Unit 2 Flashcards
What is required for successful motor control?
- Volition (movements taht accomplish goals of organism)
- Coordination (most movements require many muscle groups, which requires direction and force to be coordinated)
- Proprioception (essential to know position muscle to create and/or maintain appropriate patterns of muscle activation)
- Postural adjustments (compensates for body’s changing center of mass)
- Sensory feedback (compares desired activity with actual activity to create corrections in movement)
- Compensation (body and muscles have characteristics (mass and resistance) that must be accounted for when moving)
- Unconscious processign (many movements performed in an unconscious fashion, such as walking)
- Adaptability (growth and change in body and acquire motor skills throughout life)
Organization of Motor Control
Functional Segregation and Hierarchal Organization
Functional Segregation
- a particular function is localized to a certain region of brain
- different areas of motor system control different aspects of movement
- divide and conquer strategy
- areas are located throughout the nervous system
Hierarchal organization
- higher and lower order areas
- higher= deciding when to act, sequence of actions, coordination of numerous limbs
- lower= force and velocity of contraction of individual muscles
Motor System Hierarchy (and loops)
- Spinal Cord
- Brainstem
- Motor Cortex
- Association Cortex
Side Loops: Basal Nuclei, Cerebellum
Upper Motor Neurons
- cell bodies in higher centers
- axons descend to influence circuits in brainstem and spinal cord
- Co-ordinate activity of lower motor neurons
- Co-ordination occurs through descending motor pathways
Level 1- spinal cord: lower motor neuron
- alpha (lower) motor neurons innervate skeletal muscle
- cause muscle contractions
- release acetylcholine (Ach)
- synapse called the neuromuscular junction (NMJ)
- located in ventral horn
- only communication point between motor system and muscles
Motor Neurons and muscles: motor neuron pools
- motor neuron pools
MNs clustered into columnar, spinal nuclei - all MNs in a pool innervate a single muscle
- all MNs that innervate particular muscle are in same pool
- 1:1 relationship between a muscle and motor neuron
Motor Unit
Muscle fiber innervated by one motor neuron
- single MN can innervate many fibers
- MN + all muscle fibers innervated = motor unit
- number of fibers innervated by a motor unit = innervation ratio
- muscle fibers innervated by MN are distributed over wide area in muscle
- AP in alpha MN = contraction threshold of fibers reached
- motor unit is smallest unit of force that can be activated by muscle
Variations in Motor Units
motor units and alpha motor neurons vary in size
- small alpha MN innervates few muscle fibers, and small force generated
- large alpha MN innervates many fibers, and large force generated
type of muscle fiber innervated also changes
Type of Muscle Fibers Innervated
- small motor units innervate slow oxidative (SO) fibers: smaller, slower, less powerful contractions, fatigue resistant -Slow (S) Motor Units
- intermediate motor units innervate fast oxidative (FO) fibers: contractions aren’t as fast as FG fibers, contractions twice as forceful as SO fibers, resistant to fatigue Fast. Fatigue-Resistant (FR) Motor Units
- Large motor units innervate Fast Glycolytic (FG) fibers, contractions are fast and powerful, fatigue easily - Fast Fatiguable (FF) Motor Units
Motor Unit Activation
- slow motor units have lower threshold for activation
- tonically active during motor acts that require sustained effort - standing
- large, faster motor units reach threshold during rapid movements that require greate force - jumping
- motor unit activation is often called recruitment
Control of Muscle Force
- Rate code: increase rate AP = increased force contraction; strength of muscle contraction is able to summate up to a maximum
- size principle: not all MNs recruited at the same time; smaller MNs recruited and fire APs first; easier for smaller neurons to reach firing threshold
fused tetanus does not occur naturally, instead asynchronous fitring of APs provides steady level of input to muscle
resting membrane potential occurs when:
K+ ions diffuse out of the cell
list events occurring at neuromuscular junction in order:
1. ACh binds to recepters on postsynaptic membrane
2. acetylcholinesterase breaks down ACh
3. choline is reabsorbed by presynaptic membrane
4. depolarization of muscle cell
5. ligand-gated Na+ ion channels open
1, 5, 4, 2, 3
Which motor units are resistant to fatigue?
