The Acting Brain (midterm 5) Flashcards
•Utilization:
compulsively use objects in view (if they see a broom, they’ll start sweeping; there’s no awareness).
•Perseveration:
keep repeating actions that are no longer useful
Disorders of cerebellum and
basal ganglia result in
strategy without tactics
Summary: Execution
• Skeletal muscle consists of muscle fibers or myocytes
• Muscle fibers innervated by axons of lower motor
neurons
• Lower motor neurons have their cell bodies in spinal
cord (ventral horn)
• Upper motor neurons in primary motor cortex project
downwards via multiple pathways
▫ Main one is corticospinal tract
Cerebellum
Cerebellum
• Coordination of movement, especially multi joint, i.e.
the movement that you want to make happens at the
right time and occurs accurately (e.g. correct distance
and strength)
Many movements are “multi-joint” eg reaching to pick something up requires precise timing of individual movements and joints
Innervation of skeletal muscle
• Muscle contraction is directly controlled by lower motor neurons • Each myocyte gets innervation from only one lower motor neuron • But a single lower motor neuron innervates many myocytes
** point where lower motor neurons contact muscle cells is the neuralmuscular junction
TACTICTS
level & structures
MIDDLE : Specific actions needed to perform goal
Primary motor cortex
Premotor cortex
Basal ganglia
Cerebellum
Basal ganglia loops
• Direct pathway:
accelerator
(promotes
movements)
• Indirect pathway:
brakes (decreases
movements)
* two pathways back from the BG to the motor cortex.
ACCELERATOR - increase movements
BRAKES - decrease movements
Huntington’s disease
• Reduced output of
indirect pathway
(“brakes”) but normal
output on direct pathway
(“accelerator”)
• The shift in balance
promotes movement in
general
-Arises in middle age (~40s)
• Symptoms: Excessive movement, dance-like flailing
limbs (chorea) and contorted postures
▫ https://www.youtube.com/watch?v=OveGZdZ_sVs
Premotor cortex and Supplementary motor area (SMA)
Premotor cortex and Supplementary
motor area (SMA)
• Modulate activity of M1
• Stimulation does not
result in movement
• Involved in motor
planning
• NOTE: In textbook, premotor
cortex = lateral premotor cortex
and supplementary motor area =
medial premotor area
Supplementary motor area (SMA):
organizing internally generated actions
• SMA
▫ Receives input from brain areas that represent the
position of the limbs
▫ Learned (internally generated) action patterns that do
not require feedback from the environment, but simply
from the position of the limbs (e.g. playing a familiar
song on the piano)
Parkinson’s disease
• Degeneration of
substantia nigra
• Increase output of
indirect pathway
(“brakes”) and
decrease output on the
direct pathway
(“accelerator”)
• Results in difficulty
with self-initiated
movement
-Parkinson’s Disease
• Affects 0.15% of the population; Onset ~60 years
• Impaired:
▫ Self-initiated movements
• Symptoms
▫ Resting tremor
▫ Slow movements
▫ Difficulty initiating movements
Mirror neurons : premotor cortex
respond when?
Mirror neurons:
Premotor cortex
• Respond when:
▫ Monkey performs action
• Specific to particular actions
• Also respond when monkey
observes experimenter
performing action
• Do NOT respond when object
moves on its own
▫ Purposeful action is critical
Hierarchical organization of action
• Strategy: high-level goals and plans ( make coffee)
- Tactics: specific actions needed to achieve goals (▫“Add water to coffee pot, measure coffee, add coffee…”)
- Execution: neural signals and muscle movements needed to carry out tactics (▫E.g. muscle movements needed to grasp coffee pot)
Useful distinction: strategy vs. tactics vs. execution
Old distinction from military: described by General Sun Tzu
Strategy: big picture, aim is to win the war. Would be good to have a trench in a certain place.
Tactics: actions needed to make the trench happen. Certain number of soldiers, have to do certain things in certain order.
Execution: actually digging the trench, holding the shovel etc.
Hemiplegia
Damage to one side of the primary motor cortex results in a failure to voluntarily move the other side of the body.
