Topic 6.5 Neurons and Synapses

Question 1

Neurons

Answer 1

Specialized cells that function to transmit electrical impulses within the nervous system.
- Differ according to role, but share basic components

Question 2

Function of the nervous system

Answer 2

Converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli.

Question 3

Three basic components of neurons

Answer 3

• Dendrites: short-branched fibers that convert chemical information from cells into electrical signals
• Axon: an elongated fibre that transmits electrical signals to terminal regions for communication with other neurons/effectors
• Myelin sheath (in some): insulates the axon to improve the conduction speed of electrical impulses
• Soma: a cell body containing a nucleus and organelles, where essential metabolic processes occur

Question 4

Membrane potential

Answer 4

A charge difference of the neuron membrane caused by the unequal distribution of positively-charged ions (Na+ and K+).

Question 5

Resting potential

Answer 5

The difference in charge across the membrane when a neuron is not firing.
- Typically, the inside of the neuron is more negative relative to the outside (approx. -70 mV)

Question 6

Maintenance of a resting potential

Answer 6

An active (ATP-dependent) process controlled by sodium-potassium pumps

• The sodium-potassium pump (transmembrane protein) actively exchanges sodium and potassium ions
• Expels 3 Na+ ions for every 2 K+ ions admitted (some K+ ions will then leak back out of the cell), creating an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment

Question 7

Action potential

Answer 7

The rapid changes in charge across the membrane that occur when a neuron is firing
- Occur in three main stages: depolarization, repolarization, and a refractory period

Question 8

Depolarization

Answer 8

A sudden change in membrane potential, usually from a negative to positive internal charge.

• In response to a dendritic signal, sodium channels open within the membrane of the axon
• As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of Na+
• The influx of Na+ causes the membrane potential to become more positive (depolarization)

Question 9

Repolarization

Answer 9

The restoration of a membrane potential following depolarization (restores a negative internal charge).

• Following an influx of Na+, K+ channels open within the membrane of the axon
• As K+ ions are more concentrated inside the neuron, opening K+ channels causes a passive efflux of K+
• The efflux of K+ causes the membrane potential to return to a more negative internal differential

Question 10

Refractory period

Answer 10

The period of time following a nerve impulse before the neuron is able to fire again.

• Normal resting state: Na+ ions are predominantly outside the neuron and K+ ions mainly inside (rest)
• Following depolarization (Na+ influx) and repolarization (K+ efflux), the ionic distribution is largely reversed
• Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump

Question 11

Nerve impulses

Answer 11

Action potentials that move along the length of an axon as a wave of depolarization.

• Depolarization occurs when ion channels open and cause a change in membrane potential
• Ion channels that occupy the length of the axon are voltage-gated (responds to changes in membrane potential)
• Depolarization at one point of the axon triggers the opening of ion channels in the next segment of the axon
• Causes depolarization to spread along the length of the axon as a unidirectional ‘wave’

Question 12

Threshold potential

Answer 12

Minimum stimulus required to open voltage-gated ion channels that generate an action potential (fires a neuron).

Question 13

Oscilloscopes

Answer 13

Scientific instruments that are used to measure the membrane potential across a neuronal membrane.

• Data displayed as a graph: time (X axis), membrane potential (Y axis)
• Resting potential: state of rest at approx. -70 mV
• Depolarization: rising spike up to approx. 30 mV
• Repolarization: falling spike to approx. -80 mV
• Refractory period: returns to the level of the resting potential

Question 14

Myelin sheath

Answer 14

A mixture of protein and phospholipids produced by glial cells, functioning as an insulating fatty white layer for the axon.

• Function: to increase the speed of electrical transmissions via saltatory conduction by allowing action potentials to “hop” between gaps in the sheath (nodes of Ranvier) rather than propagate
• Takes up significant space

Question 15

Synapses

Answer 15

Physical gaps that separate neurons from other cells (other neurons and receptor or effector cells).
- Neurons transmit information across synapses by converting the electrical signal into a chemical signal

Question 16

Chemical transfer across synapses

Answer 16

• Action potential reaches the axon terminal, triggering the opening of voltage-gated calcium channels
• Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitters) with the cell membrane
• Neurotransmitters released from the axon terminal by exocytosis cross the synaptic cleft
• Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels
• Opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal
• Neurotransmitters released into the synapse are recycled (by re-uptake pumps) or degraded (by enzymatic activity)

Question 17

Neurotransmitters

Answer 17

Chemical messengers released from neurons and function to transmit signals across the synaptic cleft.

• Released in response to depolarization of the axon terminal of a presynaptic neuron
• Bind to receptors on post-synaptic cells can either trigger or prevent a response

Question 18

Effects of neurotransmitters

Answer 18

Responses depend on type of cell activated:

• Neuron: electric signal (nerve impulse) stimulated or inhibited
• Glandular cell: secretion stimulated or inhibited
• Muscle fiber: muscular contraction or relaxation

Question 19

Acetylcholine

Answer 19

Neurotransmitter utilized by the central nervous system and peripheral nervous system.

• Commonly released to trigger muscle contraction and promote parasympathetic responses
• Created in the axon terminal by combining choline with acetyl group (from Acetyl CoA)
• Stored in vesicles until release via exocytosis
• Activates post-synaptic cell and then broken down into its two component parts by enzyme AChE

Question 20

Neonicotinoid pesticides

Answer 20

Able to irreversibly bind to nicotinic acetylcholine receptors and trigger a sustained response.

• Insects have a different composition of acetylcholine receptors which bind to neonicotinoids more strongly
• Hence, neonicotinoids are significantly more toxic to insects than mammals, making them a highly effective pesticide

Question 21

Graded potentials

Answer 21

Small changes in membrane potential caused by the opening of ligand-gated ion channels.
- A result of neurotransmitters binding to neuroreceptors on post-synaptic membranes of target cells

Question 22

Nervous system

Answer 22

• Central nervous system (CNS): the brain and spinal cord

- Peripheral nervous system (PNS): peripheral nerves linking the CNS to receptors and effectors

Question 23

Central nervous system (CNS)

Answer 23

Integrates information received from peripheral nerves and coordinates bodily responses, with the majority of activity occurring in the brain.

• White matter: bundles of myelinated axons
• Grey matter: neuronal cell bodies and dendrites, along with unmyelinated nerve fibers (regions here information is processed)

Question 24

Peripheral nervous system (PNS)

Answer 24

Sends information to the CNS via sensory neurons and activates effectors via motor neurons.

• Nerves for a particular region feed into the spinal cord
• Divides into sensory (afferent) and motor (efferent) divisions
• Motor division divides into autonomic and somatic nervous systems
• Autonomic nervous system divides into sympathetic and parasympathetic divisions

Question 25

Stimulus-response model

Answer 25

• Stimulus: a change in the environment (either external or internal) detected by a receptor
• Receptors: transform environmental stimuli into electrical nerve impulses
• Impulses: transmitted via neurons to the CNS, where decision-making occurs
• When a response is selected, the signal is transmitted via neurons to effectors
• Effectors: organs (either muscles or glands) that produce a response to a stimulus
• Response: a change in the organism resulting from the detection of a stimulus

Question 26

Three types of neurons transmitting information via the stimulus-response pathway

Answer 26

• Sensory neurons: transmit information from sensory receptors to the CNS
• Relay neurons (interneurons): transmit information within the CNS as part of the decision-making process
• Motor neurons: transmit information from the CNS to effectors (muscles or glands), to initiate a response