ME03 - Motor System 1 - SC, Cortex, Brainstem Flashcards
Neural pathways that control the sequence and pattern of muscle contractions
Distributed throughout brain and spinal cord
Motor System
What constitutes a MOTOR UNIT?
Consists of a single motor neuron and the muscle fibers that it innervates
For fine control, a single motor neuron innervates only a few muscle fibers. True or False
True, Example: Eye Muscle
For larger movements, a single motor neuron may innervate thousands of muscle fibers. True or False?
True, Example: Postural Muscles/Axial Muscles
Force of muscle contraction is graded by recruitment of additional motor units
Graded Response - Size Principle of Muscle
What principle is applied in “As additional motor units are recruited, more motor neurons are involved and more tension is generated”
Size Principle of Muscle
2 Types of Motor Neurons
Alpha Motor Neurons | Gamma Motor Neurons
o Innervate extrafusal skeletal muscle fibers
o Action potentials in _ motor neurons lead to action potentials in the extrafusal muscle fibers they innervate, which results in contraction
Alpha Motor Neurons
o Innervate specialized intrafusal muscle fibers
o Adjust the sensitivity of the muscle spindles
Gamma Motor Neurons
2 Types of Muscle Fibers
Extafusal Fibers | Intrafusal Fibers
o Make up the bulk of muscle
o Innervated by alpha motor neurons
o Provide the force for muscle contraction
Extrafusal Fibers
o Smaller than extrafusal muscle fibers
o Are innervated by gamma motor neurons
o Encapsulated in sheaths to form muscle spindles
o Are too small to generate significant force
Intrafusal Fibers
Mnemonic for Motor Neuron Fiber
AEGIS (Alpha Motor - Extrafusal | Gamma Motoe - Intrafusal)
Central role in skeletal muscle control
Cell bodies are topographically arranged within the ventral horn of the spinal cord
Axons innervate skeletal muscle fibers
Alpha Motor Neurons
Cell bodies of Alpha Motor Neuron receive numerous synaptic connections from:
o proprioceptors
o higher levels of the CNS including the brainstem, basal ganglia, cerebellum, and motor cortex
Topographic Arrangement of muscles of the trunk
Medial
Topographic Arrangement of muscles of the extremities
Lateral
Topographic Arrangement of limb flexors
Dorsal
Topographic Arrangement of limb extensors
Ventral
Cells responsible for the synapse with the pool of motor neurons when stimulated
Renshaw Cells»_space; predominantly inhibitory
bring about recurrent or feedback inhibition
NTs involved in the muscle contraction
Glycine - inhibitory | Strignin - cause prolonged muscle contraction
What type of synaptic arrangement is exemplified by Renshaw cells?
One to Many
What neurotransmitter is released by Renshaw cells?
Glycine
What type of neuronal circuit is exemplified by Renshaw cells?
Divergent
Sense of awareness of POSITION of the body in space
Proprioception
Proprioception progress of the movement by sensory receptors within the muscles and joints. True or False?
TRUE
Where can proprioceptors be found?
Mechanoreceptors within muscles and joints | Two Major Proprioceptor are (1) Muscle Spindle (2) Golgi Tendon Organ
Function of proprioceptors beside the sense of awareness of position?
Provide the CNS with information regarding muscle length, position and tension (force)
Small, encapsulated intrafusal fibers
Lie in parallel with extrafusal muscle fibers
Muscle Spindle - (STRETCH)
What are the innervations of Muscle Spind (afferent, efferent)
Afferents via group Ia (primary or annulospiral endings) and group II fibers (secondary endings)»_space; respond to muscle stretch
Efferents via gamma motor neurons»_space; regulates sensitivity of the spindles
The finer the movement required, the greater the number of muscle spindles in a muscle. True or False?
TRUE
Where is y-neuron found in intrafusal fibers?
