LIFESCI 3K03: Brain Control: Unit 6

What are the three stages + areas of control of movement and what do they do?

1. Determines what needs to be done (Ready) - Prefrontal Cortex

   1. Identifies the goal of the movement and best strategy to accomplish this goal

2. Plans the specific movement (Set) - Motor Cortex

   1. Specific sequences of muscle activations and patterns required to do the movement

3. Execute the plan (Go) - Spinal Cord

   1. Activation of the motor neurons to do the movement and make minor adjustments

What is the range of complexity in the 3 stages of control of movement?

What does the Prefrontal Cortex do?

• Highest in the chain of command and greatest complexity

• Identifies a goal and determines what needs to be done to accomplish this

  • “What you need to do”

• Highly connected with sensory cortex

• Executive Function (EF)

  • Higher cognitive processes for planning, organizing, and controlling thoughts, speech, and behaviors

  • Involves a wide-range of skills

What are the two components of Executive Function and Movement?

1. Goal-directed actions

• Organizing

  • What is the goal and how does this relate to the current sensory state?

• Planning

  • What will need to be done to accomplish the goal?

• Directing

  • Sending this information to the next processing station (i.e., motor cortex)

2. Attention

• Multitasking

  • Allocating attention among tasks performed simultaneously

• Response inhibition

  • Respond effectively with distractions/irrelevant information

  • Similar to a second level of “sensory gating” (i.e., thalamus filtering)

How is EF and aging related?

Executive function declines with age

• Lesions in white matter

• Loss of gray matter

• Loss of dendritic branching

Changes are highly variable

• Decline can be minimal in healthy aging

• Influenced by things like lifestyle, education, genetics, etc.

What are the age related declines related to EF?

Aging related declines in:

• Overall processing speed

• Problem solving ability

  • Organizing, planning, directing (Goal-directed actions)

• Controlling attentional resources

  • Multitasking and response inhibition (Attention)

Declines are not to a level of dysfunction in healthy aging

What are the changes in gait parameters with age?

In general, with aging we see:

• Decrease in gait speed

• Decrease in step length

• Increase in step time

• Increase in variability in these parameters

However, healthy older adults may have little to no change

Reduced EF may be an important driver of these changes

What is Gait in relation to executive function?

• Gait is a complex motor task that uses executive function

  • Not fully managed by CPGs (especially in humans)

  • EF needed to plan, organize, and direct of movements

  • Often must also divide attention to other tasks

• EF allows effective division of attention between gait and other tasks

  • Dual-task or Multi-tasking (e.g., Walk and talk/text)

What are the important parts of the motor cortex in relation to control of movement?

• Motor cortex is involved after you have decided what you need to do, in order to decide how you will do it

  • Supplementary Motor Area (SMA)

  • Premotor Area (PMA)

  • Primary Motor Cortex (M1)

How are the PMA & SMA involved in the planning of movement?

• Created movement plans and holds them until ready to execute

  • Active just BEFORE movement occurs

  • Up to 1s before voluntary movement

• Plans must be highly integrated with sensory information

• Details of coding taking place remains unclear

Premotor Area (PMA)

• Selection of best motor plans based on current sensory information (e.g. picking up an object)

Supplementary Motor Area (SMA)

• More complex motor sequences often with bilateral connections

• May be more internally driven (remembered sequences) (e.g. performing a dance)

How was the planning of movement tested?

Through experiment with monkeys

• Measuring the planning of movement

  • Instruction stimulus

    • Red light where movement will need to be

    • PMA neuron begins firing

  • Trigger stimulus

    • Blue light tell it to act

    • PMA neuron stops firing soon after the action is made

  • Specific to the side required for movement

What is active when planning (or even watching) movement?

“Mirror neurons” exist in the PMA

• Respond when movement is imagined or watched

• May be part of an extensive brain system for understanding actions and intentions of others

Monkey’s motor neurons didn’t activate when seeing the peanut being picked up by a tool instead of the monkey hand or human hand

What does the primary motor cortex (M1) do in relation to movement?

• Transforms the motor plan into specific movement patterns

• Motor map masks the deeper complexity

  • Complex and overlapping neurons work together to control specific movements

  • Coding related to direction and force of movement

How does the coding of movement in the primary motor cortex work?

Movement direction is encoded by the collective activity of neurons

• Many neurons are active for every movement

• Activity of each cell represents a single “vote”

• Direction of movement is based on a tally (and averaging) of votes

A single cell’s vote = Direction Vector

Tally of all cell votes = Population Vector

So…A population vector is based on MANY individual direction vectors

What is a direction vector?

Recordings from a single cell in M1 create a direction vector

• Each cell has a “preferred” direction where firing rate is highest

• Firing rate reduces as movement direction changes

Direction of the arrow stays the same for a given cell, but the length changes based on the direction of movement

How do you make a population vector from 2 direction vectors?

