L1: Intro to the Motor system

Learning objectives

1. To understand the organisation of motor pathways

2. To understand the problems of controlling movements

3. To introduce neuronal networks and their function

4. To show how motor defects reflect deficits in motor system components

Motor vs sensory systems

Motor:

• balance, posture, move body, limbs, eyes and allow communication

• allos body to move through and manipulate the environment

• Transform nerual info into physical energy

Senosory:

• tranform physical energy into neural info (i.e the opposite)

Why important to understand motor systems

• artificial systems

• treatments for neurological conditions that affect movement

Sensory vs motor (why is motor more difficult to study)

• sensory→ linear chain

  • several relays (with feedback)

• motor→ loops

  • hierachy that generates outputs than in turn generate inputs

    • ‘Re-afference’

  • To study→ must break the loop and analyse parts individually

  • (at the same time as recognising how they act co-operatively)

Why is it really important to study the motor system (the most important?)

Understanding motor system is relevant to understanding behaviour

• note: its it not that it is dominant of most important

• instead: all conscious and unconscious beahviour involve the motor system

  • e.g animal with memorty of where it stored food can only use this memory if it can move to get the food→ otherwise no point)

  • e.g Movement disorder are cardinal signs of CNS damage

    • understanding it helps to identify the location and possible cause of CNS pathology

Charles Sherrington and Edgar Adrian quotes about the importance of the motor system

• Charles→ “To move is all mankind can do-whether in whispering a syllable or in felling a forest.”

• Edgar→ “The chief function of the central nervous system is to send messages to the muscles which will make the body move effectively as a whole”

not that motor system is the most important but that it is involved in every behaviour

Roles of the motor systems

1. Move in or to manipulate the world

2. To maintain equilibirium

3. Autonomic functions- respiration, gut movements

4. Communication→ speech/gesture/written

5. Sensation→ vision *saccades), Somatosensory (haptics, active touch)

   • have specific sequence of eye movement to recognise face (look at eyes then mouth etc)

   • Touch→ move object around to fully feel it

     • used to improve gripping in robots

Sensory-motor interactions→ How do motor systems rely on sensory (and evidence)

• Proprioreception→ motor requires sensory feedback of body and limb position to generate appropriate ouputs

  • Evidence: Tabes dorsalis (large fibre sensory neuropathy)→ propriorecption deficets

    • Axons that carry proprioreceptive and tactile inputs degenerate

    • RESULT: cannot sense their position or movement

      • cannot plant and control motor system

      • (unless can see thier limbs)

But sensory also relies on motor (therefore shows how they are intimately related)→ cat experiment

Held and Hein

Procedure:

• kittens moved either actively or passivele through environment

Result:

• Passive kitten→ developed visual deficits

Shows how need motor to develop visual/ visual is needed for motor to work→ is motor is passive→ no longer need visual

Using this experiment→ developed sensory subsitiution for the blind

How it works

• Use a camera and convert the visual field into tactile inputs whic can be sensed by the blind

• felt on skin or tongue

Result:

• Could only sense the environemnt if the camera was placed on the head (instead of just to the side)

• This is because the wearer could actively move the camera

  • NOT PASSIVE

What does this active movement of the camera tell us about perception

• Perception needs movement

and

• The motor system needs sensory input

• The motor system is not a passive system→ it is actively invovled in sensory perception

  • we only separate the two systems for experiments

Three basic types of movements

note: movement = variable combinaton of these three types

1. Reflex→ knee jerk, cough

   • simplest

   • limited muscle groups

   • little voluntary control

   • Rapid

   • stereotypes

   • graded function of eliciting stimulus

2. Rhythmic→ walking, running chewing

   • several muscle groups

     • coordinate limbs, joints or whole body

   • Relatively sterotyped

   • Some modification

3. Voluntary→ playing piano, manipulating objects, speech

   • most complex

   • Purposeful

   • Whole body muslce groups

   • initiated by stimuli or internal motivational systems

   • Goal directed

   • Modifiable: learnt and improve with practice

Why do we think motor control is trivial compared to perception/cognition?

1. Unconscious control→ limited focus needed

2. Invariant development→ everyone develops the same levesl of motor functions (except in disease)

   • irrespective of background (which markedly affects cognitive function)

     • (although can be improved by training)

3. Highly reliable→ only go wrong is the environment changes (wet surface)

   • compared to cognitive→ forget and make bad decisions

Why is motor control atually not trivial/ is difficult to perform

1. Significant computations

   • rapid regulation of billions of neurons and synapses

   • to control mutliple muscle groups and limbs

2. Highly flexible

3. HIghly reliable

Motor systems have overcome two paradoxes

1. Speed vs reliability

   • can change motor output quickly BUT still be reliable (e.g switch to sprint→ will still be able to know how to switch even with speed)

2. Reliability vs flexibility

   • artificial systems cannot overcome this

How is such difficult control made look so easy?

