Lecture 4 Flashcards
What do motor tracts of the spinal cord relay?
motor instruction either from motor areas of the frontal lobe, of the cerebral cortex, or some of the nuclei of the brainstem to the spinal cord
What are the 2 classifications of motor tracts of the spinal cord?
- pyramidal tract
- extrapyramidal tracts
Which tract classification of the spinal cord coneys impulses to control the skeletal muscles of the limbs and trunk (voluntary movement)?
pyramidal tract
Which motor tract classification of the spinal cord includes the the lateral & ventral corticospinal tracts?
pyramidal tract
Which tract classification of the spinal cord conducts signals that regulate body movement that are subconscious or postural in nature (involuntary movement)?
extrapyramidal tract
Which motor tract classification of the spinal cord includes Rubrospinal, Tectospinal, Vestibulospinal, and Reticulospinal tracts?
extrapyramidal tracts
Where do the pyramidal tracts’ upper motor neurons (UMNs) reside?
cerebral hemispheres’ of motor cortex
Where are the upper motor neurons (UMNs) of the extrapyramidal tracts housed?
within brainstem & cerebellar cortex
Where do the lower motor neurons (LMNs) of both the pyramidal & extrapyramidal tracts located in?
anterior horn of spinal cord
What do the axons of LMNs exit the spinal cord as? What do they target?
exit as ventral root of spinal nerves & target skeletal muscles
Which neurons begin in the cortex and terminate in the brainstem or spinal cord ?
upper motor neurons
Which neurons begin in the brainstem or spinal cord & innervate muscles?
lower motor neurons
Where does the corticobulbar (corticonuclear) tract terminate? Which cranial nerves does it synapse on?
- brainstem
- synapse on cranial nerve nuclei (III, IV in the midbrain; V-VIII in pons; IX-XII in the medulla)
Where does the corticospinal tract terminate? What does it synapse on?
terminates in the spinal cord and synapses with the motor neurons in the ventral horn of the spinal cord.
Where do the cell bodies of lower motor neurons of cranial nerves reside?
Cell body in the motor nuclei of the cranial nerves in the brainstem
Where do the cell bodies of lower motor neurons of spinal nerves reside?
cell body in the ventral horn of the spinal cord that form the motor root of the spinal nerve
What is the pathway of the corticospinal tract?
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)
What does the corticospinal tract transmit?
voluntary motor/conscious (GSE) control of skeletal muscles of the body
What are the pyramids of the medulla made up of?
decussating fibres (upper motor neuron)
What do the descending fibres of the corticospianl tract pass through? the internal capsule, the crus cerebri of the midbrain, the basilar portion of the pons, and the pyramids of the medulla
- internal capsule
- crus cerebri of the midbrain
- basilar portion of the pons
- the pyramids of the medulla
Where do most of the fibres of the corticospinal tract cross at?
decussation of pyramids (junction of medulla & spinal cord)
What do the fibres of the corticospinal tract that cross at the decussation of the pyramids form? Where do they descend?
Lateral corticospinal tract
descend in the lateral column of the spinal cord
What do the fibres that do not cross at the decussation of the pyramids form? Where do they descend?
anterior corticospinal tract
anterior column of spinal cord
Where does the anterior corticospinal tract cross?
in spinal cord before synapsing in the anterior horn
Where does the corticospinal tract recieve input from?
sensory cortex, cerebellum
Where does upper motor neuron lesions (UMNL) of the corticospinal tract occur?
inside the corticospinal tract (CNS) - crus cerebri, spinal cord, etc.
Why is damage to upper motor neurons of the corticospinal tract contralateral?
because damage happens before the cross point
Where does lower motor neuron lesions (LMNL) of the corticospinal tract occur?
motor root of spinal nerves (PNS)
Is damage of the lower motor neurons of the corticospinal tract showcase contralateral or ipsilateral signs?
ipsilateral
What are symptoms of upper motor neuron lesions?
- Spastic muscles (increased muscle tone: no inhibitory influences)
- exaggerated or pathological reflexes (i.e., Babinski sign)
- paresis (weakness) or paralysis when voluntary movement attempted
- absent fasciculations (muscle twitching)
- slow atrophy
What are symptoms of lower motor neuron lesions?
- flaccid muscles & atrophy (decreased muscle tone: no reflex input)
- loss of reflexes
- paresis or paralysis when voluntary movement attempted
- present fasciculations (muscle twitching)
- fast atrophy
The ___ __ serves an essential function in maintaining posture and regulating involuntary motor functions
extrapyramidal tracts
What does the extrapyramidal tracts provide?
- Postural tone adjustment
- Preparation of predisposing tonic attitudes forinvoluntary movements
- Performing movements that make voluntary movements more natural and correct
- Control of automatic modifications of tone andmovements
- Control of the reflexes that accompany the responses to affective and attentive situations (reactions)
- Control of the movements that are originally voluntary but thenbecome automatic through exercise and learning (e.g., in writing)
- Inhibition of involuntary movements (hyperkinesias), which are particularly evident in extrapyramidal diseases
Where is the pathway of the rubrospinal tract?
