sensorimotor system Flashcards
types of motor control (4)
voluntary (run, talk)
goal-directed (conscious, explicit, controlled)
habit (unconscious, implicit, automatic)
involuntary (eye movements, cardiac, posture)
3 response pathways (evolutionary - for defence)
all lead to the motor, autonomic, and endocrine systems for defence-related output
pain –> spinal cord –> escape reaction
e.g. move hand away from hot object
loom → sensorimotor midbrain → “avoid”
e.g. basic visual processing of large threat, therefore not in oculomotor area - more complex than pain reaction needing more processing
learned threat → cortex and limbic system → “avoid”
e.g. learned that a gun is a weapon before knowing it is a threatening thing, so know to be scared and avoid it
hierarchical control architecture –> where different stimuli is processed in the brain to produce output (7)
bad (3)
neutral (3)
cognitive analysis (1)
complexity
noxious/contact –> spinal cord –> reflexive withdrawal
sudden distal –> hindbrain –> startle response
species-specific threat –> midbrain and hypothalamus –> species specific response - fight flight or freeze
neutral –> thalamus
complex neutral –> sensory cortex
context –> hippocampus and septum
–> these 3 lead to the amygdala for conditioned emotional responses
cognitive analysis –> frontal cortex –> response suppression
more complex and sophisticated threat detection and avoidance behaviour needs more complex processing/neural systems
sensorimotor system
- top down
- bottom up
descending and ascending motor circuits for control and sensory feedback
top down - descending motor circuits
association cortex –> secondary motor cortex –> primary motor cortex –> brainstem motor nuclei –> spinal motor circuits –> motor units –> effect
also with input high up from basal ganglia (what to do) and cerebellum (how to do it)
bottom up - sensory feedback
muscles sensory systems passes messages to each of the higher up regions - and basal ganglia and cerebellum to change based on this feedback
3 types of muscle
skeletal
smooth
cardiac
antagonistic arrangement of muscle fibres
combined co-ordinated action - e.g. 6 muscles in eye allow it to move in different directions very precisely
fibres can only contract or relax
recruitment of muscle fibres
small or large motor units are used depending on what needs to happen - fine and precise movements or strong ones
what about fibres determines strength
number of fibres varies between people - doesn’t change much with time or training - due to genetics
muscle size and strength can be changed with training and is from the cross sectional area of individual fibre types
muscle contraction process
myosin cross-bridge forms as myosin head attaches to actin filament (due to influx of Ca2+ from action potential) and bends to move the actin filament
it then releases using ATP, extends again, and reattaches
during this, ACh is released, triggering biochemical cascade in muscles - Ca2+, Mg2+, ATP
sarcomere = basic contractile unit of muscle fibre made of actin and myosin
rigor mortis
release of ACh leads to Ca2+ being released
triggers myosin to bind to actin, as in muscle contraction
ATP is required to break bond between actin and myosin but isn’t present due to no oxidative metabolism in death
therefore muscles remain contracted until enzymes breakdown the actin and myosin
motor unit
one motor neuron and all the muscle fibres it innervates
number of fibres innervated depends on functional requirements of the muscle - either control or strength
often many MNs for each muscle, some only have a few
size principle of muscle units
units are recruited in order of size from smallest to largest
fine control required at low forces, but then precision is substituted for strength
therefore moving heavy objects is jerky as small motor units aren’t used
fast and slow muscle fibres (3)
slow = keep going all day - don’t fatigue e.g. for standing
fast fatigue resistant = for walking and running, use for a while before they fatigue
fast fatigable = kept in reserve for short bursts of movement e.g. jumping and so fatigues very quickly
explains why sprinters aren’t as good at long distance as they train their fat fatigable muscles not the fatigue resistant ones for endurance
LMNs
lower motor neurons
alpha = innervate extrafusal fibres for muscle movement and function
gamma = innervate intrafusal fibres for stretch detection
cell bodies found in grey matter of spinal cord or brainstem
fewer fibres in unit = greater resolution
all or nothing - LMN activates all fibres
all fibres in a unit are the same type of fibre, distributed throughout muscle for evenly distributed force, reducing effect of damage
motor pool
all LMNs that innervate a single muscle
pool contains both alpha and gamma LMNs arranged in a rod like shape in ventral horn of spinal column
cell bodies activated by:
sensory info from muscles (dorsal root) e.g. reflexes
