Session 4: The Motor System Flashcards
What are motoneurones? What are UMNs and LMNs?
In neurology, neurones constituting the motor system are collectively known as motoneurones. They provide the efferent response of the CNS and all have their cell bodies located within the CNS:
A motoneurone is a somatic efferent that supplies skeletal muscles to bring about displacement of limbs (i.e. movements) and set muscle tone.
Upper motoneurones have their cell bodies located in the brain and synapse within the CNS. They can further categorised into cortical efferents (with cell bodies in the cerebral cortex, and are also known as pyramidal motoneurones) and brainstem/bulbar efferents (with cell bodies in the sub-cortical brain areas, and are also known as extra-pyramidal motoneurones).
- There is a variety of species of upper motoneurones. All descending tracts are UMNs. All interneurons are thus UMNs. Thus there is potential for diversity of deficits arising from UMN depending on which UMN is damaged.
Lower motoneurones have their cell bodies located either in the spinal cord (in lamina IX) or in the cranial nerve motor nuclei (e.g. facial nucleus). There is only one species of lower motoneurone thus there is only one variety of motor deficits arising from LMN damage.
Are all LMNs alpha-motoneurones? And what is a motor unit?
Lower motoneurones are normally synonymous with alpha-motoneurones BUT not all lower motoneurones are alpha-motoneurones. There are other types of lower motoneurones such as beta-motoneurones and gamma-motoneurones. The cell bodies of alpha-, beta- and gamma-motoneurones are all found mixed in lamina IX of the spinal grey matter (or cranial nerve motor nuclei).
A motor unit is formed of a motoneurone and the muscle fibres it supplies.
More strictly, it comprises of 1 alpha-motoneurone + variable number of extrafusal muscle fibres it supplies
Extra-ocular muscles (10 fibres)
Quadriceps (1000 fibres).
It is the minimal functional unit of the motor system
How many UMNs can predominate over LMNs at one time? What does this mean?
Upper motor neurones can be excitatory (small proportion) or inhibitory (larger proportion), and only one (normally inhibitory) can predominate over the lower motoneurones at one time.
Lower motoneurones are those cells of the ventral horn of the spinal cord or cranial nerve nuclei that give rise to axons that supply skeletal muscles. Their cell bodies are either in lamina IX of the spinal cord or cranial nerve motor nuclei (e.g. CN VII) in the brainstem.
The result is the upper motoneurones controlling the activity of the lower motoneurones and thus the lower motoneurones acting (supplying) directly on the skeletal muscle. Their axons form the crucial “final common pathway” between the nervous system and all voluntary muscles of the body.
Lower motoneurones are the only neurones of the body that produce movements through the activation of muscles. These movements can be reflexive in response to segmental inputs or volitional as directed by higher centres of the CNS acting via upper motoneurones.
Describe the constellation of LMN lesion signs?
Flaccid Muscle Weakness
Hypertonia or atonia
Hyporeflexia or areflexia
Denervation Muscle Atrophy
Fasciculations (acute phase)
Muscle wasting
Describe the circuit set up that produces muscle tone and movement?
Both muscle tone and movement are produced by the same “circuit” set-up. Each segment of the spinal cord has its own “self-sufficient” motor machinery that generates the movement at that segment, allowing limb movements to occur at that spinal level that are not coordinated with those of other segments. The system is controlled by the brain via descending inhibitory signals shutting them down, and is always under this inhibition from the upper motor neurones. This means that there is no chance of uncontrolled limb movements.
The cortex gives permission for movements to occur by removing this inhibition and it is this removal that results in voluntary movement.
An individual who is conscious and motionless will have a large input from the descending inhibitory neurones yet the lower motoneurones have sufficient output to cause motor tone.
An individual in a deep sleep has their descending inhibition paralysing all their skeletal muscles (except those of breathing and the extraocular muscles) whereas descending inhibition is temporarily lifted in order for us to carry out voluntary movements.