- Originates from the red nucleus of the midbrain
- Crosses (decussates) the midline as it exits from nuclei in the caudal midbrain and descends through brainstem
- Travels in the ventrolateral white column of the spinal cord.
- Fibers terminate by synapsing in on lower motor neurons within grey matter of anterior grey horn
What is the function of the rubrospinal tract?
mediation of involuntary movement: muscle tone, posture, and skeletal muscle control
What is the importance of the rubrospinal tract?
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
T/F: With the corticospinal tract, the rubrospinal tract controls hand and finger movements in addition to flexor muscles
True
What is the reason for crawling before walking in babies?
indicate myelination of RS before CST
What tract was used in “four-limb” walking of animals?
rubrospinal tract
Which tract ‘swings’ the arm when walking?
rubrospinal tract
Where does the rubrospinal tract recieve input from?
- cerebellum
- cerebral cortex
- branches of corticospinal tracts
What is the pathway of the tectospinal tract?
- originates at tectum of midbrain (superior colliculus)
- decussates in the mibrain and descend through the brainstem to the anterior column of the spinal cord
- Terminate in the anterior grey horn of spinal cord cervical region only
What is the function of the tectospinal tract?
- control movements of head in response to visual stimuli
- control movements in response to auditory stimuli - CN VIII (vestibulocochlear)
The __ __ are essential pathways by which higher centers of the brain maintain the balance of the body
vestibulospinal tracts (VST)
What does the activity of the vestibulospinal tracts depend on?
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
What are the two vestibulospinal tracts?
lateral vestibulospinal tract and the medial vestibulospinal tract
What are the vestibulospinal tracts responsible for?
antigravity muscle tone in response to the head being tilted to one side and are indirectly influenced by the cerebellum and the labyrinthine system of inner ear (balance)
What is the pathway of the lateral vestibulospinal tract?
- This tract begins at the lateral vestibular nucleus (in the medulla; CN VIII)
- The fibres do not cross and descend to the anterior column of the spinal cord
- terminate at the anterior grey horn of spinal cord
What is the function of the lateral vestibulospinal tract?
The function of this tract is to regulate muscle tone (in flexor and extensor muscles) in response to movements of the head, and hence it plays a role in: balance and equilibrium.
What does the vestibulospinal tract recieve input from?
inner ear via the vestibulocochlear nerve, and the cerebellum
What is the pathway of the pontine reticulospinal tract?
- begins from the reticular formation of the pons
- remain uncrossed
- descends in the anterior column of the spinal cord
- terminates on anterior grey horn
What is the pathway of the medullary reticulospinal tract?
- begins from the reticular formation of the medulla
- both crossed and uncrossed fibers
- descends in lateral column
- terminates on anterior grey horn
What is the function of both reticulospinal tract?
- reflexes and voluntary movements (allow for inhibition of antagonist muscle when agonist is contracted)
- carry autonomic information from hypothalamus to lateral horn of T1 to L2 and S2,S3,S4 segments of the spinal cord (Sources of sympathetic and parasympathetic N.S respectively)
- muscle tone, balance, postural changes
What is the input for the reticulospinal tracts?
- cerebral cortex
- cerebellum
- hypothalamus
What is the function of the pontine RST?
controlling axial and extensor motor neurons, e.g enabling extension of the legs to maintain postural support
What is the function of the medullary RST?
Inhibits extensor motor neurons to enable modulation of the stretch reflex
Which tract carries pain information to the midbrain, synapsing in the periaqueductal gray region around the cerebral aqueduct ?
spinomesencephalic tract
What does stimulation of PAG neurons by the spinomesencephalic tract do?
- sends impulses down to the dorsal horn of the spinal cord (via the connection with the reticular formation nuclei) to block the release of substance P that is necessary to transmit signals from the first to second-order neurons of the lateral spinothalamic tract
- blocks the transmission of pain
What is a reflex?
fast, involuntary, unplanned sequence of action that occurs in response to a particular stimulus and helps to maintain homeostasis
__ may be spinal, cranial [somatic or autonomic (visceral)]
reflexes
The gray matter of spinal cord serves as an integrating center for?
spinal reflexes
A pathway followed by nerve impulses that produce a reflex is called a?
reflex arc
What are 5 components of a reflex arc?
- Receptor
- Sensory neuron
- Integrating center
- Motor neuron
- Effector
What is the function of the sensory receptor of a reflex arc?
responds to a stimulus by producing a generator or receptor potential
What is the function of the sensory neuron of a reflex arc?
axon conducts impulses from receptor to integrating center
What is the function of the integrating center of a reflex arc?
one or more regions within the CNS that relay impulses from sensory to motor neurons
What is the function of the motor neuron in a relfex arc?
axon conducts impulses from integrating center to effector
What is the function of the effector in a reflex arc?
muscle of gland that responds to motor nerve impulses
What is a monosynaptic reflex?
reflex arc that provides direct communication between sensory and motor neurons innervating the muscle (no interneuron)
What is a polysynaptic reflex?
utilizes interneurons which pass signals between the sensory and motor neurons –> creating multiple synaptic connections
What is a somatic reflex?
reflex of skeletal muscle
What is a autonomic (visceral) reflex?
reflex of smooth muscle, cardiac muscle, or glands
What are examples of somatic spinal reflexes?
