L23 - Control of Movement Flashcards
Define motor unit?
Consists of the motor neuron + all of the muscle fibers that it innervates
How does fatigue- resistance relate to a motor unit?
Fatigue depends on Number of muscle fibers innervated by a single motor neuron and size of motor units:
Slow, fatigue-resistant muscles (e.g. eye muscle): fewer muscle fibers per motor neuron
Fast fatigable muscles: more muscle fibers per motor neuron
Compare the 2 ways the Nervous system controls the force generated by muscle contractions? (coding)
- Frequency coding: controls frequency of action potential firing in the motor neuron (e.g. forces of successive muscle twitches can summate into tetanic contraction)
- Population coding: recruits increasing number of motor units in a fixed order (size principle) from weakest (slow fatigue-resistant) to strongest (fast fatigable)
Which ‘coding’ determines max force generated by a muslce?
Population coding
Which ‘coding’ determines the duration of sustained contraction?
Frequency coding
Define the 3 types of movement?
- Reflexes
- Rhythmic motor patterns
- Voluntary movements
Describe the movement in reflexes. Can it be modulated? Give examples of reflexes.
Simple, rapid, stereotyped (= not modulated), involuntary movements with predicatable outcome
Controlled, elicited by stimulus
e.g. stretch reflex, flexion withdrawing reflex
Function of reflexes?
Protective, e.g.:
Withdraw limb from a painful stimulus
Sneezing, coughing to get rid of germs
Reflexes cannot be part of a voluntary action. T or F?
False
Reflex = Essential for voluntary action
Give examples of rythmic motor patterns. Describe the movement.
repetitive movements (e.g. walking, running, swallowing)
Combining features of voluntary and reflex acts
»_space; stereotyped, repetitive, may be automatic
Rhythmic motor patterns occur with automatic initiation and termination. T or F?
False
the initiation and termination are voluntary;
once initiated, sequence of movement is stereotyped, repetitive and may be automatic
Define the 2 feature of voluntary movements.
- Complex, goal-directed (intentional)
2. Learned (performance improves with practice); e.g. playing the piano
Voluntary movements always require full conscious control. True or False?
Partially True
Voluntary movement always need conscious control
But more practice and mastery = require less conscious direction
List the 5 processes that occur in the control of movements?
- Accurate time control of contractions of different muscles (sequential)
- Associated postural adjustment (balance)
- Adjusting mechanical properties of joints & muscles
- Sensorimotor integration (posterior parietal cortex)
- Error detection, feedforward (anticipation) & feedback control
Define the brain areas with upper motor neurons and their input?
Motor regions of cerebral cortex (input from basal ganglia, cerebellum through thalamus)
Brainstem (input from motor regions of cerebral cortex, cerebellum, spinal cord)
> > Do not directly innervate muscles, only modulate motor output
Define the CNS areas with lower motor neurons and their input?
Brainstem (motor nuclei of cranial nerves, e.g. facial muscle)
Spinal cord (receives descending signal from only motor cortex + brainstem; NOT basal ganglia, thalamus, cerebellum)
> > Directly innervates muscle
Which brain area contains both UMN and LMN?
Brainstem
Define the 3 levels of motor control and their organization?
spinal cord, brainstem, cerebral cortex
Hierarchical organization of the three levels; parallel organization (each level can operate independently or together)
What brain structures modulate the 3 levels of motor control in the brain?
All 3 level:
Influenced by 2 independent subcortical structures: basal ganglion, cerebellum
Input of sensory information from environment, vestibular system, joint afferents, and muscle spindles
Describe the arrangement of ventral horn neurons?
Somatotopically arranged:
Medial neurons innervate axial musculature (e.g. back, shoulder, proximal muscle)
Lateral neurons innervate distal musculature (e.g. limb muscles): further divide:
- Ventral = extensors
- Dorsal = flexors
- Lateral = Distal limb
- Medial = Proximal limb
What types of movements can be carried out by the spinal cord without input from brain? Give examples of each
Reflexes:
• Stereotyped responses e.g. stretch reflex
• Stereotyped motor coordination e.g. flexion reflexes, flexion withdrawal crossed extension
Rhythmic movement:
• locomotor pattern e.g. walking
Define the stimulus, coordination and output of Stretch reflex (stereotyped responses)?
MONOSYNAPTIC
Stim: muscle stretch (proprioceptor)
Coordination = Interneurons in the spinal circuitry inhibit the motor neuron innervating the antagonistic muscle
Output: i.e. Flexor contract + Ia inhibitory interneuron cause antagonist to relax
Function of stereotyped responses?
Adapted for speed of operation;
allows muscle tone to be regulated quickly and efficiently without intervention from higher centers
> > protective
Define the function of stereotyped motor coordination e.g. flexion reflexes?
Protective + coordinates voluntary movement
Define the stimulus, coordination, and output of flexion reflex e.g. flexion withdrawal crossed extension?
