function of nervous system Flashcards
define nerve impulse
message that travels along nerve fibre
similarity and difference between leakage channels and voltage-gated channels
- leakage channels are open all the time
- voltage-gated channels open only when nerve is stimulated
- both allow ions to enter or leave neuron through facilitated diffusion down the conc gradient
describe sodium potassium pump
- moves 2 K+ ions into the cell for every 3 Na+ ions removed, net reduction of + ions in the cell
- carrier protein, moves Na+ and K+ against the concentration gradient through active transport using ATP
define membrane potential and hence resting membrane potential
- MP: potential difference of the inside and outside of the cell membrane
- RMP: membrane potential of unstimulated nerve cells (-70mV)
why is RMP always -70mV?
- Sodium potassium pump
- Sodium ion leakage channels allows sodium ions to enter intracellular fluid through facilitated diffusion down a concentration gradient
what does it mean for membrane to be polarised?
inside of cell membrane is more negative than outside of cell membrane
define an action potential
rapid depolarisation and repolarisation of the cell membrane of the neuron to transmit a nerve impulse
explain how membrane potential is increased slightly
- neuron is stimulated by a neurotransmitter or a sensory receptor
- some sodium channels are opened, some sodium ions diffuse into cell down conc gradient
- influx of these sodium ions cause intracellular fluid to be less negative, increasing membrane potential of the (section of the) neuron
define threshold
hence explain whether or not action potential generated
threshold: membrane potential at which the all-or-none response occurs (-55mV)
membrane potential exceeds threshold , action potential generated
define depolarisation
sudden increase in membrane potential
what happens in depolarisation?
- voltage-gated sodium channels open
- sodium ions diffuse into intracellular fluid down conc gradient
- influx of sodium ions into cell too great to be balanced out by sodium-potassium pump, removing 3 sodium ions from the intracellular fluid
- intracellular fluid is more positive than extracellular fluid
- membrane is depolarised
- voltage-gated potassium channels start to open, but take longer than sodium
define depolarised
describe peak
sodium ions entering = potassium ions leaving, no net movement of ions
define repolarisation
describe repolarisation
- sodium voltage gated channel is closed, stop the influx of sodium ions
- voltage gated potassium channels finish opening, increasing the amount of potassium ions diffusing out of the cell
- intracellular fluid more negative than extracellular fluid, decreasing membrane potential
- membrane has been repolarised
define refractory period
- short period following stimulus where a nerve cell is unable to be stimulated again
- lasts from when membrane potential reaches threshold to when it returns to RMP
why does nerve impulse only travel in 1 direction?
- membrane will not undergo an action potential if it is in refractory period
- action potentials only propagated in the regions which are not in refractory period
- nerve impulse will travel in one direction only
how does RP set the limit of frequency of impulses along a neuron
- when RP has passed, a further action potential can be carried out in the same nerve, as the first impulse has already travelled down the neuron
- refractory period sets the limit of frequency of impulses along a neuron
transmission of nerve impulse general
- single action potential occurs in 1 section of the membrane
- triggers action potential on adjacent section of membrane due to difference in electric charge between membrane where action potential is occuring and its adjacent membrane
- this process continues along the whole length of the neuron, which transmits the nerve impulse
- action potential does not travel along nerve fibre but but nerve impulse does
unmyelinated nerve impulse transmission
- action potential occuring in 1 section of the membrane causes movement of sodium ions to adjacent areas
- stimulates voltage-gated sodium ion channels open in the adjacent membrane, membrane potential rises to -55mV
- action potential is initiated in that section of the membrane
(because of the difference in electrical charge between the section where the action potential is occuring and the adjacent section of the membrane) - repeats itself along the whole length of the neuron until the nerve impulse has travelled away from the point of stimulation
myelinated transmission
- myelin sheaths insulated membrane from extracellular fluid
- ions were not able to move between inside cell and extracellular fluid
- action potentials could not be generated
- nodes of Ranvier, myelin sheaths absent, action potentials can be generated
- saltatory conduction occurs –> nerve impulse is transmitted from 1 node of Ranvier to the next
