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Neuronal networks and behaviour summary movement
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Movement chapters 16, 17, 20 & 21Learning objectives: Basal ganglia
• Explain how stretch reflexes work The basal ganglia are embedded in the forebrain and prevent
• Describe the anatomical organisation of spinal innervation of upper motor neurons from initiating unwanted movements and
muscles prepare the motor circuits for the initiation of movements.
• Explain how saccadic eye movements are controlled > Parkinson’s disease and Huntington’s disease are disorders
• Explain how walking is controlled associated with the basal ganglia.
• Describe the anatomical organisation of upper motor neurons
• Explain how voluntary movements are initiated Types of movement:
• Voluntary
The neural centres responsible for the control of movement can • Involuntary
be divided into four subsystems. • Rhytmic
1) Lower motor neurons • Learned movements
2) Upper motor neurons • Habitual movements
3) Cerebellum
4) Basal ganglia Ballistic control of movements
In ballistic control, there is a controller, which in this case is a
Lower motor neurons neuron, and a plant, which in this case are the muscles. What is
These are located within the gay matter of the spinal cord and fed into the controller is a desired result, motor commands are
the tegmentum of the brainstem. They send their axons to the sent to the plant, and something happens in the real world. The
brainstem and spinal cord to innervate skeletal muscles of the downside of ballistic control is that it is very prone to noise, for
head and body and the local circuit neurons. The local circuit example when the weight of something changes, or you get hit
neurons receive sensory input as well as descending projections while moving.
from higher centres and the circuits they form provide
coordination between different muscle groups that are essential
for organised movement.
> Even after the spinal cord is disconnected from the brain in an
experimental animal, appropriate stimulation of local circuits in
the isolated spinal cord can elicit involuntary but highly To tackle the problem of noise, the ballistic control can be
coordinated limb movements that resemble walking. improved with parametric adjustment (learning). This makes use
of feed-forward, where you take a readout of the noise, and
Upper motor neurons feed-forward it to the controller so that the controller can
The cell bodies of upper motor neurons are located in the adjust.
brainstem or the cerebral cortex. They send their axons to What is also possible is to give feedback. This way a comparator
synapses with the local circuit neurons or with the lower motor can be introduced. You see the result and then you learn from
neurons directly. The pathways that arise in the cortex are feedback and adjust the parameters, so next time you can get a
essential for the initiation of voluntary movements and complex better result.
skilled movements. Upper motor neurons originating in the
brainstem are responsible for regulating muscle tone and
orienting the eyes, head, and body concerning vestibular, somatic,
auditory, and visual sensory information. Their contributions are
also critical for basic navigational movements and the control of
posture.
Cerebellum
The cerebellum sends its axons to the upper motor neurons and
can detect a difference or “motor error” between an intended
movement and the movement performed. The cerebellum Another way to do this is by feedback guidance. This way the
mediates both real-time and long-term reductions in these motor desired result goes into the comparator, the error gets corrected,
errors, which could be described as a form of motor learning. and then send into the controller. This is how the muscles work!
> Patients with cerebellar damage exhibit in coordination with
errors in controlling the direction and amplitude of ongoing
movements.
, The stretch reflex Rate coding
There are sensors in the muscles that sense stretch and send You can see two forms of rate encoding and spacial coding. Each
inputs into the spinal cord through the dorsal root ganglion and of the lines represents a neuronal fibre. You can also see that
then directly to the alpha motor neurons that control the tone of there are more motor units recruited with higher firing rates to
the muscle. An example of this is when you hold a cup and you generate more force.
put soda into it, the muscle is stretched due to the extra weight, The lowest-threshold motor units generate the least amount of
which activates the neurons to tone the muscle to bring the cup voluntary force and are recruited first. As the individual
up. generates more and more force, both the number and the rate of
Another example is the knee-jerk reflex, where a tap of the firing of active motor units increase.
hammer on the tendon stretches the muscle.
The organisation of the spinal cord
There is also a topographical map of the spinal cord. Alpha motor
neurons are located in the grey matter in the ventral root of the
The motor unit spinal cord. Sensory neurons come in via the dorsal root into the
The motor unit is composed of alpha motor neurons, which are the spinal cord and then the motor neurons come out through the
lower motor neurons, that innervate muscles directly. They have ventral root. Meaning that the sensory neurons are on the dorsal
neuromuscular junctions, so when the neuron fires the side and the motor neurons on the ventral side. The visceral
acetylcholine in the vesicles is released and binds to the nicotinic neurons are in the intermediate region.
receptors on the muscle. This causes an influx of calcium and
contraction of the muscle.
> One alpha motor neuron innervates multiple fibres and is
controlled by upper motor neurons. This arrangement reduces the
chance that damage to one or a few alpha motor neurons will
alter the muscle’s action.
Also, the motor neurons in the lateral part of the ventral horn
3 different types of motor units can be distinguished by their are more for distal muscles, such as the fingers. Closer to the
response to a single action potential. medial part of the ventral horn are motor units that are more for
1) Slow motor units the proximal muscles.
2) Fast fatigable motor units
3) Fast fatigue-resistant motor units
Slow motor units
The smaller motor units contain red muscle fibres that contract
slowly and generate small forces. They also contain a lot of
myoglobin, mitochondria and capillary beds, and are therefore Spinal cord anatomy
resistant to fatigue. These motor units are important for activities The cervical and lumbar spinal cords are thicker because these
that require sustained muscular contraction, such as maintaining areas contain more grey matter. This is because these areas are
an upright posture. connected to our hands and feet.
Fast fatigable (FF) motor units
These larger motor neurons innervate muscle fibres that have
sparse mitochondria and are easily fatigued. These motor units
are important for exertions that require large forces, such as
running or jumping.
Fast fatigue-resistant (FR) motor units
These motor units have properties in between the other two.
They are of intermediate size.
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