descending info from the brain e.g. voluntary control to withstand reflexes or make co-ordinated movements
proprioception
ability to sense movement and action
CNS needs to know how stretched or tense muscles are
sensed by golgi tendon organs (GTO) and muscle spindles
golgi tendon organs (GTO)
sense tension in muscle - force
within tendons (where muscle joins bone)
under extreme tension, GTOs can inhibit muscle fibres via circuit in spinal cord to prevent damage
muscle spindles
sense stretch in muscle - muscle length
integral to reflex circuits - e.g. patellar reflex is caused by muscle elongating and jerking
two types of muscle fibre
extrafusal = do the work of the muscle
intrafusal = within muscle, sensory fibre containing the muscle spindle - sensory neuron connected,
intrafusal fibres
can change amount of force needed in response to changing environment (e.g. when object changes weight whilst being held)
detects strength regardless of current muscle length - it is kept to a set length
controlled by gamma LMNs so it can always detect slight changes
complex reflex - withdrawal reflex
reciprocal innervation - antagonistic muscles contract when others relax
e.g. steady self when one leg is lifted by taking more weight through the other leg, otherwise you would fall
vestibular reflex - righting reflex
detection that body isn’t upright and acceleration due to gravity from falling
combines this with visual, somatosensory, and proprioceptive sensory input to specify pattern of motor activity to restore uprightness
cerebellum = intended motor plans combined with current situation to compute desired motor activity
function of pathways in brainstem and midbrain
smooth out movement, mainly unconscious e.g. balance, postural control
speech - evolutionary brain areas
speech = primitive sound sculpted by cortex - evolutionary (grunts etc)
ancient and modern (cortical) control systems work co-operatively
primary motor cortex (M1) control
top-down control over muscles, directs action
sometimes only has one synapse in spine between cortical neuron and innervation of muscle
where does motor command in motor cortex come from
pyramidal cells layer 5-6
cell bodies in grey matter of cortex
axons can project directly or indirectly to spinal cord and LMNs
axons form the pyramidal tract
pyramidal cortical control can be in co-operation with brainstem for complex sequences of movement
e.g. excitatory effect of cerebellum and inhibitory of basal ganglia
most projections are contralateral (right controls left)
projections from motor cortex down 2 different tracts
both have direct (corticospinal route) and indirect routes via brainstem nuclei (different nuclei tho)
dorsolateral:
- indirect via red nucleus
- innervates contralateral side of one segment of spinal cord
- sometimes direct to alpha motor neurons (LMNs)
- project distally e.g. to fingers
ventromedial:
- indirect via tectum, vestibular nuclei, reticular formation, cranial nerve nuclei
- diffuse innervation projecting to both sides and multiple segments of spinal cord
- projects proximally e.g. trunk and limbs
what is the basal ganglia
group of nuclei beneath cortex:
substantia nigra (pars compacta and pars reticulata)
caudate and putamen (together = striatum)
subthalamic nucleus
globus pallidus (internal and external segments)
role in motor control isn’t fully understood
has excitatory input from many areas of cortex (glutamate)
inhibitory output (GABA)
not just motor loops - also oculomotor, prefrontal, and limbic
basal ganglia selection problem
multiple command systems spatially distributed for parallel processing but motor resources cannot complete them all
needs to resolve to decide what to do
basal ganglia disinhibitory gating of motor cortex output LOOK AT NOTION PAGE!!!!!!!
look at diagram to understand!!!!!!!
cerebellum
parallel processor for smooth coordinated movement and cognitive tasks
no direct projections to LMNs
modulates activity of UMNs - contains hald of CNS neurons and projects to almost all UMNs
cerebellum inputs
cortical (via pons) = mostly from motor cortex - copies of motor commands, also somatosensory and visual areas of parietal cortex
spinal = proprioceptive info about limb position and movement - muscle spindles and other mechanoreceptors
vestibular = rotational and acceleratory head movement - semi-circular canals/otoliths in inner ear
cerebellum function
knows current motor command from cortex
knows body position and movement from spine and vestibular
projects info back to motor cortex to compute motor error and adjust cortical motor commands accordingly
learning - along with basal ganglia and cortical circuits
adjusts cognition and movements
exoskeletons and neurorehabilitation
converting brain activity (either from electrode in brain or on scalp surface) to signals for the exoskeleton (wearable robotics) allows for movements when motor function is impaired
using these devices over a period of weeks can help motor recovery
