The Motor System (lecture based)

Question 1

What is 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

What are the 4 types of motor control?

Answer 2

Voluntary: running, walking, talking playing guitar etc

Goal-directed: conscious, explicit, controlled

Habit: unconscious, implicit, automatic

Involuntary: eye movements, facial expressions, jaw, tongue, postural muscles throughout trunk, hand and fingers, diaphragm, cardiac, intercostals (around lungs), digestive tract

Question 3

What is the hierarchical control architecture of movement from an evolutionary perspective?

Answer 3

Pain -> spinal cord
Loom -> sensorimotor midbrain
Learned threat -> cortex + limbic system

All 3 systems use motor, autonomic and endocrine systems to create a defence related output

Question 4

What is motor control governed by?

Answer 4

Upper and Lower motor neurons

Lower - begin in the brainstem or spinal cord and project to the muscles

Upper- originate in higher centres and project down to meet motor neurons

Question 5

What modulates movement in the sensorimotor system?

Answer 5

Basal ganglia= what to do
Cerebellum = how to do it

Question 6

Muscle fibres

Answer 6

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.

Question 7

What are the three different types of muscles?

Answer 7

Skeletal muscle
Smooth muscle
Cardiac muscle-

Question 8

Muscles can only relax or contract (all or nothing) so how do we get such complex movements and forces?

Answer 8

Antagonistic arrangement – combined co-ordinated action (e.g biceps and triceps work in opposition allowing you to operate your arm)

Recruitment of muscle fibres – process by which different motor units are activated to produce type of muscle contraction e.g. more recruitment for bigger muscle contraction
small and large motor units (combinations of muscle fibres)
fast/slow twitch,

Question 9

What are individual differences in muscle fibres?

Answer 9

The number of muscle fibres varies across individuals, but changes little with either time or training – appears to be genetically determined

Muscle size (+ strength) is more about cross sectional area of individual fibres and different proportions of the different types of fibre

Question 10

What is the composition of a muscle?

Answer 10

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

Question 11

How do muscles contract?

Answer 11

1. Nerve impulse reaches the neuromuscular junction
   Acetylcholine released by the motor neuron
2. Ach binds to the muscle receptor site
3. Sodium channels in the muscle open causing sodium ions to rush into the cell initiating an action potential
4. Action potential impulse causes the sarcoplasmic reticulum to release Ca+ into the cell and around the sarcomere contraction units
5. Ca+ binds to troponin releasing tropomyosin out of the actin-myosin binding site = causes myosin heads to fit into actin filaments
6. Myosin heads pull actin filaments toward the centre allowing filaments to slide past each other and shorten the sarcomere
   ATP also binds

When the muscle fibre is depolarised actin and myosin slide against each other which produce muscle contraction

Question 12

What is the myosin cross-bridge cycles?

Answer 12

The myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind actin in a new cycle. This process is known as myosin-actin cycling. As the myosin S1 segment binds and releases actin, it forms what are called cross bridges, which extend from the thick myosin filaments to the thin actin filaments. The contraction of myosin’s S1 region is called the power stroke

Question 13

What is the cause of rigor mortis?

Answer 13

Rigor mortis is a postmortem change resulting in the stiffening of the body muscles

ATP is required to break the bond between the myosin head and actin filament

ATP is produced by oxidative metabolism, which stops upon death

So the muscle becomes contracted and remain that way until enzymes begin to disrupt the actin/myosin

Therefore, if there is no ATP becuase the person is dead, the bond between the myosin head and actin filament cannot be broken so the muscles remain contracted

Question 14

What is the motor unit?

Answer 14

The unit of action of the motor system is the MOTOR UNIT rather than the muscle fibers.

Motor unit = single alpha motor neuron (lower motor neuron) + all the muscle fibres it innervates

Different motor neurons innervate different numbers of muscle fibres –>Fewer fibres means greater movement resolution - e.g. those innervating finger tips and tongue

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)

Question 15

How do you get different levels of muscle force?

Answer 15

By activating motor units that have different numbers of muscle fibres attached to them

Question 16

What two principles determine the number of muscle fibres innervated by a single motor neuron based on the functional requirements of the muscle?

Answer 16

1. Level of control
2. Strength

The size of the motor unit is what trades off to give you a balance between the level of control that you need and the amount of strength that you need

SIZE PRINCIPLE
Units are recruited in order of size (smallest first)
Fine control typically required at lower forces

The combination of motor units activated by such orderly recruitment optimally matches the physiological properties of different motor unit types with the range of forces required to perform different motor task

Question 17

What are the three different types of muscle fiber?

