week 7 motor control Flashcards
Organization of neural structures involved in the control of movement
Organization of control of movement
Motor neuron cell bodies are located and arranged how?
- located in ventral horn of spinal cord
- Arranged logically along body axis.
within the ventral horn of the spinal cord
there is a medial-to-lateral topographical map of body muscles
In addition to sensory input and motor outputs, there are many _______________ in the spinal cord that create networks to control simple motor behaviors (ex: reflexes, balance, coordination)
interneurons
define a motor unit
- an a-motor neuron and all of the muscles it innervates
- two kinds:
- small (few fibers)
- large (many fibers)
a muscle fiber is only innervated by one a-motor neuron BUT
it takes many a-motor neurons to innervate all the fibers in a complete muscle
How does the brain encode a command for force?
rate coding; the rate of AP coming from the brain
what are the three different types of motor neurons? what is different about them?
- fast fatigable
- fast fatigue resistant
- slow
- training can change the amount/proportion of these (ex: long distance runners would build more fast fatigue resistant)
With increased motor unit size, a-motor neurons exhibit:
- increase in:
- cell body size
- dendridic complexity
- short term EPSP potentiaton with repeated activation
- axonal diameter (faster conduction)
- # of axonal branches (more muscle fibers innervated)
- decrease in:
- input resistance
- excitability
- Ia EPSP amplitude
- PSP decay constant
- duration of after-hyperpolarization
a-motor neurons vary in:
- cell size
- excitability
- axon branches
- dendrites
- synapses received
- input resistance
what types of summation do motor neurons perform and what does this look like?
- temporal summation
- spatial summation (NOT mutually exclusive)
what is the size principle? why?
smaller motor units depolarize sooner than big motor units
- this is bc smaller cell bodies have higher input resistance, which makes it easier to reach AP threshold with few EPSPs
- input resistance refers to the amount of resistance a neuron puts on the flow of electrical current (higher = current cant leave = bigger change in neuron membrane potential)
smaller motor units excite _______ muscle fibers, meaning there is ____ ______.
smaller motor units excite FEWER muscle fibers, meaning there is LESS FORCE.
Small and large units can be innervated by the same ____________ ________ __________ and as firing rate ___________ , _________ motor units are recruited
Small and large units can be innervated by the same DESCENDING MOTOR PATHWAYS and as firing rate INCREASES , LARGER motor units are recruited
Motor neuron recruitment in the cat medial gastrocnemius muscle under different behavioral conditions
Effect of stimulation rate on muscle tension
The number of active motor units and their rate of firing both increase with voluntary force
(this is an entire motor neuron pool)
a-motor neurons cause __________________
MUSCLE BODIES TO CONTRACT
what do y-motor neurons do?
ADJUST LENGTH OF MUSCLE SPINDLES
what do Ia and II afferents do?
carry feedback about stretch from muscle spindle to the spinal cord a-motor neurons
Stretch reflex circuitry : How spinal circuits modulate motor unit activity.
what is the role of γ-motor and a-motor neurons in regulating muscles
- During normal movements, your brain
sends commands to both α-motor
neurons and γ-motor neurons to
coordinate them. - This ensures muscles spindles stay
sensitive and stretch reflexes don’t
interfere with intended movements. - y-motor neurons stop firing when a-motor neurons are stimulated and they are not, but when both stimulated they fire nonstop
Golgi tendon organ makeup
- made up of the capsule (outside)
- and collagen fibrils (inside)
Golgi tendon organs (GTO) and their role in the negative feedback regulation of muscle tension
- Golgi tendon organs (GTO) relay force info back to spinal cord.
- Here the GTO is also exciting an excitatory interneuron (purple) to activate antagonist muscle group.
- _________ ________ ___ _______ _________ can prevent muscle from self-injury.
negative feedback via inhibitory interneurons
spindle organs and tendon organs respond __________ to passive stretch and active force, but together…
differently; they provide sufficient information to smoothly coordinate the amount of stretch and force exerted by the muscle
muscle spindles fire _______ when the muscle is stretched, while golgi tendon organs fire more when the muscle is __________.
