Motor Physiology
By: Dr. Abdulrahman Aqra MD MSc
Thankful to the motor system, the living species sustain their needs and satisfy their instincts, and survive on this planet.
The sensory system inform us about changes in the surrounding environment, but our interaction with the surrounding environment still not enough without taking action expressed as motion.
Motion is a characteristic feature of life. All living species even the unicellular organisms move to seek food, escape danger. The multicellular organisms, involving the human also move to seek food, escape danger, or even to examine pleasure (dancing and sport competition for example). Thus the motor function is the first step in behavior.
Both central and peripheral nervous system are involved in motor functions. Motion to a great share is integrated with sensation, cognition, and emotion, so it is very difficult to view it as an independent function of the nervous system.
In this chapter we will summarize most important points in motor physiology. Here we are talking about summarization because discussing motor function of the nervous system is not that easy task keeping in mind that we still do not know a lot about this system, which is controlled by a highly sophisticated brain, with complicated enough afferent and efferent pathways, in addition to numerous skeletal muscle fibers.
Types of movement
There are two forms of movements:
1. Involuntary movement: which are manifested as reflexes on the level of spinal cord (spinal reflexes) and brain (cranial reflexes). This form involves reflexes like mastication, swallowing, micturition, stretch reflex, flexor reflex, corneal, pupillary light reflex, accommodation of lens of the eye, and many others. Although those reflexes are involuntary ones but usually our volition interferes with, for example: in adult human Micturition will not take place in unconventional situations due to the interference of the frontal lobe.
2. Voluntary (intentional) movements, which are manifested as motor skills.
Motor skills have to be learned and they develop by proceeding of time, for example the baby can set straight at 6 months of age, walk at 1st year, and jump at 2nd year...and so on. Our typing on keyboard, playing a musical instrument, dancing, praying are also motor skills.
Learning of motor skills has many stages, which includes
- Cognitive stage: emphasized as determining the goal of a given movement and determining strategies for that.
- Associative stage: determining the way to achieve the goal.
- Autonomous stage: this stage takes longer time, and means (automatically) doing the task (dancing for example).
Circuits for motor function: Motor function takes place by afferent receptors from muscle, tendon, and/or skins, stimulation of which generates action potential that will be transmitted to the motor areas in the central nervous system. In CNS interpreting and processing of nerve signals takes place to form a motor order that will be transmitted via efferent neurons (such as alpha motor neuron in spinal cord) to the peripheral skeletal muscles .Efferent motor neuron and all fibers it innervates are collectively called motor unit .
Motor functions can be aided either by inputs from sensory receptors (reflectively controlled) or by internal desire to move (volitional control). The first one is usually responsible for muscle movement related to posture, balance, and locomotion(active or passive locomotion).
The later are more involved in controlling motor skills.
Levels of organization of motor system:
level 1: (strategic level): Initiation, planning, and programming of movement are complicated tasks of the motor system. This task is accomplished by the basal ganglia and by the association areas of the neocortex.This level is influenced by sensory information ( visual, auditory, proprioceptive..etc). Such information is filtered to the basal ganglia and back to the new Cortex until the final motor decision is taken.
Level2: (tactic level):The Motor Cortex and cerebellum receive signals from the strategic level. The tactic for achieving the desired movement is determined here and translated to the next level.(( executive level).
Level 3: (executive level): The desired movement is then executed by exciting the brainstem and the spinal cord ( lower motor neurons).
Primary motor cortex (Brodmann 4) produces motor response with minimal electrical activation. It projects to lower motor neurons either directly (corticobulbar and corticospinal pathways), or indirectly via subcortical extrapyramidal tracts. It receives afferent information from cerebellum, premotor area, supplementary motor area, and cerebral sensory cortex, in addition to sensory inputs from muscle spindle.
Primary motor cortex which occupies Brodmann area 4 receives inputs from different brain region. It receives inputs from somatosensory cortex, from thalamus which transmits signals from cerebellum and vestibular nuclei in addition to somatosensory signal from proprioceptors. It is important to notice that the motor cortex also receives inputs from the corresponding motor cortex via corpus callosum.
It is not the only cortical region that is connected to spinal cord or to movement, but it is considered as a primary because it needs minimal electrical stimuli to excite its neurons. This means that these neurons have the lowest threshold to elicit movement by electrical stimuli.
Homunculus: Topographical representation of each body part being proportional to levels of motor organization in the cerebral cortex with the head represented laterally and the feet medially. As we can see in the figure the representation of the hand and speech muscles is the largest. This is due to the fact, that movement of hand and speech muscles are fine movements that are controlled as a (set of movements) not as a (set of muscle fibers), for more explaining let’s give an example
Lets imagine that a single motor complex has been excited, in a response; separate muscles will be excited in different way to cause a specific fine movement in the hand.
Histologically the primary motor cortex is agranular.
Neurons in the primary motor cortex are arranged in columns. Each neuron in different columns receives somatosensory inputs from different areas of the body. Excitation of each single neuron may excite different muscle groups.The layer V of the primary motor cortex involves very large neurons ( Pyramidal neurons). Layer V form the pathway by which the cortical motor neurons activate lower motor neurons.