Answer 17

Slow
Fast fatigable
Fast fatigue resistant

Question 18

Describe slow motor units

Answer 18

These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction, such as maintaining an upright posture

Question 19

Describe fast fatiguable motor units

Answer 19

Larger α motor neurons innervate larger muscle fibers that generate more force; however, these fibers have sparse mitochondria and are therefore easily fatigued. These units are called fast fatigable (FF) motor units and are especially important for brief exertions that require large forces, such as running or jumping.

Question 20

Describe fast fatigue-resistant motor units

Answer 20

Of intermediate size and are not quite as fast as FF motor units. They generate about twice the force of a slow motor unit and are resistant to fatigue

Question 21

What do functional distinctions between classes of motor neurons tell us?

Answer 21

Functional distinctions between the classes of motor units explain some structural differences among muscles.

For example, a motor unit in the soleus (a muscle important for posture that comprises mostly small motor units) has an average innervation ratio of 180 muscle fibers for each motor neuron.
In contrast, the gastrocnemius, a muscle that comprises both small and larger motor units, has an innervation ratio of 1000 to 2000 muscle fibers per motor neuron and can generate forces needed for sudden changes in body position.

Other differencesare related to the highly specialized functions of particular muscles.
For instance, the rotation of the eyes in the orbits requires rapid, precise movements that are generated by small forces. In consequence, extraocular muscle motor units are extremely small (with an average innervation ratio of only three fibers per unit) and have a very high proportion of muscle fibers capable of contracting with maximum velocity.

Question 22

How can training and exercise change muscle fibres?

Answer 22

Training and exercise increases the thickness of muscle fibers and can change the proportion of different fiber types

Question 23

What contributes to the regulation of muscle tension?

Answer 23

The frequency of action potentials generated by motor neurons contributes to the regulation of muscle tension.

The increase in force that occurs with increased firing rate reflects the summation of successive muscle contractions.

The muscle fibres are activated by the next muscle action potential before they have time to completely relax, and the forces generated by temporarily overalapping contractions are summed.

Question 24

What are lower (alpha) motor neurons?

Answer 24

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 25

Where does the sensory input and motor output occur in the spinal cord?

Answer 25

Sensory input = dorsal root (back)
Motor output = ventral root (front)

Sensory input is key

Question 26

What is the motor pool?

Answer 26

All the lower motor neurons that innervate single muscle = the motor pool

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 27

What is the arrangement of alpha motor neurons?

Answer 27

Tend to have more proximal muscles and their cell bodies in the centre of the spinal cord and more distal muscles towards the outside, maining this topography we see in motor systems

Both ascending and descending information for the brain at the same level- important for reflex circuits, interesting point is that the brain is regulating these circuits and not necesarily completing them

Question 28

What 2 pieces of information do good control systems (CNS) need to be able to contract or relax muscles to provide movement?

This is a key part of proprioception

Answer 28

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 (length)

Question 29

What are Golgi tendon organs?

Answer 29

Important for muscle tension (force)

Contained in the tendon where the muscle joins to the 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- regulating ongoing motor plans

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 30

What are muscle spindles?

Answer 30

Important for muscle stretch (length)

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

This information forms a key part of reflex circuits

Question 31

What are reflexes?

Answer 31

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.

Question 32

What is the reflex circuit in movement?

Answer 32

Most simple reflex- monosynaptic- e.g. the patellar tendon reflex

Within muscle fibres there is an interloper, an intrafusal muscle fiber which contains the muscle spindles, the stretch detector, alongside the extrafusal muscle fibres (muscle fibres doing the force)

Question 33

What is the stretch reflex?

Answer 33

If a muscle is stretched you get a signal from the muscle spindle that travels along the sensory nerve into the spinal cord, it can synapse with the alpha motor neurons and cause contraction of the muscle which returns it to its initial length

e.g. if you’re holding a cup and someone pours tea into it you get a passive stretch, the muscle splindle will then be activated and causing additional contraction of that muscle to retun it tp the length it was already set at.

Question 34

Why are intrafusal fibers innervated separately to extrafusal fibres and what innervates both types?

Answer 34

We need a system to detect stretch regardless the current muscle length
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 (y) motor neurons
Gamma motor neurons keep the intrafusal fibres set at a length that optimises muscle stretch detection

So, alpha motor neurons are activating your muscle fibres which are doing the muscle force, and gamma motor neurons are activiting the intrafusal fibers which is about a sensory system optimising a sensory system

Question 35

Complex reflexes in quadrupeds

Answer 35

Animal models show that local circuits in the spinal cord, called central pattern generators are fully capable of controlling the timing and coordination of complex patterns of movement and adjusting them in response to altered circumstances.