In the belly of the muscle
2 Types of Intrafusal Fibers in Muscle Spindle?
Nuclear Bag Fibers | Nuclear Chain Fibers
o Detect the rate of change in muscle length (fast, dynamic changes)
o Innervated by group Ia afferents
Nuclear Bag Fibers - Have nuclei collected in a central bag region
o Detect static changes in muscle length
o Innervated by group II afferents
o More numerous than nuclear bag fibers
o Have nuclei arranged in rows
Nuclear Chain Fibers
There is very intense period activity of primary endings (Group IA) during the stretch compared to secondary endings. True or False?
True; this is related to the tendency to overstretch “Wear & Tear” of Muscles
Role of Muscle Spindles
Comparators for maintenance of muscle length
Important during goal-directed voluntary movements
o Voluntary changes in muscle length are initiated by motor areas of the brain
o Includes changes to the set-point of the muscle spindle system
Simultaneous activation of extrafusal fibers (by alpha motor neurons) and intrafusal fibers (by gamma motor neurons)
Readjusts the sensitivity of muscle spindles continuously as the muscle shortens
Allows the muscle spindles to be functional at all times during a muscle contraction
C0-ACTIVATION
Alpha-Gamma Coactivation (Stretch Spindle/Intrafusal)
- Alpha motor neuron fires and gamma motor neuron fires.
- Muscle contracts.
- Stretch on centers of intrafusal fibers unchanged. Firing rate of afferent neuron remains constant.
Without Gamma Motor (Alpha-motor only) Neurons
- Alpha motor neuron fires.
- Muscle contracts
- Less stretch on center of intrafusal fibers
- Firing rate of spindle sensory neurons decreases.
Mechanoreceptors that lie within the tendons of muscles immediately beyond their attachments to the muscle fibers
Golgi Tendon Organ - senses tension in the tendon
Golgi Tendon Organ respond to degree of tension within muscles. True or False?
TRUE
Which fibers is responsible to relay this information to the CNS (in particular the spinal cord and cerebellum)
Group Ib afferent fibers
Follow the pathway of GTO from muscle contraction to relaxation
Muscle contracts»_space; INC tension»_space; GTO»_space; CNS»_space; Interneuron»_space; INHIBIT a-motor neurons»_space; Relaxed
Why are Group Ib fibers of GTO have short periods of intense firing?
They have short periods of intense firing since they are series with muscle fibers and well-adapted to monito muscle tension.
Rapidly executed, automatic, and stereotyped response to a given stimulus
Simplest form of irritability associated with the nervous system
Reflex
Components of a Reflex Arc
o Receptor
o Afferent neuron that synapses in the CNS
o Efferent neuron that sends impulses to an effector
o Interneurons may be present between the afferent and efferent neurons
Classify neural reflexes by EFFERENT DIVISION THAT CONTROLS THE EFFECTOR
Somatic Motor Neuron controls skeletal muscles
Autonomic Neurons control smooth and cardiac muscle, glands, and adipose tissue.
Classify neural reflexes by INTEGRATING REGION WITHIN CNS
Spinal Reflexes do not require input from brain
Cranial Reflexes integrated within the brain
Classify neural reflexes by TIME AT WHICH REFLEX DEVELOPS
Innate Reflexes genetically determined
Learned Reflexes acquired through experience
Classify neural reflexes by NO. OF NEURONS IN THE REFLEX PATHWAY
Monosynaptic Reflexes have only 2 neurons (one afferent and one efferent)
Polysynaptic Reflexes include one or more interneurons between afferent and efferent neurons.
Only somatic motor reflexes can be monosynaptic. True or False?
TRUE
All autonomic reflexes are polysynaptic because they have 3 neurons: one afferent and two efferent. True or False?
