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Psychology 110: Biological Psychology
Lecture 13: Motor control
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Outline
we will look at the basic components of the motor system: muscles,motoneurons and proprioceptive receptors.
we will then look at the hierarchy of processing that exists in the motorsystem, beginning with spinal reflexes, and continuing to look at the
primary motor cortex, and motor association cortices.
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Muscles
Muscles can exert force only by contraction. The movement produced bythis contraction is determined by the muscles points of attachment(tendons).
E.g. because of their attachments,contraction of the biceps flexes thearm, while contraction of the tricepsextends the arm (we call this
muscular synergism).
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Muscle fibers
Muscle fibers consist of many individual fibers bound together withconnective tissue.
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Mechanism of muscle contraction
Muscle fibers consist of two types of interlocking protein actin and myosin.
During contraction the myosin heads bind to actin, and then bend to slide
filaments past one another, shortening the muscle.
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Types of muscle fiber
i). Fast muscle:
reacts quickly, fatigues rapidly used for activities in which muscle tension changes frequently e.g.
locomotion white meat in poultry (used to beat wings)
ii). Slow muscle:
react slowly but resist fatigue
used to maintain posture dark meat in poultry (used for standing)
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Innervation of muscle fiber
Motoneurons leave the ventral root of the spinal cord and branch multipletimes before synapsing with individual muscle fibers at the neuromuscular
junction.
ACh is released and elicits anaction potential in the musclefiber itself.
The influx of Ca2+ and Na+into the fiber triggers themolecular changes in actionand myosin that produce
contraction.
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Innervation ratio
The innervation ratio is the ratio of motor axons to muscle fibers.
Lower innervation ratios permit more fine motor control.
For example, the muscles involved in eye movements have about one
motoneuron for every three muscle fibers (1:3 innervation ratio), whilethose for the leg have one motoneuron controlling several hundred fibers(~1:300).
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Motoneuron recruitment
Motoneurons differ in size and electrophysiological properties.
Large motoneurons have wide diameter axons to conduct impulses fast, butare difficult to initially excite.
Small motoneurons have small axons and conduct impulses slowly, but areeasy to initially excite.
SIZE PRINCIPLE:
Muscle tension is increased by recruitment of
increasing numbers of motoneurons in fixedorder according to their size. Weak stimulationactivates only the small neurons, and strongerstimulation recruits additional larger neurons.
The principle is similar to range fractionation.Henneman
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Sensory feedback (proprioception)
There are two proprioceptive sensory structures that are associated withmuscles:
i). Muscle spindles
ii). Golgi tendon organs
Proprioception refers to the detection of the position and movement of ourbody.
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Muscle spindles
A muscle spindle consists of a nerve fiber wrapped around an intrafusalmuscle fiber (extrafusal muscle fibers are responsible for the maincontraction of the muscle).
If the muscle fiber is stretched this stretches the nerve fiber and causesthe nerve to fire action potentials.
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Spinal reflexes: the stretch reflex
1. Weight is placed insubjects hand.
2. MA is stretched.
3. SNA is excited.
4. SNA connectsmonosynaptically withMNA exciting thestretched muscle.
5. Via an interneuron,SNA also inhibits MNBensuring that the
antagonistic muscle(MB) is relaxed whenMA is contracted.
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Muscle spindles: efferent nerves
Muscle spindles also receive input from efferent nerves called gamma
motoneurons (alpha motoneurons go to main muscle fibers).
Activity in gamma fibers causes a contraction in the length of the spindle,
which modifies its sensitivity.
The importance of this system is reflected in the fact that 30% of all
motoneurons are gamma.
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Golgi tendon organ
Golgi tendon organs detect stretch in the tendon, not the muscle, and aresensitive to contraction.
The Golgi tendon organ has feedback inhibition on the motoneuronsstimulating the muscle. If they detect a muscle overload (too muchcontraction) they fire, and inhibit the neuron that is causing the musclecontraction.
