Motor control 1 (LMNs, muscles, reflexes)

Question 1

What are the different types of movement?

Answer 1

Voluntary - walking, talking etc

Goal-directed - conscious, explicit, controlled

Habit - unconscious, implicit, automatic

Involuntary - some: eye movements, facial expressions, postural muscles, diaphragm, cardiac, digestive tract etc

Question 2

What are some examples of defensive reflexes and at what level they’re processed?

Answer 2

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

All with the goal of avoidance

Lower level inputs also feed into motor, autonomic and endocrine outputs

Question 3

What are some useless facts about muscles?

Answer 3

40% body weight

3 types - cardiac, smooth and skeletal

Smallest = stapedius; largest = gluteus maximus; strongest by weight = masseter

Hardest working - heart (3bn beats/lifetime), eye muscles (10,000 movements in 1hr of reading), neck (5kg head in place…)

Question 4

What does muscle size and strength depend on?

Answer 4

Cross sectional area of individual fibres + proportions of different types

Number of fibres varies across individuals but doesn’t change with time/training (genetically determined)

Question 5

What are some features about how muscles work?

Answer 5

Contract or relax

Activation of a fibre is all or none

Antagonistic arrangement = combined coordination

Recruitment = of different fibre types (fast/slow/small/large)

Question 6

What are the components of skeletal muscle?

Answer 6

Bone-tendon-skeletal muscle = several fasciculi (group of fibres/cells) = several myofibrils (actin/thin + myosin/thick myofilaments, perpendicular to the Z-line)

Question 7

What is sliding filament theory?

Answer 7

Initiated by calcium release

Myosin head bins to actin

Hydrolysis of ATP - conformational change of myosin head pulling the actin

Sarcomere (distance between two Z lines)
shortening due to sliding of filaments

Accessory proteins troponin/tropomyosin mediate Ca2+ levels

Question 8

What is a motor unit and what are some features of its activation?

Answer 8

Lower motor neuron + the fibre it innervates; LMNs may innervate multiple units

Neuron and its fibre are of the same type

Fibre types depends on the neuron

Size of unit i.e. number of muscle fibres innervated by a single α motor neuron depends on functional requirement of muscle i.e. level of control, strength, size principle – motor units recruited to an action in order of size, smallest first = gradual recruitment

Fine control often required at lower forces

The final common pathway

Question 9

What is the arrangement of alpha motor neurons in the spinal cord?

Answer 9

Ventral horn and output from the ventral roots

Proximal muscles are medial in the column whereas distal muscles are lateral

Motor neuron pools - all the motor neurons that go into innervating a single muscle

Cervical and lumbar enlargements of the spinal cord containing dense populations of neurons for innervation of upper and lower limbs

Question 10

How are cell bodies in the ventral horn activated?

Answer 10

Sensory feedback from the muscle itself (muscle spindles) + tendons (golgi tendon organs) - synapsing with interneurons

Descending information from the brain corticospinal tracts)

Question 11

What is a spinal reflex?

Answer 11

Involuntary physiological response to a stimulus ie withdrawal of limb

Unlearned/instinctive/unconditioned response

Role of muscle spindle feedback is key in stretch reflex and withdrawal reflex

Question 12

What is the stretch reflex?

Answer 12

Also known as the myotactic/deep tendon reflex (knee jerk etc)

Activation of the 1a fibre in the spindle upon stretching

1a fibre monoynapses (no interneurons) with the alpha motor neuron of that particular muscle in the spinal cord

Signal is excitatory so alpha motor neuron increases firing rate and muscle contracts

1a fibre also synapses with an GABAergic inhibitory neuron which then synapses with the alpha motor neuron of the antagonistic muscles which would inhibit the reflex response

The antagonistic muscle is inhibited and does not contract – allows for smooth movement

Example of reciprocal innervation

Question 13

What is the withdrawal reflex?

