short answer exam 2

Question 1

What comprises the “motor unit”, and how does this relate to the control of muscle force generation (i.e.
what is the size principle)?

Answer 1

• 1 a-motor neuron and all the muscle fibers it innervates
  
  • Small a-motor neurons control fine motor control
  • Large a-motor neurons control gross motor function
• The more motor units activated the more force
• The size principle : smaller motor units depolarize sooner than larger motor units (because they have higher input resistance)
• As firing rate from the descending motor pathways is increased, larger motor units are recruited (generates more force)

Question 2

What are Pacemaker neurons? What makes them fire action potentials? How might two or more different
pacemaker cells be connected to create a central pattern generator circuit?

Answer 2

• Fire bursts of APs spontaneously (without synaptic input) and rhythmically
• A leak current slowly and repeatedly depolarizes them to threshold. Size of leak current (# of ion channels) determines how frequently pacemaker cells fire an AP
• Modulatory NTs and hormones can increase or decrease the leak current, activating or altering the frequency of APs
• Inhibitory interneurons ensure that the pacemaker neurons fire out of sync with each other
• “To create a central pattern generator (CPG) circuit using multiple pacemaker cells, you would typically connect them in a network where each pacemaker cell has different intrinsic firing rates, allowing them to rhythmically activate each other through synaptic connections, often utilizing a mechanism called “reciprocal inhibition” where one cell inhibits the other when it fires, creating an alternating pattern of activity between them, resulting in a rhythmic output; this can be further enhanced by incorporating additional neurons that act as “followers” to amplify the signal generated by the pacemaker cells.”

Question 3

What is the functional difference between proprioceptive feedback from muscle spindles and Golgi tendon
organs? Where and how is this sensory feedback integrated into descending motor commands?

Answer 3

• Muscle spindles primarily signal muscle length and changes in length (Ia and II), while Golgi tendon organs signal the amount of tension or force exerted by a muscle (Ib), essentially acting as a protective mechanism against excessive muscle contraction
• Sensory feedback is integrated into descending motor commands primarily within the spinal cord, allowing for rapid adjustments to muscle activity based on the current muscle state
  Neural networks are created by connecting sensory inputs, inhibitory/excitatory interneurons, and motor neurons to produce desired behavior

Question 4

What is the most important anatomical and functional distinction between the corticospinal and
corticobulbar motor pathways? Which one evolved first?

Answer 4

• Corticospinal project from cortex directly to spinal motor neurons that control specific muscles/motor units; support more complex learned behaviors so it probably evolved after
• Corticobulbar tracts project from cortex to midbrain CPG areas that coordinate major muscle groups for things like walking and also head, face, neck; probably evolved 1st because it more basic survival skills

Question 5

What is the “receptive field” of a Betz cell in the primary motor cortex? How does this differ from the
receptive field of a neuron in premotor cortex?

Answer 5

• Betz cells = pyramidal cells of cortical layer V are the upper motor neurons of the primary motor cortex. in charge of executing movement
• receptive field = the group of muscles it controls (voluntary muscle movements)
• Best word to explain it is “directional”. Controls a group of muscles to move in a specific direction.

Question 6

The somatotopic map of the primary motor cortex is very imprecise, partly because it appears to encode
the coordinated activation of many functionally related muscle groups to perform useful actions. This
coordination is hypothesized to arise from “neural population vectors”. Give an example of what this
means and how neural population vectors can be measured?

Answer 6

• Population vectors = groups of cortical motor neurons that show coordinated activity that drive movements in a particular direction
• Refers to the combined activity of a group of neurons in the motor cortex, where each neuron contributes slightly differently to the overall movement, leading to a coordinated action rather than a single muscle activation
• Example: a monkey trained to move a joystick following a light; the activity of a specific neuron would be more discharged in a particular direction (“preferred direction”); “population vectors” would be the groups of upper motor neurons that share a preferred direction; combination of neurons would fire to coordinate the money’s hand, arm, wrist, and shoulder simultaneously

Question 7

Diagram the basic circuit of the cerebellum, showing (and labeling) a Purkinje cell, granule cell, parallel fiber, mossy fiber, climbing fiber, and a neuron in a deep cerebellar nucleus. What do the inputs and outputs represent regarding the hypothesized function of the cerebellum?

Answer 7

INPUTS
- climbing fiber (from olivary nucleus) -> purkinje cell
- mossy fiber (from pontine nucleus, cerebral cortex, spinal cord, vestibular system) -> granule cell -> parallel fibers -> purkinje cell
OUTPUTS
- purkinje cell (inhibits) -> deep cerebellar nuclei -> thalamus (motor cortex)

Question 8

What are the most significant inputs and outputs of the striatum and how do they relate to the
hypothesized general function of the basal ganglia? What is the functional distinction between the
“direct” and “indirect” pathways through the BG?

Answer 8

Inputs:
- All areas of cortex
- Substancia nigra pars compacta
Outputs:
- Globus pallidus
- Substancia nigra pars reticula
Direct: (gas)
- Cortex + SNpc -> neostriatum -> GPi -> VA/VL of Thalamus -> cortex
Indirect: (breaks)
- Cortex + SNpc -> neostriatum -> GPe -> Subthalamic Nucleus + GPe -> GPi -> VA/VL of Thalamus -> cortex

Question 9

Describe the physiological basis of Parkinson’s disease and explain what this reveals about how the basal
ganglia contribute to the control of movement. How does this inform current treatments and therapies for
Parkinson’s patients?

Answer 9

• Parkinson’s disease results from degeneration of dopaminergic neurons in the substantia nigra pars compacta, reducing dopamine levels in the neostriatum.
• This dopamine loss disrupts the balance between the direct and indirect pathways, impairing movement initiation and suppression, causing bradykinesia, rigidity, and tremors.
• Treatments like dopamine replacement therapy (levodopa) and deep brain stimulation (DBS) aim to restore motor control by targeting basal ganglia function.
• Current therapies focus on compensating for dopamine loss and modulating basal ganglia circuitry to improve movement.

Question 10

What are mirror neurons, where are they found, and what behaviors might they serve?

Answer 10

• VA sector of lateral premotor cortex
• Are active during execution of a task, but are similarly activated by watching someone else perform the same task
• Hypothesized to help in learning new tasks from observation