TRUE
Single synapse between afferent and efferent neurons
Monosynaptic Reflex
Two or more synapses between afferent and efferent neurons
Polysynaptic Reflex
Reflexes that do not need to go to the brain
Spinal Cord Reflexes
Also known as patellar tendon-tap reflex, knee-jerk reflex or Myotactic Reflexes
Stretching of a muscle stimulates the muscle spindle afferents
Plays an important role in the control of posture
Muscle Stretch Reflex
Caused by rapid stretch or unstretch/ RAPID CHANGE IN STRETCH
Transmitted from primary sensory or annulospiral endings of the muscle spindles
Oppose sudden changes in muscle length
Lasts within a fraction of a second only
Dynamic Stretch Reflex (Ia Only)
Elicited by the continuous static receptor signals
Transmitted by both primary and secondary endings
Causes the degree of muscle contraction to remain reasonably constant
Continues for a prolonged period
Static Stretch Reflex (Ia and II)
Muscle spindles prevent oscillation or jerkiness of body movements
Ensure that contraction is relatively smooth, even though the motor nerve to the muscle is excited at a slow frequency
Damping Function of Stretch Reflexes
Reinforcement technique for eliciting deep tendon reflexes
o fingers are locked together and one hand pulls against the other while reflex is evoked
Jendrassik’s Maneuver
What is the physiologic basis of Jendrassik’s Maneuver?
When one muscle is stretched, it facilitates a substantial number of alpha motor neurons
Transient increase of gamma motor neuron activity
Jendrassiks maneuver facilitates multiple alpha motor neurons. True or False?
TRUE
Oscillation of a stretch reflex
Clonus
When does Clonus occur?
Ordinarily occurs only when the stretch reflex is highly sensitized by facilitatory impulses from the brain
This is elicited by noxious stimuli
Transmitted by group II, III, IV fibers
Possesses at least one interneuron, and so the most basic flexion reflex is disynaptic
Usually many muscles are involved through polysynaptic pathways
Flexor Withdrawal Reflex
To achieve withdrawal of a limb:
Flexor muscles in the limb must contract while the extensor muscles relax
Follow the pathway for “Reciprocal Inhibition”
- Painful stimulus activates nociceptor.
- Primary sensory neuron enters spinal cord and diverges.
- One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity)
- Withdrawal reflex pulls foot away from painful stimulus.
Crossed extensor reflex supports body as weight shifts away from painful stimulus.
This ensures that the extensor muscles acting on a joint will relax while flexor muscles contract
Reciprocal Inhibition
Neuronal circuit that causes this reciprocal relation
Reciprocal innervation
What are the components of Flexor Withdrawal Reflex
Diverging circuits to spread the reflex to the necessary muscles for withdrawal
Reciprocal inhibition circuits to inhibit the antagonist muscles
Circuits to cause afterdischarge lasting many fractions of a second after the stimulus is over
Stimulation of the flexion reflex frequently elicits extension of the contralateral limb about 250 ms later
Long latency between flexion and crossed extension represents the time taken to recruit interneurons
Helps to maintain posture and balance
Crossed Extensor Reflex (More sustained, more time for afterdischarge)
What type of neuronal circuit is exemplified by prolonged afterdischarge in crossed extensor reflex?
Recurring/Reverberating
Responsible for the after discharge
Interneurons
Golgi tendon organs monitor muscle tension
Negative feedback mechanism that prevents development of too much tension on muscles
Inverse Myotactic Reflex (Instantaneous Relaxation)
A reaction when tension becomes extreme, reflex inhibitory effects lead to instantaneous relaxation of the entire muscle
Lengthening Reaction
What are the four major spinal cord reflexes?
Muscle Stretch Reflex monosynaptic; muscle stretch (stimulus); Group Ia fibers > Muscle Contraction
GT Reflex di/polysynaptic; muscle tension (stimulus); Group Ib fibers > Muscle Relaxation
Flexor Withdrawal Reflex polysynaptic; Pain (stimulus); II,II, IV fibers > Ipsilateral muscle flexion
Crossed Extension Reflex polysynaptic; Pain (stimulus); II,III,IV fibers > Contralateral Muscle Extension
Caused by transection of the spinal cord
Loss of spinal reflexes (areflexia) and flaccid paralysis below the level of the injury
Over the ensuing weeks, spinal cord activity below the level of the lesion returns as the excitability of undamaged neurons increases
May give rise to spasticity of the paralyzed muscle groups
Spinal Shock
What are the events in SPINAL SHOCK?
