Topic 3: Nervous System I Flashcards
Neurons
-Neurons are excitable (responsive to stimuli)
nerve impulse
when a neuron is stimulated (usually on cell body or dendrites) an electrical impulse may be generated and propagated along the axon
Electrical Properties of Cells due to:
- ionic concentration differences across membrane (gradients)
- permeability of cell membrane to ions
Important ions
- K+, Na+, Cl-, Ca++
- large negatively charged organic ions (org-) – are non-diffusable proteins
Na+/K+- ATPase (pump) ions concentration
- [Na+] + [K+] due to and maintained by activity of pump in cell membrane
- [K+] is higher inside cell
- [Na+] is lower inside cell
[Ca2+] low inside the cell
due to various transporters in cell and endoplasmic reticulum membranes
Cl-
repelled by org- (large, negatively charged organic ions) so is higher outside of the cell than inside
org-
- large, negatively charged organic ions
- stay inside the cell
Permeability of cell membrane to ions
-determined by ion channels - ions diffuse through them down conc. gradients
Ion channel types
- non-gated
- gated
Non-gated
- always open
- more K+ than Na+ ∴ 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)
Gated
- not involved at rest
- open in response to stimuli:
- membrane voltage changes = voltage gates
- chemicals e.g. binding of hormone or neurotransmitter (nt) = chemical gates
- temperature = thermal gates
- mechanical deformation = mechanical gates
Resting Membrane Potential (RMP
- At rest (not stimulated), a charge difference (potential difference) exists just across the cell membrane = membrane potential
- ≈ -70 mV (inside of cell is more -ve)
Factors establishing RMP
- Na+/K+-ATPase (Na+/K+ pump)
- org- inside cell – can’t cross membrane
- More non-gated K+ channels than non-gated Na+ channels (membrane more permeable to K+ than Na+ at rest ∴ K+ is the major determinant of RMP)
Na+/K+-ATPase (Na+/K+ pump)
- 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:
- -maintains concentration gradients of Na+ and K+
- -contributes a little (a few mV) to RMP (pumping more +ve ions out than in)
More non-gated K+ channels than non-gated Na+ channels (membrane more permeable to K+ than Na+ at rest ∴ K+ is the major determinant of RMP)
- K+ diffuses out of cell down concentration gradient ∴ cell loses +ve charge (inside becomes more –ve)
- unlike charges attract and K+ diffusion slows as inside becomes increasingly -ve
- Na+ diffusion into cell increases due to increasing attraction to –ve cell interior
- until -70 mV reached, +ve out (K+) is greater than +ve (Na+) in – greater K+ permeability
- once at -70mV, the amount of +ve (K+) moving out equals the amount of +ve (Na+) moving in – force on Na+ much higher than on K+
- ∴ The net movement of charge (ions) is 0 (equal in both directions): RMP of -70 mV
Electrically Excitable Cells
- ONLY muscle and nerve cells
- capable of producing departures from RMP in response to stimuli (= changes in the external or internal environment)
When a neuron is stimulated
- gated ion channels open
- MP changes = producing a graded potential. If the threshold potential is reached…
- triggers an action potential
Graded Potentials (GPs)
stimulus causes a small change in RMP, usually on dendrite or cell body (no longer at rest!) by opening gated channels (changes membrane permeability)
GPs possible results:
- more +ve than RMP = depolarization
e. g. -70 mV to -65 mV (closer to zero) - more –ve than RMP = hyperpolarization
e. g. -70 mV to -75 mV
GPs characteristics
- ions move passively (unlike charges attract (+,-)) = current flow, causing depol. or hyperpol. on adjacent membrane
- GPs are short distance signals - die away quickly (short lived)
- magnitude and distance travelled by potential varies directly with the strength of the stimulus
i. e. larger stimulus ⇒ larger graded potential that travels further - GPs can summate - 1st GP present when 2nd stim occurs ⇒ these add (sum) together to create the resulting GP
After a GP
repolarization = return to RMP after depolarization or hyperpolarization
GPs to Action Potential (AP)
- GPs are essential in initiating a nerve impulse (AP)
- if the GP causes depol. and if it is large enough i.e. caused by a critical stimulus (or multiple GPs sum to be large enough) ⇒ leads to an AP
GPs to Action Potential (AP) steps:
- critical stimulus (or summating stimuli)
- GP reaching threshold
- Action Potential