Lower Motor Circuits

What is the motor neuron pool?

the last step before the innervation of skeletal muscles → they directly connect the CNS to the skeletal muscles

• are for reflexive and voluntary movement

What is the advantage of going straight to the motor neuron pool compared to local circuit neurons?

going direct results in much fast movement because its directly controlled by the cortex

What happens when degrees of freedom increases?

It means the control is more complex

What are the basal ganglia involved in?

• facilitate desired movement and inhibit unwanted movement

• can lead to Parkinson’s or Huntington’s if there is degeneration of basal ganglia

What is the role of the cerebellum?

• they can help predict and correct motor error

What are the different types of learning?

• error based learning → brain compares expected outcome vs actual outcome and corrects the error (cerebellum involved)

• motor learning → improving movement through practice and procedure

• adaptation learning → motor system adjusts to changes in the environment or body

• unsupervised learning → brain finds patterns or structure in input without external reward or explicit error signal

• reinforcement learning → learning based on reward prediction error (basal ganglia associated)

Where are motor neurons located and what do they innervate?

• they are located in the ventral horn of the spinal cord

• each motor neuron innervates muscle fibres within a single muscle (all motor neurons innervating a single muscle is called a neuron pool for that muscle)

Why is there an enlargement in the lumbar and cervical regions?

because they control the limbs (distal regions) which require a much larger number of motor neurons and sensory neurons

where are proximal motor neurons located compared to distal motor neurons in the ventral horn?

• proximal are for more coordinated control and less freedom → its closer to the actual body so its more towards the centre of the ventral horn

• distal are for more precise movement and have more freedom → towards the distal part of the ventral horn

What do lateral motor neuron pools control? medial? axial?

medial control axial muscles and postural trunk muscles

lateral control distal limb muscles

axial control deep trunk muscles

where are long distance local circuit neurons located? what do they control?

• in medial ventral horn

• control axial muscles (proximal muscles)

• go across several layers of the spinal cord

• control the position of the body and locomotion

• bilateral

where are short distance local circuit neurons located? what do they control?

• lateral ventral horn

• control distal limbs

• unilateral (stay ipsilateral)

• mediate fine control of distal limb muscles

what are the two types of lower motor neurons?

alpha and gamma motor neurons

what are gamma motor neurons?

• small diameter

• innervate specialized muscle fibres called muscle spindles

• sensory receptors embedded in intrafusal muscle

• sense sensory information about muscle length and speed of change in length

• they do not produce force

• spindle sensitivity

what are alpha motor neurons?

• large diameter

• innervate extrafusal muscle fibers

• in striated muscle they generate force for posture and movement

• each neuron branches widely and terminates on 10-100 individual muscle fibres → controls their contraction

What is a motor unit?

the ensemble of alpha motor neuron and the muscle fibres that it innervates

• one alpha and all the muscle fibres it innervates

Why are the muscle fibres spread out within a motor unit?

because if the muscle fibres were severed then all the fibres would lose control

what does a lower innervation ratio mean? higher innervation ratio?

• lower innervation ratio → for fine movement where control is more precise

• higher innervation ratio is for control of larger and less precise movements

why does size of the motor neurons matter? (size principle)

• small alpha motor neurons innervate fewer muscle fibres forming small motor units that generate small force

  • used when we are sitting where we don’t need a lot of force → just required to keep shape of body

• large alpha motor neurons innervate larger, more powerful motor units with greater force

  • when walking or running you need more force

• the more singles that are sent, the more likely the large motor neurons will get activated

• more stimulation → recruitment of larger, higher threshold motor units → more force

what are the three different types of speed related motor neurons?

1. Slow (S) → appear red and are rich in myoglobin and mitochondria → they are resistant to fatigue and important for sustained muscular activation → LOW FORCE, LONG SUSTAINING (standing)

2. Intermediate (FR) → innervate more moderate muscle fibres with greater force than slow but less than the fast ones → INTERMEDIATE FORCE (walking or running)

3. Fast (FF) → easily fatigued, low mitochondria → LARGE FORCE, SHORT SUSTAINING (jumping)

*recruited in this order

What is motor unit plasticity?

• changing of fibre type depending on use → firing patter strongly influences this

• activity-dependant ability of motor neurons and their associated muscle fibres to change their structural and functional properties in response to training, disuse or injury

What is a twitch?

it is a singe contraction produced by an action potential within the muscle fibre itself

• it produces a small force then goes back to 0

What is an unfused tetanus?

when motor neurons fire at high frequency but individual twitches are apparent

• it produces that initial small force but instead of going back to 0 the next action potential comes and produces another amount of force

What is a fused tetanus?

the highest rate of firing individual twitches are indistinguishable → you get sustained contraction without relaxation

• but this is also not healthy for the muscle

How is contraction actually sustained if fused tetanus is not healthy?

thought the unfused tetanus at high frequency stimulation

each unit is producing force in different phases → some produce less and some produce more force at the same time → when you combine them all together you get an average steady force

What are the two types of muscle fibres that skeletal muscle composed of?