NEUROGENIC SHOCK
o arterial blood pressure falls instantly
o demonstrates that sympathetic nervous system activity becomes blocked almost to extinction
AREFLEXIA
o may last 2 weeks to several months
o order of return: stretch reflexes, flexor reflexes, postural antigravity reflexes, remnants of stepping reflexes
INCONTINENCE
o sacral reflexes for control of bladder and colon evacuation are suppressed
EXAMPLES OF EXTENT OF PARALYSIS
Tetraplegia | Paraplegia | Hemiplegia | Bulbocarvernosus Reflex
Impairment or loss of motor and sensory function in the arms, trunk, legs, and pelvic organs
TETRAPLEGIA / QUADRIPLEGIA
Impairment of function of the legs and pelvic organs
PARAPLEGIA / BIPLEGIA
Total paralysis of the arm, leg, and trunk on the same side of the body
Does not usually result from spinal cord injuries but from strokes
HEMIPLEGIA (Stroke)
Polysynaptic reflex useful in testing for spinal shock
Checks anal sphincter contraction in response to squeezing the glans penis
Bulbocavernosus Reflex
Reactions to Bulbocavernosus Reflex
Absence indicates spinal shock
First reflex to return after spinal shock
_ once this reflex has returned, all remaining neurologic deficits are considered permanent
Contains motor areas
o stimulation will elicit contralateral movements
displays somatotopic arrangement
Cerebral Cortex
Areas of the body that are capable of especially refined and complex movements (i.e. fingers, lips, and tongue) have a disproportionately LARGE area of representation. True or False?
TRUE
What are the three sub-areas of Motor Cortex?
o PRIMARY MOTOR CORTEX
o PREMOTOR AREA
o SUPPLEMENTARY MOTOR AREA
located in pre-central gyrus or Brodmann area 4
responsible for the execution of movement
o programmed patterns of motor neurons and voluntary movement
Primary Motor Cortex
PMA is somatotopically organized. True or False?
True, it is called the motor homunculus
Epileptic events in the primary motor cortex cause
Jacksonian Seizures
Immediately anterior to the lateral portion of the primary motor cortex
Forms a portion of Brodmann area 6
Responsible for generating a plan for movement
o transferred to primary motor cortex for execution
Stimulation causes activation of groups of muscles
Premotor Area
Located in the medial portion of Brodmann area 6 just anterior to the lower extremity portion of the precentral gyrus
Stimulation causes activation of bilateral muscle activation (usually upper extremities)
Programs complex motor sequences
Supplementary Motor Area
Supplementary Motor Area is active during
Mental rehearsal for a movement
o Motor speech area
o Converts simple vocal utterances into whole words and complete sentences
BROCA’S AREA
Controls conjugate eye movement required to shift gaze from one object to another
FRONTAL EYE FIELD (Brodmann area 8)
Enables movement of head correlated with eyes
HEAD ROTATION AREA
When damaged, hand movements are lost (motor apraxia)
AREA FOR FINE MOVEMENTS OF HAND
Motor Outflow of Cerebral Cortex IS carried by what tracts?
Corticospinal (pyramidal) and Extrapyramidal tracts
Where is the predominant sensory input?
Somatosensory system, which receives its input from the thalamus
Where is afferent information received?
