The Sensorimotor System ( UP TO SLIDE 42/83)

smart falling, buttered cats, robot movement. Understand the general architecture of the motor system from cortical association areas to contraction of muscle fibres Understand how lower and upper motor neurons contribute to motor control Understand the importance of integration of ascending (sensory) and descending (motor) signals in accurate motor control Understand the main roles of the motor cortex, cerebellum and basal ganglia in motor control Understand the principle descending motor pathways including their subdivisions and the functional differences between them

motor control involves…

a dynamically changing mix of conscious and unconscious regulation of muscle force

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

overview of motor control

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

what has NS been sculpted by

evolutionary realities

evolution + motor control

way we control movement and diff systems involved has a lot in common w very ancient vertebrate ancestors when first crawled on to land- lots of similiarities in type and function of brain structures and how work together

types of motor control

• voluntary

• goal directed

• habit

• involuntary

most voluntary motor control is…

goal directed

goal directed motor control

conscious, explicit, controlled

habitual motor control

unconscious, implicit, automatic

examples of voluntary motor control

running, walking, talking, playing, guitar, etc

examples of involuntary motor control

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

falling + motor system

• stops hurting important parts

• protects ur face, protects neck/core parts of body

• e.g. fall and roll forwards, step up, keep running

• scruff on shoulder or in knee, rolled

• momentum not stopped but carried

• lot unconscious process to keep safe- all uncon and complicated

hierarchical control architecture

• three levels of control

• elementary → pain → spinal cord → escape

• next level → looming threat → sensorimotor midbrain

• top level → learned threat → cortex + limbic systems

• these things → motor/ autonomic / endocrine → defense related output

hierarchical control architecture: evolutionary perspective

• pain and looming threats are evolutionary

• learned threat, related to cortical structures, is higher level and not evolutionary

hierarchical control architecture: level 1

• pain is elementary sensory signal that tells u tissue damage is happening

• response to withdraw from pain/pain source

• mediated by simple circuits in spinal cord

• basic defensive behaviours is shared by most animals, even most organisms like plants

hierarchical control architecture: level 2

• needing to move in response to something

• looming threat

• more complicated

• need visual systems to encapture lumens, objects, size, in front of you- big part of visual image effected by potential predator

• need motor coordination to be able to move away

• sensorimotor midbrain

hierarchical control architecture: level 3

• top level, learned threat

• e.g.- gun, nothing intrinsic or innate, all learned

• weapon- not a big visual image

• visual processing, learning, able to break down image and id object within it, understanding high level, cognitive function

• cortical and limbic system

• emotional parts telling threat

• high levels development, humans highly cortical animals

noxious or contact stimuli →

spinal cord

spinal cord →

reflexive withdrawal

sudden distal stimuli →

hindbrain

hindbrain →

‘startle’ responses

species- specific threat stimuli →

midbrain and hypothalamus

midbrain and hypothalamus →

species-specific responses e.g. freeze/flight/fight

neutral stimuli →

thalamus

thalamus →

amygdala

complex neutral stimuli →

sensory cortex

sensory cortex →

amygdala

context →

hippocampus. and septum

hippocampus and septum →

amygdala

amygdala →

conditioned emotional responses

cognitive analyses →

frontal cortex

frontal cortex →

response suppression

threat detection, avoidance behaviour, processing compacity

more complicated and sophisticated threat detection and avoidance behaviour → requires additional or more complex processing capacity (neural systems)

label

1. pain

2. spinal cord

3. looming threat

4. sensorimotor brain

5. learned threat

6. cortex and limbic system

7. motor, autonomic, endocrine

8. defense-related output

9. escape

10. avoidance

11. avoidance

label

1. Sensory input

2. Motor, autonomic, and endocrine output

1. Noxious or contact stimuli

2. Spinal cord

3. Reflexive withdrawal

4. Sudden distal stimuli

5. Hindbrain

6. Startle responses

7. Species specific threat stimuli

8. Midbrain and hypothalamus 

9. Species specific responses freeze/flight/fight

10. Neutral stimuli

11. Thalamus

12. Complex neutral stimuli

13. Sensory cortex

14. Context 

15. Context hippocampus and septum

16. Amygdala 

17. Conditioned emotional responses

18. Cognitive analyses

19. Frontal cortex

20. Response suppression

what is motor control governed by?

