WEEK 3 - Introduction to the Nervous System

Question 1

NS functions

Answer 1

• sensory input: information gathered by sensory receptors about internal and external changes
• integration: processing and interpretation of sensory input
• motor output: activation of effector organs producing a response

Question 2

structure of neuron

Answer 2

1. dendrites: receptive regions
2. cell body/soma: clusters of cell bodies located in CNS and known as nuclei and ganglia in PNS
3. axon/nerve fibre: conducts the impulse, bundles of it are called tracts in CNS, and nerves in PNS
4. myelin sheath: electrical insulators, can’t travel through it so they jump from gap to gap
5. myelin sheath gap
6. terminal branches
7. axons

myelinated neurons are fast and non-myelinated are slow

Question 3

3 types of neurons (classified according to number or processes extending form cell body)

Answer 3

1. multipolar
   - they are interneurons (communicates between two other neurons)
   - some are motor neurons (conduct impulses along the efferent pathway from the CNS to an effector organ)
2. bipolar
   - 2 processes coming out of the cell body
   - all bipolar neurons are sensory neurons that are located in the eye, nose and ear
3. unipolar
   - only has one process from the cell body
   - most unipolar neurons are sensory neurons that conduct impulses along afferent pathways to the CNS

Question 4

action potential

Answer 4

• it is the way neurons send signals, it is a means of long-distance neural communication
• occurs only in muscle cells and axons of neurons
• in neurons, also referred to as a nerve impulse
• involves opening of specific voltage-gated channels
• propagates down the axon, ends with a release of neurotransmitters

Question 5

differences in voltage in the membrane potential

Answer 5

• inner surface is negatively charged, outer is positive
• resting membrane potential depends on the concentration of gradients, permeability to Na+ and K+ (which depends on the channels)

Question 6

action potentials

Answer 6

• originate in the axon initial segment/trigger segment
• voltage-gated sodium and potassium channels open and close, thus altering the membrane’s permeability to sodium and potassium ions
• as they diffuse, the membrane potential changes, it depolarises from a resting state of -70mV to a peak value of +30mV, and then quickly repolarises back to -70mV after a brief period of hyperpolarization

1. resting state: no ions move through voltage-gated channels
2. depolarisation: is caused by sodium ions flowing into the cell
3. repolarization: is caused by potassium ions flowing out of the cell
4. hyperpolarization: is caused by potassium ions continuing to leave the cell

Question 7

resting state

Answer 7

• generates a resting membrane potential depending on
  1. differences in potassium and sodium concentrations inside and outside the cells
  2. differences in permeability of the PM to these ions, will they let them through or not?

RMP= -70mV
- inside of the cell is 70mV more NEGATIVE than the outside

Question 8

depolarisation

Answer 8

• depolarises local currents open voltage-gated sodium channels and the sodium ions rush into the cell
• the influx of sodium causes more depolarisation and thus activates more sodium channels
• as a result, the inside is becoming less negative
• it hits the threshold of -55mV, all sodium channels open
• results in large action potential spike
• membrane polarity jumps to -30mV

Question 9

repolarisation

Answer 9

• sodium ion channels close, and potassium ion channels open
• potassium ions exit the cell down its electrochemical gradient
• membrane returns to resting membrane potential = -70mV

Question 10

action potential

Answer 10

• it is the way neurons send signals, it is a means of long-distance neural communication
• occurs only in muscle cells and axons of neurons
• in neurons, also referred to as a nerve impulse
• involves opening of specific voltage-gated channels
• propagates down the axon, ends with a release of neurotransmitters