Received from the visual system, cerebellum, and basal ganglia
Motor Input of Cerebral Cortex is important because:
Used to refine movements, particularly to match the force generated in specific muscle groups to an imposed load
originates over a wide area of cortex including both motor and somatosensory areas
more than 80 per cent of the fibers decussate at the pyramids (CERVICOMEDULLARY JUNCTION)
Corticospinal Tract
MAIN FUNCTION OF Corticospinal Tract
predominant pathway for the control of fine skilled manipulative movements of the extremities
Hallmark feature of lesions to the corticospinal tract
Loss of precise hand movements
FLOW OF CORTICOSPINAL TRACT
Motor Cortex»_space; Corona radiata»_space; Internal capsule»_space; Cerebral peduncle»_space; Brainstem»_space; Cervicomedullary junction*»_space; Corticospinal tract (A/L)»_space; Anterior horn cell»_space; Ventral root»_space; Peripheral nerve»_space; Neuromuscular junction»_space; Muscle
conveys nerve impulses from the motor cortex to skeletal muscles of the head and neck
axons of UMNs descend from the cortex into the brain stem, where some decussate and others do not
provide input to lower motor neurons in the nuclei of cranial nerves III, IV, V, VI, VII, IX, X, XI, and XII
Corticobulbar Tract
MAIN FUNCTION OF Corticobulbar Tract
Control voluntary movements of the eyes, tongue and neck, chewing, facial expression and speech
also called CEREBROVASCULAR DISEASE
cessation of blood flow to the brain due to:
o ruptured blood vessel that bleeds into the brain
o thrombosis of a vessel, producing local ischemia
Strokes
Muscles controlled by the damaged areas show a corresponding loss of function. GIVE EXAMPLE OF LOSS OF FUNCTION
o clumsiness and loss of fine muscle control
o postural movements may not be affected
o hyperreflexia, hypertonia and spasticity occur with extension of involvement
Loss of the ability to prepare for voluntary movement
Ability to execute simple movements is retained
APRAXIA
LESION to what areas of the brain causes Apraxia
Lesions to supplementary and premotor areas
o motor neurons that originate in the motor region of the cerebral cortex or the brain stem
o main effector neurons for voluntary movement in layer V of the primary motor cortex (Betz cells)
UPPER MOTOR NEURON
UMN pathways (above anterior horn cell) include what tracts?
_ corticospinal tract
_ corticobulbar tracts
_ extrapyramidal tracts
o motor neurons connecting the brainstem and spinal cord to muscle fibers
o bring nerve impulses from the upper motor neurons out to the muscles
o begins at the level of the anterior horn cell in the spinal cord
LOWER MOTOR NEURON
Total loss of motor function associated with an increase in muscle tone
SPASTIC PARALYSIS (UMN)
Manifestation of a Spastic Paralysis
Associated with clasp-knife phenomenon and hyperreflexia
Total loss of motor function associated with a decrease in muscle tone
FLACCID PARALYSIS (LMN)
Manifestation of a Flaccid Paralysis
Associated with floppiness, areflexia or hyporeflexia
Reflex extension of the great toe with flexion of the other toes
Babinski Reflex
How do you test for Babinski Reflex?
Evoked by stroking the lateral sole of the foot
Presence of Babinski Reflex indicates what type of lesion?
indicates an UPPER motor neuron lesion
Small, local, involuntary muscle contractions visible under the skin
Arise from spontaneous discharge of a bundle of skeletal muscle fibers
Fasciculations
Presence of Fasciculations indicates what type of lesion?