Basal ganglia (vs. cerebellum: important in coordination of movements)
- Caudate, putamen, globus pallidus, substantia nigra
- Modifies activity in frontal motor structures
- Important in initiation and execution of movements
- Damage leads to excess movement or insufficient movement
▫Parkinson’s, Huntington’s
Motor loops: Cerebellum and basal
ganglia
Motor loops: Cerebellum and basal
ganglia
• Not involved in choosing what movements to make
• Set particular movement parameters (e.g. force, duration of
movement)
• Control/modulation movements in progress
How motor loops
work:
get information from
the motor cortex, do
some processing,
send information
back.
Executing Action: Primary Motor Cortex (M1)
- Located in precentral gyrus of frontal lobe
- Somatotopically organized and crossed (left hemisphere = right side of body)
- Stimulation results in movement and lesion results in paralysis (e.g. hemiplegia)
Somatotopically organized: neurons for controlling movement in adjacent parts of the body are located in adjacent areas on precentral gyrus
Some parts of the body have more area in M1 devoted to controlling them – why?
Organizing principles Hierarchical organization of action
Execution: Muscle cells, lower motor neurons, the spinal cord
Strategy vs. tactics: The brain Understanding the actions of others
Amyotrophic lateral sclerosis (ALS)
• Symptoms:
▫ Early: Muscle weakness and atrophy
▫ Late: Loss of all voluntary movement, ability to speak,
breathe, swallow
• Death of upper and lower motor neurons
Doesn’t affect intellect
Cause not well understood
MOTOR SYSTEM AND UNDERSTANDING THE ACTIONS OF OTHERS
* A DETOUR INTO SOCIAL NEUROSCIENCE
Why talk about understanding the actions of others in a class on the motor system?
Turns out that we use our own motor system to understand other peoples’ actions.
So we’ve got these lower motor neurons that directly control the muscle cells.
They have their cell bodies in the spinal cord and they send their axons out through the spinal nerves to contact the muscle cells.Where are they getting information from?
From the brain!
What do we call information leaving the brain?
Efferent
So there must be neurons in the brain that send their axons out down the spinal cord.
•Lower motor neurons receive innervation from upper motor neurons
▫Upper motor neurons have their cell bodies in the brain (primary motor cortex, M1)
▫Axons run downward through different spinal pathways that control different muscle groups
▫Most important is the corticospinal tract
Neural substrates of action :
Level || Function || Structures
High || Strategy || Prefrontal cortex
Middle || Tactics || Primary motor cortex, Premotor cortex, Basal ganglia, Cerebellum
Low || Execution || Spinal cord, Motor neurons, Muscle cells
Mycoytes and their innervation
▫AKA muscle cells or muscle fibers
- Muscle contraction is directly controlled by lower motor neurons
- Each myocyte gets innervation from only one lower motor neuron
- But a single lower motor neuron innervates many myocytes
OVERVIEW
•Motor system is hierarchically organized
▫Execution:
▫Tactics:
▫Strategy:
•We may understand the actions of others by…
▫Execution: muscle cells, lower motor neurons, corticospinal tract
▫Tactics: M1, SMA, PMC
Cerebellar and basal ganglia loops are involved in fine-tuning and coordinating actions
▫Strategy: prefrontal cortex
•We may understand the actions of others by representing/simulating the motor actions it would take to perform their behavior
• Axons run out to muscles via spinal nerves
Axons don’t leave the spinal cord at random
Effects of trans Randal stimulation over SMA
Implications of mirror neurons
•We may understand the actions of others by simulating the motor programs it would take to perform the same behavioR
- People with autism have trouble understanding others’ goals and may have impaired mirror neurons
- Embodied cognition: complex cognitive processes stem from our body’s interaction with the environment.
If I see you do something, my brain actually computes what motor commands would be needed for me to perform the same action. My goals in performing that action are associated with the movements needed to perform that action. When I was performing this action, what was my goal? Can use this to figure out someone else’s goal.
You can figure out the other person’s goals based on the movements they’re making
Embodied cognition: the mind depends on the body
Mirror neurons are about representing what you do with your body: premotor cortex
We can’t understand other people without using representations of the body.
Primary motor cortex
Responsible for execution of voluntary movements of the body.
EMBODIED COGNITION
Embodied cognition: the mind depends on the body
Mirror neurons are about representing what you do with your body: premotor cortex
We can’t understand other people without using representations of the body.
