21. Motor Systems Flashcards
Define proprioception.
The sense of the body’s position in space based on specialised receptors that reside in the muscles and tendons.
Summarise what is required for successful proprioception.
First of all, the receptors:
- Need a way to differentiate between contracted and stretched
- Need a way to tell if length of muscle is changing
Secondly, need to use this information:
- Spinal reflexes involving motor neurons and interneurons (the M1 component)
- Unconscious control of movement -> By the cerebellum
- Conscious perception and control of movement -> Passed to cerebral cortex with dorsal column system (the M2 component)
In this way, the proprioceptive information and reflexes are integrated with centrally generated motor commands to produce adaptive movements.
Summary the patellar tendon reflex.
What are the two types of proprioceptors and what is the function of each?
- Muscle spindles
- Signal muscle stretch
- Therefore involved in the muscle stretch reflex
- Golgi tendon organs
- Signal tension produced by muscle contraction
- Therefore are involved in negative feddback to prevent over-contraction
What is afferent information from proprioceptors augmented by?
Joint mechanoreceptors:
- Large fibres signal the position of the joint
- Small Aδ are activated at the extremes of movement and have a protective function
However, the influence of the mechanoreceptors is small, as evidenced by joint replacement not having a large effect.
Skin mechanoreceptors also signal about posture and speech/facial expression.
Describe the location of muscle spindles and how they are arranged relative to muscle fibres.
- In the fleshy part of the muscle
- In parallel with the extrafusal fibres and attached to muscle connective tissue
Describe the location of Golgi tendon organs and how they are arranged relative to muscle fibres.
- In the tendons of muscles
- In series with the extrafusal fibres
What are intrafusal and extrafusal muscle fibres?
- Extrafusal muscle fibres -> Comprise the bulk of muscle and form the major force-generating structure.
- Intrafusal muscle fibers -> Buried in the muscle, and they contain afferent receptors for stretch (muscle spindles), but they also contain contractile elements.
Describe the structure of muscle spindles.
- Small (2-4mm long), encapsulated, spindle shaped
- Muscle spindles are made of intrafusal fibres that are in parallel with the extrafusal fibres that provide the main contractile force
- Each spindle has 3 main components:
- Intrafusal muscle fibres that have a non-contractile middle part with a contractile part on each side
- Large diameter sensory nerve fibres -> Wrap around the non-contractile part of the muscle fibres which is sensitive to stretch of the intrafusal fibre
- Small diameter gamma motor nerve fibres -> Innervate the contractile ends of the intrafusal fibres
What are the different types of intrafusal fibres (in muscle spindles)?
Nuclear chain fibres:
- The nuclei are in chain in the fibre (so it appears thinner)
- Many of them in a muscle spindle
- All are STATIC (i.e. not adapting)
Nuclear bag fibres:
- The nuclei are in a bag in the fibre (so it has a buldge)
- Only 2-3 in a muscle spindle
- Some are DYNAMIC (i.e. fast adapting), some are STATIC
Summarise the different fibres that innervate muscle spindles.
Afferent:
- Ia fibres:
- Innervates all fibre types in muscle spindles.
- Thus reports both muscle length and length changes.
- II fibres:
- Innervates static fibres (static nuclear bag and nuclear chain fibres)
- Thus primarily reports muscle length.
Efferent:
- Gamma (γ) motor nerves -> Supply the dynamic and the static intrafusal fibres to modulate the activity of these fibres
Draw how the type Ia and II fibres react when a muscle goes from static to stretching to static again.
What afferent fibres are involved in the muscle stretch reflex?
Ia fibres (which innervate both dynamic and static) detect stretch in the muscle.
Give some experimental evidence for the muscle stretch reflex.
(Sherrington, 1906):
- Demonstrated that the muscle stretch reflex is not an intrinsic property of the muscle but rather required a feedback loop
- He did this by cutting the afferent and efferent fibres
What is the function of the muscle stretch reflex?
- Important for posture (standing upright), holding a heavy object still in one hand, etc.
- When deviation from the intended position is detected, the muscle contracts to correct this.
In the muscle stretch reflex, what does the afferent type Ia fibre synapse with?
- Motor neurons to the muscle being stretched
- Motor neurons to muscles that are synergistic to the one being stretched
- Ia inhibitory interneurons that supply the antagonistic muscles
How many synapses are involved in the muscle stretch reflex? What is the evidence for this?