In quadrupeds, sequence and precise control of limb movements appears complex and is substantially altered by speed changes (e.g. walk, trot, gallop)

But this is not under the control of a higer brain centre!

Quadrupeds will walk on treadmill if weight supported if spinal cord damaged at thoracic level
Will change to appropriate patterns of limb movement as treadmill speed is altered
Complex reflex system responding to nothing more than stretch of muscle spindles!!

Question 36

Complex reflexes- withdrawal reflex

Answer 36

Withdrawal of a limb from a painful stimulus, e.g. step on a pin =Withdrawl reflex in one leg and an extension reflex in the other leg

Even though it occurs quickly, this flexion reflex involves slowly conducting afferent axons and several synaptic links.

As a result of activity in this circuitry, stimulation of nociceptive sensory fibers leads to withdrawal of the limb from the source of pain by excitation of ipsilateral flexor muscles and reciprocal inhibition of ipsilateral extensor muscles.

Flexion of the stimulated limb is also accompanied by an opposite reaction in the contralateral limb (i.e., the contralateral extensor muscles are excited while flexor muscles are inhibited).

This crossed extension reflex provides postural support during withdrawal of the affected limb from the painful stimulus.

Question 37

What is the righting reflex/ vestibular righting reflex?

Answer 37

Vestibular system detects that the body is not upright (orientation), as well as any acceleration due to gravity (i.e. falling)

Information from the vestibular system is combined with visual, somatosensory and proprioceptive sensory input in order to specify a pattern of motor activity that will restore ‘uprightness’ and a safe landing

The cerebellum, which compares the intended motor plan with the actual situation is critical for computing the desired motor activity.

Question 38

Given that complex movements can be controlled by such simple circuits, why is it so hard to build robots that can walk, run, track moving objects with robotic eye etc….?

Answer 38

Control of gross movement patterns (which can be quite complex) can be devolved to simple spinal circuitry, but constant modulation based on sensory feedback is required to account for the unexpected- its the ADAPTATION

Higher CNS centres constantly adjust ongoing activity to resolve conflicting demands on the motor system and direct it towards goals- CORRECTING

Generating models would be too complicated

Question 39

What are brainstem systems?

Answer 39

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 40

Give an example of ancient and modern co-operation

Answer 40

Speech

Branches of vagus nerve project to larynx to control speech – this circuit richly interconnected with cerebellum and other brainstem sensorimotor systems

Primitive sounds have been sculpted by the cortex

Often the case that we are building on what is alrady there and increasing the connectivity with other systems

In many motor activities, these ancient motor control systems within the brainstem are working cooperatively with the higher motor control systems

Question 41

What is the motor cortex?

Answer 41

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

wired up to have very direct control

Question 42

What are the descending projections from cortical motor areas?

Answer 42

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

Question 43

Generally speaking what do the basal ganglia and cerebellum do in the motor cortex?

Answer 43

Basal ganglia is inhibiting activity in the motor cortex, it has an output which feeds into the motor cortex which basically dampens stuff down

Whereas the cerebellum its kind of excitatory of motor cortex activity

BUT these things are NOToperating in balance to each other, they’re doing qualitatively different things but in terms of the neurotransmitters invovled and the inputs the BG inhibits and the C produces selective excitation to regulate the motor cortex to map commands

Question 44

Overall what is the simplified role of the basal ganglia and cerebellum in motor plans?

Answer 44

When the motor cortex issues its command, it sends a copy of this command to the basal ganglia and cerebellum and they then do something with this information as well as with sensory information and can modulate the motor cortex plan.

This means that the motor cortex remains at the top of the motor cortex tree with direct control of everything going on below it, and this modulatory, regulatory tweaking of the motor plan is done by these two systems receiving a copy of this motor plan and feeding back to regulate the ongoing motor plan

Question 45

What is the Homunculus?

Answer 45

A homunculus is a topographical-organized map of the proportional representation of the contralateral somatosensory or motor neurons on the cortex or passing though a part of the brain.

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 46

What are the descending projections from the motor cortex?

Answer 46

Two main subdivisions: dorsolateral and the ventromedial tracts

The dorsolateral corticospinal tract starts on one side of the brain and innervates muscles on the other side of the body. It has a direct pathway but also an indirect pathway as well through the red nucleus.

