3. nerve cells and nerve impulses

Question 1

What are neurons?

Answer 1

nerve cells, basis structural and functional units of the nervous system, vary in size and shape but all share cell body, dendrites and axon

Question 2

What are interneurons? (connector neurons or association neurons)

Answer 2

make up most of the brain and spinal cord, have many branches to send or receive messages between adjacent neurons

Question 3

What does the cell body contain?

Answer 3

the nucleus, and a cytoplasm containing standard organelles (mitochondria, endoplasmic reticulum, ribosomes and golgi apparatus)

Question 4

What are dendrites?

Answer 4

fairly short extensions of the cytoplasm and are highly branched and carry messages into the cell body

Question 5

What is an axon?

Answer 5

a single, long extension of the cytoplasm which usually carries nerve impulses away from the cell body
- is unbranched for most of its length
Covered in a fatty myelin sheath

Question 6

What are nerve fibres?

Answer 6

‘any long extension of a nerve cell’

Usually refers to the axon, if they have a myelin sheath they are myelinated and if they dont, they are unmyelinated

Question 7

What is grey matter?

Answer 7

part of the CNS that consists of nerve cell bodies and unmyelinated fibres

Question 8

What is white matter?

Answer 8

parts of the CNS composed of myelinated fibres (myelin is lipid and is white)

Question 9

What are schwann cells?

Answer 9

from the myelin sheath on the neurons outiside the CNS, wrap around the axon, depositing layers of myelin in between coiled layers
Cytoplasm of schwann cells forms the neurilemma

Question 10

what are nodes of ranvier?

Answer 10

sit at intervals along the axon and in gaps in the myelin sheath

Question 11

what are the three functions of the myelin sheath?

Answer 11

• acts as an insulator
• protects the axon from damage
• speeds up the movement of nerve impulses along the axon

Question 12

What is the neurilemma

Answer 12

around the outermost coil of the schwann cell, a structure that helps in the repair of injured fibres

Question 13

What is an axon terminal?

Answer 13

terminal portions of the axon that release neurotransmitters

Question 14

What is the function of sensory (receptor) neurons?

Answer 14

-carry messages from receptors in the sense organs and skin to the CNS

Question 15

What is the function of motor (effector) neurons?

Answer 15

carry messages from the CNS to the muscles and glands

Question 16

What is the function of inter (connector/relay) neurons?

Answer 16

located in the CNS and link the sensory and motor neurons

Question 17

What are the features of multipolar neurons?

Answer 17

• have one axon and multiple dendrites
• most common
• most of the interneurons in the CNS
• motor neurons to skeletal muscles

Question 18

What are the features of bipolar neurons?

Answer 18

• one axon and one dendrite
• both may be very branched at their ends
• occur in the eye, ear and nose where they take impulses from receptor cells to other neurons

Question 19

What are unipolar neurons?

Answer 19

• have a singular extension (axon)
• cell body sits to one side of the axon
• most sensory neurons that carry messages to the spinal cord

Question 20

What is a synspse?

Answer 20

The junction between the branches of adjacent neurons

Question 21

What are features of a synapse?

Answer 21

• most occur between the branches of an axon of one neuron and the dendrites/cell body of another
• messages are carried across the synapse

Question 22

What is a neuromuscular junction?

Answer 22

similar to a synapse, where an axon meets a skeletal muscle cell

Question 23

What is a nerve impulse?

Answer 23

a message that travels along a nerve fibre, transmitted quickly so the body can respond rapidly to changes in the internal and external environments (electrochemical change)

Question 24

What is an electrochemical change?

Answer 24

involving a change in the electrical voltage brought about by changes in the concentration of ions in/out the cell membrane

Question 25

How is speed affected whether a nerve impulse is myelinated or unmyelinated?

Answer 25

unmyelinated= travels steady around 2m/s
myelinated= as sheath punctured by nodes of Ranvier, the impulse jumps from one node to the next (saltatory conduction) allows impulses to travel faster, between 18m/s and 140m/s

Question 26

What is membrane potential?

Answer 26

difference between the concentration of ions inside and outside the cell, theres a potential inside and outside the cell membrane

Question 27

What is the composition of extracellular fluid?

Answer 27

mainly composed of Na+ and Cl-

concentration of sodium ions is about 10x higher outside than within

Question 28

What is intracellular fluid composed of?

Answer 28

mainly potassium (+) and organic substances (-)
potassium is about 30x more concentrated inside than out

Question 29

What is the resting membrane potential?

Answer 29

the membrane potential of unstimulated nerve cells which is about -70mV, meaning the potential on the inside of the membrane is about 70mV

Question 30

What causes the resting membrane potential?

Answer 30

The differences in the distribution of potassium and sodium ions on either side of the cell membrane
Cell membrane is permeable to potassium and chloride ions, only slightly to sodium and impermeable to large organic negative ions

Question 31

How does the cell membrane maintain the difference in charges?

Answer 31

1. it actively moves ions across the membrane
   > via the sodium potassium pump (transports sodium ions out and potassium ions into the cell)
2. The cell membrane is not equally permeable to all ions
   > large numbers of negatively charged ions are trapped inside the cells (isn’t enough potassium inside to counteract the effect of negative ions)

Question 32

What happens if a strong stimulus is applied to the nerve fibre?

Answer 32

the membrane becomes more permeable to sodium ions (Na+ moves into the cell)
- movement is too great to be balanced by the outward movement of potassium ions -the membrane becomes depolarised

Question 33

What happens if stimulus if strong enough to cause a change of 15mV?

Answer 33

The movement of sodium ions is independent of the stimulus
- the size of the response is not related to the strength of the stimulus
Known as the all or none response

Question 34

What are two factors that determine the stimulus strength?

Answer 34

1. a strong stimulus causes depolarisation of more nerve fibres than a weak stimulus
2. A strong stimulus produces more nerve impulses in a given time than a weak stimulus

Question 35

What direction does a nerve impulse occur?

Answer 35

Axon -> dendrite
or
axon -> cell body

Question 36

How do most chemicals that influence the transmission of a nerve impulse work?

Answer 36

they affect the transmission at the synapse or the neuromuscular junction
Stimulants increase transmission, while depressants slow the transmission

Question 37

How does an impulse travel across a synpase?

Answer 37

At the synapse there is no membrane, special chemicals are released from the ends of the axon
Neurotransmitters diffuse across the gap and attach to receptors in the membrane of the next neuron.