- Stretch reflex
- Tendon reflex
- Flexor (withdrawal) reflex
- Crossed extensor reflex
T/F: Somatic spinal reflexes exhibit reciprocal innervation
True
What is reciprocal innervation?
contraction of one muscle and relaxation of its antagonist muscle or muscles
What is the stretch reflex?
A feedback mechanism that controls muscle length by causing muscle contraction
What does the stretch reflex prevent?
Prevents injury from overstretching because muscle contracts when it is stretched
What type of reflex arc does the stretch reflex have?
Monosynaptic, ipsilateral reflex arc
What are the events of the stretch reflex?
- stretching stimulates - sensory receptor (muscle spindle)
- sensory neuron = excited
- within integrating center (spinal cord) = sensory neuron activates motor neuron
- motor neuron = excited
- effector (same muscle) contracts & relieves the stretching
What is the reciprocal innervation of the stretch reflex?
- polysynaptic- interneuron –> antagonistic muscles relax as part of the reflex
What is a function of the stretch reflex?
maintain posture
What is the tendon reflex?
A feedback mechanism that controls muscle tension by causing muscle relaxation when muscle force becomes too extreme
What type of reflex arc does the tendon reflex have?
ipsilateral polysynaptic reflex
What are the events of the tendon reflex?
- increased tension stimulates sensory receptor (tendon organ/golgi tendon)
- sensory neuron = excited
- within integrating center (spinal cord) –> sensory neuron activates inhibitory neuron
- motor neuron inhibited
- effector (muscle attached to same tendon_ relaxes & relieves excess tension
What is protected in the tendon reflex?
both tendon & muscle
What is the reciprocal innervation on the tendon reflex?
polysynaptic –> motor neuron to ntagonistic muscles are excited –> contract
The __ __ is ipsilateral and is a protective withdrawal reflex that moves a limb to avoid pain
flexor (withdrawal) reflex
What does the flexor (withdrawal) reflex result in?
contraction of flexor muscles to move a limb to avoid injury or pain
What are the components of the flexor (withdrawal) reflex?
- stepping on tack stimulates sensory receptor (dendrites of pain-sensitive neuron)
- sensory neuron = excited
- within integrating center (spinal cord), sensory neuron activates interneurons in several spinal cord segments
- motor neurons = excited
- effectors (flexor muscles) contract & withdrawal leg
What does intersegmental reflex arc mean?
Interneurons branch to motor neurons of multiple levels of the same spinal cord
T/F: The flexor (withdrawal) reflex activates several motor fiber segments
True
The __ __ reflex, which is contralateral, helps to maintain balance during the flexor reflex
crossed extensor
What is the crossed extensor reflex?
balance-maintaining reflex that causes a synchronized extension of the joints of one limb and flexion of the joints in the opposite limb
What are the components of the crossed extensor reflex?
- stepping on a tack stimulates sensory receptor (dendrites of pain-sensitive neuron) in right foot
- sensory neuron = excited
- within integrating centre (spinal cord) sensory neuron activates several interneurons
- motor neurons = excited
- effectors (extensor muscles) contract, & extend left leg
T/F: In crossed extensor reflex pain signals stay on the same side
False cross to opposite spinal cord
What is the reciprocal innervation of crossed extensor reflex?
when extensors contract, flexors relax
What test is for dysfunctional corticospinal tract [Upper Motor Lesion (UML)]?
Babinski’s Reflex or Extensor Plantar Reflex
What does the Babinski’s Reflex or Extensor Plantar Reflex involve?
stroking the outside sole from heel to toe with a pointed object
What is the normal response of Babinski’s Reflex or Extensor Plantar Reflex?
Downward (flexor) movement of all toes
What is the abnormal response of Babinski’s Reflex or Extensor Plantar Reflex?
upward (extensor) movement of the big toe, often with extension and abduction (“fanning”) of the other toes
When do you get an abnormal response of Babinski’s Reflex or Extensor Plantar Reflex?
- in infants under 1.5 years of age (incomplete myelination in children)
- people w/ dysfunction in corticospinal tract
What is the achilles reflex (ankle jerk reflex)?
Tapping on the calcaneal tendon (Achilles Tendon) –> plantar flexion of the ankle joint
What does the achilles reflex (ankle jerk reflex) check for?
S1 nerve root are intact
What can the achilles reflex (ankle jerk reflex) be indicative of?
- sciatic nerve pathology
- disk herniation at L5-S1 (absent)
- peripheral neuropathy (reduction)
