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?
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
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
• Axons run out to muscles via spinal nerves
Axons don’t leave the spinal cord at random
Effects of trans Randal stimulation over SMA
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
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?
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?
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.
