengthening and relaxation in response to increased tension at a Golgi tendon organ

hypoactive reflex

a reflex response that is diminished or absent

dermatome

a specific segment of skin innervated by a single spinal nerve

filum terminale

a thin strand of pia mater that helps anchor the conus medullaris to the coccyx

anterior median fissure

a wider groove on the anterior surface of the spinal cord

hyperactive reflex

an abnormally strong response (reflex)

cervical enlargement

an enlarged region of the inferior cervical part of the spinal cord; contains the neurons that innervate the upper limbs

lumbosacral enlargement

an enlarged region of the mid-lumbar part of the spinal cord; contains the neurons that innervate the lower limbs

secondary neuron

an interneuron that extends from the primary neuron to either the teritary neuron or to the cerebellum

tertiary neuron

an interneuron; extends from the secondary neuron to the cerebrum

fasiculus cuneatus, fasciculus gracilis

axons from posterior funiculus-medial lemniscal pathways ascend within one of these specific posterior funiculi

terminal branches

emerge from the three cords: the axillary nerve, median nerve, musculocutaneous nerve, radial nerve, and ulnar nerve

extrafusal muscle fibers

fibers around the muscle spindle

neural canal

hollow part of a neural tube that develops into the central canal of the spinal cord

upper motor neuron

house either within the cerebral cortex, cerebral nuclei, or an interneuron that ultimately synapses directly on a lower motor neuron; either excite or inhibit the activity of lower motor neurons

lower motor neuron

housed either within the anterior horn of the spinal cord or within a brainstem cranial nerve nucleus; always exctiatory

pathways

how CNS communicates with peripheral body structures; conduct either sensory nerve signals from receptors to the CNS or motor nerve signals from the CNS to effectors

8

how many pairs of cervical spinal nerves

1

how many pairs of coccygeal spinal nerves

5

how many pairs of lumbar spinal nerves

5

how many pairs of sacral spinal nerves

12

how many pairs of thoracic spinal nerves

subarachnoid space

immediately deep to the arachnoid; CSF circulates in this space

motor nuclei

in the anterior and lateral horns and contain motor neuron cell bodies that send nerve signals to muscles and glands

somatic motor nuclei

in the anterior horn and innervate skeletal muscle

autonomic motor nuclei

in the lateral horns and innervate smooth muscle, cardiac muscle, and glands

lateral corticospinal tracts

include about 85% of the axons of the upper motor neurons that decussate within the pyramids of the medulla oblongata and then form these; innervate skeletal muscles that control skilled movement in the limbs

reflex arc

includes a sensory receptor, an effector, and the neural "wiring" between the two

gamma motor neurons

innervate intrafusal muscle fibers; detect changes within the muscle

alpha motor neurons

innervates extrafusal muscle fibers

musculocutaneous nerve

innervates the anterior arm muscles, which flex the humerus and/or flex the forearm

white commissure

interconnects the anterior funniculi

withdrawal reflex

involves muscles contracting to withdraw the body part away from a painful stimulus

medial cord

medial to the axillary artery and is formed by the anterior division of the inferior trunk; contains portions of nerves C8-T1

anterior ramus

larger of the two main branches of a spinal nerve; innervates the anterior and lateral portions of the trunk, the upper limbs, and the lower limbs

lateral cord

lateral to the axillary artery and is formed from the anterior divisions of the superior and middle trunks; contains portions of nerves C5-C7

posterior horns

left and right posterior masses of gray matter; axons of sensory neurons and the cell bodies of interneurons are located here

basal plates

lie anterior to the sulcus limitans; develop into the anterior and lateral horns, motor structures of the gray matter

alar plates

lie posterior to the sulcus limitans; by about the ninth week of development, they develop into posterior horns, sensory structures of the gray matter

epidural space

lies between the dura mater and the inner walls of the vertebra, and houses adipose and areolar connective tissues and blood vessels

posterior funiculus

lies between the posterior gray horns on the posterior side of the cord and the posterior median sulcus

arachnoid mater

lies external to the pia mater; partially composed of a delicate web of collagen and elastic fibers

thoracic part

lies inferior to the cervical part; contains the neurons for the 12 pairs of thoracic spinal nerves

sacral part

lies inferior to the lumbar part and contains neurons for the 5 pairs of sacral spinal nerves

cervical plexuses

located deep on each side of the neck, immediately lateral to cervical vertebrae C1-C4; formed primarily by the anterior rami of spinal nerves C1-C4; innervate anterior neck muscles as well as the skin of the neck and portions of the head and shoulders

anesthesia

loss of sensation or numbness

obturator nerve

main nerve of the anterior division of the lumbar plexus; innervates the medial thigh muscles

femoral nerve

main nerve of the posterior division of the lumbar plexus; innervates the anterior thigh muscles

brachial plexuses

networks of nerves that supply the upper limb; formed by the anterior rami of spinal nerves C5-T1; innervates the pectoral girdle and the entire upper limb on one side

crossed-extensor reflex

occurs in conjunction with the withdrawal reflex, usually in the lower limbs; when the withdrawal reflex occurs in one limb, this one occurs in the other

corticobulbar tracts

originate from the facial region of the motor homunculus within the primary motor cortex; extend to the brainstem, where they synapse with lower motor neuron cell bodies that are housed within brainstem cranial nerve nuclei; help form some of the cranial nerves

polysynaptic reflex

reflex that has one or more interneurons positioned between the sensory and motor neurons

anterior/posterior division

portions of each trunk divide deep to the clavicle into these two portions

posterior cord

posterior to the axillary artery and is formed by the posterior divisions of the superior, middle, and inferior trunks; contains portions of C5-T1 nerves

subdural space

potential space between the arachnoid and overlying dura mater

somatosensory pathways

process stimuli received from receptors within the skin, muscles, and joints

viscerosensory pathways

process stimuli received from the viscera

posterior funiculus-medial lemniscal pathway

projects from a somati receptor to the primary somatosensory cortex of the cerebral cortex

