In this article we will discuss about the role of motor neuron in central nervous system.
The movements of various parts of body should not only be executed, but also must be very much coordinated. It is not just voluntary movement that needs to be controlled; there should be proper regulation even in the case of reflex activities as well. For efficient execution and control over movements, it is essential that different parts of brain are able to exert their influence over motor neurons present in various parts of body.
Different parts of brain that have major role to play in initiation and smooth control over movements of body are:
1. Cerebral cortex—motor areas especially for voluntary and skilled movements.
2. Reticular formation present in brainstem
3. Cerebellum
4. Basal ganglia
5. Red nucleus
6. Vestibular nucleus, etc.
The impulses that originate in these parts of brain (upper motor neurons) will be reaching lower motor neurons present in brainstem and spinal cord (Fig. 9.24). Since lower motor neurons of brainstem are very much in the vicinity of many of controlling parts, there are no separate well-defined bundles of nerve fibers which have to traverse long distance.
On the other hand, bundles of nerve fibers from brain which have to control activity of lower motor neurons of spinal cord are quite distinct and have to traverse long distance to reach lower motor neurons.
Bundle of nerve fibers which are descending through brainstem reach spinal cord to exert their influence on lower motor neurons are termed as descending tracts. For any part of body to execute smooth movements, it is very essential that structurally and functionally both upper and lower motor neurons should be intact.
Some of the important descending tracts are (Fig. 9.25):
1. Lateral corticospinal tract
2. Anterior or ventral corticospinal tract
3. Rubrospinal tract
4. Reticulospinal tract
5. Vestibulospinal tract, etc.
The above mentioned tracts can be broadly classified into pyramidal and extrapyramidal tracts. Tracts involved in pyramidal group are lateral and anterior pyramidal tracts. But for these two, all other descending tracts are included under extrapyramidal tracts. Descending tracts are present either in lateral or anterior funiculus in spinal cord.
Lateral and anterior corticospinal tract:
A large number of fibers take origin from motor areas present in motor cortex in the frontal lobe. Nerve fibers which take origin from motor cortex but influence lower motor neurons present in brainstem regions are termed as corticonuclear or corticobulbar fibers. These fibers regulate activity of nuclei from where motor cranial nerves take origin.
Rubrospinal tract:
It takes origin from red nucleus present in midbrain and influences activity of lower motor neurons present in spinal cord.
Reticulospinal tract:
This tract takes origin from reticular formation present in brainstem and exerts control over lower motor neurons of spinal cord.
Vestibulospinal tract:
This tract takes origin from motor component of vestibular nucleus present in medulla oblongata and exerts control over lower motor neurons of spinal cord (Fig. 9.26).
Cerebellum has no direct control over lower motor neurons of either brainstem or spinal cord. However, cerebellar role in smooth movement is very much warranted. Cerebellar influence over lower motor neurons is indirect. Cerebellum control over lower motor neuron is mediated through either motor cortex or red nucleus or reticular nucleus or vestibular nucleus (Fig. 9.27).
Motor function cannot be smooth in the absence of afferent impulses coming from peripheral parts of body. Hence during any movements, there would be constant bombarding of afferent impulses from different parts of the body (Fig. 9.28) to areas of brain, like cerebellum, vestibular nucleus, sensory areas of cerebral cortex, etc. This type of feedback influence facilitate smooth movements.
Another way to classify descending motor pathways will be lateral motor system versus medial motor system.
The lateral pathways end directly on motor neurons or interneuronal groups in lateral parts of spinal cord gray matter, whereas medial pathways excite motor neurons directly. They also influence reflex arcs that control fine movements in distal limbs apart from supporting musculature in proximal limbs.
The medial pathways end on motor neurons in the medial ventral horn or interneuronal groups present in this part of gray horn. These interneurons connect bilaterally with motor neurons that control axial musculature and thereby contribute for balance and posture. They also contribute to control of proximal limb muscles.
Pyramidal Tract:
i. This tract is also known as corticospinal or cerebrospinal tract.
ii. The tracts included in pyramidal tract are lateral and ventral or anterior corticospinal tract.
iii. The tracts take origin from cerebral cortex present in precentral gyrus area no. 4. In addition to this, fibers also take origin from premotor cortex area no. 6 and 8 and some of the fibers also take origin from sensory cortex area no. 3, 1, and 2.
iv. In motor cortex, body representation is upside down (Fig. 9.29).
v. The motor cortex exerts contralateral control that is, right half of body activity is controlled by left cerebral cortex and vice versa.
vi. Representation is not for individual muscles but for movements. Movements are brought about by involvement of group of muscles.
In precentral gyrus, there are special cells present, namely Betz cells or pyramidal cells. These cells contribute for origin of pyramidal tract in addition to neurons present in premotor cortex and sensory cortex.
Axons of these fibers converge towards internal capsule to form corona radiata. Corona radiata fibers as they approach internal capsule get compactly packed. In internal capsule, fibers pass through anterior two-thirds of posterior limb and genu.