differences in myelinated and unmyelinated
- nerve impulses travel faster in myelinated
- unmyelinated –> conduction occurs across every section of the membrane along its entire length. myelinated –> conduction only occurs at nodes of Ranvier, saltatory conduction
speed of conduction
myelinated, increase diameter of axon
what happens with a stronger stimulus
depolarisation of more nerve fibres
strong stimulus produces more nerve impulses in a given time
define synapse
junction between pre-synaptic axon terminal and dendrite of post-synaptic neuron
define neurotransmitter
molecule that carries the nerve impulse across the synaptic cleft from pre-synaptic axon terminal to dendrite of post-synaptic neuron
describe transmission across synapse
1) nerve impulse reaches axon terminal which activates voltage-gated calcium ion channels on the membrane
2) calcium ions diffuse into pre-synaptic axon terminal down a concentration gradient
3) this stimulates synaptic vesicles to move towards and bind with the membrane, releasing neurotransmitters into synaptic cleft through exocytosis
4) neurotransmitters diffuse across synaptic cleft and bind to specific receptors on the dendrite of the post synaptic neuron
5) stimulates ligand-gated protein channels to open, allowing the influx of sodium ions and initiates an action potential at post-synaptic neuron
6) neurotransmitters removed from synapse by being reabsorbed by presynaptic membrane, broken down by enzymes, or diffuse away
thermoreceptors
peripheral - skin/mucous membranes, temp changes in external environment (hot and cold receptors exist)
central - hypothalamus, changes in core temperature
osmoreceptors
hypothalamus, small changes in osmotic pressure
chemoreceptors
mouth nose - tastes odours
some blood vessels - composition of body fluids, changes in blood pH, concentration of CO2 O2 in blood
touch receptors
close to skin surface - light touch
deeper - pressure, vibrations
base of hair follicles - touches that bend hair (adapt rapidly over a short time so you’re not aware of touch)
pain receptors
everywhere except brain, mostly skin mucous membranes - damage to tissues (don’t adapt, pain continues for as long as stimulus is present)
define receptor
structure that is able to detect changes in the bodies internal or external environment
stimulus
internal or external change which causes a response
cell body of neuron
contains nucleus and other organelles, controls functioning of the cell
dendrites
highly branched short extensions of cytoplasm of cell body
axon
single long extension of cytoplasm
axon terminal
where branches of axon terminate
myelin sheath
fatty material covering nerve fibre, formed by Schwann cells and oligodendrocytes which wrap around axon
- insulator
- protect from nerve damage
- increase speed of conduction
outermost coil - neurilemma - repair of injured fibres
nodes of Ranvier
unmyelinated gaps
sensory neurons
carry messages from receptors in sense organs or from skin to CNS
motor neurons
carry messages form CNS to effectors (muscle or secretory cells)
interneurons
link sensory and motor neurons within grey matter of spinal cord
unipolar
1 ext from cell body, 1 axon, 0 dendrites - all sensory neurons
bipolar
1 axon, 1 dendrite - eyes ears nose
multipolar
1 axon, multiple dendrites - skeletal motor neurons, interneurons
reflex
rapid automatic response to a change in internal or external environment
4 characteristics of reflexes
not spontaneous - stimulus required to trigger reflex
involuntary - occur without conscious thought
- rapid - only a small number of neurons involved
- stereotyped - occurs the same way each time it happens
spinal reflex
reflex carried out by spinal cord alone
reflex arc
pathway a nerve impulse follows travelling from receptor to effector
receptor (reflex arc)
(what) receptors at (where) detect the stimulus which is (what) and produces a nerve impulse
sensory neuron (reflex arc)
conducts the nerve impulse from the receptor to the spinal cord/brain
synapse (reflex arc)
impulse either passed directly onto motor neuron or is passed to 1 or more interneuron which then passes the nerve impulse onto the motor neuron
transmission across at least 1 synapse occurs in both cases
motor neuron
carries nerve impulse to the effector, which is (what) in the (where)
effector
receives nerve impulse, carries out appropriate response
brain and reflex
brain is not involved in reflex arc. but while response is occuring, impulses travel up spinal cord and into the brain. People will only be consciously aware of the situation after the response
acquired reflexes
response to a stimulus that has been learnt through practice
innate reflex
response to a stimulus that is acquired genetically and is therefore present at birth
effect of chemicals
influence transmission mostly at synapse or neuromuscular junction
stimulate or depress transmission
