C2.2 Neural Signaling

Question 1

Define:

Nervous System

Answer 1

Body system used for internal communication. It consists of nerve cells.

Question 2

Define:

Nerve Cells

Answer 2

Neurons. They help with internal communication by transmitting nerve impulses.

About 8.5 billion neurons in the human nervous system.

Question 3

Define:

Nerve Impulses

Answer 3

Electrical Signal; An action potential that starts at one end of a neuron and is propagated along the axon to the other end of the neuron.

Question 4

Describe:

Neuron Structure

Answer 4

Neurons have:
* A cell body with cytoplasm and a nucleus.
* Nerve fibres, which nerve impulses travel along.
* Dendrities
* Axons

Question 5

Define:

Nerve Fibres

Answer 5

Narrow Outgrowths

Question 6

Describe:

Dendrites

Answer 6

Short Branched Nerve Fibres

Question 7

Describe:

Axons

Answer 7

Elongated Nerve Fibres

Question 8

Define:

Membrane Potential

Answer 8

Voltage exists across the membrane, due to an imbalance between the net charge (negative or positive) of cytoplasm and the fluid outside.

When neurons transmit an impulse, its membrane potential changes suddenly.

Question 9

Why is the Membrane Potential Negative?

Answer 9

Cytoplasm is usually electrically negative compared to outside fluid. Therefore, membrane potential is expressed as a negative value (e.g. -40 mV for liver cells).

Question 10

Define:

Resting Potential

Answer 10

The membrane potential when a neuron is not transmitting an impulse; -70mV

Question 11

What factors contribute to the resting potential?

Answer 11

1. Sodium-Potassium Pumps In Membrane: For every 3 Na+ pumped out, only 2 K+ pumped in.
2. Pumped Ions Leak Back Across Membrane By Diffusion: Membrane 50x more permeable to K+ than Na+, so leakage of K+ ions is faster.
3. Negatively Charged Proteins In Nerve Fibres (Organic Anions)

1. Creates a charge imbalance and concentration gradients between both ions.
2. Increases the difference between the Na+ and K+ concentration gradients, increasing the overall charge imbalance across the membrane.
3. Contributes to the charge imbalance.

Question 12

Define:

Electronode

Answer 12

Can be used to monitor the membrane potential at one position along a nerve fibre.

Question 13

Define:

Action Potential

Answer 13

An all-or-nothing sequence of changes in membrane potential with two main phases—depolarization and repolarization.

Question 14

Define:

Depolarization

Answer 14

Change in membrane potential from negative to positive.

Both due to movement of positively charged ions (Na+ and K+) across the membrane—NOT to movement of electrons.

Question 15

Define:

Repolarization

Answer 15

Change in membrane potential back from positive to negative.

Both due to movement of positively charged ions (Na+ and K+) across the membrane—NOT to movement of electrons.

Question 16

How does depolarization occur?

Answer 16

Depolarization is due to opening of sodium channels in the membrane, allowing Na+ ions to diffuse into the neuron down the concentration gradient.
* Concentration of sodium ions outside about 10x higher than inside.
* Entry of Na+ ions reverses the charge imbalance across the membrane, resulting in the inside to be positive relative to outside.
* Raises membrane potential from -70mV to +30mV.

Question 17

How does repolarization occur?

Answer 17

Repolarization is due to closing of sodium channels and opening of potassium channels in the membrane, K+ ions diffuse out the neuron down the concentration gradient and no more sodium ions diffuse in.
* Occurs rapidly after depolarization.
* Sodium channels close (no more Na+ ions diffuse into neuron).
* Inside of neuron becomes negative again relative to outside.
* Potassium channels remain open until the membrane potential has fallen to close to -70mV.

Question 18

What happens after repolarization?

Answer 18

Diffusion of potassium repolarizes the neuron, but does not fully restore the resting potential as the concentration gradients of sodium and potassium ions have not yet been re-established.
* To restore resting potential, Na+ is actively pumped out and K+ in.
* Takes a few milliseconds.
* Must occur before another action potential.

Question 19

How is the action potential propogated?

Answer 19

Action potentials are propagated along nerve fibres, as the ion movements that depolarize one part of the fibre trigger depolarization in the neighboring part of the fibre.

This is how neural signals pass along nerve fibres.

