Physio-Neural Circuits and Reflexes (Brownell)

Question 1

What factors control nerve membrane potential?

Answer 1

-Voltage inside relative to the outside.
-Equilibrium potential:
• ECa = 120 mV
• ENa = 60 mV
• ECl = -65 mV
• EK = -80 mV
-The amplitude and duration of fluctuation depends on what types of ion channels (g’X’) are opened (or closed), the duration of opening/closing and the driving force (Vm-Ex) for each permeant ion.
-Equilibrium potentials for the 4 main permeant ions are virtually constant and set up by active pumps for Na+, K+, Cl- and Ca++ in/out of cells
-Opening/closing of membrane ion channels (“conductances”) for these ions cause changes in membrane potential (voltage inside relative to outside)

Question 2

What are the 3 main classes of channel conductances?

Answer 2

1. Voltage-activated ion channels (=>action potentials)
2. Ligand gated or metabotropic (g-protein coupled) channels (=>synaptic transmitters)
3. Sensory transducer channels

Question 3

What is responsible for an Action Potential?

Answer 3

Voltage-gated Na+ channels and delayed K+ channels.

Question 4

Where are motor neurons found?

Answer 4

The ventral horn of the spinal cord gray matter

Question 5

What does and alpha motor neuron excite? How many muscle fibers does 1 alpha MN innervate? How many neural plates are on each muscle fiber?

Answer 5

• Skeletal muscle
• 5-2000
• just 1

Question 6

How do alpha motor neurons and skeletal muscle fibers develop?

Answer 6

TOGETHER!
The neurons make connections super early, before the muscles are even really muscle fibers (just simple myotubes at that time).

Question 7

What is a motor unit?

Answer 7

one alpha motor neuron and all the muscle fibers it innervates.

Question 8

What do the medial neurons in the spinal cord correlate with?

Answer 8

Proximal muscles (lateral=distal muscles)

Question 9

What do extrafusal muscle fibers do? What are they innervated by?

Answer 9

Force contraction.

Alpha Motor Neurons.

Question 10

What are intrafusal muscles fibers part of? What are they innervated by? What reaction do they cause?

Answer 10

Part of the muscle spindle.
Innervated by Gamma motor neurons.
Cause spindles to contract.

Question 11

What do Muscle receptor organ (MRO) (Ia) and Golgi tendon organ (GTO) (Ib) afferents sense?

Answer 11

muscle length and tension

Question 12

What are the two types of intrafusal muscle fibers?

Answer 12

Dynamic and static ‘nuclear bag fibers’

Question 13

What are the 2 types of sensory afferents? What do they do?

Answer 13

• Type Ia: responds dynamically (most sensitive to changes in muscle length).
• Type II: tonic responses that support sustained output.

Question 14

What are the 2 types of gamma motor neurons?

Answer 14

dynamic and static

Question 15

What is the myotatic reflex (I)? What are the 5 steps?

Answer 15

MRO feedback acts to automatically maintain constant muscle length.
Steps:
1. Load stretches muscle.
2. MRO (spindle) stretches which excites Ia.
3. Ia conducts impulse directly to homonymous alpha motor neuron.
4. Alpha motor neuron fires which causes contraction of extrafusal muscle fibers.
5. Increased muscle tension compensates load, and restores desired muscle length.

Question 16

What are the 5 steps of central command for muscle contraction?

Answer 16

1. Nerve tract w/ descending axon excites both alpha motor neuron and gamma motor neuron.
2. Excitation of gamma motor neuron establishes a new “set point” for length that the extrafusal muscle fibers must achieve.
3. Intrafusal fibers contract which excites Ia.
4. Ia further excites homonymous alpha motor neuron.
5. Alpha motor neuron fires which shortens muscle to new set point.