The Motor System

Question 1

Definition of motor control

Answer 1

Motor control involves a 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

Goal directed

Habit

Involuntary

Question 3

Voluntary

Answer 3

Running, walking, talking, playing guitar

Question 4

Goal directed

Answer 4

Conscious, explicit, contorlled

Punching and pointing

Question 5

Habit

Answer 5

Unconscious, implicit, automatic

Question 6

Involuntary

Answer 6

Eye movements, facial expressions, throughout trunk

Question 7

Different stimuli lead to responses via different ways- heirarchal control

Answer 7

Pain —> spinal cord —> (escape) —> motor, autonomic, endocrine

Fear —> sensorimotor midbrain —> (avoidance) —> motor, autonomic, endocrine

Threat —> cortex and limbic system ->(avoidance)—> motor, autonomic, endocrine

Question 8

More Complex, Sophisticated Threat Detection and Avoidance Behaviour

Answer 8

Requires Additional or More Complex Processing Capacity (Neural Systems)

Question 9

Sensorimotor system

Answer 9

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 10

Muscle fibre activation

Answer 10

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

Muscle can only contract our relax

Question 11

Antagonistic arrangement

Answer 11

Combined co-ordinated action

Question 12

Recruitment of muscle fibres

Answer 12

Fast/slow twitch, small and large motor units

Question 13

What is the muscle fibre and strength dependant on?

Answer 13

About the cross sectional area of an individual and different proportions of different types of fibre

Question 14

How do muscles contract?

Answer 14

A skeletal muscle is attached to the bone by the tendon
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

Question 15

The myosin cross bridge cycle

Answer 15

Relaease of the neurotransmitter ACh triggers a biochemical cascade in muscle cells

Slide 18

Question 16

Rigour Mortis

Answer 16

The release of acetylcholine causes a cascade of events resulting in the release of packets of calcium from inside the muscle cell (fibre)

This causes the myosin head to change shape, enabling it to bind with the actin filament

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 17

What is a motor unit?

Answer 17

Motor unit = single alpha () motor neuron + all the 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

Question 18

The motor unit is the final common pathway for motor control

Answer 18

Activation of an alpha motor neuron depolarises and causes contraction of all muscle fibres in that unit (all or none)
Muscle fibres innervated by each unit are the same type of fibre and often distributed through the muscle to provide evenly distributed force (and may help reduce effect of damage)
More motor units fire – more fibres contract – more power

Question 19

Average number of muscle fibres innervated by single motor neuron (a motor unit) varies according to two functional requirements for that muscle:

Answer 19

1.Level of control
2. Strength
Typically a range of motor units in a muscle, some with few, some with many fibres.

Question 20

Size principle

Answer 20

Units are recruited in order of size

Fine control typically required at lower forces

Question 21

Lower alpha Motor neutrons

Answer 21

Originating in the grey matter of the spinal cord, or in the brainstem, an alpha motor neuron and the muscle fibres it connects to represent the ‘unit of control’ of muscle force

Question 22

Fast and slow muscles

Answer 22

Training and exercise lead to changes in the thickness of muscle fibres and the proportion of different muscle fibre types

Question 23

The motor pool

Answer 23

All the lower motor neurons that innervate single muscle

The motor pool contains both the alpha and gamma motor neurons (see later)

Motor pools are often arranged in a rod like shape within the ventral horn of the spinal column

Question 24

Dorsal root

Answer 24

Sensory root

Question 25

Ventral root

Answer 25

Motor output

Question 26

Innervation of the muscle- Alpha Motor Neurons originate in the Spinal Cord

Answer 26

Cell bodies in the ventral horn: activated by:
-Sensory information from muscle
-Descending information from brain

Note the closeness and prominence of sensory input to the dorsal horn indicated in this diagram

Question 27

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

Answer 27

how much tension is on the muscle; Golgi tendon organs sense tension, force in muscle

what is the length (stretch) of the muscle; muscle spindles sense stretch- detect lengthening of muscle which leads to some determination of force

Question 28

Golgi tendon organs- muscle tension (force)

Answer 28

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 (guide movement)

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 (Golgi tendon reflex/inverse stretch reflex)
1) The Golgi tendon organ will sense excessive tension on a muscle
2) sensory info is sent to brain via spinal cord through 1b sensory neurons
3) if the tension of muscle is too great, alpha motor neurons are inhibited preventing the muscle from contracting

Question 29

Muscle spindles- muscle length

Answer 29

Muscle spindles sense the length of muscles, i.e. the amount of stretch

This information forms a key part of reflex circuits…….