In brainstem, cranial nerve motor nuclei are present. When pyramidal tract fibers pass through brainstem, some of these fibers synapse on motor nuclei of cranial nerve. Pyramidal tract fibers influencing cranial nerve nuclei 3, 4, 6 are called as corticonuclear fibers and those fibers influencing cranial nerve nuclei 5, 7, 9, 10, 11, and 12 are called as corticobulbar fibers.
In lower part of medulla oblongata, majority of pyramidal tract fibers (80-85%) cross midline to reach opposite side. Crossing of fibers is known as motor deccussation. Fibers after crossing descend down as lateral corticospinal tract (LCST), and remaining fibers which have not crossed the midline descend down in spinal cord as anterior/ventral corticospinal tract (ACST/VCST).
In the anterior horn of spinal cord, lower motor neurons (LMN) are present. Lateral corticospinal tract fibers which are located in lateral funiculus of spinal cord along their course, end on LMN at every segmental level. Many fibers end directly on LMN and some of them may end through an internuncial neuron.
Anterior corticospinal tract fibers descend down in anterior funiculus of spinal cord. Just like lateral corticospinal tract fibers, fibers of anterior corticospinal tract also synapse on LMN present in anterior gray matter. Just before ending on LMN, many of anterior corticospinal tract fibers cross midline to end on LMN present in opposite half of spinal cord.
However, some fibers end on LMN present in anterior gray matter on same side. About 95% (about 85% will have crossed in medulla oblongata and another 10% will cross at every segmental level) of pyramidal tract fibers control activity of LMN present in opposite half of spinal cord. It is because of this, 95% of pyramidal tract fibers will control activity of LMN of opposite side.
Functions of Pyramidal Tract:
1. Voluntary motor activity in any part of body is because of pyramidal tract. Whenever there is damage to pyramidal tract, most of voluntary movements in distal parts of body will be lost. Loss of voluntary control over muscles activity is known as paralysis.
2. It is also concerned with regulation of skilled movements like playing a piano, hand writing, modulation of voice, etc. Hence, it plays an important part whenever refine or precision is required for movement.
3. Regulates muscle tone:
Pyramidal fibers have excitatory influence on LMN (inhibitory influence on LMN will be because of extrapyramidal fibers). Hence, if there is pure pyramidal tract lesion, loss of excitatory influence leads to decreased muscle tone and hypotonia results.
But in clinical situations, pure pyramidal tract lesions are quite rare. Apart from damage to pyramidal tract, there will be damage to extrapyramidal fibers also. Because of this, rather than hypotonia, hypertonia is seen in most of UMN lesions.
Effects of Lesion to Pyramidal Tract at Different Levels:
1. Unilateral lesion in internal capsule:
a. Loss of voluntary movement in opposite half of body that is contralateral hemiplegia. But some muscles escape paralysis. Muscles which do not get affected are muscles of upper half of face on opposite side, muscles of trunk and back (axial group of muscles). Muscles which have escaped effect have bilateral representation in motor cortex. All features (refer differences between UMN and LMN lesions) of upper motor neurons are observed in affected part of body.
b. In addition to motor fibers since sensory fibers are also passing through internal capsule, sensations in opposite half of body will be lost. So there will be contralateral hemianesthesia, homonymous hemianopia (field of vision (in one eye temporal field and in the other eye nasal field are affected), but hearing would not get affected much since hearing has bilateral pathways.
2. Crossed hemiplegia:
It is seen in a patient when there is lesion in one half of pons. Since pyramidal tracts are yet to cross, there will be contralateral hemiplegia (UMN lesion) in the other parts of body. In addition to this, cranial nerve motor nucleus of 7th nerve is damaged. Because of this, muscles of face are paralyzed on same side (LMN lesion).
3. A complete transection of spinal cord above level of C3 segment will lead to paralysis in whole body except in face. Person cannot survive due to respiratory paralysis because of involvement of both diaphragm and intercostal muscles (since phrenic fibers take origin from cervical segments 3, 4 and 5).
4. Complete transection at C6 or C7:
There will be quadriplegia (paralysis of all 4 limbs). Muscles of face and diaphragm will not be paralyzed. Since diaphragm is not paralyzed, person is able to breath on his own (diaphragmatic breathing).
5. If there is unilateral sectioning in spinal cord around C6 segment, it leads to hemiplegia on same side. It is because majority of fibers have already crossed midline at medulla oblongata.
6. Section at T12 segment:
If there is transaction of spinal cord at this level, lower limb muscles supplied by lumbosacral segments on both sides are paralyzed. Hence results in paraplegia. If lesion is only on one side above origin of lumbosacral plexus, there is paralysis of only one of lower limbs on affected side. Hence results in monoplegia.
Characteristic Features in UMN and LMN Lesions:
Lower motor neuron refers to anterior horn cell or corresponding cranial nerve motor nuclei and its axon. There are two types of LMN, namely gamma (ϒ) and alpha (α) motor neurons. Special feature of LMN is it forms final common efferent pathway from CNS to any part of body.
Upper motor neuron takes origin from cerebral cortex or subcortical regions and exerts influence over LMN.
