2.1 Motor Neuron Structure and Function

Question 1

What is an alpha motor neuron?

Answer 1

• a large, multipolar neuron in the spinal cord that innervates skeletal muscle fibers and controls their contraction

Location:
- Cell body is at the ventral horn of the spinal cord and the axons exists from the ventral root and extends to skeletal muscle

Structure:
- Has many dendrites for receiving input, and myelinated axon for fast conducton to muscle fibers. They are connected to muscle fibers to form neuromusclar junction on the muscle fibers

Question 2

What is the difference between upper and lower motor neurons?

Answer 2

Upper Motor Neurons:
- neurons that originate in the brain and control the lower motor neurons

Function: initiate and modulate voluntary movement
- Influence muscle tone and reflexes via lower motor neurons

Lower Motor Neurons:
- Neurons that directly innervate skeletal muscle fibers

Location: cell bodies in ventral horn of the spinal cord and the axons exist via ventral roots

Function:
- Produce actual muscle contraction
-Responsible for reflexes and voluntary movement execution

Question 3

What is the final common pathway? Why is the lower motor neuron called this?

Answer 3

Because it is the last neuron that conveys signals from the CNS to skeletal muscle

All input from upper motor neurons, reflex arcs, and other motor centers must go throug the lower motor neuron to produce movement.

Goes from lower motor neuron to peripheral axon to skeletal muscle

Question 4

These are comparisons between upper and lower motor neurons from slides

Answer 4

Upper Motor Neurons:
Location: Cortex
Neurotransmitter: Glutamate
Targeting: Within the CNS
Symptoms upon lesion: Spasticity

Lower motor neurons
Location: Brainstem and spinal cord
Neurotransmitter: Acetylcholine
Targeting: Outside the CNS
Symptoms upon lesion: Paralysis mmml;ooo

Question 5

What is the difference between a presynaptic and a postsynaptic neuron?

Answer 5

Presynaptic Neuron:
Location: Before the synapse (sends signal)

Function: Releases neurotransmitter from synaptic vesicles

Signal Type: Electrical to chemical (neurotransmitter)
- this is becuase when there is an action potential, then the presynaptic neuron signal is electrical so that the pre release ach and then the post comes from chemical to electrial signal

Postsynaptic Neuron:
Location: After the synapse
- Receives neurotransmitter

Question 6

What is the process of a chemical synapse?

Answer 6

1. Action potential reaches presynaptic terminal
   - Depolarizes membrane
2. Voltage-gated Ca+2 channels open
   - Calcium enters the presynaptic enuron
3. Synaptic vesicles fuse with presynaptic membrane
4. Neurotransmitter binds to receptors on postsynptic membrane
   - Usually ligand-gated ions channels
5. Postsynaptic response occurs
   - Excitaroty postsynaptic potential (EPSP) = depolarization
   - Inhibitory postsynaptic potential (IPSP) = hyperpolarization
6. Neurotransmitter is removed
   - Diffusion,, enzmatic breakdon

Question 7

what is depolarization?

Answer 7

A decrease in the membrane potential, making the inside of the neuron less negative

-Na+ chanels open and Na rushes to come in to membrane potential rises toward +30 mV, this bring closer to firing an action potential.

• if it crosses threshold (55mV) = action potential is triggered

Question 8

What is hyperpolarization?

Answer 8

An increase in the membrane potential, making the inside of the neuron more negative than the resting potential

Question 9

What does it mean by less negative and more negative?

Answer 9

Less negative: inside of the cell becomes closer to 0 mV

More negative: inside of the cell becomes farther from 0 mV

Question 10

What is the difference between EPSP and IPSP?

Answer 10

EPSP deploarizes the membrane and usually NA+ in or CA+ in

IPSP is hyperpolarizes mmebrane usually through K+ out or cl- in

Question 11

What are the phases of the action potential.

Answer 11

Resting potential: 70 mV
Depolarization: -70 + 30
Hyperpolarization: -70 to -80 mV : when K+ channels stay open too long

Return to resting: K+ channels close, pumps restore ion balance