Lower motor neuron lesion
Differentiate UMN vs LMN
UMN
INC Muscle Tone, Spastic Paralysis
Hyperreflexia
(+) Babinski, Clonus
Atrophy of Disuse
LMN
DEC Muscle Tone, Flaccid Paralysis
Hyporeflexia
(+) Fasciculations
Atrophy of Denervation
Composed of midbrain, pons and medulla
Brainstem
Special functions of Brainstem
o control of respiration
o control of the cardiovascular system
o partial control of gastrointestinal function
o control of many stereotyped movements of the body
o control of equilibrium
o control of eye movements
o way station for command signals from higher centers
Modifies and Refines the activity of the neural circuitry within the spinal cord
Descending Motor Control Pathways
Example of Descending Motor Control Pathways
o pyramidal tract
_ CORTICOSPINAL TRACT
o extrapyramidal tracts
_ RETICULOSPINAL TRACT
_ VESTIBULOSPINAL TRACT
_ RUBROSPINAL TRACT
_ TECTOSPINAL TRACT
influence mainly the muscles of the trunk and proximal parts of the limbs
important in maintenance of certain postures and in STARTLE REACTIONS
Reticulospinal Tract
Two main divisions of Reticulospinal Tract
o PONTINE or MEDIAL RETICULOSPINAL TRACT
o MEDULLARY or LATERAL RETICULOSPINAL TRACT
originates in the pontine reticular nuclei
projects to the ventromedial spinal cord
general stimulatory effect on both extensors and flexors, with the predominant effect on extensors
Pontine Reticulospinal Tract
originates in the medullary reticular formation
projects to spinal cord interneurons in the intermediate gray area
stimulation has a general inhibitory effect on both extensors and flexors, with the predominant effect on extensors
Medullary Reticulospinal Tract
originates in DEITERS nucleus
projects to ipsilateral motoneurons and interneurons
Vestibulospinal Tract
Important functions of Vestibulospinal Tract
o control the activity of extensor muscles
_ stimulation causes a powerful stimulation of extensors and inhibition of flexors
o maintenance of an erect posture
_ selectively controls the excitatory signals to the different antigravity muscles
o making adjustments in response to signals from the vestibular apparatus
originates in the SUPERIOR COLLICULUS
projects to the cervical spinal cord
decussates before entry to spinal cord
Tectospinal Tract
Lesions IN TECTOSPINAL TRACT are always
CONTRALATERAL
IMPORTANT FUNCTIONS OF TECTOSPINAL TRACT
o control of neck muscles
o controlling head and eye movements
most important extrapyramidal tract
originates in the red nucleus
afferent information from cortex, cerebellum and basal ganglia
projects to interneurons in the lateral spinal cord
decussates before entry to spinal cord
Rubrospinal Tract
Lesions in the Rubrospinal Tract are always
Ipsilateral
IMPORTANT FUNCTIONS OF RUBROSPINAL TRACT
controls both flexor and extensor muscles
o stimulation of the red nucleus produces stimulation of flexors and inhibition of extensors
Voluntary movements are impaired with lesions in the Rubrospinal Tract. True or False?
TRUE
Differentiate Descending Tracts
PONTINE RETICULOSPINAL
-Pontine reticular nuclei; Ventromedial SC projection
-Stimulates flexors and extensors
MEDULLARY RETICULOSPINAL
-Medullary reticular nuclei; Intermediate gray projection
-Inhibits flexors and extensors
VESTIBULOSPINAL
-Deiters nuclei; Ventromedial SC projection
-Stimulates flexors & extensors
TECTOSPINAL
-Superior colliculus; Cervical SC projection
-Contralateral (lesion)
-Control neck muscles
RUBROSPINAL
-Red nucleus; Lateral SC projection
-Contralateral (lesion)
-Stimulates flexors; Inhibit extensors
involuntary flexion or extension of arms and legs
occurs when one set of muscles becomes incapacitated while the opposing set is not
indicates a severe medical emergency requiring immediate medical attention
Abnormal Posturing
Types of Abnormal Posturing
o DECORTICATE RIGIDITY
o DECEREBRATE RIGIDITY
involuntary flexion of the upper extremities in response to external stimuli
arms flexed, hands are clenched into fists, legs extended and feet turned inward
Decorticate Rigidity
involuntary extension of the upper extremities in response to external stimuli
head is arched back, arms are extended by the sides, and legs are extended
Decerebrate Rigidity
Cause DECEREBRATE RIGIDITYE because of the removal of inhibition from higher centers
LESIONS ABOVE THE LATERAL VESTIBULAR NUCLEUS
Cause DECEREBRATE RIGIDITY because of the removal of central inhibition from the pontine reticular formation
LESIONS ABOVE THE PONTINE RETICULAR FORMATION BUT BELOW THE MIDBRAIN
Result in DECORTICATE RIGIDITY and intact tonic neck reflexes
LESIONS ABOVE THE RED NUCLEUS
