What are sensory receptors?
- specialised cells that detect stimuli and generate an action potential.
- most are transducers - convert one form of energy to another (electrical impulse).
- transducers adapted to detect a change in a particular form of energy.
What are Pacinian corpuscles?
- pressure receptors that detect changes in pressure on the skin.
- rings of connective tissue wrapped around the end of a nerve cell.
- rings deformed when skin is under pressure which push against nerve ending.
- when pressure is constant they stop responding.
How do Pacinian corpuscles work?
- Na/K pumps actively pump 3Na+ out for every 2K+ in, conc gradient created across membrane.
- cell is -vely charged inside compared to outside.
- membrane is more permeable to K+, some leak out. Few Na+ leak in.
- Na+ and K+ channels are voltage gated
- when membrane is deformed by changing pressure, Na channels open, allowing Na+ to diffuse into cell producing a generator potential.
How are nerve impulses generated (in Pacinian corpuscles)?
1) cell membrane in polarised when cell is inactive, -vely charged inside compared with outside.
2) Na channels open, Na+ diffuse down their conc gradient into the cell.
3) this movement creates a change in potential difference across the membrane.
4) inside cell becomes less -ve (compared to outside) than usual. this is depolarisation (of cell membrane).
5) change in potential difference across receptor membrane is called a generator potential.
6) small stimuli, few Na channels open. Larger stimulus, more gated channels open.
7) If enough gates open and enough Na+ enter cell, pot diff across cell membrane changes significantly and will initiate an action potential (impulse).
function of sensory neurones
carry ac pot from sensory receptor to CNS
function of relay neurones
connect sensory and motor neurones
function of motor neurones
carry ac pot from CNS to effector (muscle or gland)
structure (specialised features) of neurones
- long so they can carry ac pot over a long distance.
- many gated Na+, K+ channels in plasma membrane.
- Na/K pumps use ATP to actively transport 3 Na+ out and 2 K+ in.
- neurones maintain a pot diff across plasma membrane.
- cell body contains nucleus, many mitochondria + ribosomes.
- axon carries impulses away from cell body. Dendron carries impulses to cell body.
- surrounded by fatty layer (Schwann cells), insulating cell from the electrical activity in other nerve cells nearby.
How are motor neurones different?
have their cell body in the CNS and have a long axon that carries ac pot out to the effector.
How are sensory neurones different?
have a long dendron carrying ac pot from sensory receptor to cell body (outside CNS). they then have a short axon carrying ac pot into CNS
What are myelinated neurones?
neurones wrapped in several layers of membrane and thin cytoplasm from the Schwann cells.
What are non-myelinated neurones?
neurones not wrapped in myelin sheath.
Which type of neurone aren’t myelinated?
- those in CNS aren’t.
- peripheral are
What are the gaps in myelin sheath?
nodes of Ranvier
What does the myelin sheath prevent?
movement of ions across membrane, leads to saltatory conduction where impulse jumps from one node to the next for faster conduction.
How do non myelinated neurones transmit impulses?
as waves
What are the advantages of myelination?
- ac pot transmitted more rapidly
- faster response to stimuli
Why are faster transmission speeds not important non-myelinated neurones?
- neurones are shorter and carry ac pot over a short distance.
- Often used is coordinating body functions like breathing, action of digestive system.
What does it mean when a neurone is at rest?
- polarised
- not transmitting an ac pot
Describe function of neurone at rest
- plasma membrane actively pumping 3Na+ out for every 2K+ in.
- pot diff across membrane is about -60mV (resting potential).
What’s an action potential?
action potential is a brief reversal of the pot diff across membrane causing a peak of +40mV compared to resting pot of -60mV.
What’s positive feedback?
a mechanism that increases a change, taking it away from the optimum.
How is an ac pot self-perpetuating?
once it starts at a point on a neurone, it will continue along to the end of neurone.
What are the stages in creating an ac pot?
1) membrane starts in its resting state - polarised with the inside of cell being -60mV compared to the outside. Higher conc of Na+ outside than inside and higher conc of K+ inside than outside.
2) Na+ channels open and some Na+ diffuse into the cell.
3) membrane depolairses (becomes less -ve) compared to outside and reaches the threshold value of -50mV.
4) +ve feedback causes nearby voltage-gated Na+ channels to open and many Na+ diffuse in. As more enter, cell becomes more +vely charged inside compared to outside.
5) pot diff across plasma membrane reaches +40mV. Inside of cell is +vely charged inside compared with outside.
6) Na+ channels close, K+ open.
7) K+ diffuse out of cell bringing the pot diff back to -ve inside compared with outside (repolarisation).
8) pot diff overshoots slightly, making cell hyperpolarised.
9) the original pot diff is restored so that the cell returns to its resting state.
