neurophysiological approaches to motor control – with a slant to memory

Neurophysiological Approaches to Motor Control

– with a slant to memory

Components of the nervous system Basic components (neurophysiologically speaking):

• Sensory receptors• Motor units• Neurons (nerves) & synapses

How complex?• 1,000,000,000,000-100,000,000,000,000 neurons

• UK: 1 billion to 100 billion• US: 1 trillion to 100 trillion

• Each neuron has up to 10,000 connections• “More connections than there are particles in the known universe”

Components of the nervous system Basic function

• CNS, PNS• Afference, efference

CNS

PNSEfferent

information

Afferent informatio

n

Neurons and Synapses Structure and function of neurons


• Vary by # of dendrites, length of axon • Structure determines role – many types (bipolar,

multipolar, golgi I, golgi II…etc.)

Purkinje cell (in

cerebellum)Basic reflex

arc, with interneuron


• Neural communication:• Dendrites pick up electrical signal• If total signal strength exceeds a threshold, neuron

fires…pulse sent along axon• Pulse is carried more efficiently if axon is myelinated

(myelin is fatty stuff that insulates)• So signal transmission is complex, determined by total

strength of arriving signal and subsequent strength of descending signal

Neurons and Synapses Structure & function of

synapses• Joints of the nervous

system• Electrical or chemical

communication across synaptic gap

• Chemical most common, via neurotransmitter

• Can be either inhibitory or excitatory

synapse

Neurons and Synapses Structure & function of

synapses• So, typical signal transmission is:

• Electrical activity • Chemical activity • Electrical activity

• Excitation, inhibition necessary for complex patterns of communication

• Neurotransmitters necessary for signals to travel

synapse

Sensory Receptor Systems for Movement

Primarily from vision and proprioception• Purpose is to communicate information• Achieved by converting signals through several

energy types


The visual system• Light retina rods,

cones optic nerve LGN (70%) visual cortex…focal vision

• Light retina rods, cones optic nerve SC…ambient vision

• Focal vision pathway = “what” pathway

• Ambient vision pathway = “vision for action” pathway


The visual system• Rods, cones• All over retina but more

concentrated in fovea• Cones sense color,

require high light• Rods only sense B&W,

only require low light• Hence seeing at night in B

& W


The kinesthetic system• Sensory receptors are distributed through muscle,

tendon, joints and skin

Muscle receptors• Muscle spindles

• In all muscles – esp. small ones, used for fine control (think about rate control…see open & closed loop control later)

• Made up of intrafusal muscle fibers and sensory receptors• Transmits info about amount and rate of stretch in muscle




• Basically, they lie alongside the main muscle fibers, and are stretched by them(at endpoints)


• Intrafusal fibers:• Innervated by gamma motor neuron activity• Contract at end points only• Loaded with sensory connections returning signals to the spinal

cord


Type Ia afferent: connected at the non-contractile center.

Fires when entire muscle stretches

Fires on mismatch between contraction due to alpha motor neuron and contraction due to gamma motor neuron activation

Sensory Receptor Systems for Movement Tendon receptors

• Golgi tendon organs• Impulse returns via type

Ib afferent fiber• Respond to tension in the

tendon• Fire when entire muscle

shortens (contrast w/spindle)

• Function: • protection – like a fuse

in a circuit• Feedback to spinal cord

• Stiffness? Tension? – fine tuning

Exact role of spindles & GTO’s still debated

Sensory Receptor Systems for Movement Skin (cutaneous) receptors

Respond to light vibration

Responds to high

frequency vibration, & compression


Skin (cutaneous) receptors• Density of cutaneous receptors varies around the

body• Gives rise to “just noticeable differences” varying in

different parts of the skin• Loss or damage to these receptors ruins fine control of

movements• Problems w/robots lacking these senses

Sensory Receptor Systems for Movement Joint receptors

• Modified Ruffini corpuscles,Modified Pacinian corpuscles

• In joint capsule• Golgi organs

• In ligaments

Thought to signal problems w/extreme ranges of motion

Sensory Receptor Systems for Movement The vestibular system

• Signals body orientation in space• Semicircular canals, otolith organs (utricle, saccule)

Canals allow for both linear and angular acceleration sense…like internal accelerometers

Highly integrated with vision…mismatch leads to motion sickness

Sensory Receptor Systems for Movement Intersensory integration and sensory dominance

• Overall sense of what is going on dependent on information flowing from many receptors simultaneously

• Occasionally they contradict each other• Vision is dominant…can lead to some amusing

experiments (and experiences)

Effector Systems for Movement The motor unit

• Covered in Chapter 2.

