Control of Movement

One of the principle functions of generating internal representations of the world is to guide movement. Even simple actions require sensory information about what and where our actions are. Proprioreceptive information is needed.

 

The following is a general description of how movement is controlled:

Special types of movement:

 

 

Muscle Tone

tone is passive resistance to movement

 

 

Hypertonia

 

Decreased inhibition of upper motor neurons

 

 

spasticity

rigidity

 

Hypotonia

Also known as flaccidity

normally due to lower motor neuron disease, but can also follow acute sponal cord injury for a few days (spinal shock)

 

 

Planning and Initiation

The purpose of movement is determined by the dorsolateral frontal cortex.

 

Motor plans are thought up by the posterior parietal and premotor areas. The premotor cortex specifies spatial characteristics of movement based on sensory information from the parietal cortex.

 

The spatiotemporal details of muscle contractions required to execute planned actions are coordinated by motor circuits in the spinal cord.

 

 

primary motor cortex

pre-motor cortex

supplementary motor cortex

following stroke in some areas, volitional control is gone and someone can't smile symmetrically if told, though a joke will cause a symmetrical smile

 

 

Processing

 

Many structures are involved with processing motor control. These include:

 

Spinal Output

 

Descending motor tracts can synapse in the spinal cord, mediating activity there, or exit the central nervous system through the ventral horn to innervate muscle.

 

 

corticospinal tract

Motor commands for fine, learned, complex movements (ie finger/hand) arise in the motor cortex (30%); pre-motor cortex (30%), and parietal (somatosensory) cortex (40%), and descend through the corticospinal tracts.

Axons descend ipsilaterally through white matter of the internal capsule and the cerebral peduncles of the midbrain, breaking into fascicles as they enter the basilar pons. They then descend through pyramidal tracts of medulla, the site of decussation of the lateral corticospinal tract (85%), which extend the length of the spinal cord. The other 15% do not cross and descend in the anterior corticospinal tract. These upper motor neurons synapse in the ventral horn upon α motor neurons, which exit along the ventral root.

 

clinical correlate: sectioning the corticospinal tract in a monkey leads to a few days of finger dexterity loss, but after a few weeks there is little to show for it.

 

corticobulbar tract

The corticobulbar tract carries motor information from the cortex alongside the corticospinal tract, terminating on the reticular formation and on motor nuclei of the cranial nerves.

 

medial longitudinal fasciculus

The MLF is responsible for coordinating head and eye movements. It begins in the vestibular nuclei and runs along the midline of the brainstem to the cervical spinal cord, where it synapses on interneurons. Some fibres end in the abducens, trochlear, and oculomotor nuclei.

Problems in the MLF can lead to internuclear ophthalmoplegia, where horizontal gaze is disrupted in one eye past midline.

 

rubrospinal tract

arises in red nucleus in midbrain, which receives input from deep cerebellar nuclei and motor cortex

 

 

reticulospinal tract

projections from the cortex to the reticular formation; descends in lateral column

 

vestibulospinal tract

arises in vestibular nuclei, which receives input from the vestibular nerve and cerebellum

 

tectospinal tract

arises from cells in the superior colliculus in the midbrain

 

 

 

Motor Neurons

 

Descending pathways synapse in the medial and lateral alpha motoneuronal nuclei. Medial nuclei are found at almost every level, while lateral nuclei are prominent in the upper and lower limb cord enlargements.

There is somatotopy here, with flexors being more anterior and extensors being posterior.

 

Motor neurons are massive cells with a huge dendritic tree. They reside in the brainstem or the ventral horn of the spinal cord. Each lower motor neuron innervates muscle fibres of the same type.

All the neurons that innervate a single muscle form a motor neuron pool.

 

Upper motor neuron - used clinically to indicate a population of neurons with oligosynaptic input to lower motor neurons, the loss of which leads to relatively speicific motor findings. Usually refers to cells in the brain, but can also include those of the spinal cord.

Lower motor neurons originate in the brainstem or spinal cord; innervates muscles

 

Distinguishing Upper vs Lower Motor Neuron Problems:

 UMN LMN
muscle bulk normal atrophy
muscle tone increased (spasticity) normal or decreased
tendon reflexes increased normal or decreased
Babinski positive normal
other - fasciculations

 

 

 

 

 

loss of neurons (polio, ALS, etc) causes remodeling of the motor units. Giant single contractions are seen, but reduced interference patterns

 

 

Motor Units

Each motor neuron commands a group of skeletal muscle cells. Each muscle cell belongs to only one motor unit, which can vary in size depending on muscle function. Extraocular muscles can have just a few fibres per unit, while some leg muscles can have several thousand muscle fibres per motor unit.

 

 

Central Pattern Generators

Motor programs are neural circuits set in place before movement begins that can be sent to muscles with appropriate timing. This removes the need for sensory feedback during a sequence of movements.

The fact that a chicken with its head cut off will run around shows that central pattern generators are present in the spinal cord.

 

Central pattern generators mediate behaviours that include walking, running, breathing, chewing, certain eye movements, and even scratching. Central pattern generators for breathing reside in the brain stem, while those for locomotion are present in the spinal cord. Initiation and speed are controlled by the brainstem.

 

How are Central Pattern Generators Built?

 

At the core of CPGs is a set of cyclic, coordinated timing signals generated by a cluster of interconnected neurons. These affect as many as several hundred muscles that precisely contract or relax during a movement.

 

To achieve coordination among the limbs, sets of CPGs are interconnected. They are also very flexible, turning on or off as needed or adapting to variables such as terrain.

 

There is no single way to generate rhythmic patterns, and different mechanisms are used. The simplest are single neurons with membrane properties making them pacemaker cells, similar to cardiac nodal cells. These cells are embedded within interconnected circuits.

 

Other, more complex models of pattern generation include the half-centre model, which suggests two excitatory interneurons are connected to reciprocally inhibit one another. This leads to rhythmic cycling from one center to another.

 

Feedback and central pattern generators

Sensory feedback, in the form of stretch receptors, acts to terminate activity on one side and initiate it on the other. Spinal networks also reciprocally communicate with the brainstem to maintain control capacity in higher centres.

 

 

 

To Sort

monoamines modulate how motor neurons fire, in order to change the signals which are required to maintain muscle strength

injection of DOPA increases motor neuron firing frequency, as well as sustained firing after signal has been removed.

 

 

Stimulation of the MLR activates descending mono-adrenergic pathways, causing increased force in the 1B reflex (golgi tendon)