Appunti di Fisiologia (Human physiology, ENG)

LOCATION OF DESTINATION SITE OF

TRACT UPPER MOTOR (TERMINATION) DECUSSATION FUNCTION

NEURONS

CORTICOSPINAL PATHWAYS

Corticobulbar Primary motor Lower motor Brainstem Conscious motor

tracts cortex neurons of control of facial

cranial nuclei in muscles

brainstem

Lateral As above Lower motor Pyramids of Conscious motor

corticospinal neurons of medulla control of skeletal

tract ventral horns of oblongata muscles

the spinal cord

Anterior As above As above Level of lower As above

corticospinal motor neuron

tract

MEDIAL PATHWAYS

Vestibulospinal Vestibular nuclei As above None Subconscious

tracts (at the borders of (uncrossed) regulation of

pons and medulla) balance and

muscle tone

Tectospinal Tectum As above Brainstem Subconscious

tracts regulation of eye,

head, neck, and

upper limb

position in

response to

visual and

auditory stimuli

Reticulospinal Reticular As above None Subconscious

tracts formation (network (uncrossed) regulation of

of nuclei in reflex activity

brainstem)

LATERAL PATHWAYS

Rubrospinal Red nuclei of As above Brainstem Subconscious

tracts midbrain (midbrain) regulation of

upper limb

muscle tone and

movement

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The corticospinal tract (one ) is the

of the pyramidal tracts, the other being the corticobulbar tract

primary pathway that carries the motor commands that underlie voluntary movement. The lateral

corticospinal tract is responsible for the control of the distal musculature and the anterior

corticospinal tract is responsible for the control of the proximal musculature. A particularly

important function of the lateral corticospinal tract is the fine control of the digits of the hand, and in

general fine movements. 68

The rubrospinal tract is an alternative by which voluntary motor commands can be sent to the

spinal cord. It arises from the red nucleus. Although it is a major pathway in many animals, it is

relatively minor in humans. Activation of this tract causes excitation of flexor muscles and inhibition

of extensor muscles. The rubrospinal tract is thought to play a role in movement velocity, as

rubrospinal lesions cause a temporary slowness in movement. In addition, because the red nucleus

receives most of its input from the cerebellum, the rubrospinal tract probably plays a role in

transmitting learned motor commands from the cerebellum to the musculature. Red nucleus also

receives some input from the motor cortex, and it is therefore probably an important pathway for the

recovery of some voluntary motor function after damage to the corticospinal tract.

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The reticulospinal tracts (pontine and medullary) are a major alternative to the corticospinal tract,

by which cortical neurons can control motor function by their inputs onto reticular neurons. It arises

from reticular formation. These tracts regulate the sensitivity of flexor responses to ensure that

only noxious stimuli elicit the responses. Damage to the reticulospinal tract can cause excess of

sensitivity to harmless stimuli, such as a gentle touch, to elicit a flexor reflex. Reticular formation also

contains circuitry for many complex actions, such as orienting, stretching, and maintaining a complex

posture. Commands that initiate locomotor circuits in the spinal cord are also thought to be

transmitted through the medullary reticulospinal tract. Thus, the reticulospinal tracts are involved in

many aspects of motor control, including the integration of sensory input to guide motor output.

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The vestibulospinal tracts mediate postural adjustments and head movements, helping the body

to maintain balance. It arises from the vestibular nucleus. Small movements of the body are

detected by the vestibular sensory neurons, and motor commands to counteract these movements

are sent through the vestibulospinal tracts to appropriate muscle groups throughout the body. The

lateral vestibulospinal tract excites antigravity muscles in order to exert control over postural changes

necessary to compensate for tilts and movements of the body. The medial vestibulospinal tract

innervates neck muscles in order to stabilize head position during motion. It is also important for the

coordination of head and eye movements. 71

Little is known about the function of the tectospinal tract. It arises from superior colliculus, in the

tectum of the midbrain. Because of the nature of the visual response properties of neurons in the

superior colliculus (the optic tectum), it is presumably involved in the reflexive turning of the head to

orient to visual stimuli.

Motor cortex

The upper motor neurons in the cerebral cortex reside in several adjacent and highly interconnected

areas in the frontal lobe, which together mediate the planning and initiation of complex temporal

sequences of voluntary movements. These cortical areas all receive regulatory inputs from the basal

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ganglia and cerebellum via relays in the ventrolateral thalamus, as well as inputs from the somatic

sensory regions of the parietal lobe. The motor cortex can be divided into three areas:

1. Primary motor cortex. This is the main contributor to generating neural impulses that pass

down to the spinal cord and control the execution of movement. It can be distinguished from

the adjacent premotor areas both cytoarchitettonically (Brodmann’s area 4) and by the low

intensity of current necessary to elicit movements by electrical stimulation in this region.

