Lecture13 Revised

8/8/2019 Lecture13 Revised

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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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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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(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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Lecture13 Revised

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