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

2. Motor Systems Functions Guided by sensory systems Movement
Posture & balance
Communication
Guided by sensory systems
Internal representation of world & self
Detect changes in environment
external & internal ~

3. 3 Classes of Movement 1. Voluntary Complex actions
reading, writing, playing piano
Purposeful, goal-oriented
Learned
improve with practice ~

4. Continue… 3 Classes of Movement
2. Reflexes
Involuntary, rapid, stereotyped
eye-blink, coughing, knee jerk
Graded control by eliciting stimulus
3. Rhythmic motor patterns
Combines voluntary & reflexive acts
chewing, walking, running
Initiation & termination voluntary
once initiated, repetitive & reflexive ~

5. Control of movement by the brain
What are the principles of sensorimotor Function?

6. 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.

7. 3 Principles of Sensorimotor Function
Hierarchical organization
Association cortex at the highest level, muscles at the lowest
Parallel structure – signals flow between levels over multiple paths
Like a company: President (association cortex) issues general commands and lower level (motor neurons and muscles) take care of details
Advantage: Higher levels are left free to focsu on complex fucntions.

9. Continue… Motor output guided by sensory input.
Sensorimotor system monitors the external world (sensory input) and the consequences of its own actions
It acts accordingly
Learning (experience) changes the nature and locus of sensorimotor control
With regards to new tasks, after much practice at the lower levels – they mastered the task.
These well-learned tasks need little involvement from the higher level.

10. 2 Major Areas of Sensorimotor Association Cortex
Each composed of several different areas with different functions
Some disagreement exists about how to divide the areas up:
Posterior parietal association cortex
Dorsolateral prefrontal association cortex

11. A. Posterior Parietal Association Cortex
Before we respond to sensory input, we integrates information about
Body part location (Where are they?)
External objects
Receives visual, auditory, and somatosensory information
Most outputs go to secondary motor cortex.

12. 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 hemiphere.
Contralateral neglect – unable to respond to stimuli contralateral to the side of the lesion - usually seen with large lesions on the right

13. B. Dorsolateral Prefrontal Association Cortex
Input comes from posterior parietal cortex
Projects output to secondary motor cortex, primary motor cortex, and frontal eye field.
Evaluates external stimuli (i.e. characteristic, location, response fr object)
Initiates voluntary reactions – supported by neuronal responses.

15. Secondary Motor Cortex (SMC)
Input mainly from association cortex
Output mainly to primary motor cortex
At least 7 different areas of SMC in each hemisphere
2 supplementary motor areas, 2 premotor areas (i.e.dorsal and ventral); 3 cingulate motor areas
All are interconnected, All send axons to the motor circuits of the brainstem.

16. Continue… SMC Produces complex movements when stimulated
Activated before and during voluntary movements
Are active when either side of the body is involved in ta movement.
Premotor cortex: Respond → visual and touch stimuli

17. Primary Motor Cortex (PMC)
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 is devoted to body parts that make more movements (eg. face vs elbow)

18. Motor homunculus

19. The Motor Homunculus
Eg: Control of hands involves a network of widely distributed neurons on PMC → each area of PMC received feedback from muscles and joints it influences.
Stereognosis – ability of recognisation of object from touches – requires input from sensory (skin) and motor systems (feedback).
Some neurons are direction specific – firing maximally when movement is made in one direction (preferred direction)

20. Subcortical sensorimotor structures: Cerebellum and Basal Ganglia
Interact with different levels of the sensorimotor hierarchy but neither participates directly in signal transmission to the spinal cord.
Coordinate and modulate activities at various level of the sensorimotor system.

21. Cerebellum 10% of brain mass, > 50% of its neurons
Input from 1° (PMC) and 2° (SMC) 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.

22. Basal Ganglia A collection of nuclei
Part of neural loops that receive information from various part of cortex and send output back via the thalamus
Modulate motor output and cognitive functions
Eg: Sequencing of movements, expanded role in non-motor tasks (siognitive)

23. SCL: How do principle of sensorimotor system relate to our daily life?
What will happen when cerebellum is damaged?
How is Primary Motor Cortex organised and what is/are its main function(s)?

24. 4 Descending Motor Pathways:
2 dorsolateral areas of spinal cord
Corticospinal tract
Corticorubrospinal tract
2 ventromedial (each side of Spinal cord)
Cortico-brainstem-spinal tract tract
Both corticospinal tracts are direct

25. Dorsolateral Tracts
DCorticospinalT = Axons from PMC descend through Medullary pyramids → then cross, continue to contralateral dorsolateral white matter of spinal cord
Betz cells (in PMC) – synapse on motor neurons projecting to leg muscles
Wrist, hands, fingers, toes
DCorticorubrospinalT = Axons from PMC – synapse at red nucleus and cross before the medulla
Some control muscles of the face
Distal muscles of arms and legs

27. Ventromedial Tracts Corticospinal Cortico-brainstem-spinal
Descends ipsilaterally (following the same side)
Axons branch and stimulate 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

29. 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/ stimulation
Proximal muscles
Posture and whole body movement

30. REFLECTIVE BEHAVIOUR: 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 requires fewer fibers/neuron
Muscle – fibers bound together by a tendon

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

32. Muscles Movement occurs at joints Flexors – bend or flex a joint
Extensors – straighten or extend limb
Contraction & relaxation of opposing muscles

33. Movement & Muscles
Synergistic muscles – any 2 muscles whose contraction produces the same movement
Agonists: prime movers
Antagonistic muscles – any 2 muscles that act in opposition
Antagonists
counterbalance agonists
decelerate movement ~

34. Functions of muscles:
Movement control more than contraction & relaxation
Accurately time control of many muscles
Make postural adjustment during movement
Adjust for mechanical properties of joints & muscles
inertia, changing positions ~

35. 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

37. Knee-jerk reflex

38. Types of Reflexes:
Stretch reflex – monosynaptic, serves to maintain limb stability
Withdrawal reflex – multisynaptic, evoke by painful stimulus, before info reaches the brain
Reciprocal innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.

40. 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.

41. Motor equivalence
A given movement can be accomplished various ways, using different muscles.
Central sensorimotor programs is 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 (SMC)
Eg: You can sign your name with left or right hand. Signature is very similar and the SMC for preferred right hand is activated (even when left hand signs).

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

43. 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

44. Hierarchical Control of Movement
3 levels of control
Spinal cord (SC)
Brainstem
Cortex
Division of responsibility
Higher levels: general commands
Spinal cord: complex & specific
Each receives sensory input
Relevant to levels function ~

45. Hierarchical Control: Spinal Cord
Automatic & stereotyped responses
reflexes
rhythmic motor patterns
Can function without brain
Spinal interneurons
same circuits as voluntary movement
Pathways converge on a motor neurons
final common path ~

46. SCL: How does our brain control muscles? What are the neural pathways?
What are the concepts related to Central Sensorimotor program and its important functions?