What is the role of the muscle stretch reflex in neurology?
It is the template neural circuit from which all motor circuits of the body are built from
It is the minimal neural circuit that underlies all movements of muscles of the body
It is the neural circuit that sets all motor tone of the body
It is the easiest pathway to test and if anything is wrong, the defect can be readily attributed to the stretch receptors/afferents, synapse in the spinal cord, motoneurone, neuromuscular junction and muscle.
What a monosynaptic stretch reflex? What other types of reflexes are there?
Monosynaptic (myotatic) stretch reflex: it’s a subset of motor stretch reflexes that only has one synapse. It’s activated by stretch and it causes contraction of a skeletal muscle.
- Stretch receptor (in muscle spindle) => afferent fibre (alpha) => LMN => effector muscle
Other reflexes can be disynaptic, trisynaptic, quadsynaptic and oligosynaptic by the set up of additional interneurons between afferent and efferent neurones. These reflexes provide greater control to the finer movements of the body and are also inhibited by the descending tracts.
Any damage to the descending neurones can consequently cause clonus (muscular spasm involving repeated, often rhythmic, contractions) as signals are no longer inhibited, and pass round and round the other interneurons.
What is a spinal reflex? Describe the pathway of the reflex arc
A spinal reflex is an involuntary, unlearned, automatic reaction to a specific stimulus that does not require the brain to be intact. The pathway is described as a “reflex arc”. There are 5 components to the reflex arc:
- A receptor (e.g. muscle spindle)
- An afferent fibre (e.g. a muscle spindle afferent)
- An integration centre (e.g. lamina IX of spinal cord)
- An efferent fibre (e.g. alpha-motoneurone)
- An effector (e.g. muscle)
Compare alpha motoneurones and gamma motoneurones
Alpha motoneurones innervate extrafusal fibres (the normal skeletal muscle fibres). They are myelinated with large cell bodies and have a-alpha conduction velocity.
Gamma motoneurones innervate intrafusal fibres (found in muscle spindles). They have small diameter cell bodies and a-gamma conduction velocity. Activity in the gamma motor neuron causes contraction of the poles of the intrafusal fibre, which stretches the central zone and activates the peripheral process of the afferent neurone.
What are muscle spindles and where are they located?
Muscle spindles are present in skeletal muscles, being more numerous in muscles that control fine movements. Each spindle consists of a connective tissue capsule in which there are 8-10 intrafusal fibres located in a connective tissue bag, known as a fusical.
Afferent nerves of muscle spindles are permanently connected to cell bodies of lower motoneurones. This pathway is the muscle stretch reflex arc.
Efferent innervation is provided by the polar ends of the intrafusal fibres, innervated by gamma motor neurons; consequently, muscle spindles detect changes in length of the muscle.
How does the muscle stretch reflex work?
The Muscle Stretch Reflex is a stretch-activated reflex contraction of a skeletal muscle.
When a muscle is not contracted, it relaxes
A relaxing muscle is effectively lengthening (or stretching).
- When muscle length receptors detect stretch (or lengthening), they fire action potentials via afferent axons to keep the CNS appraised of muscle length at all times.
- Action potentials from muscle length receptors are sent to:
- the brain via dorsal columns
- the cerebellum via spino-cerebellar tracts
- the copy of that signal is also sent directly to spinal motoneurones
The results in reflex recruitment of motoneurones
Hence reflex contraction of the muscle that stretched.
What are Golgi Tendon Organs?
