2.1.2 Neural Homeostatic Control Pathways

Question 1

Where is the soma in a motor neuron vs sensory neuron

Answer 1

Motor - soma is at one end of the cell

Sensory - soma is in the center of cell

Question 2

Where are the dendrites connected in motor vs Sensory neuron

Answer 2

motor: dendrites connected directly to the cell body
Sensory: dendrites don’t connect directly to the cell body

Question 3

Compare the axon length of motor vs sensory neuron

Answer 3

motor: longer axon
Sensory: shorter axon

Question 4

compare the connections of the motor vs sensory neuron

Answer 4

motor: axon connects to the effector
sensory: dendrite connects to receptor

Question 5

function of the afferent neuron

Answer 5

transmits information from tissues and organs to the CNS

Question 6

function of efferent neuron

Answer 6

carry impulses to effectors. Dendrites are on the cell body

Question 7

function of interneurons

Answer 7

short neurons found in the spinal cord, linking sensory and motor neurons as well as making up nerve tissue in the brain. Dendrites are on the cell body.

Question 8

function of cell body of a neuron

Answer 8

contains nucleus and granular cytoplasm containing many ribosomes

Question 9

function of dendrites

Answer 9

thin extensions that carry impulses towards the cell body

Question 10

function of axon

Answer 10

a long membrane-covered cytoplasmic extension that transmits impulses away from the body
- it divides into branches at its end, which form synapses with other neurons

Question 11

function of schwann cell

Answer 11

produce the multi-layered myelin sheath around neuronal axons
- acts as an electrical insulator and speeds up the transmission of impulses

Question 12

function of Nodes of Ranvier

Answer 12

a small gap in the myelin sheath

allows action potentials to take place

Question 13

function of axon terminals

Answer 13

the endings of the axon

specialised to release the neurotransmitters of the presynaptic cell

Question 14

function of the synapse

Answer 14

information from one neuron flows to another neuron across synapses
contains a small gap separating neurons
consists of:
- a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles
- a postsynaptic ending that contains receptor sites for neurotransmitters
- a synaptic cleft or space between the presynaptic and postsynaptic endings

Question 15

What are the 3 types of neurons/nerve cells

Answer 15

1. afferent (sensory) neurons
2. Interneuron
3. Efferent (motor) neurons

Question 16

How is a resting potential generated?

• When a neuron is not sending a signal it is at _____
• The inside of a neuron is ____ relative to the outside (approx __)
• This is because 3 ____ are being actively moved __ of the membrane, whilst 2 ___ are actively transported ___ the axon.

Answer 16

1. ‘rest’
2. negative
3. -70mV
4. NA+
5. out
6. K+
7. into

Question 17

How is a resting potential generated?

• the negative values explain that the inside of the membrane is ____
• the membrane is inactive and _____
• at rest, there are relatively more ___ ions ____ the neuron and more ____ ions ____ that neuron.
• ____ moves ___ NA+ out for every __ K+ in.

Answer 17

1. negative
2. polarized
3. sodium
4. outside
5. potassium
6. inside
7. Active transport
8. 3
9. 2

Question 18

How is an action potential generated?
stimulus causes the sodium channels to ___
- causes ___ to flow into the cell
- causes a ____ of the membrane potential
- This affects the _________ nearby and starts an action potential
An action potential occurs when a neuron sends information down an axon
- The resting membrane potential ____

Answer 18

1. open
2. Na+
3. depolarization
4. voltage-gated sodium
5. changes

Question 19

How is an action potential generated?
When a stimulus detected:
- Na+ ions moves inside cell through the sodium pump, causing the inside of the cell to become more + and the outside to lose + ions, becoming more negative

Question 20

Describe the action potential process/graph

Answer 20

1. Stimulus (received from receptors or the previous neuron) are carried along the dendrites to the axon hillock.
   - if the stimulus is insufficient to raise the membrane potential from -70mV to -55mV they are referred to as failed initiations
2. Depolarization: If the stimulus is sufficient to raise the membrane potential from -70mV to -55mV (via input of positive charges) it triggers the opening of voltage-gated Sodium channels at the Node of Ranvier.
   - As they open, Na+ ions rush IN the axon by diffusion (down their electrochemical gradient) raising the membrane potential to approximately +40mV.
3. Na+ ions entering the axon quickly move along the axon (as + charges repel each other). As they arrive at the next Node of Ranvier, the membrane potential (-70mV) is raised to -55mV, again triggering the opening of voltage-gated Sodium channels and the entry of Na+ ions into the axon.
   - Na+ ions entering the axon quickly move along the axon (as + charges repel each other) towards the next Node. This is repeated at each node along the axon and is referred to as the saltatory conduction of action potential
4. Repolarization: As the membrane potential at a Node of Ranvier reaches +40mV, the voltage-gated Sodium channels close - stopping the entry of Na+ ions.
   - At the same time, the voltage-gated Potassium channels open (+40mV) allowing K+ ions to rush OUT of the axon (down their electrochemical gradient). This restores the difference in charge between the inside and the outside of the membrane. The membrane potential is thus repolarized. This happens at all Node of Ranvier that has been depolarized.
5. Hyperpolarization: prevents another action potential from being generated, ensuring the stimulus travels in one direction only.
   - This is when charge goes below -70mV.
6. Resting state: to restore the electrochemical gradient (more Na+ outside and more K+ inside) and the difference in charge across the membrane, the Sodium/Potassium pumps actively (i.e. ATP required) push 3 Na+ out of and 2K+ into the axon.
   - Leakage channels also contribute to restoring the resting potential - K+ leakage channels allow more K+ ions to leak out of the axon than Na+ leakage channels allow more K+ ions to leak out of the axon than Na+ leakage channels allow Na+ to leak into the axon.

Question 21

How are nerve impulses able to be transmitted across a synapse?

Answer 21

1. Action potential arrives at the axon terminal, called the synaptic knob or presynaptic terminal
2. This causes depolarization in the synaptic knob/presynaptic terminal and the opening of calcium channels
3. Calcium ions (Ca+) rush in via diffusion (down their concentration gradient)
4. Influx of Ca+ signals to synaptic vesicles (which hold the neurotransmitters) to fuse with the presynaptic membrane. Ca+ therefore causes the release of neurotransmitter by exocytosis.
5. Neurotransmitters are released into the synaptic cleft
6. Neurotransmitters bind to receptors on the sodium ion channels found in the membrane of the postsynaptic neuron (i.e. on the dendrites of the
   next neuron)
7. The binding signals sodium channels to open, allowing Na+ ions to diffuse in (down their concentration gradient). This is signal transduction.
8. The postsynaptic neuron depolarizes as a result of Na+ entry (+ charges). If depolarization is sufficient to reach the threshold (-70mV -> -55mV) a new action potential is sent along the axon of the postsynaptic neuron
9. neurotransmitters bound to the sodium channel receptors are broken down (hydrolysed)
10. This induces the closure of the sodium channel. Hydrolysed neurotransmitters are recycled back into the presynaptic knob.