Motor Flashcards
We use muscle contractions for more than just moving our limbs. Give 5 other uses.
- Moving the external world
- Moving yourself around
- To preventing movements: for example, maintaining equilibrium,
- Communication- speech/ gesture/ writing
- To move sensors: visual, somatosensory (active touch)
How did the motor system first evolve
How was sense involved
simple organisms with bilateral columns of muscles on either side of the body which were activated alternately to produce swimming (e.g. fish).
sense organs in head provided signals that guided swimming behaviour towards/away from signal
Describe the action of the spinal cord in early simple organisms with bilateral columns of muscle
largely operates autonomously (to generate swimming through the action of intrinsic spinal networks), and a guidance system from the brainstem adjusts the swimming appropriately
Why did the basal ganglia and cerebellum originally develop
to assist the operation of of the guidance system from the brainstem which adjusts the swimming appropriately
How did land-dwelling vertebrates elaborate on the motor systems of early aquatic vertebrates
What did the brainstem do here
by developing limbs, with new groups of muscles that act specifically on each limb appearing
controls the operation of this more sophisticated system in which
limbs need to be coordinated together inappropriate patterns for locomotion.
What is the functional hierarchy of the motor system
IDEA (what is my goal?) PLAN (how do I achieve it?) PROGRAM (which muscles? scale?) EXECUTION (send motor commands) MOVEMENT
How is the motor system defined in this lecture series? (may be useful for essays)
the system that plans and executes movements
What is an advantage of the negative feedback systems
automatically compensate for
unpredicted deviations from the set point (e.g. caused by noise)
What are the major limitations of negative feedback systems caused by
time delays in the feedback loop
(takes time to get to and from the brain)
leads to instability and oscillations in rapid movement control
Why might we think there was an evolutionary attempt to regulate sensory and motor systems via negative feedback systems
t the very fast conduction of sensory and motor signals represents an evolutionary attempt to minimise biological delays, and therefore these problems, in negative feedback systems.
Can we adjust ballistic movements as they are happening?
What does this mean?
no
a motor plan must be formulated in advance and then executed: there is no way that sensory information generated during the progress of the movement can influence its outcome
What is an open loop system
where sensory information is used to generate a prediction of what is needed in the future
feedforward
What does the body use to make the open- loop system work
How does this work
internal model system - : simulators that represents the mechanics of the body and the
behaviour of the external world
Such a system could learn to predict which motor commands are useful in a given situation and even to mentally rehearse movements before actually making them
How does the internal model become accurate
by comparing the actual results with the desired movement:
differences can be used to adjust the
model so that in future it becomes more accurate.
What is a system related to the internal model system that allows feedforward control
how does it work
internal feedback system via efference copies
efference copy is a prediction of the upcoming movement and so this can be compared to the desired result. the actual output is then corrected accordingly if predicted and desired outcomes do not match.
What must be noted about the model systems
depend critically on the accuracy with which they simulate the mechanical world and as such are continually being adjusted and calibrated (they are continually learning).
What is the major motor area of the spinal cord
ventral horn where alpha-motoneurons that innervate muscle fibres are located
Where are most synapses on motor neurons
from spinal interneurons located in the intermediate zone grey matter
What is a motorneuron pool?
Where is it located?
The group of 200-500 motoneurons that innervate a given muscle
close together in the ventral horn usually extending rostro-caudally over
several spinal segments.
Are motor neurons arranged somatotopically in the spinal cord?
yes
distal muscles are represented laterally
What is the basic unit of force production?
is there 1 motor neuron to 1 motor unit?
motor unit
no: Each motorneuron axon branches to innervate many muscle fibres which are distributed throughout the muscle.
How may motor neurons innervate a single muscle
several hundred
What is a key problem of motor control
how is this overcome
how the brain can generate both finely graded, low force contractions when precise control is needed and high forces when strength is needed
motor units can be divided into 3 different types with different properties
What are slow motor units ideal for
what are fast fatigueable good for
Slow motor units are ideal for continuous generation of small forces,
whereas fast fatigueable
units produce high forces, but over a short period
Why can the body not just vary motor neuron firing rate instead of having different types of motorneuron
twitches generated by muscles fuse into a tetanus at quite low frequencies
What is the Size Principle
what does this ensure
motor units are recruited in an orderly sequence as force increases, the lowest force motor units first, the highest force units last
as you increase the force of a movement the next motor unit recruited is always the one that generates the lowest force
What is the mechanism underlying the size principle
developmental plasticity
What is developmental plasticity
motoneurons with low firing threshold innervate few muscle fibres and induce them to become slow twitch, low force and fatigue resistant.