- There is one synapse in the pathway to the muscle that is being stretched
- There are two synapses (due to the interneuron) in the pathway to the antagonistic muscle
(Lloyd, 1946) gave evidence for this.
When is reciprocal inhibition in the muscle stretch reflex especailly important?
During repetitive movements, such as walking.
What is the importance of descending tracts in the muscle stretch reflex?
Descending inputs modulate all levels of the stretch reflex.
What are the different descending tracts that affect the muscle stretch reflex and what do they do?
- Corticospinal tract -> Signals voluntary movement, which can override stretch reflexes
- Vestibulospinal tract -> Primarily engages extensor antigravity muscles (e.g. for when you stumble)
- Reticulospinal tract -> Modulates intensity of reflex activity via gamma motor neurons
What are some recurrent pathways (i.e. those that affect the muscle of origin) in the muscle stretch reflex?
[IMPORTANT]
Renshaw inhibitory interneurons inhibit the muscle that is firing to prevent fatigue (feedback inhibition).
What motor fibres innervate intrafusal and extrafusal muscle fibres?
- Aα -> Extrafusal muscle fibres
- Aγ -> Intrafusal muscle fibres
Summarise how the Aα and Aγ motor neurons work together in muscles.
- When there is a load applied to a muscle so that it is stretched, the muscle stretch reflex is activated and the extrafusal muscle fibres contract due to Aα innervation (shown on the left)
- The contraction of the muscle leads to slack in the intrafusal fibres (shown on the right)
- This means that there is decreased Ia fibre firing from the muscle spindle since there is no tension
- Thus, if Aα and Aγ fibres are co-activated and fire together, then we solve this problem because the Aγ fibres stimulate the intrafusal fibres to contract and detect the stretch again
Draw a diagram to summary alpha-gamma co-activation.
What are the different gamma fibre types?
Both innervate the intrafusal fibres in muscle:
- Gamma dynamic fibres
- Gamma static fibres
What are the two components of the muscel stretch reflex?
- M1 (Short latency component) -> Monosynaptic connection (i.e. the classic reflex arc involving only one synapse) involved primarily with axial and proximal muscle control.
- M2 (Long latency component) -> This is formed by the collaterals that the afferent fibres send up to the cerebral cortex, then the descending fibres from the motor cortex. It is slower than monosynaptic but shorter than voluntary reaction time. Involved mostly in fine voluntary distal limb movements.
Give some examples of disorders of the muscle stretch reflex.
[EXTRA]
- If the motor afferent input to a muscle is severed then the muscle offers little resistance and becomes flaccid.
- If a muscle shows high resistance to stretch, it is described as hypertonic (spastic), often because of hyperactive stretch reflex. This is often due to lack of descending control of reflex pathways.
What fibres innervate Golgi tendon organs?
Ib fibres
Describe the structure of a Golgi tendon organ.
- Ib afferent fibres are interwoven with collagen fibres at the muscle-tendon junction
- Stretching the tendon compresses and stimulates the nerve
What is the function of the Golgi tendon organs?
- The GTO effect was first thought to be purely protective to prevent over-contraction of muscles.
- However, GTOs are sensitive to small changes in tension so they are also involved in fine movement
Describe the reflex arc that Golgi tendon organs are involved in.
- Ib afferents detect excess contraction in a muscle
- They synapse onto an inhibitory interneuron that provides feedback inhibition onto the a motor neurons that innervates the original muscle
Describe how Golgi tendon organ reflexes are modulated.
- Golgi tendon organs are usually involved in inhibition of the contracting muscle
- However, during voluntary movements, such as locomotion, this can be reversed by descending pathway modulation
Summarise the reflex response when you step on a painful stimulus.
[IMPORTANT]
This is the combination of the ‘flexor withdrawal reflex’ and ‘crossed extensor supporting reflex’.
(The crossed extensor reflex is mentioned in the spec)
What is the Babinski response?
[IMPORTANT]
What does an abnormal Babinski response indicate?
- An abnormal Babinski response where the toes point up may indicate an upper motor neuron lesion since the corticospinal tract is involved in modulating the reflex arc and therefore if it is damaged, then the toe extensors may dominate.
- The exception is in newborns, since they do not yet have mature descending tracts.