Primary motor cortex encodes the force, direction, extent , and the speed of movement.
Premotor cortex (Brodmann area 6): responsible for what is known as (institutional movement). It is composed of two parts: anterior and posterior. The anterior one performed a motor image for the required movement, and then the posterior one excites the muscles that are involved in the required movement. The premotor cortex transmits signals to the motor cortex either directly or indirectly via the thalamus.
It signals the preparation for movement , and various sensory aspects related to movement. It is sensitive to behavioral contents of a particular movement .Premotor cortex can signal correct and incorrect
Supplementary motor cortex acts in concert with the premotor area, but the specific in supplementary motor area is that the contraction of muscles caused by stimulation of this area is bilateral rather than unilateral (ex. grasping of an object by the two hands). It responds to sequence of movements and to mental rehearsal of sequences of movement. It also involves in transformation of kinematic information into dynamic one.
The motor neurons in the nervous system are subdivided into upper and lower motor neurons: Upper motor neurons are those found in the motor cortex and basal ganglia, while the lower motor neurons are those found in the spinal cord and cranial motor neurons.
Clinical Physiology: Disorders of motor neurons cause different pathology:
- Disorders in upper motor neurons cause spasticity, hypertonia, and hyperactivity stretch reflexes.
- Disorders in lower motor neurons usually lead to flaccid paralysis, hypotonia, hyporeflexia, , areflexia, muscular atrophy, and fasciculation.
Distal projections: Direct and indirect
Descending projections from motor cortex to lower motor areas have two major pathways: Lateral pathway and ventromedial pathway. The lateral pathway is involved in control of voluntary movement of the distal muscle(direct cortical control), while the ventromedial tract is involved in control of posture and locomotion ( brain stem control)
- Pyramidal Pathways: involves:
- Corticospinal tracts: Fibers project from the neurons of the motor area to the lower motor neurons in the spinal cord. They originate from pyramid-shaped cells in layer V of the motor cortex. 31% of these neurons originate from primary motor cortex, while 29% originate from the premotor cortex and supplementary motor area. 40% of the corticospinal fibers originate from the primary somatosensory cortex. The fibers of corticospinal tract are subdivided into. Lateral corticospinal tract: forms 80% of the fibers which are numerous and cross the medullary pyramid to the contralateral side. They form monosynaptic connection with motor neurons involved in skilled movements. Anterior corticospinal tract form 20%, descend ipsilateral to the cervical and thoracic and then decussate to synapse with contralateral motor neurons.
- Corticobulbar tracts: Fibers that project from the motor neurons of the motor cortex to the motor nuclei of the cranial nerves involved in motor control of muscles of face and neck such as facial, trigeminal and hypoglossal nuclei . These fibers innervate the motor neurons bilaterally with some exceptions, for example, upper motor neurons to the hypoglossal nerve are only contralateral , while to the facial nerves are contralateral only to the lower quadrant of the face below the eyes.
II. Extrapyramidal pathway: Spinal motor neurons also receive fibers from the motor centers in the brain stem, which control involuntary muscle movement, muscle tone, and posture. They could be summarized as follow:
- Reticulospinal tract: from medulla oblongata and pons: The pontine reticulospinal fibers are primarily excitatory, while the fibers of medullary corticospinal tract are primarily inhibitory. Both of them connect to neurons at all levels of the spinal tract to control posture and muscle tone, especially fibers, connected to gamma motor neurons.
- Vestibulospinal tract: Involves medial and lateral tracts. The medial one originates from medial and inferior vestibular nuclei and descend bilaterally to connect to spinal motor neurons in the cervical spinal cord, that are involved in controlling neck muscles. The lateral one originates from lateral vestibular nucleus and descends ipsilaterally to connect to motor neurons in all levels of spinal cord in order to activate antigravity muscle that maintain posture and control balance.
- Tectospinal tract: originate from the superior colliculus and descend to the contralateral spinal cord to control eye movement and head movement.
- Rubrospinal tracts: originate in red nucleus and descend to the spinal cord to connect to interneurons and to contribute to controlling of distal limb muscle as the corticospinal. It is considered from the evolutionary point of view that in human the rubrospinal tract has been replaced by the corticospinal one. The fibers of corticospinal tracts also connect to interneurons of the spinal cord. This specific connection is important in modulation of muscular groups.
2. Basal ganglia:
As we mentioned above basal ganglia form collection of gray matter mass situated within the cerebral hemispheres. They translate intention and desire into movement. In addition to their motor role, they are also involved in cognitive and behavioral functions.
Basal ganglia involve:
- caudate nucleus, putamen and globus pallidus (both are called lentiform nucleus), dorsal striatum, parts of substantia nigra, amygdaloid nucleus,and the subthalamic nucleus.
- Neostriatum (caudate nucleus and putamen) receives information from the whole cortex. It receives mainly input from motor cortex via corticostriatal pathway and from thalamus via thalamostriate pathway.
- Striatum also receives dopaminergic input from pars compacta of substantia nigra, as sends GABA output to pars reticulata of the striatum.
