Assignment for lecture 4 (motor control)
Please fill in the blanks. In some cases there may be two or more options, and in this
case circle the correct one.
A motor unit is the smallest part of a muscle that can be independently controlled is
the group of muscle fibres connected to one motor nerve fibre. It consists of a single
motor nerve fibre, together with the group of muscle fibres that it controls.
Motor units are of several types and can be grouped into two broad groups named fast
and slow based on their rate of contraction. Their characteristics are:
Fast:
Strength of contraction (large)
Susceptibility to fatigue (susceptible)
Metabolism (mostly anaerobic)
Found in larger numbers in which kind of athlete (sprinter)
Slow:
Strength of contraction (small)
Susceptibility to fatigue (resistant)
Metabolism (mostly aerobic)
Found in larger numbers in which kind of athlete (marathon runner)
Aerobic metabolism is a very efficient way of converting glucose into energy in the
form of ATP. It generates 36 molecules of ATP per molecule of glucose, compared to
only 2 in the case of anaerobic metabolism. Aerobic metabolism takes place in
organelles called mitochondria. The waste product of aerobic metabolism, carbon
dioxide, is removed quickly from the muscle in the circulation; the main waste
product of anaerobic metabolism, lactic acid, remains in the muscle and this leads to
pain because it lowers the pH (low pH is a strong stimulus for some nociceptors; see
lecture 3).
We control the force generated by a muscle in two ways. Firstly, we can vary the
number of motor units that are active. Each motor unit adds an increment of force to
the total muscle force. The other way that we control muscle force is by varying the
frequency of action potentials. If this is low, i.e. there is a long gap between action
potentials, the motor unit can relax completely between “twitch” contractions and not
much force is generated. When action potentials become more frequent, so that
relaxation between them is incomplete, the “twitches” start to add. At very high rates
of activation, the fusion of twitches leads to a state called tetanus, not to be confused
with the disease of the same name.
The simplest movements are reflexes, and the simplest of these is the knee jerk reflex;
it can be elicited by tapping on the patellar ligament, just below the kneecap. The
afferent (sensory) arm of this begins with the sensory receptor in the quadriceps
muscle that detects stretch of the muscle, which is called the muscle spindle. The
sensory axon arrives in the spinal cord and makes a synapse with a motor neurone
which returns to the same muscle. Activation of the muscle stretch receptors thus
causes the muscle to contract.
Another reflex, with a protective function, involves the Golgi tendon organ, situated
between the muscle and tendon. This detects muscle force and, if muscle force
becomes excessive, the reflex inhibits motor neurone activity. This is possible
because the reflex involves an inhibitory interneurone (a neurone that connects two
other neurones) in the spinal cord.
A third reflex is the withdrawal and crossed extensor reflex. When we stand on an
object that causes pain, the reflex activates (flexor) muscles in the same leg, and
inhibits (extensor) muscles in that leg. This causes us to lift the leg off the ground. In
order to support our weight on the other leg, the other leg must be stiffened, and this
is done by activating (extensor) muscles in the other leg.
Walking involves alternation between two phases: the stance phase, which involves
activation of (extensor) muscles, and the swing phase, which involves activation of
(flexor) muscles. The part of the central nervous system that generates the walking
rhythm is the spinal cord.
Four brain structures involved in movement are the motor cortex, cerebellum,
brainstem and basal ganglia. Initiating movement as well as ensuring muscle
relaxation at rest are the functions of the basal ganglia, and damage to this area results
in Parkinson’s disease. The motor cortex is responsible for generating the “motor
plan” that defines the whole of a movement, and the cerebellum receives a copy of the
motor plan and compares it with sensory information about what actually happened,
in order to correct the motor plan while it is in progress, and refine it for future
occasions.