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Core
In the somatic (voluntary) motor system of the body, the neural pathways are made up of two neurones in a chain.
The upper motor neurone remains within the central nervous system. In the cortical voluntary-motor pathway described here, many cell bodies lie in primary motor cortex, in the precentral gyrus (the strip of cortex at the back of the frontal lobe). Other cortical areas and brainstem centres also give rise to descending motor pathways. These descending signals combine excitatory corticospinal output with regulation of spinal excitatory and inhibitory circuits.
The second neurone in the chain is called the lower motor neurone. Its cell body is located in the central nervous system (specifically in the brainstem or the ventral horn of the spinal cord). Its axon leaves the central nervous system and reaches skeletal muscle in the peripheral nervous system, where it forms a synapse with the muscle.
Diagram - The Relationship of Upper Motor Neurones and Lower Motor Neurones
SimpleMed original by Dr. Maddie Swannack
Lower motor neurones are the neurones that connect the central nervous system to the muscles. Their axons run in spinal or cranial nerves. They are controlled by upper motor neurones, which descend through the cord or brainstem and synapse on LMNs.
The cell bodies of lower motor neurones are found in the ventral horn (the front part of the grey matter) of the spinal cord, and in cranial nerve motor nuclei (clusters of cell bodies such as the oculomotor nucleus, trochlear nucleus and trigeminal nucleus). The locations that the cell bodies of LMN can be found are shown in colour on the image below. Some of the cranial nerve nuclei have eponymous names (names that honour a person), but for general understanding it is fine to know them by the cranial nerves they supply.
Diagram - The Different Potential Sites for Cell Bodies of Lower Motor Neurones
SimpleMed original by Dr. Maddie Swannack
A spinal reflex can occur without input from upper motor neurones, because the reflex arc lies within the spinal cord. Upper motor neurones do regulate how strong a reflex is.
Consider the patellar reflex as a monosynaptic reflex arc (a reflex with a single synapse between the sensory neurone and the motor neurone). The patellar tendon is struck with a hammer. The stretch is detected by muscle spindle fibres (stretch sensors within the muscle), and the quadriceps contract, straightening the leg. This is shown in the image below, with the sensory neurone shown in red and the LMN shown in green.
Diagram - The Monosynaptic Reflex Arc of the Patellar Reflex (lumbar spinal levels L3 and L4)
SimpleMed original by Dr. Maddie Swannack
Quadriceps excitation is monosynaptic. A branch of the same spindle afferent (sensory fibre) activates an inhibitory interneurone (a small connecting neurone within the spinal cord) that inhibits hamstring motor neurones. This is disynaptic (two-synapse) reciprocal inhibition: the opposing muscle relaxes as the quadriceps contracts. The sensory fibre therefore excites the quadriceps motor pathway while recruiting inhibition of the antagonist; it does not directly excite both muscles.
Diagram - Monosynaptic quadriceps excitation with disynaptic reciprocal inhibition of the hamstrings. The knee jerk mainly tests L3–L4, especially L4. The L5 label depicts a schematic hamstring motor pool (the group of motor neurones supplying that muscle) and must not be read as an additional knee-jerk root.
SimpleMed original by Dr. Maddie Swannack
Lower motor neurones can be damaged anywhere along their path from the cell body (within the CNS), along the axon to the neuromuscular junction (where the nerve meets the muscle fibre). The most common site for lower motor neurone injury is the axon, which is also known as a peripheral nerve injury.
The following signs can be seen in the muscles supplied by the damaged lower motor neurones:
- Weakness (due to denervation, meaning loss of the nerve supply to the muscle)
- Areflexia (absent reflexes, due to denervation)
- Wasting (due to loss of the supportive influence that the LMN normally has on the muscle)
- Hypotonia (floppiness, meaning less resistance when the limb is moved passively; muscle tone depends on the nerve supply and its spinal reflex circuits, which are interrupted)
- Fasciculation (visible muscle twitching, due to spontaneous firing of the affected lower motor neurone or its axon)
Upper motor neurones originate in cerebral cortex or brainstem and influence lower motor neurones through descending pathways, directly or via interneurones. The following pathway focuses on cortical corticospinal output, including fibres from primary motor cortex in the precentral gyrus.
