Neuroanatomy Lecture 4: Spinal cord motor pathways & asynch Flashcards
Pyramidal tract:
conveys impulses to control the skeletal muscles of the limbs & trunks
Includes: lateral & ventral corticospinal tracts
Extrapyramidal tracts:
conducts signals that regulate those body movements that are subconscious or postural in nature.
Rubrospinal, tectospinal, vestibulospinal, & reticulospinal tracts
Both pyramidal and extrapyramidal tracts:
two sets of neurons:
-upper motor neurons (UMNs): reside in cerebral hemispheres motor cortex
-lower motor neurons (LMNs): located in anterior horn of spinal cord. Axons exit the spinal cord as ventral root of the spinal nerve & target the skeletal muscles.
Upper Motor Neurons (UMNs):
begin in the cortex & terminate in the brainstem or spinal cord
Corticobulbar (corticonuclear) tract: terminates in brainstem; synapse on cranial nerve nuclei (III, IV in the midbrain/ V-VIII in pons/ IX-XII in medulla)
Corticospinal tract: terminates in the spinal cord & synapses w/ the motor neurons in the ventral horn of spinal cord.
Lower Motor Neurons (LMNs):
begin in the brainstem or spinal cord and innervate muscles:
Lower motor neuron of cranial nerves; cell body in the motor nuclei of the cranial nerves in the brainstem.
Lower motor neuron of spinal nerves; cell body in the ventral horn of the spinal cord that form the motor root of the spinal nerve
PNS:
Corticospinal tract pathway:
Neuron 1: Upper motor neuron (UMN)
-originates in primary motor cortex of frontal lobe (pre-central gyrus)
-Descends via large white matter tract within diencephalon
-Tract decussates at medulla oblongata
-Travels in antero-lateral white columns of spinal cord
-Synapses in anterior grey horn of spinal cord
Neuron 2: Lower motor neuron (LMN)
-cell body originates in anterior grey horn of spinal cord
-Exits cord via ventral root of spinal nerve
-Travels via peripheral spinal nerve to reach target (skeletal muscle)
Corticospinal tract info:
1) Pyramidal pathway UMNs reside in motor cortex
2) Descending fibres pass: internal capsule, the crus cerebri of midbrain, basilar portion of pons, & pyramids of medulla
3) at junction of medulla & spinal cord, most of the fibres cross at the decussation of the pyramids.
4) crossed fibres form lateral corticospinal tract & descend in the lateral column of the spinal cord
5) fibers that don’t cross descend in the anterior column of the spinal cord as anterior corticospinal tract
6) Fibres of the anterior corticospinal tract cross the spinal cord before synapsing in the anterior horn
7) Both anterior & lateral fibres are responsible for coordinated, precise, & voluntary skeletal muscle movements
8) CST receives input from the sensory cortex, cerebellum etc.
Upper motor neuron lesions (UMNL):
corticospinal tract (contralateral signs)
-Spastic muscles: (increased muscle tone: no inhibitory influences)
-Exaggerated or pathological reflexes: (babinski sign)
-Paresis (weakness) or paralysis when voluntary movement is attempted
Lower Motor Neuron Lesions (LMNL):
motor root of spinal nerves
-Flaccid muscles & atrophy: (decreased muscle tone: no reflex input)
-Loss of reflexes
-Paresis of paralysis: when voluntary movement attempted
Clinical correlate: Lesions of motor neurons:
EPS:
1) Postural tone adjustments
2) Preparation of predisposing tonic attitudes for involuntary movements
3) Performing movements that make voluntary movements more natural & correct
4) Control of autonomic modifications of tone & movements
5) Control of the reflexes that accompany the responses to affective & attentive situations (reactions)
6) Control of the movements that are originally voluntary but then become automatic through exercise and learning
7) Inhibition of involuntary movements (hyperkinesia), which are particularly evident in extrapyramidal diseases.
Rubrospinal tract pathway:
-Originates from the red nucleus of the midbrain
-crosses the midline as it exits from nuclei in the caudal midbrain and descends through brainstem
-Travels int he ventrolateral white column of the spinal cord
-Fibres terminate by synapsing in on lower motor neurons within grey matter of anterior grey horn
-Involves in mediation of involuntary movement
=muscle tone, posture, & skeletal muscle control
Rubrospinal tract info:
-The importance of the tract lies in the maintenance of muscle tone and in the regulation of rudimentary motor skills that are refined by corticospinal control
-With the corticospinal tract, the rubrospinal tract controls hand and finger movements in addition to flexor muscles
-Used in 4-limb walking
-Babies: crawling before walking could indicate myelination of RS before CST
-Flex arm (swinging) when walking is automatic: have to stop consciously
-Since it is involved in posture, it is not surprising that this tract receives input from the cerebellum
-It receives input from cerebral cortex & branches of corticospinal tracts
Tectospinal tract:
1) Begins @ tectum of midbrain (where sensory fibres carried by the spinotectal tract terminated
2) Fibres cross soon after their origin & descend through the brainstem to the anterior column of the spinal cord
3) Terminate in the upper cervical segments of the spinal cord
4) Controls movements of the head in response to visual stimuli.