Stim: e.g. pain
Coordination: POLYSYNAPTIC:
- Descending control from the brain adjust the strength of the spinal reflexes
- Multiple interneurons in spinal cord
Output:
- Opposite leg’s extensor contract for support, flexor relax
- Withdrawal of stimulated leg by flexor muscle contraction, extensor relax
Stimulus, Coordination and output of rhythmic locomotor pattern (e.g. walking)?
Stim = mesencephalic locomotor region (MLR) in the brainstem initiates walking
Coordination = Central pattern generator (neural circuit within spinal cord) can produce rhythmic stepping without brain input
Output: Walking (voluntary action without conscious direction)
Function of the UMN and LMN in brainstem?
LMN = motor innervation to head and neck region
UMN modulate activities of spinal motor neurons and interneurons via the descending pathways»_space; e.g. Controls posture by integrating visual, vestibular with somatosensory information
Define the subtypes of motor functions of lower motor neurons in the brainstem?
- General somatic motor neurons
- Special visceral motor neurons
- General visceral motor neurons
Give examples of CN with general somatic motor functions?
Extraocular muscles (CN III: oculomotor nucleus)
Intrinsic muscles of tongue (CN XII: hypoglossal nucleus)
Give examples of CN with special visceral motor functions?
CN5,7,9,10,11
Chewing (CN V: trigeminal (masticator) motor nucleus)
Facial expression (CN VII: facial nucleus)
Larynx (CN X, XI: nucleus ambiguus)
Pharynx (CN IX, XI: nucleus ambiguus)
Give examples of CN with general visceral motor functions?
CN 3,7,9,10
Glands: salivation (CN VII: superior salivatory nucleus, CN IX: inferior salivatory nucleus)
Smooth muscles: pupillary light reflex (CN III: Edinger-Wesphal nucleus, CN X: dorsal motor vagal nucleus)
List the tracts that the brainstem use to modulate activities of spinal motor neurons and interneurons?
4 descending output pathways to spinal cord:
Lateral brainstem > distal muscle:
- Rubrospinal tract
Medial brainstem > axial muscle:
- Reticulospinal tract
- Tectospinal tract
- Vestibulospinal tract
What is the overall motor control exerted by the medial and lateral brainstem pathways?
Lateral brainstem pathways control distal limb muscles (e.g. fingers)
Medial brainstem pathways control axial muscles, posture
List the origins and inidividual function of the 4 descending motor pathways from the brainstem?
- Rubrospinal (from magnocellular part of red nucleus): manipulating object, goal-directed movement
- Tectospinal (from tectum): coordinate eye, head orientation
- Vestibulospinal (from lateral, medial vestibular nuclei): control posture
- Reticulospinal (from medial reticular formation): control posture
Define the 3 parts of the motor cortex?
- Primary Motor Cortex (M1)
- Premotor Area (PMA)
- Supplementary Motor Area (SMA)
Define the location of the 3 parts of motor cortex?
- Primary Motor Cortex (M1)
= Covers precentral gyrus of frontal lobe, from mid-saggital sulcus to lateral sulcus (Brodmann area 4) - Premotor Area (PMA) = Located immediately anterior to M1 cortex (Brodmann area 8)
- Supplementary Motor Area (SMA)
= Forms the medial part of Brodmann area 6 (buried within the longitudinal fissure)
List all the output tracts from the motor cortex?
- Lateral and ventral corticospinal tract (direct to spinal cord)
- Corticobulbar tract (cortex > brainstem)
- Modulation circuit (striatum, thalamus, pons, basal ganglia, cerebellum)
Define functions of lateral and ventral CST?
- Lateral corticospinal tract»_space; acts on lateral spinal motor neurons directly»_space; control of contralateral distal muscle after decussation (e.g. fingers)
- Ventral corticospinal tract »_space; acts on spinal motor neurons indirectly via brainstem»_space; medial spinal motor neurons»_space; control of ipsilateral axial and proximal muscles for posture
Define function of the Corticobulbar tract?
acts on lower motor neurons in the brainstem to control head and neck muscle
Control non-ocular CN motor function: innervate brainstem nuclei: Motor trigeminal, Facial, Nucleus Ambiguus, Hypoglossal nuclei
Which of the motor cortex areas has the lowest stimulaton threshold? What does this imply?
Primary motor cortex
Lowest stimulation threshold to induce movement compared to PMA and SMA, implying that M1 has strong connections with spinal motor and interneurons
> > INITIATE MOVEMENT
Describe the functional organization of primary motor cortex?
Motor homunculus (like S1):
Somatotopical organization related to degree of motor control
Body parts involved in tasks requiring precision, fine motor control have larger representation (e.g. face, hands)
Inputs to M1?