(Check this -> I thought descending tracts are usually inhibitory)
Give some forms of altered reflex activity and what they may indicate.
What are primary and secondary spindle afferent fibres?
[IMPORTANT]
- Primary -> Type Ia fibres
- Secondary -> Type II fibres
What are the different spinal interneurons you need to know about?
[IMPORTANT]
- Ia inhibitory interneurons -> Inhibit the motor neurons that innervate the antagonist in the muscle stretch reflex
- Ib inhibitory interneurons -> Inhibit the over-contraction muscle in the Golgi organ tendon reflex
- Renshaw interneurons -> Inhibit the muscle that is firing in the muscle stretch reflex to prevent fatigue
Describe the arrangement of motor neurons in the grey matter of the spinal cord.
What are upper and lower motor neurons?
[IMPORTANT]
- The upper and lower motor neurons form a two-neuron circuit.
- The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body.
Summarise the consequences of upper and lower motor neuron lesions.
[IMPORTANT]
Lower motor neuron (LMN) lesions:
- Muscle flaccidity/weakness
- Decreased tone
- Profound muscle atrophy
- Fasciculations present
- No Babinski sign
- Decreased muscle stretch reflex
Upper motor neuron (UMN) lesions:
- Spasticity
- Increased tone
- Minimal muscle atrophy
- Fasciculations absent
- Babinski sign present
- Increased muscle stretch reflexes
What is meant by the medial and lateral descending systems? What is the function of each?
[IMPORTANT]
- Lateral descending system (top left in red) -> These are the descending tracts that innervate distal limb muscles.
- Medial descending system (bottom left in red) -> These are the descending tracts that innervate axial and proximal limb muscles.
This is easy to remember because the lateral descending system innervates more lateral muscles.
What are the medial and lateral descending systems made up of?
Lateral:
- Lateral corticospinal tract
- Rubrospinal tract
- Reticulospinal tract (medullary)
Medial:
- Anterior corticospinal tract
- Reticulospinal tract (pontine)
- Vestibulospinal tract
- Tectospinal
Compare the medial and lateral descending systems in terms of what they connect to and their function.
Lateral:
- Decussate
- Often end on α motor neurons
- Control distal limb muscles for precision grip and palpation
Medial:
- End, bilaterally, on interneurons
- Very few monosynaptic endings on α motor neurons
- Control posture, locomotion, and proximal components of reaching movements via supplying the mainly axial and proximal muscles
For the corticospinal tract, state:
- Origins
- Course (inc. decussation)
- Terminations
- Functions
[IMPORTANT]
- Origins: Motor cortex (area 4) and premotor cortex (and also somatosensory area for modulation)
- Course: Internal capsule -> Crus cerebri -> Pons -> Pyramids of medullary pyramids -> Spinal cord
- Decussation at the bottom of the medulla:
- Lateral corticospinal tract decussates and descends as the contralateral lateral funiculus (90% of fibres)
- Anterior corticospinal tract doesn’t decussate and descends as the anterior funiculus (10% of fibres)
- Terminations and functions:
- Lateral corticospinal tract -> Terminates mostly on alpha motor neurons and controls distal limb muscles
- Anterior corticospinal tract -> Terminates mostly on interneurons and controls axial muscles bilaterally
What is the consequence of these lesions?
The key is understanding whether the lesions are upper or lower motor neuron lesions.
What does the circled bit of this diagram show?
- This is a cross-section of the midbrain.
- It shows the location of the corticospinal & corticobulbar fibres form cerebral peduncles, which make up the cerebral peduncles.
What is decerebration and what is the result?
[IMPORTANT]
- The elimination of cerebral brain function in an animal, often by cutting across the brain stem.
- The vestibulo-/reticulospinal tracts remain intact, however.
- Depending on the level of this cut, this is essentially a upper motor neuron lesion of various descending tracts.
- Thus it often leads to spastic rigidity (hyper-reflexive state where both arms and legs extend).