Slide 32

Question 30

Reflexes

Answer 30

Reflexes can be quite simple or quite complex. They can operate without engaging with the brain, and are critical for the avoidance of injury and effective motor control

Many adjustments come from reflexes

Question 31

Reflex circuit

Answer 31

Slide 34-36

Stretch reflex
1) Muscle is stretched/lengthened suddenly
2) this is detected by the muscle spindles
3) a sensory signal is sent from the intrafusal fibres to the motor pool through gamma motor neurons
4) this synapses with an alpha motor neurone at the motor pool
5) an efferent impulse is sent to the extrafusal muscle fibres
6) this causes a muscle contraction that opposes the stretch

Question 32

We need a system to detect stretch regardless the current muscle length

Answer 32

If intrafusal muscle fibre is controlled by same motor neurons as extrafusals, when muscle is slack (or taught), the system won’t be sensitive to slight changes
So, intrafusal fibres are innervated separately, by gamma () motor neurons
They keep the intrafusal fibres set at a length that optimises muscle stretch detection

Question 33

Muscle spindle feedback

Answer 33

An efficient motor control system needs to know how much each muscle is stretching – information provided by muscle spindles

Muscle sensory receptors = muscle spindles

Embedded within most muscles
Composed of intrafusal fibers
Detect stretch regardless of the current muscle length
Sensory fibres are coiled around the intrafusal fibers
Intrafusal fibers are innervated separately, by gamma () motor neurons
They keep the intrafusal fibers set at a length that optimizes muscle stretch detection

Question 34

Reciprocal Innervation- withdrawal reflex

Answer 34

Principle described by Sherrington (also called Sherrington’s Law of reciprocal innervation)

Reciprocal innervation of antagonistic muscles explains why the contraction of one muscle induces the relaxation of the other

Permits the execution of smooth movements

When, for eg pulling one leg away from a needle, there is a need to increase tension in the other leg to hold the weight of the rest of body- extension of one leg and withdrawal of the other

One pathway results in the Motor neurone to cause flexor muscle to withdraw limb
Another pathway inhibits the motor neurone that normally activates the opposing extensor muscle of the limb to prevent the extensor muscle trying to get rid if the withdrawal of the limb
1) Afferent fibres synapse with motor neurons in the spinal cord
2) causes ipsilateral flexion (the fibres of affected limb contract)
3) also causes contra lateral extension (extensors relax)
4) the limb then withdraws from the stimuli

Question 35

Brainstem structures

Answer 35

Pathways and nuclei within the brainstem (and midbrain) connect sensory input to motor output in quite direct ways, providing an evolutionarily ancient but still very important control system

Question 36

Ancient brainstem motor control

Answer 36

Without cortical involvement e.g. balance and postural control, also orienting, gross limb movement/positioning

Sophisticated e.g. branches of vagus nerve project to larynx to control speech – this circuit richly interconnected with cerebellum and other brainstem sensorimotor systems
Control of respiration

Speech: primitive sounds sculpted by cortex
Primitive vocalisations are driven by control of the respiratory circuit and some control or head or neck muscles

Question 37

Motor cortex

Answer 37

Primary motor cortex exerts quite direct, top down control over muscular activity, with as few as one synapse (in the spine) between a cortical neuron and innervation of muscle cells

Upper motor neurone originates in primary motor cortex and in many cases projects straight down to synapse with a lower motor neurone
So theres a single synapse between the cortical command and movements of a limb- very direct level of control

Question 38

Descending projections from cortical motor areas- complex sequences of movement

Answer 38

Motor command originates in motor cortex pyramidal cells (in layer 5-6, grey matter).

These are the upper motor neurons

Pyramidal cell axons project directly or indirectly (e.g. via brainstem) to spinal cord, where they synapse with lower motor neurons

The axons of these upper motor (pyramidal) neurons form the pyramidal tract

Most cortical projections innervate contralateral motor units- one side of brain control movement in opposite side of body

Question 39

Overview of motor control

Answer 39

Slide 11

The cerebellum and the basal ganglia are critical for regulation of movement, for initiation of movements, for modulation of movement, so that movements are successful and free
Also allows us to learn different movement patterns

The way that both of these systems feed into
motor control is by inputting directly into the motor cortex
rather than adjusting the commands as they come down through motor cortex primarily

Question 40

The motor cortex: The Homunculus

Answer 40

Homunculus is a reasonable representation, but an oversimplification: damage to a single finger area doesn’t mean loss of voluntary control of that finger.