However, some of the other motor areas in the brain also play a role in this function. It is

located in the precentral gyrus.

2. Premotor cortex. It is responsible for some aspects of motor control, possibly including the

preparation for movement, the sensory guidance of movement, the spatial guidance of

reaching, or the direct control of some movements with an emphasis on control of proximal

and trunk muscles of the body. Located anterior to the primary motor cortex

3. Supplementary motor area (SMA). It has many functions including the internally generated

planning of movement, the planning of sequences of movement based on prior experience,

and the coordination of the two sides of the body such as in bi-manual coordination. Located

on the midline surface of the hemisphere anterior to the primary motor cortex. It can be also

divided into:

• Posterior parietal cortex. It is sometimes also considered to be part of the group of

motor cortical areas; however, it is best to regard it as an association cortex rather

than motor. It is thought to be responsible for transforming multisensory information

into motor commands, and to be responsible for some aspects of motor planning, in

addition to many other functions that may not be motor-related.

• Primary somatosensory cortex. Especially the part called Brodmann’s area 3a,

which lies directly against the motor cortex, it is sometimes considered to be

functionally part of the motor control circuitry.

The primary motor cortex has particular multipolar neurons called Betz cells, also known as

giant pyramidal cells (due to their pyramidal shape of the soma), located within the fifth layer of

the gray matter in the primary motor cortex. Pyramidal neurons are the primary excitation units of

the prefrontal cortex and the corticospinal tract. 73

The gray matter of the cerebral cortex is divided in six layers. The pyramidal cells of cortical layer V

are the upper motor neurons of the primary motor cortex. Their axons descend to the brainstem and

spinal motor centers in the corticobulbar and corticospinal tracts, passing through the internal

capsule of the forebrain to enter the cerebral peduncle at the base of the midbrain. Then, they run

through the base of the pons, where they are scattered among the transverse pontine fibers and

nuclei of the pontine gray matter, coalescing again on the ventral surface of the medulla where they

form the medullary pyramids. 74

The components of this upper motor neuron pathway that innervate cranial nerve nuclei, the reticular

formation, and the red nucleus pathway (that is, the corticobulbar tract) leave it at the appropriate

levels of the brainstem. At the caudal end of the medulla, most of the axons (but not all) in the

pyramidal tract decussate to enter the lateral columns of the spinal cord, where they form the lateral

corticospinal tract. A smaller number of axons enters the spinal cord without crossing: these axons,

which comprise the ventral corticospinal tract, terminate either ipsilateral or contralateral sides of the

spinal cord, after crossing in the spinal commissure. The ventral corticospinal pathway arises

primarily from regions of the motor cortex that serve axial and proximal muscles. The lateral

corticospinal tract forms the direct pathway from the cortex to the spinal cord and terminates primarily

in the lateral portions of the ventral horn and intermediate zone. The indirect pathway to lower motor

neurons in the spinal cord runs, as already described, from the motor cortex to two of the sources of

upper motor neurons in the brainstem: the red nucleus and the reticular formation. In general, the

axons to the reticular formation originate from the parts of the motor cortex that project to the medial

region of the spinal cord gray matter, whereas the axons to the red nucleus arise from the parts of

the motor cortex that project to the lateral region of the spinal cord gray matter.

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The motor cortex has a somatotopic arrangement of the body musculature in the primary motor

cortex and somatosensory cortex, called homunculus. The most medial parts of the motor cortex are

responsible for controlling muscles in the legs; the most lateral portions are responsible for

controlling muscles in the face. The disproportional representation of various portions of the body

musculature in the homunculus represents parts of the body that exhibits fine motor control

capabilities (such as the hands and face) occupying a greater amount of space than those that

exhibit less precise motor control (such as the trunk).

Cerebellum

The cerebellum can be subdivided into three main parts based on differences in the source of input.

As a rule of thumb, each term is constituted by the suffix –cerebellum and the prefix indicating the

source of input signals:

1. Cerebrocerebellum. It is the larger of the three subdivisions and is concerned with the

regulation of highly skilled movements, especially the planning and execution of complex

spatialized temporal sequences of movement, including speech.

2. Vestibulocerebellum. As the phylogenetically oldest part of the cerebellum, this portion

comprises the caudal lobe of the cerebellum and includes the flocculus and the nodulus.

As suggested by the name, the vestibulocerebellum receives inputs from the vestibular nuclei

in the brainstem, and is primarily concerned with the regulation of movements, underlying

posture and equilibrium.

3. Spinocerebellum. It occupies the median and paramedian zone of the cerebellar

hemispheres and is the only part that receives input directly from the spinal cord. The lateral

part of the spinocerebellum is primarily concerne