Question 20

Why do nerve impulses always move in one direction?

Answer 20

Nerve impulses always move in one direction, as it is only initiated at one terminal of a neuron and can only be passed on to other neurons or different cell types at the other terminal.
There is a refractory period after depolarization that prevents propagation of an action potential backwards along an axon.

Question 21

Refractory Period

Answer 21

A state of recovery that occurs after a neuron has fired an action potential.

During this period, another action potential cannot be easily produced. This encourages unidirectional flow of action potentials because they cannot travel backwards to inactive neurons.

Question 22

How does diameter affect the speed of nerve impulses?

Answer 22

Larger diameters of nerve fibres reduce resistance, so impulses are transmitted quicker (e.g. squids). However, not all animals have this space available.

Question 23

Define:

Myelination

Answer 23

Coating of nerve fibres consisting of a series of Schwann cells, with gaps between them called nodes of Ranvier.

Question 24

How does myelination affect the speed of nerve impulses?

Answer 24

Modification of nerve fibres that increase the speed of nerve impulses. In myelinated nerve fibres, the impulse can jump from one node of Ranvier to the next, speeding up the transmission to as much as 100 metres per second.

Question 25

Define:

Synapse

Answer 25

Junction between two cells in the nervous system. Signals only pass in one direction across a synapse.

Question 26

List:

Three Main Types of Junctions

Answer 26

• Synapses between sensory receptor cells and neurons, in sense organs.
• Synapses between neurons, in both the brain and spinal cord.
• Synapses between neurons and muscle fibres or gland muscles.

Muscles and glands are called effectors, because they effect (carry out) response to a stimulus.

Question 27

Define:

Presynaptic Neuron

Answer 27

Brings the signal to the synapse in the form of a nerve impulse or action potential.

Question 28

Define:

Postsynaptic Neuron

Answer 28

Carries the signal away from the synapse, again in the form of a nerve impulse.

Question 29

Define:

Neurotransmitters

Answer 29

Chemicals that carry signals across a narrow fluid-filled gap (synaptic cleft) between the presynaptic and postsynaptic neurons.

Question 30

Describe:

Release of Neurotransmitter From a Presynaptic Membrane

Answer 30

Synapse Transmission: Occurs rapidly, in this sequence:
* Nerve impulse is propagated along the presynaptic neuron until it reaches the end of the neuron and the presynaptic membrane.
* Depolarization of the presynaptic neuron causes calcium ions (Ca2+) to diffuse through the channels in the membrane into the neuron.
* Influx of Ca2+ causes vesicles containing neurotransmitters to move to the presynaptic membrane and fuse with it.
* Neurotransmitter is released into the synaptic gap by exocytosis.

Question 31

Describe:

Triggering of an Action Potential in the Postsynaptic Neuron

Answer 31

Release of neurotransmitter from a presynaptic neuron leads to a series of events that trigger an action potential in the postsynaptic neuron:
* Neurotransmitter molecules diffuse across the synaptic gap.
* Neurotransmitter binds to receptors in the postsynaptic membrane, causing ion channels to open.
* Ions diffuse down their concentration gradient into the postsynaptic neuron, causing the membrane potential to change.
* Mostly, potential rises (becomes less negative). This is called an excitatory postsynaptic potential.
* If excitatory postsynaptic potential is strong enough, it triggers an action potential which propagates away from the synapse.
* Neurotransmitter rapidly broken down and removed from the synaptic gap.

Question 32

Describe:

What is the role of acetylcholine as a neurotransmitter?

Answer 32

• When it binds to receptors in the postsynaptic membrane, a channel opens in the receptor. Sodium ions diffuse through this channel and into the postsynaptic membrane, causing an excitatory postsynaptic potential.
• Acetylcholine only remains bound to the receptor for a short time and only one action potential is initiated in the postsynaptic neuron.
• Because enzyme (acetylcholinesterase) is present in the synaptic gap and rapidly breaks acetylcholine down into choline and acetate.
• The choline is reabsorbed into the presynaptic neuron, where it is converted back into acetylcholine by recombining with an acetyl group.

In the presynaptic neuron, choline (absorbed from the diet) is combined with an acetyl group produced by aerobic respiration. This produces acetylcholine, which is loaded into vesicles and then released into the synaptic gap during synaptic transmission.