lower and upper motor neurons

lower motor neuron: location

begins (has its cell body) in brainstem or spinal cord and projects to the muscle

upper motor neuron: location

originate in higher centres and project down to meet the other category of motor neurons

what is at the bottom of the sensorimotor system

spinal cord and motor circuits

what is at the top of the sensorimotor system

association cortex

what does the association cortex sit between

visual and motor cortices

association cortex: location

top of sensorimotor system

between motor and visual (above) cortices

interaction between

describe overview of sensory motor system layout

association cortex. ← → basal ganglia X sensory feedback ←

secondary motor cortex. ←. → basal ganglia X sensory feedback ← and ^

primary motor cortex ← → cerebellum X sensory feedback ← and ^

brainstem motor nuceli X sensory feedback ← and ^

spinal motor circuits X ← → muscle ^ sensory feedback ← and ^

motor units X ← → muscle ^. sensory feedback ← and ^

label

1. Association cortex

2. M2

3. Primary motor cortex

4. Brainstem motor nuclei

e1.      Spinal motor circuits

e2.      Motor units

i) basal ganglia

ii) cerebellum

iii) muscle

1. Secondary motor cortex

2. Descending motor circuits

3. Sensory feedback

basal ganglia function

selecting programmes of motor activity

cerebellum function

fine control and correction of motor activity

describe sensorimotor system

• lots of interaction between different parts

• bottom- spinal cord and motor circuits

• top- association cortex

• motor cortex below, and secondary and primary corticies

• controlling neurons as go down

• all feedback into system at lots of different levels

• signals at top, going down to bottom

• other systems may interfere with or optimise signals as go down

• basal ganglia and cerebellum

what type of system is the sensorimotor system

a descending control system with lots of ascending feedback

label, including:

red arrows show… a

blue lines show… b

a) Descending control system

b) Ascending feedback

1.a. Association cortex

2.a. Motor cortex

2.b. Cerebellum

2.c. basal ganglia

3.a. Brainstem circuits

4.a. Spinal circuits 

5.a. Motor unit

5.b. Sensory systems

5.c. Sensory systems

6. Effect on the World

describe

sensorimotor system is a descending control system coming down through these different layers, but with lots of feedback, effect guides movement

what manner do muscle fibres act in

all or none

what does control of muscle force depend on?

the way in which lower motor neurons activate different types of muscle fibre, bc individual muscle fibres act in an all or none manner

why are individual muscle fibres acting in an all or none manner

bc neurons are all or none- action potential (fires) or it does not

postsynaptic potentials and other stuff happen, but re signal- it either fires or does not fire

muscle fibres are the same

what do individual muscle fibres do

they either contract or do not contract- not a continuum

how are muscle fibres controlled, as an implication of knowing they how individual ones work

individual muscle fibres either contract or do not contract

indicates muscle control is not about more or less work, but about number of and type of muscle fibres

how many and what type are activated

what % of body weight is muscle

40

how many types of muscle

three

types of muscle

1. cardiac

2. smooth

3. skeletal

smallest muscle

stapedius, in inner ear

focus on dampening sound

largest muscle

gluteus maximus, in hip/buttock

strongest (based on weight) muscle

masseter, in jaw

skeletal muscle

• focus of our content

• controls movement

• attached to skeleton

• skeleton is frame around which muscles pivot you to move

smooth muscle

• lots in body

• does lots of diff things

• lines digestive system

• blood vessels

• e.g. blood vessels in brain decide how much goes to which parts

cardiac muscle

• heart muscle

• diff type designed to not stop working

label

1. skeletal muscle

2. smooth muscle

3. cardiac muscle

how do we achieve such a range of movements and forces?

