lecture 22 - nervous system 2: movement & motor control Flashcards
what does movement depend on?
coordinated muscle contraction and relaxation
muscles contract to operate around a joint to bring about flexion and extension
what muscles does flexion require?
flexor muscles
• bicep brachi
• branchialis
what muscles does extension require?
extensor muscles
• tricep branchi
• anconeus
what happens in flexion?
contract flexors
relax extensors
when happens in extension?
contract extensors
relax flexors
what are synergists?
muscles that pull in the same direction
what are antagonists?
muscles that pull in opposite directions
what nerves control the muscles?
neurons that innervate muscle
• alpha motor neurons
• located in the ventral horn of the spinal cord
• responsible for making muscle fibres contract
muscle units
• motor neurones and the muscle fibres they innervate
what is a large motor neurone?
neurone innervating many muscle fibres
provides strong forces but less precision
e.g. antigravity muscles (legs)
what is a small motor neurone?
neurone innervating few fibres
fine control, more precision
eg. finger
types of muscle contraction
twitch
summation
unfused tetanus
fused tetanus
twitch
individual brief contraction
muscle shortens and relaxes quickly
summation
increased frequency of APs
2nd twitch begins before 1st relaxes
building 1 twitch on top of another
unfused tetanus
no full relaxation
increasing APs
fused tetanus
remains contracted
no ripple in contraction
eg. lifting something heavy
what are the 3 sources of input to motor neurones?
upper motor neurones
interneurons in spinal cord (local circuits, largest input)
receptors: muscle spindles and Golgi tendon organs
upper motor neurones
found in the cerebral motor cortex and brainstem
basis of voluntary control of movement
provide input to alpha/lower motor neurone
pathway activated when movement needs to be initiated
interneurons in spinal cord
excitatory and inhibitory
spinal movement programs
receptors: muscle spindles and Golgi tendon organs
sensory receptors in muscles and tendons
feedback to alpha motor neurones
important in the control of movement
muscle spindles
intrafusal muscle fibres in parallel with extrafusal
innervated by 1a afferents that go to the spinal cord
activated by muscle stretch
muscle contraction unloads spindle
gamma motor neurons contract intrafusal fibres - keep spindle in operating range
Golgi tendon organs
activated by muscle tension
innervated by 1b nerve afferents that go to the spinal cord
muscle receptor structure
extrafusal muscle fibres do the work of contraction
intrafusal muscle fibres run parallel to them
at the base of the muscle there are Golgi tendon organs
tendon attaches muscle to bone extrafusal fibres surround the intramural fibres which are sensory terminals for the 1a afferent
muscle spindles have their own motor neurone
what motor neurones do muscle spindles have?
gamma motor neurones
send their information to the intramural muscle fibres from the CNS
can stretch the central region of the myofibrils
what does the 1a afferent do?
sends signals about stretching (increase in APs) to the spinal cord
what happens to the spindle when it contracts?
spindle is unloaded so ceases to fire
role of gamma motor neurones in controlling muscle spindles
motor centres predict force required for task
firing of alpha and gamma motor neurones together keeps spindle firing constant even though muscle contracts
but if the force required is more than predicted, spindle is stretched and 1a fibres from spindle activated leading to greater force of contraction
whats it called when the alpha and gamma neurones fire together?
alpha-gamma co-activation
what are the 3 types of movement?
reflex
voluntary
rhythmic
reflex movement
involuntary
simple movement limited to 1 or 2 muscles
initiated by external stimuli via receptors
voluntary movement
under conscious control
complex
may be learned
can become unconscious - muscle memory
rhythmic movement
initiated and terminated by conscious control
activates an autonomic pattern generated within spinal cord
intermediate complexity
what are reflexes?
simplest unit of movement
reflex arc involving at least 1 sensory and 1 motor neurone
involuntary
what are the origins of reflexes?
spinal
cranial
what are monosynaptic reflexes?
only 1 synapse (2 neurones) in the arc
what are polysynaptic reflexes?
several neurones in the arc
what happens in the stretch reflex?
maintains contraction with increased load - monosynaptic
1) contraction applied isn’t enough
2) arm dips and muscle stretches
3) muscle spindle stretches
4) 1a muscle fibres activated
5) muscle contracts more slowly
6) can hold a heavier load
what is the Golgi tendon reflex?
protection against excessive load
also important for control of posture
it detects tension
what happens in the Golgi tendon reflex when too much load is added?
1) too much for muscle to support
2) when we stretch the muscle, tension goes up and signals back to the spinal cord
3) 1b afferent goes back to the spinal cord but contacts interneurone (not motor neurone)
4) it is an inhibitor neurone that switches off the motor neurone reducing firing of the muscle
5) muscle relaxes and drops load
what is the knee jerk reflex?
example of a stretch reflex
lower leg jerks forward after striking tension below the knee cap (patella)
what is the flexion and cross extensor reflex?
polysynaptic refelx - involves multiple interneurons in the spinal cord
painful stimulus to foot - withdrawal of the limb and extension of opposite limb
helps support body on 1 leg
what happens in the flexion and cross extension reflex after stimulus to the foot?
1) activates nocireceptors in the sole which signal back to spinal cord
2) input from the sensory neurone will go via a chain of interneurons to drive the alpha motor neurones
3) for the limb that made contact: inhibit extensors and excite flexors - this lifts the foot away from painful stimulus
4) opposite for the other leg - strengthens contraction as leg has to support whole body
5) basic pattern of walking - alternate contraction and relaxation
what areas in the brain are involved in planning and organising voluntary movement?
cerebral cortex
• planning, coordinating and initiating
basal ganglia
• planning and selection of movements
cerebellum
• fine tunes movement bases on experience and sensory feedback
brainstem
• control of posture
