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
Action Potential (AP)
- 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 and bipolar neurons; just past dendrites of unipolar neurons - once K+ channels close ⇒ MP returns to RMP (e)
- *NOTE: Na+/K+-ATPase always working to maintain gradients
- takes 10,000s of APs to cause a measurable change in [ion] in the cell
AP phases
- depolarization phase
- repolarization phase
- After-hyperpolarization phase (below the RMP)
Depolarization phase
- voltage-gated Na+ channels respond to MP change (ie GP) and open – greatly increases Na+ permeability
- as gates open more Na+ diffuses in (further changing MP) ⇒ causes even more Na+ gates to open (a +ve feedback mechanism)
- Na+ diffuses in causing depolarization to +30 mV (inside membrane becomes +ve)
Repolarization phase
- Na+ channels close, become inactivated (decreased Na+ permeability) ⇒ Na+ movement returns to resting levels
- voltage-gated K+ channels open (increased permeability) ∴ K+ diffuses out (+ve charges (K+) move out – decreases MP)
After-hyperpolarization phase (below the RMP)
i. K+ channels are slow to close and remain open longer than necessary
ii. Na+ channels are reactivated – can respond to stimuli at this point
Refractory Periods of an AP
- Absolute Refractory Period (prevents AP summation)
- Relative Refractory Period (region d)
Absolute Refractory Period (prevents AP summation)
- NO AP can be generated, regardless of stimulus size
- results from either:
- -all Na+ channels being open (region b) or
- -all Na+ channels being inactivated (can’t open until MP reaches RMP, region c)
Relative Refractory Period (region d)
- period when an AP can be generated but only by a greater than normal stimulus
- Na+ channels are reactivated when MP passes RMP ∴ they are closed but can be reopened if threshold reached
- K+ channels are open & membrane hyperpolarized
- further to go to get to threshold ∴ need larger stimulus
All-or-none Principle of APs
- ALL: if threshold reached AP is produced - same every time (same max depol. etc)
- NONE: below threshold ⇒ no AP
Action Potential Propagation
- to act as a communication device an AP must be propagated along the axon’s entire length
- depolarization during AP (Na+ in) ⇒ +ve ions move toward more –ve on adjacent membrane ⇒ adjacent membrane depolarizes to reach threshold (voltage-gated Na+ open) ⇒ get AP on adjacent membrane
- movement of charge occurs in both directions but APs move in 1 direction because preceding membrane is in the absolute refractory period
- ∴ get sequence of APs along membrane, each one the same
- AP propagates along its entire length to the axon terminal ends
Rate of propagation depends on
- fiber (axon) diameter
- myelination
Fiber (axon) diameter
-larger diam. = faster propagation b/c less resistance to ion flow (= current)
Myelination
- unmyelinated fibers - APs all along the fiber (Na+ channels are adjacent to each other) = continuous conduction = slower
- myelinated fibers - AP occurs at nodes of Ranvier (ion channels only present here) = saltatory (leaping) conduction ⇒ fast
Fiber Types
- Type A
- Type C
Type A
- large diameter
- myelinated
- propagate APs @ ~130 m/sec
- most sensory neurons & motor neurons to skeletal muscles
Type C
- small diameter
- unmyelinated
- propagate APs @ ~0.5 m/sec
- found in autonomic NS (ANS) and some pain fibers
Synaptic Transmission (ST) at Neuronal Junction
- NS depends on chains of neurons connected by junctions called synapses
- presynaptic neuron to postsynaptic neuron transmission
Synaptic Transmission (ST) at Neuronal Junction Steps
- AP arrives at axon terminal (synaptic end bulb)
- Ca++ voltage gates open (due to AP) and Ca++ enters (higher [Ca++] outside!)
- rise in Ca++ triggers exocytosis of neurotransmitter (nt) containing vesicles
- nt crosses synaptic cleft, binds to specific receptors on postsynaptic membrane
- -receptors = chemically-gated ion channels ⇒ open in response to nt
- gated ion channels open – allowing movement of ions into (or out of) postsynaptic membrane
- -creates a graded potential (GP) called a postsynaptic potential (PSP).
Postsynaptic Potentials (PSPs)
- PSPs may be:
- -Excitatory PSPs (EPSPs) = GP ⇒ depolarization.
- -Inhibitory PSPs (IPSPs) = GP ⇒ hyperpolarization
- PSPs occur on cell body or dendrites
Excitatory PSPs (EPSPs)
- due to opening of Na+ (or Ca++) channels, or closing of K+ channels
- nt is often acetylcholine (ACh) or glutamate
Inhibitory PSPs (IPSPs)
- due to opening of K+ or Cl- channels
- -inhibits neuron from reaching an AP
- nt is often glycine or GABA
PSPs occur on cell body or dendrites
- many neurons can synapse onto one ⇒ if many EPSPs ⇒ summation ⇒ large area of membrane depolarized ⇒ spreads to axon hillock ⇒ if (sum of) EPSPs reaches threshold, get AP
- However, some may be IPSPs ⇒ the sum of all EPSPs & IPSPs determines if AP will occur at the axon hillock
Synaptic Transmission (ST) at Neuromuscular Junction
-junction between axon terminal of neuron & an individual muscle fibre
Synaptic Transmission (ST) at Neuromuscular Junction Steps
- neurotransmitter (nt) released = always ACh
- Na+ chemical gates on muscle motor end plate (=sarcolemma of muscle fiber) open
- -causes GP (= end plate potential (EPP)) on sarcolemma
- EPP triggers AP on sarcolemma
- -lots of ACh released in (a) ∴ always get an AP from an EPP