Corticospinal output is excitatory and modulates spinal excitatory and inhibitory circuits. Loss of descending regulation helps explain the altered tone and reflexes that can develop after an upper motor neurone lesion.
The axons of these cortical upper motor neurones descend from the motor cortex through the following structures, in order. Knowing the route helps you work out where a lesion lies from the pattern of weakness:
- Corona radiata: white-matter axons beneath the motor cortex
- Internal capsule
- Cerebral peduncle in the midbrain (the upper part of the brainstem)
- Pons
- Medullary pyramids (two ridges on the front of the medulla, the lowest part of the brainstem)
- Decussation (crossing over) of the pyramids (in the caudal medulla, the lower end of the medulla)
- Lateral corticospinal tract (in the lateral funiculus, a column of white matter on the side of the cord)
- Ventral horn of the spinal cord
- Influence lower motor neurones in the ventral horn, through both spinal interneurones (small connecting neurones within the cord) and direct corticospinal connections onto the lower motor neurones. These circuits include both excitation and inhibition.
Diagram - A simplified cortical corticospinal pathway. The red UMN terminal contacts a separate blue LMN in ventral-horn grey matter, whose axon supplies muscle. Spinal interneuronal connections also occur; the direct relay shown is a schematic example.
SimpleMed original by Dr. Maddie Swannack
The lateral corticospinal tract is involved with fine motor control in the limbs.
The primary motor cortex contains corticospinal cell bodies arranged somatotopically (each body part has its own area, laid out like a map of the body), with leg representation medially and hand representation more laterally. Corticospinal fibres also arise from other cortical areas.
Cortical descending axons pass through the corona radiata, the fan-shaped white matter beneath the cortex, and converge into the internal capsule. The corona radiata is not the location of the cortical cell bodies. Motor pathways that start in the brainstem do not pass through the corona radiata.
The lateral corticospinal tract is particularly important for skilled distal limb movement. The anterior corticospinal tract contributes to axial and proximal voluntary control, with substantial bilateral influence. Parallel brainstem pathways, including the reticulospinal and vestibulospinal tracts (descending tracts that start in the brainstem), also contribute to posture and muscle tone. The lateral and anterior corticospinal tracts differ mainly in the muscles they supply: the limbs and the trunk respectively.
Distal Muscles - Lateral Corticospinal Tract
From the internal capsule, the axons of the upper motor neurones enter the brainstem through the cerebral peduncles of the midbrain. The axons descend through the midbrain, the pons and most of the medulla. In the caudal region (inferior region) of the medulla, most of the axons of the upper motor neurones decussate (cross the midline) to form the lateral corticospinal tract and descend on the contralateral side of the spinal cord. A smaller proportion does not cross here and continues as the anterior corticospinal tract.
The corticospinal fibres end at the spinal level where the LMN will exit the CNS, and act on the LMN directly or through interneurones.
Descending motor output acts on both excitatory and inhibitory spinal circuits. Corticospinal transmission is excitatory; its effects on lower motor neurones depend on the connections recruited.
Muscle contraction depends on activation of its motor units. Motor control coordinates the activation and relaxation of muscles and their reflex responses. Upper motor neurones do not simply hold muscles back from contracting.
After an upper motor neurone lesion, useful voluntary drive is reduced and descending regulation of spinal motor and reflex circuits is disturbed. As the syndrome develops, weakness is typically accompanied by increased tone and exaggerated reflexes. These signs are not explained simply by the loss of one uniformly inhibitory pathway.
Cortical motor commands for the face arise from motor cortical regions and descend through corticobulbar pathways to brainstem motor nuclei. They do not follow the spinal corticospinal route through the pyramidal crossing to the ventral horn.
The corticobulbar fibres to the face descend through the internal capsule and enter the corticobulbar tract, where they synapse with the facial nerve nucleus in the pons. The pattern of decussation applies to the upper motor neurone inputs rather than the lower motor neurone axons.
The facial motor nucleus is a special case of a cranial nerve motor nucleus because it is split into two halves. The top half contains the cell bodies of the LMNs that supply the muscles of the upper face. The bottom half contains the cell bodies of the LMNs that supply the lower half of the face.