Representations are more complex and overlapping

After all, few motor commands require isolated activation of a single motor unit

Question 41

Descending projections from motor cortex

Answer 41

Slide 13

Two projections down the spinal column

There’s quite a few different tracts; 2 main
-Dorsolateral tracts
-Ventromedial tracts

Question 42

Dorsolateral tracts vs Ventromedial tracts

Answer 42

Dorsolateral corticospinal tracts is positioned dorsally and laterally in the spinal cord

Travels straight from cortex to spine

Primarily involved in controlling distal limb muscles
Two versions of Dorsolateral tract:

-Dorsolateral corticospinal tract:
Direct pathway for trying to work on straight down to lower motor neurones in the spinal cord

-Dorsolateral corticorubrospinal tract
Other version has a synapse via the red nucleus

Ventromedial corticospinal tract innervates more proximal midline muscles and muscles of the trunk
More involved in posture movements rather than fine control movements
Two types:
-Ventromedial corticospinal tract
-Ventromedial cortico-brainstem- spinal tract

Slide 16

Question 43

Modulation of movement by the basal ganglia and cerebellum

Answer 43

This diagram indicates that the basal ganglia has a primarily inhibitory input over motor cortex, whereas the types of input that the cerebellum brings into the motor cortex is often of a excitatory nature so glutamatergic transmission

But don’t look at this and think one system is
increasing motor activity and the system is decreasing motor activity in that simple sense, because they’re both using their inputs, whether it’s inhibitory or excitatory, to modulate motor patterns

Basal ganglia works primarily by adjusting the amount of inhibitory input to different parts of the motor cortex, whereas the cerebellum works by adjusting the amount of excitatory input to different parts of the motor cortex

Question 44

Definition of Basal ganglia

Answer 44

A group of subcortical structures that act as a ‘gate keeper’ for control of the motor system (muscles)

The basal ganglia are a group of nuclei lying deep within cerebral hemispheres

Question 45

The basal ganglia role

Answer 45

Role in motor control not fully understood

Basal ganglia dysfunction implicated in many disorders

Receives excitatory input from many areas of cortex (Glutamate)

Output goes back to cortex via the thalamus

Output is mainly inhibitory (GABA)- the cortex is sending information to the basal ganglia about kind of the motor plans and the basal ganglia via the thalamus is inhibiting that activity

Slide 23

Complex internal connectivity involving 5 principle nuclei:
Substantia Nigra (pars compacta & pars reticulata)
Caudate & Putamen (together=striatum)
Globus Pallidus (internal and external segments)
Subthalamic Nucleus

Question 46

The selection problem

Answer 46

Multiple command systems

Spatially distributed

Processing in parallel

All act through final common motor path

[Cannot do more then one thing (well) at a time]

Question 47

The Basal Ganglia: disinhibitory gating of motor cortex output

Answer 47

At rest the striatum is fairly quiet and therefore theres continues inhibitory output activity from the globus pallidus which is causing inhibition of the thalamus therefore the levels of active excitation in the motor cortex is dampened down

Can have a transient burst of excitation in the striatum that can cause transient inhibition of the basal ganglia output, globus palidus in this case
This enables a disinhibitory of burst activity in the thalamus and therefore the motor cortex and then on the level of the motor neurones

Question 48

Not just motor loops

Answer 48

Basal ganglia also important in regulating cognitive and affective resources- how you focus your cognitive resources and regulation of emotion

Question 49

Cerebellum

Answer 49

The cerebellum is a large brain structure that acts as a ‘parallel processor’, enabling smooth, co-ordinated movements (regulating errors and perfection of movement)

It may also be very important in a range of cognitive tasks

Like basal ganglia, no direct projection to the lower motor neurons – instead modulate activity of upper motor neurons

Contains approx half total number of CNS neurons

Just 10% of total brain weight

Projects to almost all upper motor neurons

Question 50

Inputs and Outputs of the Cerebellum (simplified)-
CORTICAL

Answer 50

Mostly from motor cortex (copies of motor commands)

Also somatosensory and visual areas of parietal cortex

Question 51

Inputs and Outputs of the Cerebellum- Spinal

Answer 51

Proprioceptive information about limb position and movement (muscle spindles, other mechanoreceptors)

Question 52

Inputs and Outputs of the Cerebellum- Vestibular

Answer 52

Rotational and acceleratory head movement (semicircular canals / otoliths in inner ear)

Question 53

Cerebellum function

Answer 53

It knows what the current motor command is

It knows about actual body position and movement

It projects back to motor cortex

Computes motor error and adjusts cortical motor commands accordingly

Not just motor control, but motor learning too, in collaboration with basal ganglia and cortical circuits

Functional brain imaging studies have demonstrated that the cerebellum is involved in a wide variety of non-motor tasks

Question 54

Bypassing the lower motor neurons

Question 55

Agency of action

Answer 55

sense of agency give us a perceptual shock

Evidence suggests that sense of agency is retrospectively created

Connections between frontal areas that develop motor plans for voluntary action and the parietal (association) areas that monitor outcomes play a key part in computing sense of agency

The delay between motor command, and perceived outcome is crucial – too short or too long can disrupt sense of agency