Neuroscience 7a - Neuromuscular, spinal cord & Brainstem function

Anil Chopra

  1. Synapses, the neuromuscular junction and synaptic transmission.
  2. The motor unit, motor unit types, recruitment & trophism.
  3. Stretch reflex and its descending control.
  4. Flexion (withdrawal) and crossed extension reflexes.

Synapses

Synapses are found throughout the nervous system and allow contact between neurones and themselves or muscles.

The contact ratio (i.e. the number of neurones that are in contact with others) can range from 1:1 to 1: 1000.

Central synapses allow for multiple inputs to a single cell. They have 2 types of transmission at the post-synaptic terminal:

-EPSP: Excitatory Post-Synaptic Potentials

-IPSP: Inhibitory Post-Synaptic Potentials

The graded potentials involve summation to the threshold potential at which point an action potential is fired.

EPSP => closer to action potential firing

IPSP: => further from action potential firing (hyperpolarisation)

Net output = EPSP + IPSP

NeuromuscularJunction

This is specialised to allow axons to stimulate the contraction of muscle fibres. The structure formed by the axon and all the muscle fibres it stimulates is a motorunit.There are three types of muscle fibre there are three types of motor unit – all physiologically and functionally different. Muscles have different ratios of the three types of motor unit/motor fibre depending on their function. The ratios and numbers can be altered with training

The three types are:

S = slow, fatigue resistant (I) – Intermediate type, less mitochondria and capillaries than IIA, but more than IIB

FR = fast, fatigue resistant (IIA) – dense number of mitochondria and capillaries

FF = fast, fatigable (IIB) – few mitochondria, few capillaries, large and pale

FF are used for quick explosive movements and are what is developed during weight training

FR can be developed and trained and are found in people such as long distance runners and migratory birds

During voluntary movement you get early recruitment and de-recruitment of motor-units

Muscle force can therefore be regulated y the number of muscle units recruited

Motor units are recruited and de-recruited in the same specific order as the reflex drive is always S→FR→FF, FF only being recruited during maximal force

Muscle force can also be regulated by the firing rate of motor units – fused tetanus

Therefore muscle force is regulated by:

  • Recruitment of motor units
  • Rate of firing of motor units

NB: α motoneurons innervate muscle cells

End Plate Depolarisation

At rest there is a miniature end-plate potential of around 1mV of depolarisation which occurs constantly = isolated end plate depolarisation

Nervous activation in the NMJ leads to an all or nothing action potential which propagates along the muscle fibre

Arrival of action potential → depolarisation of pre-synaptic terminal → opening of voltage dependant Ca2+ channels and influx of C2+ → phosphorylation and alteration of presynaptic calcium-binding proteins → liberation of transmitter containing vesicle from presynaptic membrane → crosses cleft binds to receptor proteins on postsynaptic membrane → change in postsynaptic membrane potential leading to a miniature end-plate potential being formed → once hreshod is reached an action potential is generated.

The diagram shows the effect of neurotrophic factors on motor unit properties.

During Muslce contraction, the number of motor units recruited is changed according to the amount of voluntary force is needed:

Muscle force can also be altered by the frequency of motor unit firing:

Trophism: The specific nerve supplying a motor unit will influence its properties – neurotrophic factors. If the nerves of an S and FR unit are switched then the units will still work but their function will be altered, S will become faster and FR slower.

Reflexes

Reflexes can be:

Simple:

Monosynaptic i.e. 1 synapse, 1 afferent → 1 efferent

Basic structure: sensory neuron → motor neuron → output

Can be divergent or convergent but still only 1 synapse

Complicated:

Polysynaptic

Connection between afferent and efferent is not direct, afferent → intermediate → efferent

Use interneurons of the CNS and can have more than 1 efferent, and afferents

The Stretch Reflex

Also knows as the myotatic reflex. It is monosynaptic.

E.g. Knee Jerk

This is a postural reflex. A tap below the patella causes stretch of the quadriceps muscle. Stretch of the muscle causes muscle spindle afferents to discharge. Excitatory postsynaptic potentials in the in the alpha motoneurons cause discharge; the action potential travels to the neuromuscular junctions and causes the muscles to contract. It also results in the inhibition of the motoneurons supplying the antagonistic muscle (the lower thigh compartment). This is called reciprocal inhibition.

Higher centres in the central nervous system have both inhibitory and excitatory action on the stretch reflex neurons. Normally, the inhibitory action dominates. If there is a lesion which involves decerebration for any reason, it can lead to the excitatory action being revealed and can result in rigidity.

Facilitation from higher centres acts:

  1. On the motoneurone, increasing its sensitivity to afferent input, or
  2. Indirectly via gamma motoneurones and the muscle spindle, increasing afferent input to the alpha motoneurones.

Higher centres & pathways involved are:

Cortex – corticospinal,

Red nucleus – rubrospinal,

Vestibular nuclei – vestibulospinal, Recticular nuclei – recticulospinal.

Flexion Reflex

Also known as the withdrawal reflex, the flexion reflex is a polysynaptic reflex as it involves a number of different interneurons. Its role is protective, but it has greater central control and can be suppressed voluntarily.

Being a polysynaptic reflexes the different muscle groups on the different limbs work together. At the same time the crossed extensor reflex results in the stabilisation of posture, thus we don’t fall down!