nervous system Flashcards
what are the important ions
Na,K, Cl, Ca
large negatively charged organic ions which are non diffusible proteins
permeability of cell membrane to ions
determined by ion chennels - ions diffuse through then down concentration gradient
what are the types of ion channels
a) non-gated
- always open
- more K than Na in a neuron
these channels are important in establishing the resting membrane potential (RMP)
b) gated
- not involved at rest
- open in response to stimuli
resting membrane potential
at rest (not stimulated) a charge difference (potential difference) exists just across the cell membrane = membrane potential
what are the factors establishing RMP
a) Na/K - ATPase: not a channel
- breaks down 1 ATP and uses energy to pump 3 Na out and 2 K in
b) org- inside cell e.g. protein - connot cross membrane
c) more non-gated K than Na at rest
electrically excitable cells
only in muscle and nerve cells
- capable of producing departures from RMP in response to stimuli (= changes in the external or internal environment)
what happens when a neuron is stimulated
a) gated ion channels open
b) MP changes, producing a graded potential. if the threshold potential is reached then it triggers an action potential
graded potential
-stimulus causes a small change in MP, usually on dendrite or cell body ( no longer at rest) by opening gated channels ( changes membrane permeability)
what ae the results of GP
a) more +ve than RMP=depolarization
b) more -ve than RMP = hyperpolarization
what are the characteristics of GP
a) ions move passively (unlike charges attract =)Current flow causing depol or hyperpol on adjacent membrane
b) GPs are short distant signals - die away quickly (short-lived)
c) magnitiude and distance traveled by potential varies directly with the strength of the stimulus
d) GPs can summate - 1st GP present when 2nd stim occurs creating larger GP
what happens after a GP
repolarization = return to RMP after depolarization or hyperpolarization
GPs - action potential
GPs are essential in initiating a nerve impulse (AP)
- if the GP causes depol and if depol. is large enough or multiple GPs sum to be large enough leads to action potential
what are the steps of GPs - action potential
a. critical stimulus
b. GP reaches threshold
c. action potential
action potential
- a nerve impulse (signal)
- large change in MP that propagates along an axon with no change in intensity
- initiates at trigger zone
what are the action potential events
a. =GP - membrane potential at the axon hillock reaches -55mV
b, c, d = action potential
what are the phases of action potential depolarization phase.
I) voltage-gated Na channels respond to MP change and open-greatly increases Na permeability
ii) as gates open more Na diffuses in causing even more Na voltage gates to open
iii) Na diffuses in causing depolarization to +30 mV
phases of repolarization phase
I)Na channels close become inactivated (decreased Na permeability) - Na movement returns to resting levels
ii) voltage-gated K channels are open (increased permeability) K diffuses out decreasing MP
phases of after-hyperpolarization
I) K channels are slow to close
ii) Na channels are reactivated - can respond to stimuli
what happens when K channels close
MP returns RMP
how many APs causes a measurable change in the cell
10,000
refractory period of an AP
a) absolute refractory period ( prevents summation)
- no AP can be generated
- results from either
1. all voltage gated Na channels are open
2. Na channels are inactivated cant open until MP reaches RMP
b) relative refractory period
- period when an AP can only be generated by a greater than normal stimulus
- K channels are open & membrane is hyperpolarized
all-or-none principle of APs
all: if threshold is reached, AP is produced
none: below threshold - no AP
action potential propagation
- to act as a communication mechanism an AP must be propagated along the axons entire length
- depolarization during AP positive ions move towards more negative ion on adjacent membrane
how many directions do APs move in
1 direction because the preceding membrane is in the absolute refractory period
what does the rate of propagation depend on
a) fibre diameter
b) myelination
unmyelination fibres
APs all along the fibre = continuous conduction = slower
myelinated fibres
AP occurs at nodes of ranvier = salutatory (leaping) conduction - fast
what are the types of fibres range from
a) type A
- large diameter
- myelinated
- propagate APs at ~ 130 m/sec
b) type C
- small diameter
- unmyelinated
- propagate APs at ~ 0.5m/sec
- found in autonomic NS and some pain fibres
synaptic transmission at neural junction
NS depends on chains of neurons connected by junction called synapses
- presynaptic neuron to postsynaptic neuron transmission
what are the steps from presynaptic to postsynaptic neuron
a. AP arrives at axon terminal
b. Ca voltage gates open (due to AP) and Ca enters
c. rise in Ca triggers exocytosis of vesicles containing nt
d. nt crosses synaptic cleft binds to specific receptors on postsynaptic membrane
e. gated ion channels open allowing movement of ins into postsynaptic membrane creating a GP
postsynaptic potentials
may be:
a. excitatory PSPs
- due to opening of Na channels or closing K channels
- often acetylcholine or glutamate
b. inhibitory PSPs - hyperpolarization
- due to opening of K or Cl channels inhibits neuron from reaching threshold
- often glycine or GABA
synaptic transmission at the neuromuscular junction
-junction between axon terminal & an individual muscle fibre
steps
a. nt released =always ACh
b. chemical gates on mucle motor end plate open and Na enters. causes GP on sarcolemma
c. EPP triggers AP on sarcolemma, lots of ACh released in a.