Motoneurons with the highest firing thresholds (which are recruited last)
innervate many muscle fibres and induce them to become fast twitch and high force, but fatiguable
Does the brain have to control each motor neuron independently?
no - , inputs randomly distributed to all of the motoneurons innervating a given muscle will automatically recruit motoneurons in the order lowest force first to highest force last (Size Principle)
What does motor neuron damage lead to
flaccid paralysis
Give 3 ways to damage motor neurons and an example of when each would happen
Motorneuron degeneration - e.g. in ALS (motorneuron disease)
Peripheral nerve damage - (auto immune disease (e.g. Guillain-Barre Syndrome) or peripheral nerve damage (example in dog coming)
Spinal cord damage - e.g. vertebral column dislocation
What are the 3 sources of synaptic input to motor neurons
give examples of each
• muscle spindle afferents (ONLY from muscle spindles!)
•Descending fibres (direct pathways from brain stem or cerebral cortical structures, relatively
rare, with an important exception in primates)
•Spinal interneurons (most numerous, in most cases receiving inputs from both sensory
pathways and descending pathways from the brainstem and/or cerebral cortex)
What is the simplest form of motorneuron activation
through reflexes driven by sensory signals
How can spinal reflex actions be divided
distinctions between reflex actions targeted at specific small groups of muscles and involve in regulation of their force (examples are stretch reflexes and associated reciprocal and recurrent inhibition), and more complex reflexes that generate functional movements that involve multiple muscles (eg nociceptive withdrawal reflex).
What are the different types of proprioceptors
(the receptors from the surface of the body - cutaneous receptors - are exteroceptors,
those that sense the environment at a distance – eyes, ears & nose- are teloceptors
What are the 3 major groups of proprioceptors
- muscle spindle afferents are muscle stretch receptors
- Golgi tendon organ afferents are muscle tension receptors
- joint receptors signal joint position and movement, especially at the extremes
What are muscle spindles
spindle-shaped structures embedded in muscles that give rise to afferents that signal muscle length and change in muscle length.
Typically a muscle will contain between 20 and 100 spindles
Describe the structure of a muscle spindle
encapsulated bundle of small specialised intrafusal muscle fibres – (literally within-spindle muscle fibres).
The ends of these fibres are striated, but they are very small compared to the force-generating extrafusal fibres
Intrafusal fibres are
very small: their contractions generate no tension at the tendons but affect the sensitivity of sensory nerve endings in the spindle.
What are 2 morphological types of muscle spindle intrafusal fibre
bag fibres: swollen central region containing may nuclei and has contractile ends
chain fibre:
uniform diameter and uniformally contractile along its length
Which intrafusal fibre will a typical muscle spindle contain
several of each type
What are the 2 types of sensory fibre that attach to intrafusal muscle fibres
primary (Ia) spindle afferents
secondary (II) spindle afferents
Describe primary spindle afferents
very large (and thus very fast conducting) axons which have terminal branches that end in coils (annulospiral endings) around the central region of the intrafusal muscle fibres
Describe secondary spindle afferents
Smaller, slower conducting afferent fibres end adjacent to the central region of the intrafusal muscle fibres.
true or false Both primary (1a) and secondary (group II) sensory endings on muscle spindles are activated by stretch
true but in different ways
Describe the contractile properties of intrafusal bags
have contractile ends, which are
visco-elastic, but the central bag region is not
contractile, but is elastic.
What happens when rapid stretches are applied to bag fibres
What is the result
immediately elongate the central bag
region (providing a rapid strong activation of the
afferents at stretch onset - a dynamic response).
Subsequently this stretch is relieved as the viscous
ends the fibres elongate reducing stretch in the
central region.
during the stretch the central region is greatly elongated, generating a
strong initial response but this declines to be
distributed across the whole fibre at the end of the stretch
True or false
chain fibres have different mechanical properties across their length
What is the purpose
false have uniform mechanical properties throughout their length, so the sensory endings on them (especially secondary endings) signal muscle length approximately linearly (a static response).