• Basic movement is not effortless and unconscious because it is trivial but reflect the sophisticated motor systems that have evolved

• Better than the most sophisticated robotic systems

What does Moravec’s paradox highlight

• The ascept humans find easy (motor system) are difficult to mimic in artificial systems

Whilst

• the aspects we find hard (memory) are easier in artificial

  • Deep blue chess master

Motor control is difficult to study why

1. Movement is end point of series of commands

2. Same command different result

3. Different command same result

4. Motor equivalence problem

5. Neuronal networks are complex

1. Movement is end point of series of commands

• need to trace back to CNS to understand how it was generated

• BUT→ pathway is dynamic and non-linear

  • so difficult to infer features of upstream command

2. Same command different result

• muscular contraction depends on previous activity

  • ‘thixotropy’

• e.g if tired, drunk, cold→ get different output for same input

3. Different command same result

• same output but from different command

4. Motor equivalence problem

• every movement could in principle be unique

• because joints have many degrees of freedom

  • directions and angles of limb movement

5. Neuronal networks

• assemblies of neurons generating specific outputs/functions

  • individual spinal motor neurons integrate 1000s of inhibitory and excitatory synaptic inputs

  • e.g retina

• Understanding these actually reflect difficultis of understing any nervous system function (not just motor!)

What is needed to know to understand neuronal networks

1. Identify network neurons

2. Determine their connectivity

3. Characterise their cellular and synaptic properties

Why is identifying synapses and their functional properties really difficult

1. large numbers of compoents

2. Inverse relationship between neuron size and number

   • more neurons = smaller the size

3. Not just large numbers→ other difficulties

   • e.g seen in example of pyloric network in lobster: STG)

     • we KNOW all the individual neurons and their connections

     • yet we still do not understand how it works!

Example further→ STG network in lobster

• controls movements of stomach

• simplest and best characterised network

  • → We know ALL the three criteria listed above

    1. <30 large neurons

    2. relatively limited number of synaptic connections

    3. All properties characterised

• HOWEVER: it is still not clear how the STG generates its output

What does this example show

• SHOWS: difficulty of understanding how individual network compoentns interact

Furhter:

• if such as simple network is hard to understand→ it is very hard in mammalian → billions of cells/synapses

  • even insect locomotion is order of magnitude more components than STG

understandingneuronal networks is the biggest problem in neuronscience/science

Organisation of motor systems: the motor system as a hierarchy

• Highper components specifify effects at lower levels

  • But: does not mean that the lower levels are less important

Three general levels in the vertebrate motor system

1. Spinal cord

2. Brain stem

3. ‘higher’ centres (motor cortex, cerebellum, basal ganglia)

Motor system in invertebrates (insects)

Consists of segmented ventral nerve cord

• runs the legnth of the animal

What does the nerve cord consist of

Chain of ganglia

• (sometimes fused together)

• Ganglia contain→ a few dozen 100s thousands of neurons

• Ganglia in each segment→ joined to their neighbours by interganglionic connectives

Invertebrate vs vertebrate

• Anatomical differences

but

Similarites→ in terms of functions

• the different regions have same specific roles:

  • comparator, generator, output, feedback, stored program etc

therefore: invertbrates are useful to study to understand vertabreate motor control

However→ Is there really an Anatomical hierachy??

• Not really→ not just descening from high to low

Motor system is a series of loops

• there is feedback from every level

what other kind of hierachy could describe it?

Functional (again each stage is not more important than the other)

1. Idea→ what is the goal

2. plan→ how do i achieve it

3. Program→ what muscles and how much

4. Execution→ send out motor commands

5. Movement

As go down these→ from cortical areas to the spine etc→ have idea→ movement

Issue with functional hierachy

• e.g the spine can also the thing to PLAN and PROGRAM (as well as execute)

• e.g the motor cortex can execute (not just plan)

i.e the funtional roles are not always in the same order as the anatomical hierachy

Therefore the organisation can be consiered as

Heterarchy

1. Multiple feedforward and feedback pathways

2. no fixed upper or lower level

3. upper is context dependent

   • e.g step on a pin→ command = spinal cord which then sends signals to cortex to modify the next movements

   • e.g conventrional_ >from cortex→ to spine

Interactions between components

1. Output/negative feedback mechanisms

2. Internal feedback-efferent copy

3. Anticipated Motor commands

1. Output/negative feedback mechanisms

1. feedback signal is subtracted from the reference (desired output) by a comparator

2. feedback error signal increases or decreases the ouput of generator

3. reduces the error

• See everywhere in bio and artifical

• also be positive

1. Issue with negative feedback

Delays

• by the time the correction starts, the error has increased further

• causes oscillations due to changes above and below desired set point

1. example of this problem

Catching a ball

• take 700ms to respond to visual clue

• but

• movement takes 150-200ms

When is it used

• sensory inputs can only be used to set up the initial conditions

• e.g position of the arm and hand to catch the ball

  • based on predictions of the balls trajectory

2. Feedforward control

• movements are programmed in advance

• How:

  • predict when to use already stored (learn) motor programs

Exmaple 1 of this

• you prepare to walk differntly on icy slope

• from previous experinece

• so you adapt motor output before you even go on the ice

Example 2 of this

hitting a ball

• Visual inputs will already set up limb positions and muscle tone for the predicted position and speed of the ball

• Requires→

  • acquired internal models of ball trajectory

  • muscular skeletal properties from previous experience of where ball will land/ounce

3. Efferent copy

Form of internal feedback

• Motor output is send to other areas of motor system to inform it of the intended movement before or as it is being made

• →minimising delays with external feedback

• Actual movement can be compared to the planned and any erros corrected

All these mechanism together

• not isolated

• combine in different circumstances

• to optimally generate coordinated motor outputs

Key points

1. Motor systems fulfil diverse roles beyond just movement.

2. Motor systems consist of several structures that influence outputs through parallel distributed pathways.

3. Motor and sensory systems are closely related, and highly dependent on each other.

4. Because of delays, feedback control is limited to slow movements.

5. Fast responses use feedforward prediction.