- Premotor area and supplementary motor area
- Sensory areas within the parietal lobe and the thalamus
- Cerebellum and basal ganglion
> > Collectively provide the M1 the set of instructions required for execution of skilled movements
Explain how M1 neurons fire in ways that dictate the motor output? **
Discharge frequency of single neuron»_space; encodes FORCE of movement
Specific Populations of motor neurons (not single cells)»_space; encodes DIRECTION of movement
Which 2 areas form the premotor area?
premotor cortex + SMA
Inputs into premotor cortex? Role of each input?
- Posterior parietal cortex (imply premotor areas participate in motor control through visual and somatosensory cues)
- Prefrontal cortex (motivation and intention)
Compare the stimulation threshold and mapping of Premotor cortex vs Primary motor cortex?
Premotor cortex:
• Stronger stimulation threshold (compared to M1) • Motor homunculus not as well defined as the map in M1
Summarize the overall output function of the Premotor cortex?
SMA + PMA = Produce complex movements
- PMA = intention and planning of goal-directed movement (not initiation)
- SMA = programming complex sequence and coordinate bilateral movement (self-initiated planning, not execution)
Clinical manifestations of M1 lesions?
- Interferes with motor proficiency & manual dexterity (can’t initiate, plan direction and force)
- Independent control of fingers may be lost
Clinical manifestations of PMA lesions?
PMA = intention and planning of goal-directed movement (not initiation)
- Weakness of axial and proximal muscles
- Slow execution of complex motor sequences
- Visually guided movements are severely affected (posterior parietal cortex input)
Clinical manifestations of SMA lesions?
- Interferes with goal-directed behaviors that require planning and execution of complex motor sequences
- Produces deficit in bimanual coordination
- Deficiency in the initiation and suppression of movements (utilization behavior: compulsively grab objects)
How is the cerebellum involved in motor control?
1) Feedback control: correct ongoing movements when they deviate from intended movements
2) Feedforward control: modify central programs subsequent movements can fulfill their goal with fewer errors
Clinical manifestations of lesions in cerebellum?
Unsteady gait, ataxia
Delay in initiating responses
Errors in range, force of movement (dysmetria, past-pointing, intention tremor)
Errors in the rate, regularity of movements (dysdiadochokinesia, dyssynergia)
List the components of basal ganglia?
- Putamen
- External globus pallidus , Internal globus pallidus
- Caudate nucelus
- Subthalamic nucleus
- Substantia nigra
Striatum = caudate nucleus + putamen
Input of basal ganglia? Output to which structure?
Input:
Cortical areas (prefrontal, premotor, supplemental, primary motor, somatosensory)
Intrinsic nuclei
Output:
Project via thalamus (ventroanterior, ventrolateral nuclei) back to frontal cortex (concerned with motor planning, coordinating movement of body parts)
Detail pathways in the output of basal ganlgia?
Overall output is balance between:
- Direct pathway: promote movement: reduce G.P. internal stimulation, remove tonic inhibition on thalamus
- Indirect pathway: suppress movement: enhance G.P internal stimulation = more tonic inhibition on thalamus
Sequence of direct pathway in the output of basal ganglion?
Cerebral cortex > Striatum and nucleus accumbens > Globus pallidus internal, Substantia nigra p.r. > VA/VL thalamus > Frontal cortex
Sequence of indirect pathway in the output of basal ganglion?
Cerebral cortex > Striatum > Globus pallidus external, Substantia nigra p.c. > subthalamic nucleus > globus pallidus internal, Substantia nigra p.r. > VA/VL thalamus > prefrontal cortex
Overall functions of the Basal ganglia?
processing of information needed for:
Planning, triggering self-initiated movements
Organizing associated postural adjustment
Clinical manifestations of lesion in Basal ganglia?
Parkinson’s disease
Huntington’s chorea
Hemiballismus
Unintentional tremor
Relate the role of Substantia nigra in basal ganglia circuit to Parkinson’s disease?
S.N. has D1 and D2 neurons to the Striatum
- D1 enhances direct pathway»_space; more inhibition of cortex
- D2 suppresses indirect pathway»_space; more inhibition of cortex
Degeneration in substantia nigra = loss of dopaminergic neurons = cannot promote direct pathway, cannot inhibit indirect pathway = more inhibition of motor output
Symptoms of Parkinson’s disease? Treatment?
hypokinetc disorder
Bradykinesia, rigidity, tremor, disturbance of posture and gait
Treatment: L-DOPA increases dopamine level
Relate the basal ganglia circuit to Huntington’s disease?
(hyperkinetic disorder): due to degeneration in caudate nucleus, putamen
= inhibit indirect pathway
= less tonic suppression of thalamus
= Promote movement
Cause of Hemiballismus? Relate the basal ganglia circuit to Hemiballismus?
resulting from stroke involving subthalamic nucleus
Subthalamic nucleus excites internal globus pallidus to inhibit thalamus (indirect pathway)
Loss in subthalamic nucleus = less indirect pathway = increase activity in thalamus
