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Introduction

  • “From the felling of a tree to the whisper of the wind, all are motor”
    • Everything you do involves the motor system
  • The nervous system functions:
    • Take information about the environment in o Process/store this information
    • Interact with the environment

Organisation of motor units

  • Basal ganglia and cerebellum
    • Involved in planning movements and feedback control
  • The nervous system functions:
    • Take information about the environment in
    • Process/store this information
    • Interact with the environment

Antagonist muscles

  • Flexion vs extension
  • Eg: elbow
    • Flexion – biceps contracts, triceps relaxes
    • Extension – triceps contracts, biceps relaxes
  • Hence you need coordination of the antagonist muscles: one to contract and one to relax to carry out movement

Contraction

  • The motor unit is a motoneuron and all the muscle fibres innervated by its axon branches
    • The smallest functional motor system unit
    • One motoneuron always has multiple muscle fibres
      • Each muscle fibre belongs to one motor unit (unlike in development)
      • Vary in size from 3 muscle fibres to over one thousand muscle fibres in each unit (i.e. each motor unit has a wide influence)
    • Extraocular motoneurons have 2-5 muscle fibres to allow fine control
  • Contractile force is regulated by:
    • The frequency of the action potentials in a motor unit (rate code) - e.g. tetanic contraction
    • The number of motor units activated (population code) – regulated by the cortex
  • The “size principle”
    • Small motor units are recruited first so as to allow the greatest range of forces possible, and thus the greatest control
    • Bijection between motor neurons and motor units: hence there are strong and weak motor units
    • Most efficient way is to recruit the small one first then the big one later, otherwise less scope to vary the force we can generate

The ventral horn

  • Motor neurons are organised according to their function
    • There is a topographic plan that organises the locations of control (like a map)
      • Even within one spinal level there is a topographic organisation, with motor ventral and sensory dorsal. Trunk = medial, and extremities = lateral. Flexors tend to be dorsal and extensors and ventral
  • Inputs to the lower motor neurons (only three types)
    • (1) Upper motor neurons – from the cortex (descending input)
    • (2) Spindle afferents – sensory (important in reflexes)
    • (3) Spinal interneurons – make synapses in the same area and provide local connections
  • Hence the wiring is easier so we have a grid map where we can choose what needs to be activated to have an effect.

Muscle spindle afferents and gamma motor neurons

  • Somatosensory
    • Proprioceptors allow us to know the position of our joints
      • Rely on sensory endings of the motor fibres – muscle spindles
    • In the exercise where you put fingers to nose/together, fingers are guided by proprioception
      • Nose to head is easier since there is the additional guidance by the vestibular system
      • With two arms there are many variables
  • In every muscle fibre (the extrafusal fibre) there is an internal intrafusal fibre
    • Extrafusal fibres are innervated by alpha motor neurons (big and fast), intrafusal by gamma motor neurons
    • Stretch receptors contain ion channels that are deformed by stretch and activate – connected to muscle spindles that provide sensory information to the spinal cord
  • Intrafusal have a contractile end portion and a non-contractile middle portion, so when they contract they stretch the middle portion (where all the sensory neurons are: primary annulospiral endings of afferent fibres)

Spindle afferents modulate their firing in response to muscle stretch

  • Spindle afferents are sensitive to the length of the muscle
    • phasic component: frequency of action potentials indicates the rate of stretch (d(length)/d(t))
    • tonic component: frequency of action potentials indicates the steady muscle length; steady state (e.g. shorter muscle length = lower firing rate than longer muscle length)

Alpha and gamma motor pathways

  • purple = gamma motor neuron to intrafusal distal ends
  • blue = alpha motor neuron to extrafusal muscle
  • green = spindle afferent from middle of intrafusal
  • gamma motor neuron doesn't get spindle input, only the alpha motor neuron does
  • Although we could just use the alpha motor neuron for everything, in practice we don't. In slow movements, we use the gamma motor neurons, because it can drive what the alpha motor neuron does through this loop. For ballistic movement (fast movements), we activate both pathways together.
  • (1) Sensory (afferent) input from the muscle spindle fibre
  • (2) Alpha motor neuron output to extrafusal motor fibre
  • (1-2) The stretch reflex pathway
  • (3) Gamma motor neuron output to the intrafusal muscle spindle fibre
  • (4) Cortical pathways leading to the alpha and gamma motor neurons
  • Note:
    • Sensory receptor register stretch regardless of muscle length
    • Gamma neurons maintain the spindle stretch and thus the level of muscle activity
  • Process:
    • gamma motor neurons and alpha neurons receive innervation from the cortex
    • gamma neurons contract/relax the intrafusal muscle spindle fibre
    • afferent inputs from the sensory ends of the muscle spindle fibre input on the alpha neurons
    • alpha neurons contract/relax the extrafusal skeletal muscle accordingly
    • as the extrafusal skeletal muscle fibre extends, the spindle is stretched and afferent fibres fire faster causing alpha neurons to contract the extrafusal fibre

