Nervous system

Question 1

What is the Nervous system?

Answer 1

• Communication and control centre.
• Allows reaction to stimulus and control in the environment.
• Reactions are automatic and involuntary.

Question 2

Difference between neurons, nerve fibres and nerves?

Answer 2

Nerve fibres: Long extension from cell body (axon).

Neuron: A nerve cell.

Nerve: A collection of neurons bundled together.

Question 3

Dendrites

Answer 3

Short extensions/branches off cell body. Receive messages from sensory receptors and carry them towards cell body.

Question 4

Cell body

Answer 4

Contains nucleus, cytoplasm with other organelles.

Question 5

Axons

Answer 5

Single long extension from cell body. End divides into branches, each ending at an axon terminal. Carries impulse away from cell body.

Question 6

Mylein sheath

Answer 6

Layer of lipid material covering axon. Axons with myelin sheath and myelinated, ones without are unmyelinated. Formed by Schwann cells which wrap around axon. Functions: Insulation, Protection, Speeds up nerve impulses.

Question 7

Nodes of Ranvier

Answer 7

Gaps in myelin sheath.

Question 8

Neurilemma

Answer 8

Outermost coil of Schwann cell. Helps repair injured fibres.

Question 9

Grey matter

Answer 9

Grey areas of neurons. Unmyelinated fibres.

Question 10

White matter

Answer 10

White areas due to lipid myelin. Myelinated fibres.

Question 11

Structural types of neurons

Answer 11

Multipolar - Motor, Interneurons - One axon and multiple dendrites.

Bipolar - Sensory - Cell body in middle, one axon, one dendrite.

Unipolar - Sensory - One extension, an axon. Not found in humans.

Pseudounipolar - Sensory - Cell body is to one side of axon.

Question 12

Functional types of neurons

Answer 12

Afferent - Carry messages from receptors in sense organs/ skin towards CNS.

Interneurons, Relay, Connector, Association - Located in CNS. Link between Sensory and Motor neurons. Motor,

Efferent - Carry messages away from CNS towards effectors in muscles and glands.

Question 13

Nerves in a resting state

Answer 13

A group of separated positive and negative charges have potential to come together and release energy. This potential can be measured in Volts.

In intracellular and extracellular fluids there are positively and negatively charged ions.

Extracellular fluid - High concentration of Sodium ions and Chlorine ions.

Intracellular fluid - High concentration of Potassium ions and various negative ions.

Question 14

Potential difference

Answer 14

Difference between concentrations of positive and negative ions either side of cell membrane. Potential difference is called Membrane potential. Membrane potential of unstimulated nerve cell is -70 mV.

Question 15

How does cell membrane maintain potential difference?

Answer 15

1. Sodium potassium pump - Active transport. Moves 3 Na+ ions out of cell and 2 K+ ions into cell using Na+ and K+ protein pumps.

2. Cell membrane is not equally permeable to all ions (many negatively charged ions trapped inside cell). - Highly permeable to K+, Slightly permeable to Na+, Impermeable to various other large negative ions.

Question 16

Polarised membrane

Answer 16

Inside membrane is negative, outside is positive. While K+ is positive, there is not enough inside cell to counteract large number of negative ions.

Question 17

Depolarisation

Answer 17

Occurs if level of stimulation exceeds a certain threshold (-15mv), lifting resting membrane potential to -55mv. Movement of Na+ is too great to be balanced by an outward movement of K+. Inside membrane becomes positive, and outside becomes negative.

Question 18

Repolarisation

Answer 18

Inside cell returns to an overall negative charge, outside cell returns positive.

Question 19

Hyperpolarisation

Answer 19

K+ channels remain open longer than what is needed, results in potential dropping lower than resting (-70mv).

Question 20

Action potential

Answer 20

Rapid depolarisation and repolarisation across a membrane.

Question 21

Steps of an action potential

Answer 21

Step 1 - A sufficiently strong stimulus occurs (15mV, reaching resting membrane potential to -55mv). All or none response, action potential commences.

Step 2 - Sodium channels in axon open. Na+ diffuses into cell. Depolarisation occurs, Intracellular fluid becomes positive, extracellular becomes negative. At 15mV sodium channels close, stopping movement of Na+ into cell.

Step 3 - At 15mV potassium channels open. K+ diffuses out of cell. Repolarisation occurs, intracellular fluid becomes negative, extracellular becomes positive. At -70mV potassium channels close, and K+ stop moving out. However, some K+ leaks through membrane, causing hyperpolarisation.

Step 4 - Sodium-Potassium Pump restores resting membrane potential. It moves 3 Na+ molecules back outside cell and 2 K+ molecules back inside cell.

Question 22

Refractory period

Answer 22

Period between threshold being reached -55mv and resting membrane potential being stored -70mv. Occurs for a short period after an action potential, so part of nerve fibre cannot be stimulated again. Ensures impulse travels in only one direction.

Question 23

Conduction along unmyelinated fibres

Answer 23

Depolarisation of one channel stimulates depolarisation in adjacent channel (next to original stimulus). Continuous propagation.

Question 24

Saltatory conduction

Answer 24

Action potential cannot occur in myelin sheath. It diffuses along axon through cytoplasm until next node of ranvier. Action potentials are said to ‘jump’ from node to node. Less distance to cover = much faster

Question 25

All or none response

Answer 25

If level of stimulation reaches threshold of 15mV membrane will become more permeable to Na+ ions, so they diffuse into cell. This causes inside of cell to become more positive, increasing potential difference. Movement of Na+ ions are independent to strength of stimulus.

Question 26

Define Nerve impulse?

Answer 26

An electrochemical change that travels along a nerve fibre, brought about by movement of ions. Movement of an action potential along a nerve.

Question 27

Synapse

Answer 27

Gap between two neurons. Between axon of one and dendrite of another. Transmission of nerve impulses across a synapse will only occur in one direction.

Question 28

Neuromuscular junction

Answer 28

Axon terminals meet a skeletal muscle cell.

Question 29

Neurotransmitters

Answer 29

Chemical messengers that transmit a signal from a neuron across synapse to another neuron.

Question 30

Neurotransmitters in PNS and Brain

Answer 30

Acetylcholine, Noradrenaline – PNS Dopamine, Histamine, Serotonin – Brain

Question 31

Transmission of a nerve impulse across a synapse

Answer 31

1. Nerve impulse reaches axon terminal which activates voltage-gated ion channels in pre-synaptic membrane.
2. Calcium ions flow into pre-synaptic axon terminal and binds with vesicles (containing neurotransmitters).
3. This causes vesicles to migrate to edge of cell and attach to cell membrane.
4. Exocytosis occurs, releasing neurotransmitters into synaptic cleft.
5. Neurotransmitters diffuse across synaptic cleft and attach to receptors on post- synaptic membrane.
6. This stimulates ligand-gated protein channels to open. Influx of sodium ions in dendrite, initiates nerve impulse in next neuron.

Question 32

How are neurotransmitters cleared from synaptic cleft?

Answer 32

Diffusion: High to low concentration.

Enzyme’s degradation: Enzymes inactivate neurotransmitters. Released by post-synaptic membrane when neurotransmitter has been received.

Active Reabsorption: Reabsorbed back into pre-synaptic axon terminal by active transport.

Question 33

Label a neuron

Answer 33

Question 34

Movement of action potential steps on graph

Answer 34