Cerebellum loops & ataxia
Touch nose with finger
Think about what’s involved: move shoulder, elbow, wrist simultaneously
Cerebellar patients can’t do this: might do the movements sequentially, or might overshoot or come up short
Clumsiness due to alcohol: depression of cerebellar circuits
Skeletal muscle
• Muscles are attached to the skeleton by tendons • Cause movement by contracting ▫ Muscles can only pull; cannot push ▫ Often organized into antagonistic pairs so joints can be moved in different directions • Made up of many myocytes ▫ AKA muscle cells or muscle fibers
Types of muscle
• Smooth muscle ▫ Lines digestive tract, arteries ▫ Controlled by autonomic nervous system • Striated muscle ▫ Cardiac Heart muscle ▫ Skeletal Voluntary movement Muscle around joints, eyes, respiration, facial expression, speech
How do neurons in primary motor cortex code for movement?
If there are many neurons with many
preferred directions, how is a single
movement executed?
Does one neuron decide?
Cannot be: preferred directions of
each neuron are much broader
than precise movements)
Does a population of neurons
decide?
antagonistic pairs
Define Add slide
REPRODUCING ACTION
- Mimicry: Watch what they do, make the same movements
- Imitation: Watch what they do, figure out their goals, reproduce actions based on goals
- Can one person be imitating another if they don’t perform exactly the same actions?
Premotor cortex: acting with objects in the environment
- Receives input from dorsal visual stream
- Actions triggered by the environment
▫E.g. “pull handle when light is blue, rotate handle when light is red”
▫Lesions prevent these associations from being formed
•Single-cell recordings: Premotor cortex responds when an external cue requires movement, but not when monkey moves spontaneously
****
Whereas SMA is involved in organizing internally generated actions, premotor cortex organizes acting with objects in the environment
Acting with objects in the environment (e.g. reaching for a coffee cup —- Dorsal visual stream: location of objects in the environment
STRATEGY (FUNCTION AND STRUCTURE)
THE PRREFRONTAL CORTEX
•Prefrontal cortex: everything anterior to SMA and premotor cortex
high-level goals and plans
•Prefrontal damage results in tactics without strategy
(no problem executing actions, but have a problem with the goals)
PREFRONTAL CORTEX
- Not directly involved in programming actions
- Instead, involved in selection of goals and corresponding actions
- Lesions can result in disorganized actions that don’t fit goals (but actions themselves are normal)
- Perseveration: keep repeating actions that are no longer useful
- Utilization: compulsively use objects in view (if they see a broom, they’ll start sweeping; there’s no awareness).
E.g. trouble making coffee: have to set up and maintain a series of subgoals to achieve your main goal —- *But no problems with the actions themselves
Perseveration: deck of cards, sort by suit, then instructed to sort by color; keep sorting by suit
Utilization: people believe they are acting normally
• Lower motor neurons have their cell bodies in the spinal cord ▫ Ventral horn: motor ▫ Dorsal horn: sensory
**** SPINAL CORD - ADD IMAGE {important)
Structure of a skeletal muscle
ADD SLIDE
Spinal Cord - Ventral and Dorsal Horns
Spinal cord consists of grey matter and white matter
So are the cell bodies are in the in the grey matter part
Horizontal cross section of spinal cord
Central core of grey matter (H shape)
Projections are called horns
Ventral horns: motor
Remember this because you usually move forward and ventral is towards your stomach
Dorsal horns: sensory
If the cell bodies are in the spinal cord and the axon terminals are on the distant muscles, what route do the axons follow?
The neuromuscular junction
• Specialized synapse that connects lower motor neurons and skeletal muscle • Lower motor neurons send action potentials along their axons • Releases the neurotransmitter acetylcholine onto muscle fibers, resulting in contraction • Some arrow poisons block acetylcholine ▫ How does this kill?
Population vector
The sum of the preferred tunings of neurons multiplied by their firing rates.
Population coding in M1
Different cells have different tuning curves
Movement direction can be
coded by the summed
activity of multiple motor
cortex neurons (i.e. the
population vector), each
contributing their own vote.
Here, 241 cells from motor
cortex each contribute a
vote.
*Predictive power of
population coding is
beginning to be harnessed
in use of prosthetics.
