Download Neuroscience 7a – Neuromuscular, spinal cord

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
Neuromuscular Junction
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 motor unit. 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!