- Striatum also projects to external and internal Globus Pallidus. In turn external Globus Pallidus project to the striatum.
- Globus Pallidus projects to the thalamus (inhibitory projection) , the thalamus projects to the motor cortex (excitatory projection).
- Substantia nigra projects to the thalamus.
A simplified connection between the cortex and basal ganglia could be presented as follows:
Cortex-> Striatum -> Globus Pallidus -> VLo of the thalamus -> supplementary motor areas of the cortex.
The input from the cortex to the striatum (putamen) is an excitatory one. While the input from striatum to the Globus Pallidus is an inhibitory. Globus Pallidus has an inhibitory effect on the VLo of the thalamus. The VLo of the thalamus has an excitatory input to the Supplementary motor areas(SMA).
At rest the the neurons of Globus Pallidus are spontaneously firing and thus inhibits the SMA
When there is an intention to move the excitation of the frontal cortex will excite the putamen which will inhibit the Globus Pallidus and thus release the inhibition from VLo and excite SMA.
The role of basal ganglia is not well understood, but researches reveal that they convert the processed sensory information into some form of programmed movements (planning and programming of movement). Basal ganglia-especially caudate nucleus- are also involved in some cognitive functions and memory.
Clinical physiology:
- Parkinson's disease: Degeneration of substantia nigra input to the putamen. Neurons in substantia nigra release dopamine that activates the cells of the putamen normally. Their degeneration will NOT release the thalamus from the Globus Pallidus induced inhibition. This will cause : Bradykinesia: (slowness of movement), akinesia ( inability to initiate movement) and rigidity ( increased muscle tone).
- Huntington’s syndrome: a hereditary caused by destruction of neurons of putamen , nucleus caudatus and the cortex.The thalamus will not be inhibited by Globus Pallidus ف and thus the patient suffer from hyperkinesia, dyskinesia, chorea and dementia.
- dystonia: A damage in basal ganglia that cause uncontrollable muscle contraction. It could be focal or generalized.Examples of focal dystonia includes: blepharospasm, oromandibular and others.
2. Coordination of movement: the cerebellum. The cerebellum has a motor memory which enables it to compare between intended movement descended from the cerebral cortex and the actual movement, occur by muscle to correct timing of muscle activation during complex movements. Cerebellum also by its motor memory is responsible for learning new movements. It contributes to maintaining of balance and postures, for this reason, patients with cerebellar lesion develop stereotyped postural strategies to compensate to this problem.
Recently, we believe that functions of the cerebellum extend the motor functions and involved in some cognitive functions- similar to basal ganglia- especially the cognitive function of language.
Cerebellum is located beneath the occipital and temporal lobes of the brain. Although it forms only 10% of the whole brain size but it contains more than 50% of the brain cells.
Gross anatomy: Cerebellum is subdivided into:
1. Cerebellar nuclei: Also called deep cerebellar nuclei form the only output from the cerebellum. For that a lesion in the nuclei has the same consequences as a lesion for the cerebellum itself. These nuclei are:
- Fastigial nucleus: receives inputs from the Vermis and the cerebellar afferents. It projects to the vestibular nuclei and reticular formation. This nucleus is most medially located.
- Interposed nuclei: receive from the intermediate zone of the cerebellar cortex and cerebellar afferents. They project to the red nucleus. They are located peripherally to the Fastigial nucleus.
- Dentate Nucleus: The largest nucleus of the cerebellum. It receives input from lateral hemisphere and cerebellar afferents, as projects to contralateral red nucleus and ventrolateral nucleus of the thalamus. Located lateral to interposed nuclei.
- Vestibular nuclei: Found in the medulla, but functionally involved in the cerebellum.
2. Cerebellar cortex: composed of anterior, posterior, and flocculonodular lobe, and the Vermis. The primary fissure separates the anterior and posterior lobe. The dorsolateral fissure separates the flocculonodular lobe.
The cerebellar cortex involves almost all of the cerebellar cells. The Vermis is located in the middle of the cerebellum, lateral to Vermis is called is called intermediate zones (bilaterally) , while lateral to the intermediate zones are called lateral hemispheres.
Histologically the cortex is subdivided into three layers that are distinguished by the cells they contain:
- Innermost layer (granule cells layer): contain granular cells. This layer contains most cells of the cerebellum and more than half cells of the brain.
- Middle layer (Purkinje cells layer): single cell layer. Purkinje cells have huge dendritic trees
- Molecular layer: the outermost layer: contains the axons of granule cells and dendrites of Purkinje cells.
- In addition to granule cells and Purkinje cells the cerebellar cortex also involves other cell types like: Golgi, basket, and stellate cells.
Inputs to cerebellum occur via two bundles of fibers:
- Excitatory fibers (Mossy fibers) : originate in pons, vestibular nuclei, reticular formation, and spinal cord and project to the cerebellar nuclei and granular layer of the cerebellar cortex.-Divergence: each fiber connect synaptically with hundreds of granule cells.
- Parallel fibers: connect granule cells to the cells in molecular layer and along their way connect to Purkinji cell with also high divergence (each parallel fiber connect to hundreds Purkinji cells(.