Alpha gamma coactivation

  • If alpha motor neurons cause a contraction, there is reduced firing of the spindle afferent (bad because it isn't telling us what the muscle is doing; the spindle afferent is telling us what contractile state our muscle is)
  • Hence we need to contract both alpha and gamma so that we maintain our signal from the spindle afferents
  • If you lose your spindles, it's almost impossible to walk or do any task (the feedback is essential)

Golgi tendon organ

  • Lies in series with the extrafusal muscle; wrapped around the tendon itself; if the extrafusal muscle fibres contract, we experience the force of contraction in the tendon organ. While afferent spindle fibre is in parallel, the golgi tendon organ is in series
    • Signals muscle tension
    • Afferent is sometimes called 1b based on its diameter (same way the spindle is sometimes called 1a)
      • If there is stress on the tendon, there is increased firing which causes shortening of the muscle being stressed and activation of the antagonist muscle
    • Known as the ‘safety fuse’
      • Stops tearing of muscles

Reflexes

  • A reflex is:
    • Rapidly executed
    • Automatic (unconscious)
    • Stereotyped – roughly the same every time (i.e. on the same side)
  • A reflex may be:
    • Graded in strength – knee jerk, kick proportional to force applied (stronger stim = bigger response)
    • Subject to conscious modulation – can voluntarily be overridden
  • A reflex arc has:
    • Sensory receptor (responds to stimulus)
    • Afferent neuron
    • Integration centre
    • Efferent neuron
    • Effector organ (produces response)

Myotatic reflex with reciprocal inhibition

Reflex.png

  • If you tap the tendon, the quads are stretched slightly
    • This rapid change in length is fast, thus causes a reflex (muscle spindles detect rate of change as well as length)
  • (1) a monosynaptic reflex – excitatory
  • (2) inhibition, less contraction of the antagonist muscle (biceps)
    • ie: the reciprocal reaction, contraction and relaxation of the antagonist
  • the stubby of beer reflex
  • Use of patellar tap reflex: feedback loop (look at the loop - you have a set point for muscle length; allowing you to stand up etc without conscious input: if you fall one way, reflex is activated)

Golgi tendon organ reflex

  • Inhibits own motor neuron and also has a reciprocal set-up (opposite to the myotatic reflex: this activates the opposite muscle and inhibits its own muscle)
  • Paralysis:
    • Flaccid paralysis
    • Clasp-knife/cogwheel paralysis – resistance
      • Muscle is contracting because spindles are opposing the change in length
      • If you push hard enough, the golgi tendon kicks in and the muscle collapses to prevent damage
  • This is a protective function: if the muscle is exerting more and more force (e.g. hold something and then something else is dropped on it), then golgi tendon organ reflex kicks in and stops muscle tearing, so we've got
  • Most obvious in patients with spasticity, they have no higher control, only reflexes
    • When you try to move their arm, it will be stiff because the spindle afferents will be stretched (contract the muscle to oppose change in length). Without central control, they are hyperreflexive. If you push hard enough, the golgi reflex gives way (tension gets high enough)
      • Hard to fold first, then finally slots into place easily - clasp-knife response

Flexion/withdrawal reflex with crossed extensor reflex

  • Triggered by a pain sensor (nociceptor)
  • (1) excitation
  • (2) reciprocal
  • (3) excitation of extensors to maintain balance – crossed component
  • (4) reciprocal
    • + opposite upper limbs for balance
  • Elaborate reflex: it also crosses the spinal cord to cause extension of the contralateral side (as one leg is lifted, then the other one will be extended to maintain balance)
    • There is also extension of right arm - for four-legged animals (vestigial reflexes with diagonal stance)
      • Hence the reflex not only crosses the spinal cord, it also can ascend and descend
  • The same signals underlie walking, because of reflexes (no nociceptor involved; it's driven by spindles and golgi. This feeds into a similar reflex network in the spinal cord to cause the coordinated movement needed for walking. Hence we don't need to waste brain space on these functions - it's done in the spinal cord)