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