• Based on testing we know cell 1 and cell 2’s preferred directions

• Measure the firing rate in cell 1 & cell 2 in response to a particular motion

• Create a population vector that represents the actual movement

Population vectors are created from many cells

What forces in relation to population vectors?

Population vectors generated in the direction in which force is needed

Population vectors based on normal movement

Population vectors based on moving against a resistive force (pushing in a specific direction)

Neural decoding in M1

• We have a greater understanding of signals in M1

  • compared to prefrontal, SMA, and PMA

• Still, decoding these overlapping signals remains extremely complex:

  • Requires interpreting inputs from potentially thousands of neurons

  • Relating to multiple muscle movements

What is the basal ganglia?

Group of subcortical nuclei which supports the selection and initiation of willed movements, while preventing unwanted movements

What are the 2 motor pathways of the basal ganglia?

Direct Pathway

• Facilitates movement

• Helps to select motor plans

Indirect Pathway

• Inhibits movement

• Help to suppress competing or inappropriate motor plans

Need a healthy balance between the two

What is the direct pathway of the basal ganglia?

What is the indirect pathway of the basal ganglia?

What is the direct and indirect balance of the basal ganglia?

Direct = facilitates appropriate movements (Releasing the brake)

Indirect: Inhibits inappropriate movements (Riding the brake)

What is Parkinson’s Disease?

A disorder of the direct pathway

• Difficulty stimulating wanted movement

  • Reduced direct pathway release of thalamus inhibition

  • Arises from a loss of dopaminergic neurons acting on the striatum in the direct pathway

What are hypokinetic disorders?

Reduced voluntary motor activity due to a disorder of the direct pathway

• Bradykinesia - slowness of movement

• Akinesia - lack of movement (e.g., freezing of gait)

How does a direct pathway disorder work?

• Loss of dopaminergic neurons from Cerebral Cortex to Striatum (and from the substantia nigra to the striatum)

• Levodopa supplements help maintain the pathway

What is Huntington’s disease?

A Disorder of the indirect pathway

• Difficulty suppressing unwanted movements

  • Reduced indirect pathway = reduction in thalamus inhibition

  • Arises from a loss of striatum neurons acting on the globus pallidus external in the indirect pathway

  • Rare genetic disorder (<0.01% of people)

  • Symptoms arise in 30s or 40s

  • Neuron loss will also occur in other areas of the cerebral cortex, leading to:

    • Dementia

    • Personality changes

    • Death (~20 years after diagnosis)

What are hyperkinetic disorders?

Disorders of the indirect pathway that involve excessive involuntary motor activity

• Chorea - spontaneous and uncontrollable movements

How does an indirect pathway disorder work?

Loss of neurons from striatum to GPe

• Final overall inhibition cannot occur

What is the cortico-cerebellar loop and what are the 3 parts?

Proper execution of planned, voluntary, multi-joint movements

• Necessary to fine tune the sequences (i.e., timing) of muscle contractions

3 parts:

• Cortex to cerebellum

• Within cerebellum

• Cerebellum to cortex

What is the cortico-ponto-cerebellar pathway of the cortico-cerebellar loop?

• Sensory and motor cortex axons form massive projection on pons

• Pontine nuclei relay information to cerebellar cortex

What is ‘within the cerebellar cortex’ of the cortico-cerebellar loop?

Granule cells

• Most numerous cells in the cerebellum

• Excitatory/inhibitory output on Purkinje cells

Purkinje cells

• Largest cells in cerebellum

• Received thousands of synaptic inputs

• Excitatory/Inhibitory output to deep cerebellar nuclei

Deep cerebellar nuclei

• Excitatory or inhibitory output to thalamus

What is the ‘cerebello-thalamo-cortical pathway’ in the cortico-cerebellar loop?

• Deep cerebellar nuclei relay to thalamus (VL)

• VL relays information back to M1

  • Basal Ganglia was area 6 (premotor)

  • Here, M1 (area 4) signifies last chance tuning (timing, direction, force)

What results from cerebellar lesions?

Ataxia: uncoordinated and inaccurate movements

Summary of Brain Control

What are the two major groups of pathways in descending spinal tracts?

• Lateral motor pathways

  • Commands for voluntary movements

• Ventromedial motor pathways

  • Posture and reflex movements

What is the corticospinal tract (CST)?

Primary pathway for voluntary motor control

• Neck to feet

• One of the largest and longest pathways

Pyramidal cells arising from the motor cortex

• M1, but also premotor and other areas

Can be further divided into:

• Lateral (limbs) vs. anterior (trunk, neck, shoulders)

Lateral vs. Anterior CST

Lateral CST

• 90% of CST axons

• Decussate in medullary pyramids

• Control proximal/distal muscles (i.e., limbs)

Anterior CST

• 10% of CST axons

• Decussate within spinal cord

• Control axial muscles

Check slides for all diagrams

What is the lateral vs. anterior corticospinal tract pathway? (Draw it out)

How are the three different pathways impacted in Brown-Sequard Syndrome (Draw)?