For the vestibulospinal tract, state:
- Origins
- Course (inc. decussation)
- Terminations
- Functions
[IMPORTANT]
- Origins: Lateral, medial & inferior vestibular nuclei
- Course:
- Medial vestibulospinal tract -> Runs ipsilaterally (in ventral funiculus)
- Lateral vestibulospinal tract -> Runs bilaterally (in ventral funiculus)
- Terminations and functions:
- Medial vestibulospinal tract -> Innervates axial muscles of neck
- Lateral vestibulospinal tract -> Innervates antigravity (extensor) motor neurons for posture
For the tectospinal tract, state:
- Origins
- Course (inc. decussation)
- Terminations
- Functions
[IMPORTANT]
- Origins: Superior colliculus (of midbrain)
- Course: Fibres decussate immediately and descend in the contralateral ventral funiculus of the upper spinal cord
- Terminations and fucntions: Terminates on axial neck and upper thorax muscles to control neck movement in the direction of gaze
For the reticulospinal tract, state:
- Origins
- Inputs
- Course (inc. decussation)
- Terminations
- Functions
[IMPORTANT]
- Origins:
- Medial reticulospinal tract -> From pontine reticular formation
- Lateral reticulospinal tract -> From medullary reticular formation
- Inputs: Ascending tracts, motor/premotor cortex, vestibular apparatus, tectum
- Course:
- Medial (pontine) reticulospinal tract -> Descends ipsilaterally along the ventral funiculus
- Lateral (medullary) reticulospinal tract -> Decussates immediately and then descends contralaterally along the anterolateral funiculus
- Termination and functions:
- Medial (pontine) reticulospinal tract -> Facilitates spinal reflexes, increases voluntary movements and increases muscle tone.
- Lateral (medullary) reticulospinal tract -> Inhibits spinal reflexes, decreases voluntary movements and decreases muscle tone.
For the rubrospinal tract, state:
- Origins
- Inputs
- Course (inc. decussation)
- Terminations
- Functions
[IMPORTANT]
- Origins: Large neurons of the red nucleus (midbrain)
- Inputs: Motor cerebral cortex, Cerebellum, Globus pallidus, Reticular formation, Ascending tracts
- Course: Decussates in the midbrain -> Pons -> Medulla -> Lateral funiculus of spinal cord
- Terminations and Functions: Controls distal limb muscles (but function is not clear)
What is the corticobulbar tract and what are the fucntions?
- The descending tract that is responsible for control of cranial nerve motor nuclei (III, IV, VI, Vmandib, VII, IX, X, XI, XII)
- It has a similar initial course to the corticospinal tract.
How is the cortex involved in reflexes?
- Cortical motor areas participate in and regulate reflexes
- They form part of the long loop M2 stretch reflex, where cutaneous and proprioceptors in finger project via dorsal column system and VPL thalamus to sensory cortex and then via corticospinal tract to homonymous motor neurons.
- However, this has long latency.
Summarise the main brain centres and descending pathways involved in motor control.
[IMPORTANT as a summary]
Give some experimental evidence demonstrating the roles of the prefrontal cortex.
[EXTRA]
These tasks may be impaired in PFC lesions.
What are some of the subdivisions of the prefrontal cortex and what are their functions?
[EXTRA]
- Lateral prefrontal cortex: Inhibitory control, executive function, task switching, working memory
- Orbitofrontal PFC: Motivated decision, emotional & autonomic responses: taste, smell, reward (medial), loss chasing behaviour (lateral)
Give some experimental evidence surrounding the posterior parietal cortex.
[EXTRA]
Where is the premotor cortex and what Brodmann area is it?
Lateral side of Brodmann area 6
Give an example of an experiment demonstrating the function of the premotor cortex.
[EXTRA]
This shows that the premotor cortex is involved in programming movements in response to sensory guidance and cues.
What is interesting about the connections between the posterior parietal cortex and premotor cortex?
Inputs to the premotor cortex are along distinct channels that encode different functions (e.g. reaching vs grasping).
What are mirror neurons?
[EXTRA]
Where is the supplementary motor cortex and what Brodmann area is it?
Brodmann area 6
Experimentally, what are the effects of a lesion of the supplementary motor area?
[EXTRA]
(Brinkman, 1981):
- Unilateral lesion of the supplementary motor area disrupts cooperative behavior of the two hands, even if the monkey receives visual feedback.
- Normally, the monkey can push a morsel of food out of the hole and catch it with the other hand, but not after the lesion.
Give some experimental evidence for the role of the supplementary motor area in movement sequences and mental rehearsal.
[EXTRA]
(Roland, 1980):
- Found that these areas were activated during these tasks
Where is the motor cortex and what Brodmann area is it?
Brodmann area 4
Draw the homunculus in the motor cortex.