Golgi tendon organs are high-threshold receptors located at the junction of muscle and tendon, and the golgi tendon organ consists of a large myelinated fibre, that enters the connective tissue capsule of a tendon and subdivides into many unmyelinated receptor endings that intermingle and encircle of the collagenous fascicles. Active contraction of the muscle or stretching of the muscle activates the Golgi tendon organ, thus they are sensitive to increases in muscle tension caused by muscle contraction
Golgi tendon organs are innervated by Ib sensory axons. The firing rate of the Ib afferent fibre increases when the tendon organ is stretched, with greater outputs for active contraction rather than passive stretching of the muscle. The Ib axons branch extensively in the spinal cord and synapse on several interneurons in the ventral horn. Some of these interneurons make inhibitory synapses with alpha motor-neurones that innervate the same muscle the Ib axon is originating from. As the muscle contracts, the tension through the Golgi tendon organs increases, causing increased inhibition on the AMNs, which ultimately reduce AMN firing patterns and therefore reduces muscle contraction.
What is meant by alpha-gamma co-activation?
Alpha-gamma coactivation. The muscle starts at a certain length, encoded by the firing of a Ia afferent. When the muscle is stretched, the muscle spindle stretches and the Ia afferent fires more strongly. When the muscle is released from the stretch and contracts, the muscle spindle becomes slack, causing the Ia afferent to fall silent. The muscle spindle is rendered insensitive to further stretches of muscle. To restore sensitivity, gamma motor neurons fire and cause the spindle to contract, thereby becoming taut and able to signal the muscle length again.
How is Motor Tone produced?
Motor tone is produced by a (involuntary) minimal force of contraction by the muscles. Motor tone is produced by the tonic contraction of lower motoneurones in their muscle fibres, resulting in background minimal contraction of the muscle.
This minimal muscle contraction gives the muscle a small amount of force which is called muscle/motor tone. Motor tone allows us to maintain body posture and hold our heads upright.
Testing motor tone is a major part of a neurological examination
Muscle fibres contract randomly throughout the muscle to produce sufficient tone yet prevents muscles from becoming fatigued; an orderly recruitment pattern for additional muscle fibres when tone needs to be increased is caused by ‘size principle’, whereby the smaller motor units are recruited first and the larger motor unit recruited last.
Feedback from muscle spindle afferents results in reflex contraction of the muscles in which the spindle itself innervates, allowing for muscle tone and an ability to judge passive displacements; muscle tone rises and falls depending on the number and size of motor units recruited by the respective muscle stretch reflexes.
Where is Motor Tone present?
Muscle tone is present but low in-utero. It is suppressed in the newborn yet returns within months after birth (development informs the absence or presence of brain injury to the baby). Muscle tone is present in all skeletal muscles yet is inhibited during REM sleep in nearly all muscles (except muscles of breathing, extra-ocular muscles, urinary sphincter anal sphincter etc)
Describe the Flexion/Withdrawal Reflex
The flexion reflex is primarily mediated by pain receptor (nociceptors) stimulated by noxious stimuli.
When a noxious stimuli is applied and the free nerve endings are stimulated, the resulting impulses are conducted through myelinated afferent fibres (group 3 fibres) and unmyelinated afferent fibres (group 4 fibres).
These fibres directly synapse with alpha-motoneurones in the spinal cord (normally polysynaptic so 3 or 4 interneurones are involved), resulting in contraction of the ipsilateral flexor muscles.
The net result is to withdraw the limb in response to the noxious stimuli.
Describe the autogenic inhibition reflex and the crossed extensor reflex
Autogenic Inhibition reflex: fibres from Golgi tendon organ (beta) => inhibitory interneurone => relaxation of antagonist muscle.
Crossed-extension reflex: works with flexor reflex; contralateral III afferents cause muscle contraction to allow maintenance of balance as body weight centre is shifted.
- The flexion withdrawal reflex protects limbs against potentially noxious stimuli detected by cutaneous structures. The flexors of the affected limb contract and the extensors are relaxed. This withdraws the limb away from the noxious stimuli.
- At the same time, a crossed extensor reflex is elicited in the contralateral limb where the extensors are contracted and the flexors relaxed. This provides postural support during withdrawal of the stimulated limb.
What is the motor system? Which part of the brain is responsible for skeletal muscles?