The lower-face region of the facial motor nucleus receives predominantly contralateral cortical input.
The UMN supply for the superior portion of the facial nerve nucleus (supplying the upper half of the face) is bilateral, meaning it has inputs from both sides of the brain.
Diagram - The Nature of Inputs to the Facial Nerve Nuclei
SimpleMed original by Dr. Maddie Swannack
This knowledge can be used to distinguish between a facial nerve lesion (LMN) and a brain injury (UMN).
A peripheral facial nerve lesion usually weakens both the upper and lower face on the same side, because both sets of fibres are affected downstream of the nucleus. Bell’s palsy specifically denotes an idiopathic (of unknown cause) peripheral facial palsy (weakness of the face from a problem in the facial nerve itself), not every facial nerve lesion.
A hemispheric upper motor neurone lesion commonly weakens the contralateral lower face. Bilateral cortical input to upper-face motor neurones often preserves forehead movement, producing relative forehead sparing. This pattern differs from a peripheral facial nerve lesion affecting the ipsilateral upper and lower face.
Diagram - Facial motor pathway lesions. A right supranuclear lesion (a lesion above the facial nerve nucleus, in the upper motor neurone pathway) weakens the left lower face with relative forehead sparing; a right peripheral facial nerve lesion affects the whole right face. Minor ipsilateral lower-face cortical inputs are omitted from this simplified schematic.
SimpleMed original by Dr. Maddie Swannack
Upper motor neurone injury reduces voluntary motor drive and disturbs descending regulation of spinal circuits. Depending on timing and lesion extent, signs can include:
- Weakness (due to loss of direct excitatory inputs onto LMNs from UMNs)
- Hypertonia (increased muscle tone), including spasticity (muscle stiffness that resists passive movement), as descending regulation of stretch-reflex circuits is disrupted
- Hyperreflexia (same origin as hypertonia - an overactive reflex arc)
- Extensor plantar reflexes (see below)
Primitive reflexes reflect the developing nervous system. Maturation suppresses these automatic responses; abnormal persistence or reappearance later can indicate loss of normal cerebral control. They should not all be explained as simple loss of an inhibitory UMN-to-LMN connection.
One example of this is the plantar reflex, which is characterised by the direction of movement of the toes when the sole of the foot is stimulated. The normal, adult response is for the toes to flex down, almost as if they are trying to protect the sole of the foot. In babies, and in those with upper motor neurone lesions, the response is to extend the toes back. With age, usually during the first year or so of life, the immature, upgoing response turns into the mature, downgoing response, although the timing varies between infants. This means that testing the direction of the plantar reflexes is a good way of checking for upper motor neurone lesions.
When a baby experiences a sudden loss of support, their arms move away from the body to the sides (abduction and extension) and are then drawn back in towards the chest (adduction and flexion). The reflex begins to fade by about 12 weeks and has usually gone by 6 months. This is another primitive reflex that occurs in babies. It is lost as the cortex (the outer layer of the brain) matures and the nervous system comes to inhibit (dampen) the reflex. It is a primitive, protective response to a sudden loss of balance or an extreme stimulus. A Moro reflex that is exaggerated, or still present after about 6 months, may point to a neurodevelopmental problem (a problem with the child's development).
The palmar grasp is an automatic response in which touching an infant’s palm causes finger flexion around the stimulus. It is normally suppressed as the nervous system matures and purposeful hand use develops. A grasp response that persists beyond infancy, or reappears in an adult, can indicate loss of normal control by the brain, for example frontal lobe damage or widespread brain dysfunction.
Spinal shock is a phenomenon that occurs immediately after an acute spinal cord injury, when spinal cord function below the injury is suddenly lost or impaired. It can last days to weeks, and sometimes months. Initially there is flaccid paralysis with areflexia (as in LMN lesions), but tone then increases (becoming hypertonia) and reflexes become exaggerated (hyperreflexia). The mechanisms are not fully understood, but changes in the spinal cord after the injury (neuroplasticity, the ability of the nervous system to reorganise itself) are thought to be involved.
Edited by: Dr. Marcus Judge
Reviewed by: Adrian Judge
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