1. antagonistic arrangement

2. recruitment of muscle fibres

antagonistic arrangment

combined, coordinated action

recruitment of muscle fibres

fast/slow twitch, small and large motor units

active muscle state

contracted

passive muscle state

relaxed

explain muscle arrangement

• antagonistic arrangment

• oppose one another

examples of antagonistic arrangement

• biceps and triceps

• muscles above, below, and to either side of eye, that allow full degrees of freedom

what does the number of muscle fibres vary across?

individuals

what does the number of muscle fibres change very little across?

lifetime, with either time or training- appears to be genetically determined

what is muscle size and strength dependent on?

cross sectional area of individual fibres and different proportions of the different types of fibre

how much does the number of muscle fibres between individuals vary?

lots, with lots of reasons why people have different amounts

impact of time and training on muscle fibres

• does not really change number an individual has

• the thickness of the muscle fibres- their individual size- tends to change

• also the type of muscle fibres tends to change

explain how muscles contract

• fibres arranged in a way where they are interlocking

• when contract, muscle fibres are interlacing, and walking towards eachother close up

• muscle fibres walk up against one another and close the distance between eachother, thus shortening the muscle

• actin and mysoin filaments, esp myosin head, are walking the muscle fibres against one another

myosin cross bridge cycle

• myosin head has little bend in

• head attaches to actin filament

• head bends at neck, which pushes it along actin filament

• molecules- calcium ions, magnesium ions, and atp (energy) releases

• pushes it along

• repeats

• molescules flying around enable this to happen

• action potentials

• causing bend in protein at end of myosin filament, causing it to change chape- bend in neck

• bend in beck supplying muscle contraction

whole process supplied by neurotransmitter acetylcholine

explain how muscles contract

release of ACh triggers biochemical cascade in muscle cells

molecules like atp, magnesium ions, and calcium ions release due to ACh

cycle of:

1. ACh releases calcium from inside fibre

2. myosin head changes shape

3. myosin head binds with actin filament

4. ions causes bend in protein at end of myosin filament

5. myosin head bends at neck

6. pushes itself along actin filament

7. release of things like ADP and Pi

8. pushes it along

9. atp breaks bond between myosin head and actin filament

repeats ^^^^

explain rigor mortis

1. release of ACh → release of calcium from inside muscle cell (fibre)

2. → myosin head change shape so it can → bind with actin filament

3. ATP is required to break bond between myosin head and actin filament, so it can happen again

4. ATP produced by oxidative metabolism, which stops upon death

5. so muscles become contracted and remain that way until enzymes begin to breakdown the actin/myosin

6. depleted energy and muscles stiff bc filaments stuck together

why does rigor mortis happen?

stiffness of body after death is bc no energy supply to muscle which means myosin head constantly stuck to actin filament bc of lack of atp released so lack of energy

what does atp do

provides energy for cells

what is atp produced by

oxidative metabolism

motor unit

average number of muscle fibres innervated by a single motor neuron

what do motor units vary by

two functional requirements for that muscle:

1. level of control

2. strength

what is typical for motor units in a muscle

a range of them, some with few and some with many fibres

size principle

motor units recruited in order of size (smallest first)

what type of control is typically required at lower forces

fine

fine control is typically required at…

lower forces

when would you have very small motor units

when precise, fine control is needed

when would you have very large motor units

when large amounts of force but not precise control is needed

explain how motor units work

motor neuron activated → fires action potential → ACh released

motor neuron is attached to X number of muscle fibres

they activate when innervated by their motor neuron, forming a motor unit

motor unit definition

a single alpha (lower) motor neuron and all the muscle fibres it innervates

innervation rule

each individual muscle fibre recieves signals from only one motor neuron

recruitment (Henneman’s Size Principle)

nervous system activates motor units in order from smallest (fine control) to largest (more force) ensuring smooth, graded contractions

less fibres in a motor unit means…

more precise

more fibres in a motor unit means…

more force

in the arm, it is useful to have…

different motor units

how are motor units recruited

into order of size

increasing force, starting with smaller motor units, going up and up