1. TOPIC 6 The Sensorimotor System
How You Do What You Do

2. 3 Principles of Sensorimotor Control
The sensorimotor system is hierachically organized
Motor output is guided by sensory input
Learning can change the nature and locus of sensorimotor control

3. 3 Principles of Sensorimotor Function
1. Hierarchical organization
Association cortex at the highest level, muscles at the lowest i.e from general goals (cortical level) to specific details of action (lower levels).
Parallel structure – signals flow between levels over multiple paths
Information flow is down, while in the Sensory system informtion flows through the hierarchy.
2. Motor output guided by sensory input
Sensory feedback plays an important role in the control of movement (exception: ballistic movements).
3. Learning (experience) changes the nature and locus of sensorimotor control.
E.g. from Conscious behavior to automatic. From conscious control (cortical level) to "Automatic Pilot" (lower levels).

5. Major Areas of Sensorimotor Association Cortex
Sensory information is integrated in Association cortex.
Two Major areas of Sensorimotor Association Cortex are:-
Posterior parietal association cortex
Dorsolateral prefrontal association cortex
Each composed of several different areas with different functions

6. Posterior Parietal Association Cortex
This cortex receives input from
The somatosensory system,
 The visual system and
 The auditory system.
This information specifies the initial conditions for the programming of action:
The original position of the body parts to be moved.
The position of external objects
  Damage to the posterior parietal cortex causes Apraxia and Contralateral neglect.
Apraxia: difficulty in executing a movement when ordered to do so, but able to do it when not thinking about it. Lesion is often on the left side.
Contralateral Neglect: Patient does not respond to sensory stimulation from the side opposite to the lesion of the parietal cortex (usually on the right side).
The output of the Posterior Parietal Association Cortex goes to the Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2).

7. Posterior Parietal Association Cortex
This cortex receives input from
The somatosensory system,
 The visual system and
 The auditory system.
This information specifies the initial conditions for the programming of action:
The original position of the body parts to be moved.
The position of external objects
  Damage to the posterior parietal cortex causes Apraxia and Contralateral neglect.
Apraxia: difficulty in executing a movement when ordered to do so, but able to do it when not thinking about it. Lesion is often on the left side.
Contralateral Neglect: Patient does not respond to sensory stimulation from the side opposite to the lesion of the parietal cortex (usually on the right side).
The output of the Posterior Parietal Association Cortex goes to the Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2).

8. Posterior Parietal Association Cortex
Integrates information about
Body part location
External objects
Receives visual, auditory, and somatosensory information
Outputs to motor cortex

9. What affect does damage to the posterior parietal area have?
Apraxia – disorder of voluntary movement – problem only evident when instructed to perform an action – usually a consequence of damage to the area on the left
Contralateral neglect – unable to respond to stimuli contralateral to the side of the lesion - usually seen with large lesions on the right

10. Dorsolateral Prefrontal Association Cortex
Input from posterior parietal cortex
Output to secondary motor cortex, primary motor cortex, and frontal eye field
Evaluates external stimuli and initiates voluntary reactions – supported by neuronal responses

12. Secondary Motor Cortex
Input mainly from association cortex
Output mainly to primary motor cortex
At least 7 different areas
2 supplementary motor areas
SMA and preSMA
2 premotor areas
dorsal and ventral
3 cingulate motor areas

13. Secondary Motor Cortex
Subject of ongoing research
May be involved in programming movements in response to input from dorsolateral prefrontal cortex
Many premotor neurons are bimodal – responding to 2 different types of stimuli

14. Primary Motor Cortex Precentral gyrus of the frontal lobe
Major point of convergence of cortical sensorimotor signals
Major point of departure of signals from cortex
Somatotopic – more cortex devoted to body parts which make many movements

15. Motor homunculus

16. The Motor Homunculus
Control of hands involves a network of widely distributed neurons
Stereognosis – recognizing by touch – requires interplay of sensory and motor systems
Some neurons are direction specific – firing maximally when movement is made in one direction