• extrafusal fibers

• intrafusal fibers

What are extrafusal muscle fibres?

large diameter muscle fibres which produce movement via the large diameter alpha motor neuron axons that innervate them

• in parallel with spindle

• in series with golgi tendon organ

What are intrafusal muscle fibres?

• small diameter muscles that are much shorter than extrafusal fibres

• they are innervated by gamma motor neurons which regulate muscle length at all times

• 8-10 intrafusal fibres are grouped together in a connective capsule tissue called the muscle spindle

• the spindle is in parallel to the extrafusal muscle fibres

• intrafusal muscle fibers stretch on extension and shorten on muscle flexion

What are the two types of intrafusal fibres?

• nuclear chain

• nuclear bag

What are the two types of stretch receptors in muscle spindles?

• spindle primary ending = Ia afferents → they spiral up the bag and chain fibres and respond to small stretches and dynamic changes

  • form mono an poly synaptic excitatory connections on alpha motor neurons

• spindle secondary endings = II afferents → mainly on the chain fibres and respond to static

  • form polysnaptic connections on alpha motor neurons

What are the two types of gamma motor neurons that the muscle spindle is innervated by?

• dynamic → terminates on dynamic bag intrafusal fibres

• static → terminates on static bag and chain fibres

What is alpha-gamma co activation?

during voluntary movement the brain activates alpha motor neurons (movement) and at the same time gamma motor neurons are activated to keep the spindle sensitive during contraction

essentially when a muscle contracts, the extrafusal fibres shorten → if the gamma motor neurons were not activated at the same time, the intrafusal fibres would become sloppy and the muscle spindle would stop sensing signals about stretch

what is the stretch reflex?

• stretching a muscle spindle leads to increased activity in the Ia afferents and activity of alpha motor neurons that innervate the same muscle

• monosynaptic

reciprocal intervention:

• ispilateral extensor gets activated (agonist)

• ipsilateral flexor gets relaxed (antagonist)

What is the steady level of tension in muscles mediated by?

group II sensory afferents (also called muscle tone)

how do gamma motor neurons regulate the gain of the stretch reflex?

they adjust the level of tension in the intrafipal muscle fibres

What does the activation of both alpha and gamma motor neurons prevent?

prevents a decrease in Ia firing during muscle shortening slowing muscle spindles to operate at any muscle length

if the muscle spindle falls silent during contraction we won’t have a report on the length of the muscles

What is the Golgi tendon organ?

• important for reflex regulation of motor unit activity

• located at the junction of a msuscle and a tendon

• innervated by Ib sensory afferents

• they respond to force

What happens to the GTO when a muscle contracts?

the force increases tension in the collagen fibres in the GTO compressing the sensory nerve endings

• they are most sensitive to active muscle contraction not passive stretching

What happens to Ib afferents during contraction? (inverse stretch reflex)

• Ib sensory afferents sense high tension → synapse in the spinal cord on inhibitory interneurons → they then synapse on alpha motor neurons decreasing their activity (agonist)

  • prevents damage from excessive force

  • when muscle fatigues → less force → less inhibition → then you get more force again

• the same Ib sensory afferents synapse on excitatory interneurons → they then synapse on alpha motor neurons increasing activity of the antagonistic muscles

• at lower levels of force, the GTO maintain a steady tension level

What happens during passive stretching?

• muscle spindles signal rapidly

• GTO signal slowly

what happens during active muscle contractions?

• the spindle us unloaded (activity decreases)

• GTO increases firing rates

what is the flexion reflex pathway? (withdrawal reflex)

stimulation of nociceptors in the foot leads to withdrawal of the limb by excitation of ipsilateral flexor and inhibition of ipsilateral extensor

• contralateral limb does the opposite to compensate so you don’t fall over

What are rhythmic movements controlled by?

local circuits in the spinal cord

• central pattern generators

What happens with locomotion when you are walking or running?

its a cycle consisting of two phases:

• stance: extending muscle → limb that’s in contact with ground

• swing: flexing muscle → limb that’s off the ground

Speed is increased by completing a cycle more rapidly → reduce stance while swing remains relatively constant

how do we know that locomotion is organized by central pattern generators?

We know that locomotion is organized by central pattern generators (CPGs) from a combination of experimental evidence, mostly in animals, showing that rhythmic movements can occur without sensory input or brain signals

• Animals (like cats) with the brain disconnected from the spinal cord can still produce walking-like movements when supported on a treadmill.

• This shows the spinal cord itself can generate rhythmic patterns