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Outline
we will begin by looking at the basic components of the motor system,including muscles, motoneurons and proprioceptive receptors.
we will then look at the hierarchy of processing that exists in the motorsystem, beginning with spinal reflexes, and continuing to look at theprimary motor cortex, and motor association cortices (premotor cortexand the supplementary motor area.
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Spinal reflexes
An animal with damage to its spinal cord is still capable of many complexbehaviors. For example, the animal can often stand, and if stimulated, walkas well.
Spinal cord contains a simple oscillator circuit, called a central patterngenerator, that can produce simple locomotion.
At first cell A transmits thesame number of spikes ascell B
Negative feedback reaches Bout of phase with the inputbecause of synaptic delays,
causing rhythmic output.
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Corticospinal pathway
Consists of motoneurons that travel from the motor cortex, through thebrainstem, and down the spinal cord.
Cross over in the medulla so that the left motor cortex controls the right sideof the body and vice versa.
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Primary motor cortex (M1)
The hands, lips and tongue are over represented in M1.
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Spot the difference?
SENSORY HOMUNCULUS MOTOR HOMUNCULUS
In general, areas with many motor outputs also receive a lot of
somatosensory input. There are some subtle differences, e.g. human earsreceive little motor output, and our eyes have few somatosensory inputs.
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Primary motor cortex (M1)
lesions of M1 cause paralysis
electrical stimulation elicits twitches and jerks in limbs
M1 neurons show direction selectivity for movements
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Muscles or movements?
Monkey must move joystick from central location to one of eight peripherallocations (indicate by yellow dot), but by altering the monkeys handorientation, one can alter which muscles are used to make the movement.
One third of M1 neurons encode which muscles are used, one half encode themovement direction irrespective of the muscles used.
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Premotor cortices
Supplementary motor area (SMA) a.k.a. medial premotor cortex (MPC)
Premotor cortex a.k.a. lateral premotor cortex (LPC)
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LPC: involvement in conditional learning
Conditional tasks involve learning an arbitrary relationship (If X then Y).
TASK 1: Color instructs either pull or turna lever.
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LPC: involvement in conditional learning
Conditional tasks involve learning an arbitrary relationship (If X then Y).
TASK 1: Color instructs either pull or turna lever.
TASK 2: Color instructs which shape tochoose.
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LPC: involvement in conditional learning
(UC = unoperated control)
Lesions of LPC impair conditional tasks, butonly when the stimulus instructs a specificmotor act.
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Monkeys were trained to raise theirarms for a peanut. Monkeys with MPClesions were poor at doing this, failing toraise their arms as many times perminute as unoperated controls.
Medial premotor cortex (MPC)
NO CUE CUED
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N i i l
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Neuroimaging results
simple movements activate M1
increasing the motor complexity of the task extends the area of
increased blood flow to include MPC
when the sequence is indicated by external cues, lateral premotor cortex
is activated instead
when the subject mentally rehearses the complex movement, the area of
activation is restricted to MPC
M i i f i l hi h
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Motor system organization: functional hierarchy
PREFRONTAL
Rules, strategies
MEDIAL PREMOTOR
Internally generatedactions
LATERAL PREMOTOR
Externally cued actions
PRIMARY MOTOR
Movements, muscles
B i hi i t f
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Brain-machine interface
B i hi i t f (BMI)
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Brain-machine interface (BMI)
Matt Nagel was stabbed in the neckwhile breaking up a fight at a fireworksdisplay. Paralyzed from the neck down,he was implanted with an electrode
array in his motor cortex (similar to themonkeys).
With this array he was able to control a
cursor on a computer screen (here heuses it to draw a circle) and type. Hewas also able to check email, surf theinternet and play some video games.
Future hopes are that such devices willbe able to control prosthetic and roboticarms and control wheelchair. A further
aim is to make the implants wireless.
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