Topic 3: Nervous System 1

Question 1

Neurons Review

Answer 1

• Neurons are excitable (responsive to stimuli)
• When stimulated (usually on cell body or dendrites) an electrical impulse may be generated and propagated along the axon = nerve impulse

Question 2

Electrical Properties of Cells

Answer 2

Due to:

1) Ionic concentrations differences across the cell membrane (gradients)
2) Permeability of Cell Membrane to ions

Question 3

Important Ions

Answer 3

• Na+, K+, Cl-, Ca2+
• Large, negatively charged ions (org-) -are non-diffusible proteins
• [Na+] & [K+] due to and maintained by activity of Na+/K+-ATPase (pump) in cell membrane
• [Ca++] due to various transporters in cell and ER membrane
• Cl- repelled by org-, so is higher outside the cell than inside
• org- stay inside the cell

Question 4

Permeability of Cell Membrane to Ions

Answer 4

• Determined by ion channels - ions diffuse through them down their conc. gradients

Question 5

Ion Channel Types

Answer 5

1) Non-gated

2) Gated

Question 6

Non-gated Channel

Answer 6

• Always open
• More K+ than Na+ in a neutron - cell membrane more permeable to K+ at rest (no stimulus)
• These channels (especially K+ - more numerous) are important in establishing the resting membrane potential (RMP)

Question 7

Gated Channel

Answer 7

• Not involved at rest
• Open in response to stimuli: e.g.
  i) membrane voltage changes = voltage gates
  ii) Chemical e.g. binding of hormones or neurotransmitter (nt) = chemical gates
  iii) Temperature = thermal gates
  iv) Mechanical deformation = mechanical gates

Question 8

Resting Membrane Potential (RMP)

Answer 8

• At rest (not stimulated) a charge difference (potential difference) exists just across the cell membrane = resting membrane potential
• equals about -70mV - i.e. inside of cell = 70 mV more negative than outside

Question 9

Factors establishing RMP: Factor 1

Answer 9

• Na+/K+ - ATPase (Na+/K+ pump) - not a channel - breaks down 1 ATP and uses energy to pump 3 Na+ out and 2 K+ in –> both ions are pumped against their concentration gradients = active transport
• Effects:
  1) Maintains concentration gradients of Na+ and K+
  2) Contributes a little (a few mV) to RMP (pumping more positive ions out than in)

Question 10

Factors Establishing RMP: Factor 2

Answer 10

org- inside cell e.g. proteins -cannot cross membrane

Question 11

Factors Establishing RMP: Factor 3

Answer 11

More non-gated K+ channels than non-gated Na+ channels (membrane more permeable to K+ than Na+ at rest: K+ is MAJOR determinant of RMP)

Question 12

Why K+ is a Major Determinant of RMP

Answer 12

1) k+ diffuses out of cell down its concentration gradient - cell loses +ve charge (inside becomes more negative)
2) Unlike charges attract and K+ diffusion slows as inside becomes increasingly -ve
3) Na+ diffusion into cell increases due to increasing attraction to -ve cell interior
4) Until -70 mV reached, +ve moving out (K+) is greater than +ve moving in (Na+) - greater K+ permeability
5) At -70mV, the amount of +ve (K+) moving out equals the amount of +ve (Na+) moving in
6) The net movement of charge (ions) is 0 (equal in both directions): RMP = -70mV

Question 13

Electrically Excitable Cells

Answer 13

• ONLY muscle and nerve cells
• Capable of producing departures from RMP in response to stimuli (=changes in the external or internal environment)
• When a neutron is stimulated:
  1) Gated ion channels open
  2) MP changes, producing a graded potential. If the threshold potential is reached…
  3) Triggers an action potential

Question 14

Graded Potentials (GPs)

Answer 14

• Stimulus causes a small change in MP, usually on dendrite or cell body (no longer at rest) by opening gated channels (changes membrane permeability) = GP
• Possible Results:
  1) More +ve than RMP = depolarization
  e. g. -70mV to -65mV (closer to zero)
  2) More -ve than RMP = hyperpolarization
  e. g. from -70 mV to -75 mV

Question 15

Characteristics of Graded Potentials

Answer 15

1) Ions move passively (unlike charges attract (+,-)) = current flow, causing depot. or hyperbole. on adjacent membrane
2) GPs are short distance signals - die away quickly (short-lived)
3) Magnitude and distance traveled by potential varies directly with the strength of the stimulus (larger stimulus –> larger graded potential that travels further)
4) GPs can summate - 1st GP present when 2nd stem occurs –> these add to create the resulting larger GP

Question 16

After a Graded Potential

Answer 16

Repolarization = return to RMP after depolarization or hyperpolarization

Question 17

GPs –> Action Potential (AP)

Answer 17

• Without GP, we can’t get AP!!
• Gps are essential in initiating a nerve impulse (AP)
• If the GPs causes deploy. and if deploy. is large enough or multiple GPs sum to be large enough (i.e. there is a critical stimulus) –> leads to an action potential

Question 18

Steps for GP–>AP

Answer 18

1) Critical Stimulus (large GP or summation of multiple Gps)
2) GP threshold
3) Action Potential

Question 19

Action Potenital (AP)

Answer 19

• A nerve impulse (signal)
• Large change in MP that propagates along an axon with no change in intensity
• Initiates at trigger zone
  e. g. axon hillock of multipolar neurons and bipolar neurons; just past dendrites of multipolar neurons