Answer 13

Crossed extensor reflex falls under this reflex - retraction to avoid stepping on a pin; also an example of reciprocal innervation

On the foot stepping on a pin - thigh flexors tense/extensors relax; synapses with contralateral flexors/extensor neurons in the spinal cord (having travelled through the ventral commissure) and produces the opposite picture, allowing you to stay standing

Question 14

How is the sequence and control of limb movements in quadrupeds governed?

Answer 14

Quadrupeds with spinal cord transections made to walk on treadmills (science is fucked) whilst supported shows coordinated limb movement is still possible and will change in character to fit with the speed of the treadmill - appears conscious but all dependent on muscle spindle stretch

Circuits supporting movements of this type are called central pattern generators (in the spinal cord) (can also be initiated cortically using the mesencephalic locomotor region)

Question 15

What are the different classes of muscle fibres?

Answer 15

Slow twitch (red fibres - myoglobin) = type one, oxidative, fatigue resistant

Fast twitch = fatigue rapidly but generate large peak of muscle tension: 2A type – gylcolytic and oxidative (intermediate – fast-fatigue resistant); 2B type – glycolytic (pale)

Question 16

What are the differences between axons of the medial local circuit neurons compared to the lateral?

Answer 16

Medial - supply LMNs in medial ventral horn, axons project to several spinal cord segments ie between cervical and lumbar enlargements - coordination of rhythmic movements or entire length of cord - management of posture; some contralateral control

Lateral region of the intermediate zone - shorter axons, typically extending fewer than 5 segments, mostly ipsilateral - finer, more differentiated control over muscles of distal extremities

Question 17

What is the relationship between firing rates of skeletal muscles and power?

Answer 17

Frequency is proportional to muscle tension/power - muscle fibres activated by overlapping action potentials leading to a summation of muscle contractions - the greater the frequency, the grater the force

At highest firing rates = ‘fused tetanus’ = tension produced in motor units no longer has peaks and troughs caused by individual twitches

Question 18

What are the two types of intrafusal fibres and their functions?

Answer 18

Nuclear bag fibres (dynamic vs static) and Nuclear chain fibres

Differ in nuclei arrangement, myofibril architecture and dynamic sensitivity to stretch

Muscle spindles = 2-3x bag + >4-6x chain

1a afferents coil around middle region of fibres forming annulospiral primary endings

2 afferents form endings on mainly nuclear chain fibres = ‘flower-spray’ endings

Question 19

How do 1a and 2 afferents respond to stretch?

Answer 19

1a - phasically to small stretches; transduced by the dynamic type nuclear bag fibres - sensitive to velocity of fibre stretch

2 - tonically to sustained fibre stretch, rate proportional to the degree of stretch using static type nuclear bag fibres

Question 20

What does the gain of the myotactic reflex refer to?

Answer 20

Gamma motor neuron activity = gamma bias/gain

In the myotactic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibres - gain high in a reflex = small stretch on intrafusal fibres will produce a large increase in number of alpha motor neurons recruited and a large increase in their firing rates, increasing the tension of the extrafusal fibres (converse for low)

High gain in rapid/precise movements

Question 21

Why is stretching important in sport?

Answer 21

Reduction of gain of the myotactic reflexes - facilitates lengthening of muscle fibres - less likely to damage when worked due to overly quick/powerful response to a change in muscle tension during sport

Question 22

What else effects the gain of the stretch reflex?

Answer 22

Level of efferent alpha neuron excitability

Local circuits in the spinal cord - excitation/inhibition of alpha or gamma interneurons - themselves influenced by UMNs + neuromodulation from brainstem/cortex

Inhibitory interneurons forming axo-axonal synapses on 1a afferent terminals

Question 23

How do lower motor neuron lesions present?

Answer 23

Paralysis, paresis/weakness, loss of reflexes, loss of tone, fibrilations and fasciculations (spontaneous twitches) and later - atrophy