3.6.2 nervous coordination Flashcards
what is a nerve impulse?
not a flow of electrons
like an electric current
brief changes in the distribution of electrical charge across the CSM
caused by rapid movement of sodium and potassium ions in and out of the cell
what is the resting potential of neurones?
studied historically using giant squid axons and microelectrodes
all living cells have electrical differences across their membranes
potential difference between inside + outside = overall membrane potential
what is the sodium potassium pumps role?
- the phospholipid bilayer prevents sodium (Na+) and potassium (K+) ions from diffusing across it
- phospholipid bilayer contains intrinsic channels which form ion channels
some channels remain open all the time, allowing Na+ and K+ to move across
some channels open and close to control movement across the membrane
where does the resting potential arise from?
sodium-potassium pumps in the neurones CSM constantly move sodium ions out of the cell and potassium ions into the cell
movements against the concentration gradient
use active transport = ATP
describe the movement of ions through a sodium-potassium pump
SODIUM OUT
POTASSIUM IN
moves three sodium ions for every two potassium ions it brings into the cell
results in an excess of positive charge outside the cell membrane (electrochemical gradient)
difference in charge is = -65mV.
becomes +40mV = depolarisation
inside is more negative than outside
K+ diffuses out faster than Na+ diffuses back in, contributes to potential difference
describe the process of an action potential
when a stimulus is received, its energy causes a temporary reversal of the charges across the axon membrane
at resting potential, most voltage gates for sodium are closed
when the receptor is stimulated, sodium voltage-gated channels open
sodium gets rapidly flooded down the electrochemical gradient
triggers a reversal in potential difference
potassium vaulted channels open and potassium ions diffuse out of the axon
many potassium ions leave the axon so the membrane becomes more negative (hyperpolarisation)
potassium gates close + sodium-potassium pump restores resting potential (depolarisation)
describe the transmission of an action potential
temporary depolarisation causes a local circuit to be set between the depolarised region and resting areas
depolarises adjoining regions + generates AP
AP sweeps along membrane in one direction
new AP is only generated ahead of the AP, due to region still recovering
time it takes to recover = refractory period
what is a refractory period?
- ensured AP are propagated in one direction only
- produces discrete impulses, a new impulse cannot start immediately behind previous one
- limited frequency of AP, therefore limits strength of stimulus that can be detected
describe a saltatory (jumping) conduction
acts as electrical insulator, at nodes of Ranvier only
local circuits stretch from node to node (only points where sodium ions can flood in)
what is a saltatory conduction?
saltatory conduction speeds up transmission up to 50 times
achieves a considerable increase in conduction velocity up to -100ms-1
has enabled mammalian nerve fibres to be much smaller but still have the same conduction velocity as larger neurones
factors that affect the speed at which an AP travels?
- whether or not the neurone is myelinated
- the diameter of the axon = the greater the diameter, the faster the AP
- temperature = in ectothermic organisms, the higher the temp, the faster the AP
what is the all or nothing principle?
a stimulus below the threshold value will not trigger an AP
any stimulus above the threshold will trigger an AP of the same size
how can an organism perceive the size of a stimulus?
- the larger the stimulus, the higher the frequency of impulses
- different neurones have different threshold values so more neurones are stimulated by a strong stimulus
what are the two principles of coordination?
- nervous system = nerve cells pass electrical impulses and secrete chemicals known as neurotransmitters
- hormonal system = produces chemicals that are transported in the blood plasma to their target cells.
what is the structure of a motor neurone?
- cell body = contains all cell organelles, including a nucleus and large amounts of RER. associated with the production of proteins and neurotransmitters
- dendrons = extensions of the cell body which subdivide into dendrites (carry nerve impulses away from the body)
- axon = long fibre that carries nerve impulses away from the cell body
- Schwann cells = surround the axon and provide electrical insulation. Carry out phagocytosis and play a part in nerve regeneration
- myelin sheath = membranes of Schwann cells. myelin is a lipid
- nodes of Ranvier = constrictions between adjacent Schwann cells where there is no myelin sheath
