Sensorimotor Systems

Question 1

Sensorimotor hierarchy within the brain

Answer 1

1. Association cortex
2. Secondary motor cortex
3. Primary motor cortex
4. Brain stem motor nuclei
5. Spinal Motor Circuits

Question 2

Sensorimotor Association Cortex

Answer 2

• Top of the sensorimotor hierarchy
• Abstract, goal-directed, planning, deliberate intentions
• Two main areas:
  
  • Posterior Parietal Association Cortex
  • Dorsolateral Prefrontal Association Cortex

Question 3

Posterior Parietal Association Cortex

What happens when you stimulate the PPC during neurosurgery?

Answer 3

• Desmurget et al., (2009)
  
  • Low intensity stimulation (5 mA) - strong intention and desire to move contralateral limbs or facial muscles
  • High intensity stimulation (8 mA) - illusory experience of actual movement

Question 4

Posterior Parietal Association Cortex

What happens when you stimulate the PPC during neurosurgery? - Apraxia

Answer 4

• Apraxia - Usually caused by damage to posterior parietal cortex (e.g., stroke, brain injury, neurodegenerative disease).
• Difficulty planning to perform requested voluntary movements “on command”
• Deficit in voluntary, “willed” action, rather than a general motor deficit
• Challenges particularly prevalent when a movement is required out of context, or if the movement itself is imagined rather than real

Question 5

Posterior Parietal Association Cortex – Contralateral Neglect

Answer 5

• Posterior parietal damage → disrupted ability to attend to stimuli on one side of the body
• Typically caused by unilateral damage to cortex
• Ego-centric bias
• Items on the opposite side of the body are not attended to by the person.
• Seems to be a bias in conscious attention

Question 6

Dorsolateral Prefrontal Association Cortex (DLPFC)

Answer 6

• Receives projections from PPC
• Inputs to secondary motor cortex, primary motor cortex, and frontal eye fields
• In monkeys, neurons in the DLPFC tend to fire in anticipation of overt motor responses, and before firing happens in the primary motor cortex
• DLPFC might facilitate decisions to make overt actions

Question 7

Secondary Motor Cortex

Answer 7

• Primarily receives input from association cortex and feeds information into the primary motor cortex
• In the organisation of the motor hierarchy, might be thought of as middle management.
• Takes general instruction from association cortex and creates more complex movements
• Includes supplementary motor area, premotor area, and cingulate motor area
• Sends majority of input to primary motor cortex
• Stimulation produces physical movements on both sides of the body
• Recordings show activity both before and throughout overt actions
• Generally programs complex patterns of movements, after taking instruction from association cortex

Question 8

Primary Motor Cortex

Answer 8

• Major point of convergence for cortical sensorimotor signals
• The primary departure point of motor signals from the cerebral cortex
• Less concerned with abstract planning, and more concerned with sending specific motor commands to the body

Question 9

Neural Cartography - Wilder Penfield

Answer 9

• Penfield (1940s) pioneering work with epileptic patients and brain stimulation
• Neural cartography = mapping brain function
• Electrical stimulation → subjective experiences & movement
  
  • Temporal lobe stimulation → vivid recall of memory
  • Primary motor cortex stimulation → movements of specific muscles, and sometimes the neighbouring muscles

Question 10

Motor humunculus

Answer 10

• Motor Homunculus (“little man”):
  
  • Somatotopic layout (areas of our body that are near each other are also close to each other in the brain)
  • Contralateral activation of muscles
  • Scaled to intricacy of movement, rather than size of the corresponding body part

Question 11

Conventional view of primary cortex function

Answer 11

• Early studies recorded brain activity from the arm area of the motor cortex while people made arm movements in various directions.
• Individual neurons responded to the specific direction of hand movements
• Conventional view: each neuron in the primary motor cortex codes the direction of movement

Question 12

Updated view of primary cortex function

Answer 12

• Later studies used longer amounts of stimulation and found that primary cortex stimulation produced complex movements, such as eating and drinking behaviours.
• Updated view: each neuron in the primary motor cortex is loosely related to an individual muscle group, but will also engage actions of coordinated muscles to produce “species typical movements” (e.g., eating, grooming, mating behaviours and so on)

Question 13

Brain area outside teh cerebral cortex that are involved in the control of movement

Answer 13

• Cerebellum

Question 14

Cerebellum: Structure and Connectivity

Answer 14

• Latin for “Little Brain”
  
  • While small, the cerebellum has > half of the brains neurons.
  • That means the cerebellum has more computational power than both hemispheres of the cerebral cortex!
• Receives diverse inputs:
  
  • Primary and secondary motor cortex
  • Brain stem motor nuclei
  • Feedback from somatosensory and vestibular systems (maintaining balance)

Question 15

Cerebellum: function

Answer 15

One hypothesis: Thought to compare various inputs and correct movements online, as they happen.