Both slow and fast fatigue-resistant
Descending Motor Pathways
- both serial and parallel organization in motor system; parallel organization is protective
- descending pathways arise from numerous brain regions and synapse with alpha and gamma MNs and interneurons
Topographical Organization of Spinal Cord
Flexor-extensor rule: flexor MNs are posterior to extensor MNs
Proximal-Distal Rule: distal MNs are lateral to proximal MNs
Two major group pathways for descending motor pathways
- lateral pathways
- medial pathways
Lateral Pathways
- control proximal and distal muscles
- voluntary movement of arms and legs
- Tracts involved = lateral corticospinal tract & Rubrospinal tract
Medial Pathways
- Control axial movement
- Posture and balance
- Tracts involved: vestibulospinal tracts, reticulospinal tracts, tectospinal tract, & Ventral Corticospinal Tract
Corticospinal Tract
- originate in motor cortex
- motor projection neurons collect in internal capsule
- pass through cerebral peduncle (crus cerebti) in the midbrain
- At caudal medulla, corticospinal tract splits in two- 1. 90% cross to form the lateral corticospinal tract 2. 10% axons do not cross and form ventral corticospinal tract
Function: corticospinal system controls motor neurons and interneurons in spinal cord
Corticobulbar tract
this system controls brainstem nuclei that innervate cranial muscles
Rubrospinal Tract
- originate in red nucleus of midbrain
- function: alternative route for voluntary motor commands to be sent to the spinal cord
- major pathway in many animals but minor in humans and primates
Vestibulospinal Tracts
- Two division: medial and lateral
- Functions: mediate postural adjustments, head movements, help maintain balance
Reticulospinal Tracts
- two divisions: pontine and medullary
- originates in brainstem reticular formation
- function: major alternative to corticospinal tract
Tectospinal tract
- originates in deep layers of superior colliculus
- not much known about function- assumed to be involved in orienting reflex
Outputs of motor pathways
- voluntary movement: movements initiated in cerebral cortex and motor commands transmitted through descending pathways to muscles
- pathways: corticospinal tracts, rubrospinal tract, reticulospinal tracts
Outputs of motor pathways: Reflex modulation
adaptiveness of spinal reflexes can change depending on behavioral context
Outputs of motor pathways: gamma bias
- gamma motor neurons innervate muscle spindles
- activity of alpha and gamma motor neurons needs coordinating (alpha-gamma coactivation)
- adjustment of spindle sensitivity is gamma bias
Motor Cortex
- frontal lobe
- 3 different areas
- anterior to central sulcus
- primary motor cortex (M1)
- premotor cortex
- supplementary motor area
electrical stimulation results in movement
Primary motor cortex
- stimulation of M1 - contraction of contralateral muscles
- spatial representation occurs in M1 (topographic map) - discovered through cortical surface stimulation (certain part of cortex corresponds to movement of certain body part)
- intracortical microstimulation = smaller, focal stimulation of cortex
Topographic organization of motor cortex
- might predict that individual muscles are clustered together
- now known not to be true
- primary motor cortex homunculus represents movements of individual body parts (often require coordinated activity of large groups of muscles throughout body)
Primary Motor Cortex: microstimulation
- perform intracortical microstimulation and record muscle electrical activity
- results: small current excites several muscles, simultaneous relaxation of others
- conclusion: organized movements represented in map of M1 (not individual muscles), local circuits link areas
Premotor cortex
- encodes complex patterns of motor output
Cortical Influences of Descending Pathways
Descending tracts:
1. corticorubral tract modulates rubrospinal tract
2. corticotectal tract modulates tectospinal tract
3. corticoreticular tract modulate reticulospinal tract
Side loop processing:
1. corticostriate tract innervates basal ganglia
2. corticopontine tract innervate cerebellum
Cortical areas:
1. corticocortical pathways
2. corticothalamic pathways
Architecture of motor cortex and corticospinal tract
- 6 layers
- agranular cortex
- layer 5 most distinctive: pyramidal cells are upper motor neurons of M1
- contains Betz cells: large pyramidal neurons
- Betz + other projection neurons = 30% corticospinal tract- 30% premotor cortex, 30% somatosensory cortex, 10% posterior parietal cortex
which area of spinal cord can axons of upper MNs that supply lower motor neurons for skilled movements be found?
lateral white matter
at which level of the nervous system does pyramidal decussation occur?
medulla
come from motor cortex
Lateral corticospinal tract innervates alpha motor neurons that are important for which task?
writing
lateral = smaller motor units
What experimental technique has been most helpful in mapping primary motor cortex?
cortical stimulation
preserves tissue, so person can be alive when doing it
Encoding movement in motor cortex
- M1 doesn’t control individual muscles
- controls movements and sequences of movement
- lower MNs in spinal cord are ones that encode force contraction of muscle fibers using rate code and size principle
five aspects of movement encoded in M1
- movement
- force of movement
- direction of movement
- distance of movement
- speed of movement
Encoding movement in M1: movement
- record activity of neurons in M1 as animals perform different tasks
- M1 neurons fire 5-100msec before oneset of movement
- these neurons are involved in relaying motor commands for movement, not responding to movement
Encoding movement in M1: Force
- how mcuh force is needed? force of movement is different to force of muscle contraction
- Bowling ball vs. balloon
- neurons in M1 encode force needed for movement
- lower MNs translate this for muscle contraction
- more force required = more neuronal firing
Encoding Movement in M1: direction
Neurons in M1 are selective for a direction of movement (specific neurons have certain direction of movement that elicits higher firign rate)- directional tuning
Encoding movement in M1: Speed
- targeted movements follow bell-shaped curve of velocity vs. distance
- some neuron firing rate correlates with pattern
- info about speed is encoded in spike trains of certain neurons in M1
Encodign Movement in Premotor Cortex
- stimulate premotor cortex = more complex movements
- selection of appropriate motor plans (M1 involved in execution of demands)