The neural apparatus that generates, executes, maintains and terminates movements is known as the motor system. It is hierarchically organized with motor areas of the cerebral cortex highest, brainstem nuclei and the cerebellum intermediate, and motoneurones of cranial and spinal nerves lowest.
Somatic muscles of the body are controlled by dedicated neurones of the primary motor cortex in the pre-central gyrus of the frontal cortex.
The brain commands muscles to either:
Rest
Or Contract
Levels of contraction vary depending on the requirements of the body
Resting muscle tone (maintenance of posture) (minimal contraction)
Displacement of joints (movements) (large forces may be required)
What is the clinical relevance of decussation?
Most of the brain’s motor commands of muscles are decussated (BUT THERE ARE EXCEPTIONS). Irrespective of site of origin of motor commands,
- Skeletal muscles of the left side of the body are commanded to move or rest by the right cerebral motor cortex
- Skeletal muscles of the right side of the body are commanded to move or rest by the left cerebral motor cortex
Describe the Pyramidal System
The pyramidal system has direct (monosynaptic) contact with lower motoneurones supplying distal muscles of extremities (e.g. hand) whilst the extra-pyramidal system has indirect contact with the rest of the motoneurone pools. Broadly speaking the pyramidal pathways provide voluntary and the extrapyramidal pathways provide involuntary movements.
Pyramidal Motor Pathways
The pyramidal motor pathways pass through the medullary periods and control the voluntary movements.
They are very slow in their development and so only fully complete myelination by around 17-18 years.
Describe the two types of cortical descending tracts
Neurones of the pre-frontal, supplementary motor cortex and somatosensory cortex terminate on the Basal Ganglia or Cerebellum (they do not terminate in lower motoneurones).
Neurones with cell bodies of the motor strip (also known as the pre-central gyrus of the frontal cortex) terminate on cell bodies of alpha-motoneurones (i.e. lower motoneurone lesions) of muscles of the contralateral side of the body.
Cortical Descending Tracts Terminating in the Basal Ganglia
- They account for the majority of purposeful motor outputs of the brain.
- They carry motor programs of the brain’s intended motor output to the body’s musculature.
- Disorders of degeneration of the brain result in disturbance of this motor system (particularly an issue in old age).
Cortical Descending Tracts Terminating on alpha-motoneurones
- There are two broad classes of cortical fibre tracts from the motor strip of the frontal cortex.
- One class terminates in cranial nerve motor nuclei of the opposite side (aka cortico-bulbar tracts).
- The other class terminates on cell bodies of spinal motor nuclei of the contralateral side (aka corticospinal tracts).
What happens as the corticospinal tract splits into two?
Around 75-90% of fibres decussate to the contralateral side at the medulla, known as pyramidal decussation, to form the lateral corticospinal tract. The lateral corticospinal tract neurones form the medullary pyramids and travel in the lateral funiculus of the spinal cord. They will terminate in the ventral horn (30% synapsing with lower motoneurones and 70% with interneurons)
The rest of the fibres (about 10-25% of CST fibres in total) stay ipsilateral to form the anterior (ventral) corticospinal tract. The axons travel via the internal capsule and stay on the same side of origin until they reach the spinal neural segments of their intended target (ventral horn of the cervical and upper thoracic levels) where they decussate to the opposite side (last minute, spinal decussation). They travel in the ventral funiculus of the spinal cord.
What are the brainstem motor nuclei that project to the spinal cord? What are the extrapyramidal tracts?
- Red Nucleus – Rubrospinal Tract
- Reticular Formation – Reticulospinal Tract
- Tectum – Tectospinal Tract
- Vestibular Nuclei – Vestibulospinal Tract
Their fibre tracts do not project directly to cell bodies of the spinal motoneurones (do no synapse with lower motoneurones directly). Instead, their fibre tracts project to interneurons of spinal reflex pathways of the spinal cord.
Their main actions are to modulate the machinery of spinal reflex circuits.