Differences between upper and lower motor neuron lesions have been enumerated in Table 9.2.
Features of Reaction of Degeneration:
1. Alteration in chronaxie:
For galvanic current (long duration current), there will slow sluggish movement (worm-like movement). For faradic current, there will not be any response.
2. Fibrillation:
Involuntary contraction of individual muscle fibers occurs. This can be made out by EMG.
3. Fasciculation:
Involuntary contraction of a group of muscle fibers will also be present.
Muscle Tone Maintenance and Regulation:
How are structures in muscles organized to bring a proper control over movement of muscles?
It is basically due to stretch reflex. This reflex is an example for deep reflex. The functional integrity of muscle is maintained as long as stretch reflex is normal.
The details of stretch reflex are as follows:
i. Receptors in muscles are muscle spindle (intrafusal fiber), which is arranged parallel to extrafusal fiber (Fig. 9.30). The extrafusal fiber contains contractile unit of muscle namely sarcomere. In addition to muscle spindle, another receptor present in muscle is tendon end organ (Fig. 9.31).
ii. The intrafusal fibers are of two types, namely nuclear bag fiber (attached to extrafusal fiber) and nuclear chain fiber which is attached to nuclear bag fiber (Fig. 9.32).
iii. In nuclear bag fiber, dilated central region is known as equatorial zone. This part acts as receptor area and part has many nuclei.
iv. In nuclear chain fiber, nuclei are arranged serially.
v. Ia primary afferent fibers carry information from central portion of both nuclear bag and nuclear chain fibers.
vi. Nuclear chain fiber has another afferent nerve supply coming from II (secondary) afferent fibers.
vii. The polar region (peripheral regions) of intrafusal fibers has contractile elements which are supplied with static y efferent.
viii. Also there are dynamic y efferent fibers supplying predominantly to nuclear bag fiber.
Role of muscle spindle with respect to functioning of extrafusal fiber (EFF):
Extrafusal fiber (EFF) refers to muscle fiber contraction that brings about various actions in body.
EFF gets motor nerve supply from spinal cord through alpha motor neuron, whereas motor neuron supplies polar region of intrafusal fiber (IFF).
Afferent innervation to intrafusal fibers:
Ia primary afferent fibers carry impulses from nuclear chain and nuclear bag fibers.
IIa secondary afferent fibers carry impulses from nuclear chain fibers (Fig. 9.32).
Whenever EFF is stretched, primary afferent fibers are stimulated due to stretching of equatorial zone of intrafusal fibers. Primary afferent fibers carry afferent information from IFF about both, the amount of stretch of IFF, and velocity or rate at which stretch is taking place.
II afferent fibers are able to detect amount of stretch only.
Tone of muscle is partial state of contraction of muscle even at rest or resistance exerted for passive movement.
How is this Tone of Muscle Maintained?
Anterior horn of spinal cord has both α and ϒ motor neurons. Supraspinal influences from cerebral cortex and subcortical regions act on ϒ motor neuron present in anterior horn of spinal cord. The ϒ efferent fibers when carry impulses (Fig. 9.33) to IFF bring about contraction of polar ends of IFF (as they have contractile elements).
This leads to stretch/distortion of equatorial zone of IFF. When this happens, receptors present at equatorial zone of IFF get stimulated. This leads to production of action potentials which is carried to spinal cord through la primary afferent fibers. These afferent impulses in turn stimulate alpha motor neuron in anterior horn of spinal cord.
When alpha motor neuron is stimulated, it brings efferent impulses to sarcomere of EFF, giving rise to contraction of EFF. As IFF is attached parallel to EFF, when the muscle contracts, stretching of IFF will decreases. Since stretching of equatorial zone is decreased, receptors in muscle spindle will not be stimulated any more. This brings about no action potential development in afferent fiber.
Because of this, impulses going to spinal cord along la fibers is decreased. When there are no impulses along la fibers, stimulation of alpha motor neuron also stops (Fig. 9.34). Now muscle starts relaxing. This type of alternating contraction and relaxation goes on continuously and is responsible for maintenance of muscle tone.
This whole mechanism is known as α- ϒ linkage. Some of impulses from higher parts of CNS may predominantly stimulate alpha motor neuron and some may stimulate ϒ neuron only.
Increase in the tone of muscle is seen whenever there is mid collicular section. Increase in muscle tone in this situation is due to increased y discharge. This is known as Sherrington’s animal or condition is also known as decerebrate rigidity.
If posterior nerve root is also sectioned (deafferentation) in decerebrate rigidity, tone of muscles is decreased. Decrease in muscle tone now is due to interruptions in α- ϒ linkage (that is la primary afferents are unable to carry impulses to spinal cord through posterior nerve to stimulate alpha motor neurons).
Pollock-Davis animal is due to increased stimulation of alpha motor neurons and results in rigidity. When there is ischemia of cerebellum it results ischemic decerebration. This type of rigidity will not be lost by deafferentation (that is sectioning of posterior nerve roots). It is because in this condition increased muscle tone is due to hyperactivity of alpha motor neurons itself and not due to gamma motor neuron hyperactivity.