What do the properties of bag fibres allow it to signal
primary spindle endings on the
bag fibres a largely rapidly adapting response (i.e. have a dynamic sensitivity).
Thus, their responses signal changes in muscle length
Do primary spindle afferents connect to bag or chain fibres?
What do they therefore signal
both
have both static and dynamic components to their reponses
Do muscle spindles have efferent neural connections
yes
there is an efferent control of the sensitivity of the terminals
What happens to muscle spindles when the motor neurons innervating them are activated
Since the ends of intrafusal muscle fibres are contractile, they shorten when activated by the motoneurons that innervate them
Which motor neurons innervate muscle spindles
In most cases intrafusal muscle fibres are innervated by a group of smaller and more slowly conducting motoneurons than the alpha motoneurons, the gamma motoneurons, the result of which is to stretch the central region of the fibres where the receptors are located. gamma motorneuron firing thus increases both the firing and the sensitivity the receptors: this complicates the sensory signal.
What does motor neuron innervation of the contractile parts of muscle spindles
provides a mechanism to allowing the receptor sensitivity to be adjusted – for example to allow the spindles to be able to signal length changes from different starting muscle lengths, rather than to become saturated when stretched and too short to signal on the muscle is contracted
essentially adaptation
What suggests motor neuron innervation of intrafusal muscle spindles are more complex than we realise
different gamma – motoneurons (dynamic and static) innervate bag and chain intrafusal fibres, which enhance the dynamic and
static responses of spindle afferents respectively.
What happens if the brain decreases the drive to gamma motoneurons to intrafusal fibres
How does this affect sensitivity
they will relax,
reducing sensivity to stretch
How will contracting the parent muscle affect the muscle spindle
spindle becomes slack
What are different things it could mean if a muscle spindle changes afferent firing
How does the brain differentiate
could be generated by muscle length changes OR by altered gamma motoneuron activity (or, more likely, by combinations of both!)
efference copy
What are Golgi Tendon Organs
a second type of
proprioceptor located in tendons that are activated by tension in the tendon
How does passive stretch of a relaxed muscle affect a Golgi tendon organ
How does this differ from a muscle contraction?
tension in the tendon does not rise much due to the elasticity of the muscle fibres
generate tension directly in the tendon and make tendon organs fire briskly - i.e. they signal active tension (signal will be proportional to load on muscle)
Which signals underlie the stretch reflex
muscle spindle signals
What are stretch reflexed
do they work if the muscle is paralysed
Most muscles respond to being stretched by
contracting. This reflex is generated by the nervous system:
disappears if the muscle nerve is cut (or if the muscle is paralysed).
Why might we consider stretch reflexes to be homeostatic
maintaining muscle length in the face of an imposed stretch (stretch activates spindles, which excite alpha-motoneurons, which generate contraction that counteracts the stretch).
When might homeostatic stretch reflexes be useful
in postural control: if the body sways then the stretched
leg muscles will automatically contract to counteract the sway, helping maintain an upright stance.
What is the neuronal circuit of the stretch reflex
Monosynaptic connections from muscle spindle afferents to the alpha-motoneurons that innervate the same muscle underly the stretch reflex
True or false
there are multiple monosynaptic reflexes
false
the stretch reflex is the only monosynaptic reflex
all others involved spinal interneurons
Do all muscles have a stretch reflex?
The large majority of muscles have a stretch reflex, but eye and tongue muscles are notable exceptions.
Do muscle spindles only affect its own mother muscle?
muscle spindles also excite the motoneurons of close synergists (e.g. stretch of brachialis at the elbow produces a reflex in
biceps as well as brachialis).
also excite interneurons that inhibit antagonist muscles (reciprocal inhibition), preventing further stretch (e.g. stretch of brachialis and or biceps will produce inhibition of triceps motorneurons, and vice versa).
What is the benefit of a negative feedback system
How might this apply to the stretch reflex
they automatically compensate for deviation from a set point
compensate for unexpected loads that stretch a muscle: deviation of the muscle length from a pre-set value will be detected by muscle spindle afferents and will rapidly generate excitation of motorneurons, that could restore the pre-set length
Why might eye muscles not have a stretch reflex
they do not encounter unexpected loads, which seems to support a role for the stretch reflex in counteracting unexpected loads.