The extrapyramidal pathways have an indirect contact with lower motoneurones. There are four main extrapyramidal pathways (called extrapyramidal as they do not pass through the medullary pyramids).
Describe the Reticulospinal Tract
The reticulospinal tract arise from the reticular formation (aka tegmental fields of the brainstem). There are two sites of origin:
- The medial reticulospinal tract originates from the Pontine Reticular Formation. It facilitates voluntary movements and increases muscle tone.
- The lateral reticulospinal tract originates from the Medullary Reticular Formation It inhibits voluntary movements and reduces muscle tone.
They descend bilaterally to all levels of the spinal cord (decussating partially in the brainstem).
They act to facilitate extensor spinal reflexes.
The reticulospinal tract is normally under inhibition of the corticospinal tract. Its release from inhibition (e.g. due to trauma) results in decerebrate rigidity
- Unopposed extension of the head and limbs
- Indicative of damage to the brain at or below the red nucleus
- Very rare nowadays due to imaging technology
Describe the Rubrospinal Tract?
The rubrospinal tract arises from neurones of the red nucleus, a midbrain structure and decussates in the midbrain and descend contralaterally in the spinal cord.
It travels in the lateral funiculus, in close association to the lateral corticospinal tract.
Most of its fibres terminate in the cervical spine
The function of the rubrospinal tract is to facilitate flexor motoneurones and inhibit extensor motoneurones – its main targets are motoneurones of the flexors of the upper limb.
It is small and rudimentary in humans, not considered to be significant.
Cells of the red nucleus are rich in iron and appear red in freshly-cut brain slices due to formation of haemtite/rust from iron.
Describe UMN and LMN lesions
Lesions anywhere in the lower motoneurone will produce flaccid paralysis (or atonia), atrophy, fasciculations and hyporeflexia (if not areflexia) of muscles supplied by that motoneurone.
In contrast, lesions of upper motoneurones give rise to complex signs depending upon whether the pyramidal or extra-pyramidal system is affected. The cardinal signs of upper motor neurone lesions are spastic paralysis, hypertonia (spasticity, cog wheel rigidity) => muscle weakness, hyperreflexia, clonus (involuntary, rhythmic muscle contractions), minimal or no atrophy, no fasciculations and clonus.
A further sign indicative of lesions of the extra-pyramidal system may include the generation of unwanted or uncontrollable movements (chorea).
In the case of the pyramidal system, a positive Babinski sign (extension of the big toe) will be seen following stroking of the lateral aspect of the sole of the foot
Damage to the extrapyramidal tracts
- Extrapyramidal tract lesions are commonly seen in degenerative diseases, encephalitis and tumours. They result in various types of dyskinesias or disorders of voluntary movement.
Describe the clinical importance of the muscle stretch reflex and motor unit. Which spinal root values are you testing for each tendon reflex?
Testing muscle reflexes by brisk tapping of tendons is a common procedure in examination of the motor system. Although such a test is simple, requires only a tendon hammer and makes use of a small number of neurones connected together, the versatility of the muscle stretch reflex is not to be under-estimated, first in terms of its importance in the building of limb and postural reflexes.
Secondly, when it is used in the clinical examination of the nervous system, it reveals unambiguously definitive information on the state of health of the nervous system.
Most of the exam is based on probing the various elements of the stretch reflex
- Integrity of connections of neurones of the stretch reflex
- Health status of neurones of the reflex
- Health status of synapses of the reflex
- Health of wider circuits built from the stretch reflex etc
- Visual assessment of the MSK system System at rest
- Appearance of skeletal muscles of the body
- Symmetry of muscle bulk between left and right sides
- Presentation of the limbs and other muscle systems
- Any other visually noteworthy features of muscles of the body.
Testing for the patient’s ability to move
- Can reflex movements be evoked from the patient?
- Can a limb or muscle be displaced on command (movement)?
“Quality” and “Nature” of the movement will also be of interest
- Can motor power be generated?
- Is there muscle/motor tone?