Generally speaking we have innervation of limb muscles on the other side of the body and it tends to be the distal limb muscles, so feet, hands forearms etc

The other tract, the ventromedial tract tends to innervate more central musclees, so muscles of the trunk, important postural control and mostly its branches and innervates the muscles on both sides of the body, it has bilateral innvervation and that makes sense as often we need some degree of symmetry e.g. for posture, so it makes functional sense

Question 47

What are the similarities/differences between the dorsolateral and ventromedial tracts?

Answer 47

Similarities:
Both contain a direct corticospinal route

Both contain an indirect route via brainstem nuclei
-> via red nucleus for dorsolateral
->Via tectum, vestibular nuclei, reticular formation & cranial nerve nuclei for ventromedial

Differences:

Dorsolateral:
Innervate contralateral side of one segment of spinal cord
Sometimes project directly to alpha motor neuron
Project to distal muscles, e.g. fingers

Ventromedial:
Diffuse innervation projecting to both sides and multiple segments of spinal cord
Project to proximal muscles of trunk and limbs

Question 48

What is the Basal Ganglia?

Answer 48

A group of structures beneath the cortex 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
Widely studied
Role in motor control not fully understood
Basal ganglia dysfunction implicated in many disorders especially disorders than have an element of motor control disturbance

Question 49

What is the function of the Basal Ganglia in terms of the motor cortex?

Answer 49

Receives excitatory input from many areas of cortex (Glutamate)
Output goes back to cortex via the thalamus
Output is mainly inhibitory (GABA)
WIthin it we have a double negative, although its output is inhibitory, you can inhibit the inhibition from the striatum

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 50

What is the selection problem and the BGs role as a motor loop?

Answer 50

The basal ganglia is regulating between competing demands on the motor system e.g. may be a threat, may be dehydrated, may be hungry

Although you can do more than one thing at a time in the motor system, to do one thing well and of high survival value, you often need to devote your entire motor system to it

The idea is that the basal ganglia is resolving competition between these distributed command systems that are operating in parallel but that need to act through a final comman pathway to decide to e.g. run away well or find food well etc

Its role is to say dont do anything except this one thing I am going to remove inhibition from, relating to a particular type of behaviour

Like a motor loop

Question 51

The basal ganglia: describe the disinhibitory gating of motor cortex output

Answer 51

At rest the basal ganglia nuclei are quiet and because they’re at rest they are not inhibiting the globus pallidus which is an output structure and becuase they are not inhibiting it, that means it is active, it has a tonic activity (firing away) and that means that its passing its inhibition onto the thalamus because it is active, its inhibiting stuff, which keeps the motor cortex within a particualr set of circumstances with a relatively reduced set of excitation

However, if you have a transient burst of excitation into the striatum, that transient burst of excitation can give a transient inhibition of the globus pallidus and because the globus pallidus is noramally inhibiting the thalamus, we then get a transient excitable thalamic state and a burst of excitation becomes possible in motor cortex and therefore behaviour

What is leading to the burst is a dopamine input from the substantia nigra. If you lose the dopaminergic input from the substantia nigra, you remain stuck in an inhibited state, where a loss of the dopaminergic pathway is one of the key symptoms of something like PD, a movement disorder

Question 52

What is the cerebellum?

Answer 52

The cerebellum is a large brain structure that acts as a ‘parallel processor’, enabling smooth, co-ordinated movements. 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

Question 53

What are the inputs and outputs of the cerebellum? How does this explain its basic/simplified function in motor control?

Answer 53

Cortical:
Mostly from motor cortex (copies of motor commands)
Also somatosensory and visual areas of parietal cortex

Spinal:
Proprioceptive/sensory information about limb position and movement (muscle spindles, other mechanoreceptors)

Vestibular:
Where the body is in space
Rotational and acceleratory head movement (semicircular canals / otoliths in inner ear)

SO
It knows what the current motor command is
It knows about actual body position and movement
It projects back to motor cortex

FUNCTION
The Cerebellum computes motor error and adjusts cortical motor commands accordingly

Question 54

What is the function of the cerebellum beyond just motor correction control?

Answer 54

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 55

How can cerebellar function be used in robots?

Answer 55

Could recordings of cortical electrical activity be used to directly control lower motor neurons, thus bypassing (possibly damaged) spinal pathways?

Idea we can bypass lower motor neurons

Benabid et al. (2019)
2 Years learning to control an avatar in VR
Gradually increased number of movement parameters controlled (degrees of freedom)
The decoding model requires recalibration after 6-7 weeks

This approach has great potential because the motor cortex is a relatively direct controller of voluntary muscle activity (the cerebellar and basal ganglia input is ‘factored in’).

But the decoding is hard. And the movements are carried out without benefit of all the lower level automatic circuitry (including sensory feedback, reflexes, etc.)