(similar tract receives info from cochlear nuclei & starts from inferior colliculi & terminates in Upper cervical segments of spinal cord for auditory reflexes)
Vestibulospinal tract (VST):
essential pathways by which centers of the brain maintain the balance of the body.
Activity depends upon the position of the head and neck, aided by balance receptors in the inner ear as well as sensory information from the cerebellum.
Lateral and medial VSTs: both responsible for antigravity muscle tone in response to the head being tilted to one side and are indirectly influenced by the cerebellum & the labyrinthine system of inner ear.
Lateral vestibulospinal tract:
1) begins at the lateral vetibular nucleus (in medulla; CN VIII)
2) Fibres do not cross and descend to the anterior column of the spinal cord.
3) Function: regulate muscle tone in response to movements of the head, and hence it plays a role in balance and equilibrium
4) receives input from: inner ear via vestibulocochlear nerve & cerebellum
Reticulospinal tract:
1) pontine reticulospinal tract begins from the reticular formation of the pons
2) medullary reticulospinal tract begins in reticular formation of medulla
3) Pontine tract remains uncrossed, medullary tract has some fibres cross and others that don’t
4) pontine tract descends in the anterior column of spinal cord. Medullary tract descends in lateral column.
5) Both tracts influence reflexes & voluntary movements (allows inhibition of antagonist muscle), and carry’s info from hypothalamus to lateral horn of T1 to L2 & S2, S3, S4.
6) Input for these tracts come from cerebral cortex & cerebellum. Tracts also carry autonomic fibres, and receive input from the hypothalamus.
Spinomesencephalic tract:
carries pain info to the midbrain, synapsing in the periaqueductal gray region around the cerebral aqueduct.
Reflex arc:
pathway followed by nerve impulses that produce a reflex.
5 components:
1) receptor
2) sensory neuron
3) integrating center (monosynaptic vs. polysynaptic reflexes)
4) Motor neuron
5) Effector (somatic reflex vs. autonomic (visceral reflex)
Stretch reflex (Patellar reflex):
-A feedback mechanism that controls muscle length by causing muscle contraction. (prevents injury from overstretching because muscle contracts when it is stretched
-Monosynaptic, ipsilateral reflex arc
-Events of the stretch reflex:
-Reciprocal innervation:
1) polysynaptic-interneuron
2) antagonist muscles relax as part of the reflex
-The stretch reflex can also maintain posture
Tendon relfex:
-A feedback mechanism that controls muscle tension by causing muscle relaxation when muscle force becomes too extreme.
-Ipsilateral polysynaptic reflex
-Golgi tendon organs are in tendon:
1) activated by stretching of tendon
2) inhibitory neuron is stimulated (polysynaptic)
3) Motor neuron is hyperpolarized and muscle relaxes
-Both tendon & muscle are protected
-Reciprocal innervation (polysynaptic)
Flexor (withdrawal) reflex:
ipsilateral & is protective withdrawal reflex that moves a limb to avoid pain.
This reflex results in contraction of flexor muscles to move a limb to avoid injury or pain.
Flexor (withdrawal) reflex pathway:
1) Step on tack (pain fibres send signals to spinal cord)
2) Interneurons branch to different spinal cord segments, intersegmental reflex arch
3) Motor fibers in several segments are activated
4) More than one muscle group activated to lift the foot off of tack
Crossed extensor reflex pathway:
-Lifting right foot requires an extension of the left leg to maintain one’s balance
-Pain signals cross to the opposite spinal cord
-Contralateral extensor muscles are stimulated by the interneurons to hold up the body weight
-Reciprocal innervation: when extensors contract flexors relax, etc.
Babinski’s reflex or extensor plantar reflex:
Test for dysfunctional corticospinal tract (upper motor lesion)
Test involves stroking the outside sole from heel to toe with a pointed object.
Normal response: Downward flexor movement of all toes (Image A)
Infants under 1.5 years of age (incomplete myelination in children) & people with dysfunction in the corticospinal tract, this causes an upward (extensor) movement of the big toe, often with extension & abduction (fanning) of the other toes (Image B)
Achilles reflex (ankle jerk reflex):
-Tapping on the calcaneal tendon (achilles tendon) would cause plantar flexion of the ankle joint.
This reflex checks if the S1 nerve root are intact and could be indicative of sciatic nerve pathology.
This reflex is usually absent in disk herniation at the L5-S1 level.
A reduction in the ankle jerk reflex is also indicative of peripheral neuropathy.
