motor control 1

Question 1

what does motor control involve

Answer 1

dynamically changing mix of conscious and unconscious regulation of muscle force, informed by continuous and complex sensory feedback, operating in a framework sculpted by evolutionary pressures

Question 2

types of motor control

Answer 2

• Voluntary: walking, talking ect
• Goal-directed: conscious, explicit, controlled
• Habit: unconscious, implicit, automatic
• Involuntary: eye movements, facial expressions, postural muscles, diaphragm ect

Question 3

sensorimotor system key concepts

Answer 3

• Motor control governed by lower and upper motor neurons.
• The lower motor neuron begins (has its cell body) in brainstem or spinal cord and projects to the muscle
• The upper motor neurons originate in higher centres and project down to meet the lower motor neurons

Question 4

muscle and muscle fibre activation

Answer 4

Individual muscle fibres act in an ‘all-or-none’ manner, and so control of muscle force depends on the way in which lower motor neurons activate different types of muscle fibre
antagonistic arrangement - combined co-ordinated action

Question 5

skeletal muscle - muscle contraction

Answer 5

• A skeletal muscle comprises several muscle fasciculi (group of muscle fibres)
• A muscle fasciculus comprises several muscle fibres (= muscle cells)
• A muscle fibre is constituted of several myofibrils
• Myofibrils contain protein filaments: Actin and Myosin myofilaments
• When the muscle fibre is depolarised actin and myosin slide against each other which produce muscle contraction
• When a muscle contraction occurs a nerve impulse is sent to the neuromuscular junction that causes the release of acetylcholine – causes biochemical cascade.

Question 6

rigor mortis

Answer 6

• ATP (provides energy for cells) is required to break the bond between the myosin head and the actin filament
• ATP is produced by oxidative metabolism, which stops upon death
• So the muscle become contracted and remain that way until enzymes begin to disrupt the actin/myosin

Question 7

motor unit

Answer 7

• Motor unit = single alpha (α) motor neuron + all the extrafusal muscle fibres it innervates
• Different motor neurones innervate different numbers of muscle fibres – fewer fibres means greater movement resolution - those innervating finger tips and tongue
• The motor unit is the final common pathway for motor control
• More motor units fire – more fibres contract – more power

Question 8

size principle

Answer 8

• Units are recruited in order of size (smallest first)
• Fine control typically required at lower forces
• E.g. eye = 3 motor units, trunk = over 1000 units

Question 9

fast and slow muscle fibres

Answer 9

• Slow – e.g. posture
• Fast fatigue resistant – e.g. running/walking
• Fast fatigable – e.g. jumping
• Training and exercise lead to changes in the thickness of muscle fibres and the proportion of different muscle fibre types

Question 10

the motor pool

Answer 10

• All the lower motor neurons that innervate single muscle
• The motor pool contains both the alpha and gamma motor neurons
• Motor pools are often arranged in a rod like shape within the ventral horn of the spinal column

Question 11

cell bodies in the ventral horn activated by:

Answer 11

• Sensory information from muscle

* Descending information from brain

Question 12

sensing in muscles - key part of proprioception

Answer 12

Muscles can be contracted or relaxed to provide movement, but a good control system (the CNS) needs to know two things:

1. how much tension is on the muscle; - Golgi tendon organs sense tension
2. what is the length (stretch) of the muscle – muscle spindles sense stretch

Question 13

Golgi tendon organs – muscle tension (force)

Answer 13

• The GTO is within the tendon (where the muscle joins to bone)
• Mostly, it sends ascending sensory information to the brain via the spinal cord about how much force there is in the muscle
• Critical for proprioception
• Under conditions of extreme tension, it is possible that GTOs act to inhibit muscle fibres (via a circuit in the spinal cord) to prevent damage

Question 14

reflexes

Answer 14

• Stretch detected – travels along muscle spindle – sensory neuron – synapses – signal sent across alpha motor neurons to extrafusal muscle fibres
intrafusal fibres are innervated separately, by gamma motor neurons
• They keep the intrafusal fibres set at a length that optimises muscle stretch detection

Question 15

withdrawal reflex

Answer 15

• Reciprocal innervation of antagonistic muscles explains why the contraction of one muscle induces the relaxation of the other
• Permits the execution of smooth movements