Download Mind, Brain & Behavior

Mind, Brain & Behavior
Friday
February 28, 2003
Movement
Chapters 26 & 28
Three Kinds of Movement

Reflex responses (knee-jerk) – rapid,
stereotyped, involuntary responses.


Rhythmic motor patterns (walking, running,
chewing) – part reflex, part voluntary.


Graded in response to eliciting stimulus.
Only starting and stopping are voluntary.
Voluntary movements – purposeful (goaldirected) and learned (skilled, practiced).
Types of Movement




Extension – takes limb away from body (opens
penknife)
Flexion – brings limb toward body (closes penknife)
Muscles can only pull not push so any movement
requires coordination
Synergists -- muscles that work together


Agonists – prime movers
Antagonists – muscles that pull in opposite direction to
agonists, help brake movement.
Parts of the Motor System

Motor control operates at three levels,
organized hierarchically, operates in parallel.



Cerebral cortex motor areas – plan and control
voluntary movement, affect spinal cord neurons
directly & through brain stem.
Brain stem – two systems that regulate spinal
cord interneurons, one for posture, one for
voluntary movement.
Spinal cord – mediates automatic and stereotyped
movements.
Parts of the Cortex Motor System
Premotor area – plans the activity
 Motor cortex (M1) – initiates motor activity

Basal ganglia loop (near thalamus) gives the
“go” signal
 Cerebellar loop – tells the motor cortex how to
carry out the planned activity

 Controls
movement direction, timing and force by
activating populations of motor neurons in learned
programs.
Planning Movement
Goal directed movement involves many
cortical areas that communicate with Area 6 in
Frontal lobe.
 Area 6 has two parts:

PMA (premotor area)
 SMA (supplemental motor area)


Area 6 plans an action and stays active until it
is executed (“go” signal).
Direction of Movement

Movement is directed using frequency and
population codes:
Motor cortex (M1) neurons fire at different rates
depending on the desired direction.
 Firing rates are averaged across populations of M1
neurons.


When contributing neurons are inhibited,
resultant direction changes.

Cerebellum controls the sequence of movements.
See Chapter 29 for details
Types of Muscles
Smooth – digestive tract, arteries
 Striated:

Cardiac – accelerates or slows heart rate
 Skeletal – moves bones around joints, moves eyes,
facial expression, respiration, speech


Skeletal muscles are the somatic motor system
and are under voluntary control.
Motor Units
Each muscle fiber is innervated (controlled) by
an alpha motor neuron.
 Bundles of fibers form large and small motor
units.



Small motor units act first, fine motor movement.
Fast contracting, fast fatiguing white fibers
form “fast” motor units (slow ones are red).

Alpha neuron firing rate makes a fiber/motor unit
fast or slow.
Reflexes
Reciprocal inhibition – cannot flex and extend
the same muscle
 Myotatic (knee-jerk)

Opposes gravity
 Uses spindle sensory feedback


Reverse myotatic (knife-clasp)
Relaxes overloaded muscle
 Responds to Golgi tendon organ feedback

More Reflexes
Flexor reflex – response to pain
 Crossed-extensor reflex – compensates for
flexor reflex

One side extends as the other flexes
 The circuit for coordinated control of walking
resides in the spinal cord.
 Circuits called “central pattern generators” give
rise to rhythmic motor activity.

Two Pathways from the Brain

Two corticospinal pathways:
Lateral tract – voluntary movement, crosses
 Ventromedial tract (brain stem pathways) –
posture, descends without crossing


Lateral pathways control fractionated
movement of distal muscles, especially
flexors:
Corticospinal – new (higher mammals)
 Rubrospinal – from red nucleus, old

Ventral (Medial) Pathways

Tectospinal – orients eyes (fovea) on image


Vestibulospinal – maintains stability of head
and turns it, balance


Receives input from superior colliculus
Input from labyrinth of inner ear
Reticulospinal – originate in pons and medulla
Pontine – resists gravity and maintains posture
 Medullary – liberates muscles from anti-gravity
control
