Physiology of the Nervous System

Question 1

voltage

Answer 1

• the measure of potential energy generated by separated electrical charges
• measured in volts or millivolts (1mV = 0.001 V)
• always measured between 2 points
• called the potential difference or the potential

Question 2

current

Answer 2

• the flow of electrical charge from one point to another
• can be used to do work

Question 3

resistance

Answer 3

• the hindrance to charge flow provided by substances through which the current must pass

Question 4

What 2 factors does the amount of charge that moves between 2 points depend on?

Answer 4

voltage and resistance

Question 5

chemically gated channels

Answer 5

• also called ligand-gated channels
• open when the appropriate chemical (neurotransmitter) binds

Question 6

voltage-gated channels

Answer 6

• open and close in response to changes in membrane potential

Question 7

mechanically gated channels

Answer 7

• open in response to physical deformation of the receptor (sensory receptors for touch and pressure)

Question 8

electrochemical gradient

Answer 8

determine the direction an ion moves (into or out of the cell)

Question 9

What are the 2 components of the electrochemical gradient?

Answer 9

• concentration gradient
• electrical gradient

Question 10

concentration gradient

Answer 10

ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration

Question 11

electrical gradient

Answer 11

ions move towards an area of opposite electrical charge

Question 12

Which ion plays the most important role in generating the membrane potential?

Answer 12

potassium (K+)

Question 13

At resting membrane potential, what causes the negative interior of the cell?

Answer 13

Due to a much greater ability for K+ to diffuse out of the cell than for Na+ to diffuse into the cell

Question 14

sodium-potassium pump

Answer 14

• ejects 3 Na+ from the cell
• transports 2 K+ back into the cell
• stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium

Question 15

What can produce a change in membrane potential?

Answer 15

• anything that alters ion concentrations on the 2 sides of the membrane
• anything that changes membrane permeability to any ion

Question 16

graded potentials

Answer 16

• usually incoming signals operating over short distances that have variable (graded) strength
• short-lived, localized changes in membrane potential, usually in dendrites of the cell body
• can be either depolarizations or hyperpolarizations
• these changes can cause current flows that decrease in magnitude with distance
• their magnitude varies directly with stimulus strength
• triggered by some change in the neuron’s environment that opens gated ion channels

Question 17

action potential

Answer 17

• long-distance signals of axons that always have the same strength
• buried reversal of membrane potential with a total amplitude of ~ 100 mV

Question 18

depolarization

Answer 18

decrease in membrane potential

Question 19

hyperpolarization

Answer 19

increase in membrane potential

Question 20

receptor potential / generator potential

Answer 20

• produced when a sensory receptor is excited by its stimulus

Question 21

postsynaptic potential

Answer 21

• produced when the stimulus is a neurotransmitter released by another neuron
• neurotransmitter released into the synapse

Question 22

Why can graded potentials only act as signals over very short distances?

Answer 22

because the current dissipates quickly and decays with increasing distance from the site of initial depolarization

Question 23

What is the principal way neurons send signals over long distances?

Answer 23

by generating and propagating action potentials

Question 24

Which cells can generate action potentials?

Answer 24

cells with excitable membranes (neurons and muscle cells)

Question 25

step 1 of generating an action potential

Answer 25

resting state: all voltage-gated Na+ and K+ channels are closed

Question 26

step 2 of generating an action potential

Answer 26

depolarization: voltage-gated Na+ channels open

Question 27

step 3 of generating an action potential

Answer 27

repolarization: Na+ channels are inactivating, and voltage-gated K+ channels open

Question 28

step 4 of generating an action potential

Answer 28

hyper-polarization: some K+ channels remain open and Na+ channels reset

Question 29

How is stimulus intensity coded?

Answer 29

it is coded for by the number of impulses per second (by the frequency of action potentials) rather than by increases in the strength (amplitude) of the individual action potentials

Question 30

absolute refractory period

Answer 30

• begins with the opening of the Na+ channels and ends when the Na+ channels begin to reset to their original resting state
• ensures that each AP is a separate all-or-none event
• enforces one-way transmission of the AP
• because the area where the AP originated has just generated an AP, the Na+ channels in that area are inactivated and no new AP is generated there

Question 31

relative refractory period

Answer 31

• follows the absolute refractory period
• most Na+ channels have returned to their resting state
• some K+ channels are still open
• repolarization is occurring
• axon’s threshold for AP generation is elevated, so a stimulus that would normally generate an AP is no longer sufficient
• only an exceptionally strong stimulus can reopen the Na+ channels that have already returned to their resting state and generate another AP

Question 32

Where are nerve fibres that transmit impulses most rapidly (100 m/s or more) found?

Answer 32

found in neural pathways where speed is essential (such as those that mediate postural reflexes)

Question 33

What do the axons that conduct impulses more slowly serve?

Answer 33

typically serve internal organs (gut, glands, blood vessels) where slower responses are not a handicap

Question 34

What 2 factors does the rate of impulse propagation depend on?

Answer 34

• axon diameter
• degree of myelination

Question 35

axon diamter

Answer 35

• the larger the axon’s diameter, the faster it conducts impulses
• larger axons offer less resistance to the flow of local currents

Question 36

degree of myelination

Answer 36

• the presence of a myelin sheath dramatically increases the speed of propagation
• conduction velocity increases with the degree of myelination

Question 37

What are the 2 ways that action potentials can be propagated?

Answer 37

• continuous conduction
• saltatory conduction

Question 38

continuous conduction

Answer 38

• action potential propagation in nonmyelinated axons occurs by continuous conduction
• voltage-gated channels in the membrane are immediately adjacent to each other
• relatively slow

Question 39

saltatory conduction

Answer 39

• when an AP is generated in a myelinated fibre
• local depolarizing current does not dissipate through the adjacent membrane regions, which are non-excitable
• the current is maintained and moves rapidly to the next myelin sheath gap (distance of ~ 1mm) where it triggers another AP
• APs are only triggered at the gaps
• about 30 times faster than continuous conduction

Question 40

group A fibres

Answer 40

• mostly somatic sensory and motor fibres
• serve the skin, skeletal muscles, and joints
• have the largest diameter
• thick myelin sheaths
• conduction impulses at speeds up to 150 m/s

Question 41

group B fibres

Answer 41

• lightly myelinated
• intermediate diameter
• transmit impulses at ~15 m/s

Question 42

group C fibres

Answer 42

• smallest diameter
• non-myelinated
• incapable of saltatory conduction
• conduct impulses at a leisurely pace (~1m/s)

Question 43

What do B and C fibres have in common?

Answer 43

B and C fibre groups include:
- autonomic nervous system motor fibres serving the visceral organs
- visceral sensory fibres
- smaller somatic sensory fibres that transmit sensory impulses from the skin (such as pain and small touch fibres)