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)