ch13 - neurones and action potential

what two systems is homeostasis monitored and controlled by?

Nervous and Hormonal system

what is a stimulus

a change in the internal or external environment

what are the 5 components of the nervous pathway which an electrical impulse is carried through

receptor > sensory neurone > central nervous system > motor neurone > effector

what is the role of the receptor 

to detect and receive stimuli 

what is the role of the sensory neurone

to carry electrical impulse from receptor to CNS

what is the role of the CNS

contains relay neurones that carry electrical impulse from sensory to motor neurone 

what is another name for relay neurone

interneuron

what is the role of motor neurone 

to carry electrical impulse from CNS to effector

what is the role of the effector

muscle or gland which will carry out a response

what are the typical components of a neurone

dendron/ dendrites

axon

cell body

nodes of ranvier

myelin sheath

what is the function of the dendron/ dendrites 

to receive signal and carry impulse towards cell body 

what is the difference between dendrons and dendrites

dendrites are shorter and highly branched

what is the function of the axon

to carry impulse away from cell body 

what is the function of the cell body

it controls the cells activities and releases neurotransmitters according to the impulse it receives

what is the function of the nodes of Ranvier

gaps between the myelin sheath 

what is the myelin sheath made of and why is it important

its made of layers of plasma membrane which provides electrical insulation allowing for saltatory conduction

what is saltatory conduction and why is it important

electrical impulses will jump between the nodes of Ranvier which speeds up rate of nerve transmission 

what forms the myelin sheath

myelin sheath contains Schwann cells which forms it

image of the 3 types of neurones

what is a sensory receptor 

specialised cells that detect stimuli

why are sensory receptors called transducers

because they convert one form of energy (e.g. thermal, light) into another (always electrical)

what are the 4 types of sensory receptors

mechanoreceptors

chemoreceptors

thermoreceptors

photoreceptors

what do mechanoreceptors detect and where are they found

detect = pressure and movement

located = skin, muscle

what do chemoreceptors detect and where are they found

detect = chemicals and changes in blood conc.

located = nose, tongue, blood vessels e.g. carotid arteries and aorta

what do thermoreceptors detect and where are they found

detect = temperature changes

located = skin (detects external temp) & hypothalamus (internal temp)

what do photoreceptors detect and where are they found

detect = light

located = retina - by the cones and rods

how do photoreceptors work

1. light rays focus onto the lens by the retina

2. photoreceptors produce electrical impulse which travels through the optic nerve to the brain 

what is an example of a mechanoreceptor

Pacinian corpuscle

what are the stages when a Pacinian corpuscle is stimulated (add picture)

1. at resting state the stretch-mediated sodium ion (Na+) channels are too narrow for Na+ to enter maintaining a resting potential -70mV

2. when pressure is applied - corpuscle is stimulated - corpuscle deforms causing the membrane to stretch, which widens the Na+ channels allowing Na+ to diffuse in down the electrochemical gradient

3. membrane depolarises becoming less -ve which initiates a generator potential

4. if the generator potential reaches the threshold an action potential is triggered

when a cell is at resting potential what state is it in

it is polarised/charged and in state in which it can be stimulated

why is the resting potential of all our cells -70mV

due to the uneven distribution of charge, it is more -ve inside than outside due to there being a higher concentration of +ve ions outside 

what does potential difference mean 

when there is a difference in charge between two areas - this is caused by the uneven distrubution of charge

what does generator potential mean

a change in the potential difference due to a stimulus

what are the 6 stages for generating an action potential

1.resting potential

2.stimulation and passing threshold level 

3.depolarisation

4.repolarisation

5.hyperpolarisation

6.refractory period 

how is resting potential maintained

-Sodium-potassium ion pump = it is a carrier protein, uses ATP energy to actively transport 3Na+ out and 2K+ in neurone
-non VG Potassium ion (K+) channel = allows K⁺ to leave neurone by facilitated diffusion down its concentration gradient

-VG Sodium ion (Na+) channel = channels are closed preventing Na⁺ entering neurone

-permanent -ve ions 

when an action potential occurs what state is the cell in

state in which membrane is being stimulated

what happens when a receptor detects and receives a stimulus

first few VG sodium ion channels (Na+) will open allowing sodium to enter axon by facilitated diffusion causing membrane to become less negative

if a stimulus is strong enough what happens

the adjacent VG sodium ion channels along will open causing Na+ conc. to increase reaching threshold potential of -55mV 

what does depolarisation mean

when the membrane becomes less -ve, the inside is more positive relative to the outside

what happens in depolarisation

once threshold potential -55mV is reached the membrane becomes depolarised, so more VG sodium ion channels will open = BIG INFLUX of Na+

what is the maximum potential difference 

depolarisation will continue until +40mV is reached

why must cells repolarise after an action potential

if the action potential occurs across the whole neuron membrane at once it can lead to seizures 

what occurs during repolarisation

the membrane will start becoming more -ve because VG sodium channels close and VG potassium channels open allowing potassium to diffuse out the axon by facilitated diffusion

during repolarisation why is it important that the VG sodium channels close

to prevent depolarisation again

what occurs during hyperpolarisation

an excess of potassium ions leave the axon causing the membrane to become so negative it drops below the -70 mV resting potential

what occurs during refractory period

Sodium potassium ion (Na+ and K+) pumps reopen and reaches refractory period to restore the membrane back to -70mV resting potential

what are the 2 reasons why the refractory period important

-cant generate another AP due to Na+ being closed (depolarisation cant occur) ensuring AP dont overlap

-ensure that impulses only travel in one direction preventing backflow

why is the regeneration of an action potential an example of positive feedback mechanism

the initial sodium ion influx depolarises the axon membrane causing more Na⁺ channels to open, leading to greater influx of Na⁺ so further depolarising the membrane

why are action potentials referred to as ‘all or nothing principle’

once threshold potential -55mV is reached an action potential will ALWAYS be triggered regardless of stimulus strength, if threshold potential is not reached then an AP will not be triggered 

what is the misconception and the truth about a stronger stimulus

misconception = stronger stimulus will increase the size AP > this is wrong all AP are the same size

truth = stronger stimulus will increase FREQUENCE of AP generated 

why is the propagation of an AP known as a ‘wave of depolarisation’

as each new section of the neurone membrane depolarises the next adjacent polarized section a ‘wave’ is formed, with the previous part entering refractory period

what are the three factors that affect speed of transmission of an AP

-myelination

-axon diameter/width

-temperature

how does myelination affect speed of transmission

Myelin sheath provide electrical insulation allowing for saltatory conduction = electrical impulses jump between the nodes of Ranvier = faster nerve impulse transmission

what is the disadvantage of unmyelinated neurones

they have slower nerve impulse transmission due to AP having to depolarise the whole neurone membrane

how does axon diameter/width affect speed of transmission

• A larger axon diameter means there is less resistance to ion flow, so the wave of depolarisation travels faster along the axon.

• Therefore, broader axons transmit impulses faster.

wider axon diameter = less resistance to the flow of ions (due to there being more space for ions to travel) = faster nerve impulse transmission

how does temperature affect speed of transmission

Higher temperature = faster diffusion of ions = faster depolarisation = faster nerve impulse transmission

what is the issue if temperature is too high

causes proteins to denature which results in slower impulse transmission due to membrane damage