Next Lesson - The Cerebellum: Circuits, Coordination and Clinical Signs
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. 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, and these cross from their cell body located in the central nervous system (specifically in the brainstem or the ventral horn of the spinal cord), to their synapse with skeletal muscle in the peripheral nervous system.
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 - they are spinal or cranial nerves. They are controlled by upper motor neurones, which descend through the cord or brainstem and synapse on LMNs.
Their cell bodies are found in the ventral horn of the spinal cord, and in cranial nerve motor nuclei (oculomotor nucleus, trochlear nucleus, trigeminal nucleus etc). 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, 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
Lower motor neurones participate in spinal reflexes without the involvement of upper motor neurones.
When considering the patellar reflex as a monosynaptic reflex arc, the patellar tendon is struck with a hammer, this is detected by the stretch of the muscle spindle fibres, 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 (L3-L4)
SimpleMed original by Dr. Maddie Swannack
Quadriceps excitation is monosynaptic. A branch of the same spindle afferent activates an inhibitory interneurone that inhibits hamstring motor neurones, producing disynaptic reciprocal inhibition. 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 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. 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)
- Areflexia (due to denervation)
- Wasting (due to loss of nutritional support to the muscle from the LMN)
- Hypotonia (due to loss of the background action potentials and very small contractions within the muscle that result in tone)
- Fasciculation (due to up-regulation of nicotinic acetylcholine receptors (nAChRs) in the muscle to try to compensate for denervation)
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.
Their axons descend from the motor cortex through the following structures:
- Corona radiata: white-matter axons beneath the motor cortex
- Internal capsule
- Cerebral peduncle in the midbrain
- Pons
- Medullary pyramids
- Decussation of the pyramids (in the caudal medulla)
- Lateral corticospinal tract (in the lateral funiculus of the cord)
- Ventral horn of the spinal cord
- Influence lower motor neurones in the ventral horn, usually through spinal interneurones, with some direct corticospinal connections. 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, 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, and brainstem-origin motor pathways do not all follow this cortical route.
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 reticulospinal and vestibulospinal tracts, also contribute to posture and muscle tone; the division is not based on fine control versus endurance.
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, 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.
The UMN synapses with the LMN at the level that the LMN will exit the CNS.
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. Muscles are not naturally in continuous contraction awaiting blanket inhibition from upper motor neurones; motor control coordinates activation, relaxation and reflex responses.
After an upper motor neurone lesion, useful voluntary drive is reduced and descending regulation of spinal motor and reflex circuits is disturbed. Weakness may coexist with increased tone and exaggerated reflexes as the syndrome develops, rather than being explained by loss of a 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.
They in fact 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, in that it is split into two halves - the top half of the nucleus contains the cell bodies of LMN that supply the muscles of the upper face, and the bottom half of the nucleus contains the cell bodies 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 peripheral facial palsy, 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 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, including spasticity, 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 around when the child starts walking, the immature, upgoing response turns into the mature, downgoing response. 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 will splay out to the sides as if to catch themselves. This is another example of an upper motor neurone reflex that occurs in babies and that is lost as the baby ages and the inhibition provided by upper motor neurones matures. It is a primitive, protective reflex that is lost with age as the child can move themselves out of danger more easily.
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 prominent grasp response in an adult can indicate loss of normal cerebral control; a normal neonatal response does not establish a caregiver-protection purpose.
Spinal shock is a phenomenon that occurs immediately after an acute UMN lesion and can last days to weeks. Initially there is flaccid paralysis with areflexia (as in LMN lesions), but tone then increases (becoming hypertonia) and reflexes become exaggerated (hyperreflexia). The mechanism of this is unclear, but is related to neuroplasticity in the spinal cord.
Edited by: Dr. Marcus Judge
Reviewed by: Adrian Judge
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