Golgi tendon organ

proprioceptor that detects change in tension in a muscle tendon when a muscle contracts; composed of sensory nerve endings within a tendon or near a muscle-tendon junction

muscle spindle

proprioceptor that detects changes in stretch of a muscle

spinal cord meninges

protect and encapsulate the spinal cord

reflexes

rapid, preprogrammed, involuntary responses of muscles or glands to a stimulus

visceral sensory nuclei

receive nerve signals from blood vessels and viscera, such as stretched smooth muscle within viscera

somatic sensory nuclei

receive nerve signals from sensory receptors, such as pain or pressure receptors in the skin

ipsilateral reflex

reflex in which both the receptor and effector organs are on the same side of the spinal cord

visceral reflex

reflex involving cardiac muscle, smooth muscle, or a gland as the effector

somatic reflex

reflex involving skeletal muscle as the effector

cranial reflex

reflex involving the brain

spinal reflex

reflex involving the spinal cord

anterior corticospinal tracts

represent the remaining 15% of axons of upper motor neurons that extend through the medulla oblongata; do not decussate at the level of hte medulla oblongata, instead they decussate at the level of a spinal cord segment through the anterior gray commissure; innervate axial skeletal muscle

proprioceptor

resides in a muscle or tendon and detects any change to that structure

primary neuron

resides in the posterior root ganglion of a spinal nerve, and the axon extends to a secondary neuron

clonus

rhythmic oscillations between flexion and extension, when the muscle reflex is tested

reciprocal activation

sensory neurons communicating with other interneurons in the spinal cord that stimulate alpha motor neurons for the antagonistic muscles

lumbar part

shorter segment of the spinal cord that contains the neurons for the 5 parts of lumbar spinal nerves

tracts (fasciculi)

small structural units that the axons within each white matter funiculus are organized into

posterior ramus

smaller of the two main branches of a spinal nerve; innervates the deep muscles of the back and the skin of the back

sciatic nerve

the largest and longest nerve in the body; formed from portions of both the anterior and posterior divisions of the saral plexus

anterior horns

the left and right anterior masses of gray matter; primarily house the cell bodies of somatic motor neurons, which innervate skeletal muscle

decussate

the left side of the brain processes sensory input form and motor output to the right side of the body and vice versa

coccygeal part

the most inferior tip of the spinal cord

sensory pathways (ascending pathways)

the nerve signals transmitted from sensory receptors ascend through the spinal cord to the brain; relay sensory input from receptors to the brain; "informs" the brain about limb proprioception, touch, temperature, pressure, and pain

conus meullaris

the tapering inferior end of the spinal cord

medial lemniscus

the tracts within the brainstem

dura mater

the outermost layer of meninges; composed of dense irregular connective tissue; consists of only ONE layer; provides stability to the spinal cord and at each intervertebral foramen, it extends between adjacent vertebrae and fuses with the connective tissue layers that surround the spinal nerves

contralateral

the relationship to the opposite side

cauda equina

the rootlets of the more inferior spinal nerves including L2-L5, S1-S5, and Co1 extend inferiorly from the conus medullaris and are collectively called this

ipsilateral

the same side

cervical part

the superiormost part of the spinal cord; continuous with the medulla oblongata; contains neurons whose axons contribute to the 8 pairs of cervical spinal nerves

median nerve

travels along the midline of the arm and forearm and deep to the carpal tunnel in the wrist; innervates most of the anterior forearm muscles, the thenar muscles, and the lateral two lumbricals

radial nerve

travels along the posterior side of the arm and then along the radial side of the forearm; innervates the posterior arm muscles and the posterior forearm muscles

funiculus

three regions of white matter on each side of the cord

rubrospinal tracts

tracts in the lateral pathway that originate in the red nucleus of the midbrain

tectospinal tracts

tracts in the medial pathway that conduct motor output from the superior and inferior colliculi in the tectum of the midbrain to help regulate reflexive positional changes of the upper limbs, eyes, head, and neck as a consequence of visual and auditory stimuli

reticulospinal tracts

tracts in the medial pathway that originate from the reticular formation in the midbrain; help control reflexive movements related to posture and maintaining balance

vestibulospinal tracts

tracts in the medial pathway that originate within vestibular nuclei of the brainstem; regulate reflexive muscular activity that helps maintain balance during sitting, standing, and walking

motor pathways (descending pathways)

transmits nerve signals that descend from the brain through the spinal cord to muscles or glands; pathways in the brain and spinal cord that control effectors

reciprocal inhibition

when the sensory nerve signals reach the CNS, some of the sensory axons synapse with interneurons; these interneurons synapse with alpha motor neurons that inhibit antagonistic muscle contraction