17. Cerebellum and Basal Ganglia
Interact with different levels of the sensorimotor hierarchy
Coordinate and modulate
May permit maintenance of visually guided responses despite cortical damage

18. Cerebellum 10% of brain mass, > 50% of its neurons
Input from 1° and 2° motor cortex
Input from brain stem motor nuclei
Feedback from motor responses
Involved in fine-tuning and motor learning
May also do the same for cognitive responses

19. Basal Ganglia A collection of nuclei
Part of neural loops that receive cortical input and send output back via the thalamus
Modulate motor output and cognitive functions

20. 4 Descending Motor Pathways
2 dorsolateral
Corticospinal
Corticorubrospinal
2 ventromedial
Cortico-brainstem-spinal tract
Both corticospinal tracts are direct

21. Dorsolateral Tracts Most synapse on interneurons of spinal gray matter
Corticospinal - descend through the medullary pyramids, then cross
Betz cells – synapse on motor neurons projecting to leg muscles
Wrist, hands, fingers, toes
Corticorubrospinal – synapse at red nucleus and cross before the medulla
Some control muscles of the face
Distal muscles of arms and legs

23. Ventromedial Tracts Corticospinal Descends ipsilaterally
Axons branch and innervate interneuron circuits bilaterally in multiple spinal segments
Cortico-brainstem-spinal
Interacts with various brain stem structures and descends bilaterally carrying information from both hemispheres
Synapse on interneurons of multiple spinal segments controlling proximal trunk and limb muscles

25. Dorsolateral Vs Ventromedial Motor Pathways
one direct tract, one that synapses in the brain stem
Terminate in one contralateral spinal segment
Distal muscles
Limb movements
Ventromedial
one direct tract, one that synapses in the brain stem
More diffuse
Bilateral innervation
Proximal muscles
Posture and whole body movement

26. Motor Units and Muscles
Motor units – a motor neuron + muscle fibers, all fibers contract when motor neuron fires
Number of fibers per unit varies – fine control, fewer fibers/neuron
Muscle – muscle fibers bound together by a tendon

27. Muscles
Acetylcholine released by motor neurons at the neuromuscular junction causes contraction
Motor pool – all motor neurons innervating the fibers of a single muscle
Fast muscle fibers – fatigue quickly
Slow muscle fibers – capable of sustained contraction due to vascularization
Muscles are a mix of slow and fast

28. Muscles Flexors – bend or flex a joint
Extensors – straighten or extend
Synergistic muscles – any 2 muscles whose contraction produces the same movement
Antagonistic muscles – any 2 muscles that act in opposition

29. Receptor Organs of Tendons and Muscles
Golgi tendon organs
Embedded in tendons
Tendons connect muscle to bone
Detect muscle tension
Muscle spindles
Embedded in muscle tissue
Detect changes in muscle length

31. Knee-jerk reflex

32. Reflexes
Stretch reflex – monosynaptic, serves to maintain limb stability
Withdrawal reflex – multisynaptic
Reciprocal innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.

34. Central Sensorimotor Programs
Perhaps all but the highest levels of the sensorimotor system have patterns of activity programmed into them and complex movements are produced by activating these programs
Cerebellum and basal ganglia then serve to coordinate the various programs

35. Motor equivalence
A given movement can be accomplished various ways, using different muscles
Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task)
Sensorimotor programs may be stored in 2° motor cortex

36. The Development of Central Sensorimotor Programs
Programs for many species-specific behaviors established without practice
Fentress (1973) – mice without forelimbs still make coordinated grooming motions
Practice can also generate and modify programs
Response chunking
Shifting control to lower levels

37. The Development of Central Sensorimotor Programs
Response chunking
Practice combines the central programs controlling individual response
Shifting control to lower levels
Frees up higher levels to do more complex tasks
Permits greater speed