• Cerebellum damage:
  
  • Loss of precise control over speed, force, and direction of movement
  • Difficulty maintaining steady posture, balancing, speech, and eye-movements
  • BUT, deficits are not exclusively sensorimotor.
  • Cerebellum damage also → cognitive, sensory, and emotional deficits.
  • Broad deficits make it hard to tie the cerebellum to a particular process
  • In truth, the cerebellum is still quite poorly understood.

Question 16

Spinal cord basics

Answer 16

• Descending (efferent) pathways carry motor information to the muscles, ascending (afferent) pathways carry sensory feedback to brain and interneurons
• The spinal cord itself has some computational complexity
• Some movements and reflexes can be executed entirely within the spinal cord

Question 17

Motor Neurons, Motor Units, and Muscles

Answer 17

• Motor neuron = a neuron with its cell body located in spinal cord that projects to organs of movement (e.g., muscles).
• Motor units = smallest unit of motor activity
  
  • 1 motor neuron + the muscle fibers it innervates
  • Motor neuron firing → simultaneous contraction of connected muscle fibers
  • Each muscle = Group of muscle fibers held together by a tendon
  • Each muscle can only contract in one direction
• Motor pool = All of the motor neurones that innervate a single muscle

Question 18

The relationship between muscles matters

Answer 18

• Felxors - Extensors
• Synergistic muscles - Antagonistic Muscles

Question 19

Reciprocal Innervation

Answer 19

• Key principle of spinal cord circuitry
• Problem: Smooth limb movements would not be possible if two antagonistic muscles were fully active at once
• Reciprocal innervation means that the activation of one of the antagonistic muscles leads to the inhibition of the other

Question 20

Reciprocal Innervation: Withdrawal Reflex

Answer 20

• Reciprocal innervation ensures that the reflex occurs smoothly.
• Reflex is carried out quickly by computations conducted by interneurons in the spinal grey matter
• This is highly adaptive

1. Thumbtack produces a burst of firing in sensory neurons
2. The burst of firing in the sensory neurons excites excitatory spinal interneuons that excite biceps motor neurons
3. At the same time, the burst of firing in the sensory neurons excites inhibitory spinal interneurons that inhibit triceps motor neurons
4. The simultaneous contraction of the biceps muscle in relaxation of the triceps muscle cuase a rapid flexion of the elbow joint

Question 21

Grillon (1985)

Answer 21

• Cat’s spinal cords separated from the brain via transection
• Cats then held on a sling over a moving treadmill
• Movement from treadmill generates a sense of sensory feedback that normally occurs during walking and the cat starts to make walking movements
• Intriguing suggestion: Walking also appears to be a reflex to stimulation. The movements underlying walking do not rely on hierarchical input from the brain.

Question 22

Central Sensorimotor Programs

Answer 22

• Most complex processes are a combination of well-established simpler processes performed by the lower-level components of the sensorimotor system.
  
  • Idea that all but the very highest levels of sensorimotor hierarchy have certain patterns of activity stored into them.
  • Higher level areas contain more abstract motor programs that activate more specific actions in lower areas.
  • Once central sensorimotor programs activated, learned, and stored, lower areas can operate independently, without top-down input

Question 23

The Development of Central Sensorimotor Programs

Answer 23

• Some sensorimotor programs develop without practice (Fentress, 1973)
  
  • Typical Mouse grooming behaviour: Coordinated movements of the forelimbs and shoulders, tilt of the head, closing on the eyes, and mouth movements to clean the face.
• Other sensorimotor programs develop with practice.
• “It takes 10,000 hours to become an expert at a given skill” (Malcolm Gladwell).
• Shifting Control to Lower Levels
  
  • This move frees higher level brain structures to deal with more demanding or abstract aspects of performance
  • Speed Responses occur faster because responses can be prepared while others are being executed

Question 24

Sensorimotor acitivites can be achieved within lower levels of the hierarchy using

Answer 24

Central sensorimotor programs

Question 25

Sensorimotor programs can be

Answer 25

• Both innate or learned