Motor system

Fall 2002

The motor system refers to those parts of the nervous system contributing to movement. We will start with a description of the muscles and work back to the brain.

There are two major types of muscle:

1. Striated muscles. There are two types: a. Skeletal muscles move the bones
b. Cardiac muscle is found exclusively in the heart 2. Smooth muscle lines the intestines and other organs to move substances through the body The only thing muscles can do is to contract. That is, muscles pull things in or relax. They can not push. These contractions are controlled by the nervous system. In particular, skeletal muscle contracts when motor neurons leaving the spinal cord or medulla release ACh on the muscle. This connection between motor neurons and muscles is called the neuromuscular junction.
 

Bones and muscles work like levers. Skeletal muscles work in pairs to move the levers. Contraction of one muscle causes antagonistic muscle to relax. To do this, muscles are organized as (Figure 13.2):

Flexors which bring bones together (e.g. bicep); and
Extensors which extend bones (e.g. tricep)
  For example, muscles can make your arm do three things:  
1. Make your bicep contract. Bicep pulls the forearm up.
2. Make your tricep contract. Tricep pulls the forearm down.
3. Make bicep and tricep contract. Holds forearm rigid. Two types of neurons in the ventral horn of the spinal cord coordinate flexors and extensors. Motor neurons send axons to the muscles (Figure 13.3).
Interneurons coordinate the activity of the motor neurons.

These neurons allow the spinal cord to coordinate movements even without a brain.


The simplest reflex circuit is the stretch reflex (e.g., the knee jerk reflex). This is called the monosynaptic reflex because it only requires one synapse in the CNS. 1a afferents from the muscle synapses on motor neurons which project back to the same muscle causing it to contract (Figures 13.16 & 13.17). At the same time the antagonistic muscles are inhibited. This is made possible by 1a afferents activating interneurons in the ventral horn that inhibit antagonistic muscles (Figure 13.23).
 

There are many other automatic reflexes:

Pain reflexes--lift one leg and plant the other (Figure 13.24).
Walking--no thinking necessary once initiated, just reflex circuits. The brain allows a greater complexity of movements beyond reflexes.

Primary Motor Cortex (M1) is located at the posterior border of the frontal lobe (also called the precentral gyrus; Figure 14.7). Pyramidal cells, located in layer V of M1 cortex, project all the way to motor neurons in the ventral horn of the spinal cord (Figure 14.3). This pathway is called the corticospinal tract (also called the pyramidal system).

In the medulla, 80% of the fibers cross to the contralateral body. These neurons control movements of the extremities (arms & legs).

20% of the neurons stay on the ipsilateral side. These neurons control the axial muscles (torso)
 

 Electrical stimulation of M1 cortex causes muscle twitches on the opposite side of the body (Figure 14.8). However, movements require coordination of many muscles. This organization is carried out by the premotor cortex. The premotor cortex is located just anterior to M1 in the frontal lobe and provides a major projection to M1. The premotor cortex prepares complex movements, but does not send this message to M1 until it receives a "go" signal from the basal ganglia.
 

The Basal Ganglia are a collection of subcortical nuclei that contribute to movement (figure 14.11). It includes:

The substantia nigra is located in the midbrain. These neurons project to and excite neurons in the caudate nucleus by releasing DA.

The caudate nucleus, along with the putamen and internal capsule, is part of the striatum. Neurons in the caudate nucleus inhibit neurons in the globus pallidus

The globus pallidus inhibits neurons in the ventrolateral thalamus.

The VL thalamus sends the "go" signal to the premotor cortex so that planned movements can be executed.
 

A linear view of this circuit would look like (+ and - indicates excitatory and inhibitory connections):
substantia nigra + caudate nucleus - globus pallidus - VL thalamus + premotor area + M1 cortex + spinal ventral horn This is an important circuit because it explains a number of diseases.

A. Parkinson's disease (see page 390)

Symptoms: difficulting initiating movements, tremors at rest, rigidity
Cause: The death of substantia nigra neurons causes a decrease in DA in the caudate nucleus.

Treatment:

  1. Administration of L-DOPA can help boost DA levels, but this is a temporary treatment associated with many side effects.
  2. Lesion the globus pallidus. In Parkinson's Disease caudate nucleus neurons are less active because of the loss of DA input. Because the caudate nucleus normally inhibits the globus pallidus, this lack of activity means that the globus pallidus is overactive. Destroying these neurons reduces this activity, which allows the VL thalamus to be active thereby alleviating some of the symptoms of Parkinson's disease.
 B. Tardive Dyskinesia Symptoms: Spontaneous uncontrolled movements, typically of the mouth.
Cause: Develops in some people after prolonged block of DA receptors (DA antagonists are used to treat schizophrenia). Prolonged block of DA in the substantia nigra to caudate nucleus appears to produce a supersensitivity to DA. That is, caudate nucleus neurons become hyperactive causing spontaneous movements.
Treatment: Switch patient to more selective DA antagonists. The Cerebellum also contributes to movement (Figure 14.17). The cerebellum contains 50% of the neurons in the brain (over 50 billion)
The cerebellum receives a huge input from the somatosensory and motor cortex (20 million axons) The cerebellum coordinates the direction, timing & force of movements. It also corrects movements by timing the sequence of muscle activation. Damage causes a loss of coordination. Joints move individually instead of as part of coordinated movement.

Many movements happen too fast for feedback. Cerebellum predicts movements based on past experience. This is called motor learning (e.g., writing with your non-preferred hand).
 

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