Why is the idea of stretch reflexes being a negative feedback system attractive?
the brain needs to both calculate the geometry of movements (position and velocities of movement), but different loads will require different muscle forces to achieve these positions/velocities. If an automatic system produces the appropriate force levels regardless of load, all the brain need do is determine the muscle lengths needed – the spinal cord could take care of the forces needed.
What are the 2 issues with the stretch reflex being a negative feedback system used to deal with unexpected loads
elaborate briefly on each
gain (For this to work there must be a compensatory contraction for any given stretch.)
delays (would lead to oscillations)
If the stretch reflex were designed to maintain muscle length in a negative feedback style, what would the gain have to be
what is gain in reality
The stretch reflex should have a gain of 1 (any given amount of stretch will elicit contraction to precisely counteract it with the same amount of contraction)
IRL = <1
What happens to the stretch reflex when the descending motor systems are damaged
What causes this
What can these changes be interpreted as
spasticity
the stretch reflex is exaggerated (a high gain) - brisk responses are evoked to tapping tendons and muscles have tonic contraction
• Oscillating muscle contractions follow stretch (MYOCLONUS or just CLONUS).
they are characteristic following motor cortex damage (eg stroke or cerebral palsy)
what does most of the evidence on the stretch reflex suggest about its role
despite its attractiveness,
the stretch reflex cannot be strong enough to fully compensate for unexpected loads as a perfect negative feedback system without becoming unstable.
Can you change the gain of a stretch reflex
yes - by adjusting sensitivity of the muscle spindle
What did Prochazka’s recordings in cats suggest about the stretch reflex
the brain can control the strength of the stretch
reflex to suit the circumstances: in slow movements and situations where precision is needed then
gain may be set high (high spindle sensitivity and therefore a strong stretch reflex), whereas for
fast movements such a gain might generate clonus, so gain may be set low
What is kinesthesia
a sense of position,
movement & effort
How is kinesthesia sensed by different parts of the body
highly integrated with major contributions from visual, vestibular, tactile, and efference copy information, as well as from proprioceptors.
Different parts of the body may use this information differently – for example eye position (which has to be know precisely for visuomotor control) is principally signalled by efference copy, whereas hand position depends to a great extent on cutaneous sensation.
What is the most important role of proprioceptors?
provide information for supraspinal motor systems, which are involved in predictive feedforward control of movement (e.g. cerebellum and motor cortex) using model systems
Where is the main projection of proprioceptors to
cerebellum via spinocerebellar pathways
What does proprioceptive information to the cerebellum do
informs internal model systems about the current state of play at the outset of movement (on which model predictions can be based), and assessment of the outcome after the movement (critical for modifying the model systems ensuring that they are accurate).
What is reciprocal inhibition
What does it parallel and what are the neurons involved
parallels the stretch reflex.
Muscle spindle primary afferents excite glycinergic inhibitory interneurons that inhibit motoneurons of antagonist
muscles when a muscle is stretched.
These are effects are mediated by a group of Ia Inhibitory interneurons associated with each motoneuron pool.
What mediates recurrent inhibition
Renshaw cells
What is recurrent inhibition
Motoneuron axons have branches (recurrent collaterals) within the spinal cord that innervate inhibitory Renshaw cells - inhibit the original motor neuron
What is the purpose of recurrent inhibition
regulate the timing of motoneuron firing, preventing synchrony
How are the stretch reflex, reciprocal and recurrent inhibition different from most reflexes
these apply to the muscle in which the stretch or activation occurs.
Most reflexes activate specific patterns of movement via groups of muscles, (like nociceptive withdrawal reflexes)
Give 4 general principles about spinal reflexes
most reflexes are:
1) context dependent
2) multi-joint responses
3) Different types of afferents can contribute to the same reflex pathway
4) Reflex pathways are controlled by descending pathways
What does it means to say that most reflexes are context dependent
the same stimulus may evoke different reflexes depending on the behavioural context
What are withdrawal reflexes
What interneurons mediate these reflexes
Noxious stimuli generate a coordinated pattern of muscle contraction that moves the stimulated part of the body away from the stimulus
interneurons that process information
relayed from the dorsal horn
What are 2 properties of withdrawal reflexes that reflect basic properties of reflexes generally
spatial and temporal summation - stimuli at adjacent sites or closely timed sum to give larger responses
local sign - different reflexes are evoked from different locations
Give an example to illustrate how withdrawal reflexes differ based on location
Stimulation of the plantar surface of the foot evokes leg flexion, stimulation of the foot dorsum evokes leg extension: both remove the skin surface from the source of the stimulus.