MORE ; CONTRACTED
myotatic and inverse myotatic reflexes
_________ ________ are simple networks controlling simple behaviors
spinal reflexes
_________ _________ are created by connecting sensory inputs, inhibitory and excitatory interneurons and motor output neurons together to produce desired behaviors
Neural Networks
Spinal cord circuitry for the flexion–crossed extension reflex
define Central Pattern Generators
- simple neural networks that control rhythmic motor patterns, often involving the alternating activation of antagonistic muscle groups.
- Ex: locomotion (swimming, walking), breathing, or even chewing gum.
CPGs often include…
Pacemaker Neurons, which fire bursts of action potentials spontaneously (without synaptic inputs) and rhythmically.
Pacemaker cells (neurons or cardiac muscle) function like other neurons except
- that a leak current slowly and repeatedly depolarizes them to threshold
- size of this leak channel (# of ion channels) determines how frequently cell fires an AP
- neurotransmitters and hormones can increase or decrease the leak current, activating or altering the frequency of action potentials
what affects the pacemaker cells leak channels
- neurotransmitters and hormones can increase or decrease the leak current, activating or altering the frequency of action potentials
Pyloric circuit of crustacean stomatogastric ganglion.
Breathing requires a CPG that…
alternately activates inspiratory or expiratory muscles.
Inhibitory interneurons (blue) ensure that…
the pacemaker neurons fire out of sync with each other
Spontaneously firing (Pacemaker) neurons in medulla that project to motor neurons controlling breathing.
Locomotion in the Leech and the Lamprey
CPGs don’t have to be limited to antagonistic muscle pairs: they can control sequences, such as during leech swimming.
The mammalian cycle of locomotion is organized by central pattern generators in the spinal cord
- Highly conserved locomotor pattern in tetrapods controlled by spinal CPGs
- Spinal cord injury prevents volitional activation of walking, but does not eliminate spontaneous CPG activation by sensory feedback (proprioceptive feedback in legs).
Conceptual framework for mammalian CPGs controlling locomotion.
Organization of neural structures involved in the control of locomotion
Overview of descending motor control
- there are ____________ ______ of the body in the spinal cord
topographical maps
fill in the blanks
Motor Representation on the Cerebral Cortex
the numbers refer to Brodmann’s areas
fMRI Images of Human Primary Motor Cortex
in what layer of the primary motor cortex are Betz cells found
5
Cytoarchitectonic appearance of the primary motor cortex in the human brain
Corticobulbar tracts project from…
cortex to midbrain central pattern generator areas that coordinate major muscle groups for things like walking.
Corticospinal pathways project from…
cortex directly to spinal motor neurons that control specific muscles and motor units, thereby supporting more complex learned behaviors.
Direct pathways that activate hand movements also engage… that ensure maintenance of posture and balance
indirect pathways through brainstem
???
The influence of single cortical upper motor neurons on muscle activity
What is the “receptive field” for an M1 neuron?
Early models described M1 as ___________ ________ _____but couldn’t explain ___________ __________
topographical (somatotopic) map ; functional organization
Functional connectivity patterns make it difficult to envision ….
how the system works as a whole
How are coordinated movements encoded by motor cortex?
Neurons in motor cortex show activity related to specific movement directions:
Yellow indicates increased firing rates just before and during moving.
Blue indicates shows suppressed activity for opposite directions.
Population Vectors:
groups of cortical motor neurons show coordinated activity that drives movements in a particular direction (vector).
Mirror Neurons:
- are active during execution of a task, but are similarly activated by watching someone else perform the same task.
- Remains unclear what the function of these premotor neurons do, but hypothesized that they are important for learning new tasks from observation.
Minds and Machines : the challenges to create Brain-Machine Interfaces (BMI)