Motor units containing smaller muscle fibers are recruited first – size principle

Motor Control Functions of the Spinal Cord

Structure of the spinal cord• 2 basic functions:

• Two-way communications route• Maintains movements in progress (&

prevents bad things getting worse)• Mesencephalic cat• 31 pairs of nerves attach – see figure• It’s protected by bone along it’s

route, and is a massive information superhighway

Surprisingly functional

Spinal reflexes• 4 components:

• Sensory receptor• Afferent neuron• Efferent neuron• Effector

• The stretch reflex• The simplest example of the above 4 components• Knee-jerk reflex is an example• Muscle is stretched, reflex makes muscle contract in

response to the stretch



Spinal reflexes• The flexion reflex

• Typical response to pain or threat• Limbs flex as a consequence of reflexive contraction of

flexor muscles & inhibition of contraction of extensor muscles


Spinal reflexes• The crossed

extensor reflex• Works with flexion

reflex to get away from the pain/threat source

• Opposite pattern of contraction


Spinal reflexes• The extensor thrust reflex

• Responds to pressure on sole of foot by contracting extensor muscles in leg

• Aids balance, walking• Spinal reflexes for gait control

• Overall, these types of reflex show patterns of reciprocal inhibition that set limbs to work in cycles and initiate rhythm…leads to spine as…

• Central pattern generator (mes. cat again)


The role of reflexes in voluntary movement control• All paths lead to α–motor neuron activation• α-γ coactivation

• The γ activation of the intrafusal fibers serves as a reflexive check on the α activated extrafusal fibers

• If there’s a match, all is well• If there’s a mismatch, the α–motor neuron fires some

more

• Basic idea: reflexes are incorporated in voluntary movement control

Motor Control Functions of the Brain

brainstem

Basal ganglia (deep)


The motor cortex• Proportional

representation

• Mostly opposite to side of body

• As you move to the left, planned movements are more gross, and involve more muscles

• Takes in signals from a # of sources

• Sends out final plan to spine


The motor cortex• Pyramidal tract:

• primary path to muscles (via α–motor neurons and interneurons)

• Damage results in paralysis• Extrapyramidal tract:

• Secondary pathway, via basal ganglia, cerebellum, thalamus, brainstem

• Primarily inhibitory function (required for coordination, of course)

• Damage results in spasticity

Motor Control Functions of the Brain The cerebellum

• “the seat of coordination”

• Receives input from all over…cortex, brainstem, vestibular apparatus, sensory receptors from spine

• Output to thalamus and brainstem

Motor Control Functions of the Brain The basal ganglia

• Input from motor cortex & brainstem

• Output to thalamus & brainstem

• No direct link to α-motor neurons, but still important in regulation of movements• Parkinson’s disease• Huntington’s disease

Motor Control Functions of the Brain The brainstem

• Pons, medulla, reticular formation

• Integration of both afferent & efferent information

• Regulates many long loop reflexes (righting reflex, tonic reflexes)

• Also tunes lower reflexes, regulates arousal

Integrative Brain Mechanisms for Movement

Understanding how all systems work together in movement regulation is incredibly difficult…final understanding is a long way off

So there are a number of theories that deal instead with a different level of analysis• Cognitive science – see next chapter

Memory – development and type

Memory types (From Squire and Zola-Morgan, 2003)

Memory types

Memory as Association Answer these 2 questions:

• What continent is Kenya in?• What are the two colors of the pieces in a game

of chess?

Association, and spread

• Name any animal

neurophysiological approaches to motor control – with a slant to memory

Documents

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pathwayambient vision