What does it mean to say tendon organ reflexes illustrate context dependence
Tendon organs generate reflexes via different groups of spinal interneurons, which are active depend on the motor state
In static or resting postures activation of tendon organs it evokes inhibition of the parent muscle, but during locomotion the effect is reversed to excitation, where it supports contraction against a load (in locomotion produced by the body weight).
Do tendon organs mediate the clasp-knife reflex
When can this specific reflex be seen
no
after a stroke
Name 3 neonatal reflexes that change with development
- Grasp reflex
- Babinski’s sign (plantar reflex)
- Reflex stepping
How does Babinski’s sign differ between children and adults
scratch sole of the foot
infant: toes curl up
adult: toes curl down
Why is Babinski’s sign clinically useful
Following brain damage changes in spinal reflex patterns occur, and neonatal reflexes may reappear
Give 3 examples of changes to spinal reflex patterns following brain damage
Exaggerated stretch reflexes (Spasticity). Muscles are tense and stiff. Stretch elicits strong reflexes and clonus (a negative feedback system oscillating)
- Babinski’s sign – toes turn up in response to plantar stimulation: reappearance of the neonatal reflex
- Clasp knife reflex the limbs snap into extension or flexion
True or false
the spine can only produce simple motor outputs without the brain’s input
false
Even after being separated from the brain, the spinal cord can produce complex motor
outputs.
Locomotion can be spinally generated in most vertebrates, including most
mammals
What are motor central pattern generators
Spinal circuits that have the capacity to
generate a detailed motor pattern appropriate to produce stepping
Why do we think CPGs do not require brain input
Their activity can be seen generating a complex locomotor pattern after removal of the brain, indicating that this is not a brain-generated pattern. (headless chicken)
also survive the removal of sensory inputs, by cutting the dorsal roots of the spinal cord, indicating that an intrinsic spinal mechanism is responsible for this pattern
Can functional locomotion be generated after spinal transection in man
no
Can babies walk?
Although babies make stepping movements in the first few months after birth, these movements have a different pattern to the stepping that appears at about 12 months when walking first appears
What is the general pattern between encephalisation and spinally generated locomotion
greater encephalisation (a larger cerebral cortex) is associated with a weaker ability to generate locomotion in the spinal cord: nonmammalian vertebrates such as Fish and Amphibia routinely generate strong locomotion after removal of the brain.
Mammals are less prone to do so.
In young mammals this ability is stronger than in adults
What are the 2 major descending motor locations in the spinal cord?
both in white matter
dorsal lateral and ventromedial
Which of the descending motor pathways is evolutionarily older
What does this pathway do
VENTROMEDIAL PATHWAYS are evolutionarily ancient, and exist in all vertebrates
control axial (trunk) and proximal limb muscles and play a role in whole body movement (locomotion/posture). These tend to produce stereotypic whole-body movements and postures.
What is the largest of the descending motor ventromedial pathways
Where does this originate
Reticulospinal pathway
reticular formation
What is the reticular formation
a wide expanse of grey matter running throughout the entire brainstem
What is the reticulospinal pathway important for?
What is it controlled by?
principal route through which spinal
central pattern generators are activated and is also an
important pathway for characteristic whole body postures and movements
cerebral cortex
Where does the vestibulospinal system begin, and where does this origin receive input from?
originate from the vestibular nuclei, which receive inputs from the vestibular organs of balance.
What are the vestibulospinal systems especially important for
antigravity actions
Where does the tectospinal pathway begin?
What is the purpose of this pathway?
from superior/rostral colliculus
pathways through which rapid sensory orientation movements are generated
What is the rostral/ superior colliculus important for
visual information processing
What are the main ventromedial descending motor pathways
reticulospinal pathway
vestibulospinal system
tectospinal pathway
What are the dorsolateral spinal systems important for
they are the most important route through which goal directed movements of the limbs are driven, especially movements of the hands, feet and face (and other prehensile structures, like the lips). These systems control the more individuated movements of the limbs, rather than whole body postures and movements
Where does the rubrospinal pathway originate
Red Nucleus in the midbrain
important in cats, dogs and monkeys
vestigial in humans
