Sensorimotor System Flashcards
Introduction - Sensorimotor system
Responsible for movement, important for life. Found everywhere e.g. tongue.
3 Main Principles of Sensorimotor Function.
- Sensorimotor system is hierarchically organised - Directed by goal-base commands which go from the association cortex to muscles, allowing higher levels to perform complex tasks.
- Motor output guided by sensory input in which the system monitors the effects of activities and adjustments made by lower levels of hierarchy.
- Learning can change the nature and focus of sensorimotor control. During the initial stages, a response is performed under conscious control, after performed without conscious regulation.
Describe the role of the posterior parietal cortex
Receives input from diff areas and integrates this info. Important before movements where need to gather and integrate info before carrying out movement.
Not directing movement, but planning the goals of the system.
Tested by stimulation to explore neurofunction.
- Low Intensity Stimulation - Strong desire to move contralteral limbs/facial muscles, shows that it sets intentions for movement
- High intensity stimulation = shows an illusory experience of actual movement, indv thinks they said something but didn’t - Area involved in production and intention of movement
Explain what happens when the posterior parietal cortex is damaged
Apraxia
- Caused by stroke, brain injury
- Difficulty in planning to perform a requested voluntary movement on command
- Indv has fully functioning movement, can perform movement under natural conds, but not out of context or if imagined.
Contralateral Neglect
- Damage to right cerebral hemisphere
- Disrupted ability to attend to stimuli on one side of the body - Items on opposite side of body not attended to e.g. might not eat half of meal.
- Corbetta & Shulman (2011) - Indvs do not respond to the left sides of objects regardless of where objects are in visual field. Some fail to respond even when presented horizontally or upside down.
- Suggested that info is being unconsciously perceived.
- Geng & Behrmann (2002) - Objects repeatdly presented at same sport. Tended to look at same spot in future trials even though unaware of object.
Describe the role of the dorsolateral prefrontal association cortex
- Receives projections from the posterior parietal cortex and sends projections to areas of the secondary motor cortex and frontal eye field.
- Neurons fire first rather than what most cortical motor neurons do which is fire in anticipation of motor activity.
- Suggested that decisions to initiate voluntary movements are made, but they depend on the interactions with the posterior parietal cortex and other frontal areas.
Describe the secondary motor cortex
Receives input from the association cortex, feeds into into primary motor cortex.
Consists of multiple brain areas
- Supplementary motor area
- Premotor area
- Cingulate motor area
Stimulation of area produces physical movements on both side of body compared to association cortex. Recordings show that neurons become active prior to imitation of a voluntary movement and continue to be active throughout movement.
Areas thought be involved in the programming of specific patterns of movements after taking general instructions from dorsolateral.
Describe mirror neurons. Which part of sensorimotor system are they attributed to?
Secondary motor cortex
Fire when an indv performs a particular goal-directed hand movement or when observe the same goal directed movement by another person.
Discovered by Rizzolatti, Fogassi & Gallese (2006) when studying a ventral premotor neuron of a monkey that fired when the monkey reached for one object, but not for another. Also fired when experimenter picked up same object. Important bc provide possible mechanism for social cognition as mapping actions of others shows degree fo social understanding.
Controversial in humans, invasive. Many studies using fMRI (blood-flow, not neurons)
Describe the primary motor cortex
- Point of convergence for cortical sensorimotor signals located in the precentral gyrus of the frontal lobe.
- Major point of departure for sensorimotor signals from cerebral cortex.
Sensorimotor system plastic Damage
- Disrupt ability to move 1 body part - Astereognosia
- Reduce speed, accuracy + force of movements
- Lesions do not eliminate voluntary movement bc parallel pathways
Describe the role of the cerebellum in the sensorimotor system
Receives input from primary and secondary motor cortex, brain stem motor nuclei and feedback from somatosensory and vestibular systems.
One hypothesis is that its function is to compare inputs and correct movements as they happen.
Damage leads to…
- Loss of precise control over speed + force of movement
- Difficulty in maintaining steady posture, speech, eye-movements
- Also gives cognitive, sensory and emotional deficits - Broad deficits hard to tie it to a particular process
Describe the role of the basal ganglia in the sensorimotor system
Organised by collection of interconnected nuclei Part of the neural loops that receive cortical input from multiple cortical areas and transmit it back to cortex via thalamus Role in habit learning - motor learning acquired gradually
Explain the dorsolateral descending motor pathway
There are 4 pathways in which neural signals are conducted from the primary motor cortex to motor neurons of the spinal cord.
Dorsolateral pathways
Dorsolateral corticospinal tract
- Axons descend from the primary cortex through medullary pyramids which descends into white matter
- Notable neurons = betz cells - large pyramind neurons
- Most axons synapse on small interneurons of spinal gray matter which synapse on the motor neurons of distal muscles of wrist etc.
Dorsolateral corticorubrospinal tract
- Second group of axons that descends from the PMC in the red nucelus of the midbrain.
- Axons synapse on interneurons that synapse on motor neurons that project into distal muscles of arms and legs.
Compare both the dorsolateral motor pathways and the ventromedial motor pathways
Similarities
- Each composed of 2 major tracts
Differences
- Ventromedial tracts more diffuse. Axons innervate interneurons on both sides of spinal gray matter in diff segments. Dorsolateral tracts terminate in the contralateral half of one spinal cord segment
- Motor neurons activated by ventromedial tract project to proximal muscles of the trunk and limbs whereas motor neurons activated by dorsolateral tracts project to distal muscles e.g. finger muscles
Explain reciprocal innervation
Principle of spinal cord circuitry. Ensures reflex occurs smoothly. Means that activation of one of the antagonistic muscle leads to inhibition in other as smooth limb movements would not be possible if two antagonistic muscles fully active at once.
Grillon (1985) Do you need a brain to walk?
Cats spinal cords separated from brain. Severed efferent and afferent signals, not communicating. Held on sling over treadmill. Cat makes walking movements. Recieved sensory feedback that accompanies walking. Due to neurons in spinal cord, not brain.
Shows that walking can be controlled by circuits in the spinal cord. Suggests that there are central sensorimotor programs. All but the highest levels of sensorimotor hierarchy have certain patterns of activity stored in them. Once activated, learned and stored, lower areas can operate independently.
Describe how central sensorimotor programs are developed
Fentress (1973)
- Some sensorimotor programs develop without practice, born with.
- Typical mouse grooming behaviour - coordinated movements of forelimbs and shoulders, tilt of head.
- Had 2 groups, one grew up without front paws and one normal.
- Without paws would still try to produce species typical behaviour, present from birth. -
- Sensory feedback important as forelimbles mice would interrupt grooming to lick cage mate.
Other programs develop with practice. Jenkins et al. (1994)
- Comparison of PET activity during newly-learned vs well practiced tasks.
- Activated areas during newly learned sequences = a lot of areas. Almost every area of sensorimotor hierarchy.
- Well-practiced - few areas activated. Low activity.
Describe the neural circuits that control muscles
Muscles generate force by contraction. Force only in 1 direction.
Motor pool =All motor neurons that innervate the fibres of a single muscle 2 categories of skeletal muscles
- Flexors - Bend or flex joint
- Extensors - Straighten or extend
