| the role of acetylcholine | the neurotransmitter used in the parasympathetic nervous system |
| myelin sheath | produced by schwaan cells
acts as an electric insulator |
| what is a neuron? | nerve cells
it conducts nerve impulses
3 types of neurons: sensory, motor and relay |
| sensory neuron function | neuron that transmits impulses from receptors to relay neurons in the CNS |
| motor neuron function | a neuron that transmits impulses from the CNS to the effectors |
| relay neurons function | found in the CNS
connects the sensory to the motor neurons |
| sensory neuron structure | one long dendron and one short axon
cell body in the middle of the neuron |
| motor neuron structure | one long axon and many short dendrites
cell body at the end of the axon |
| relay neuron structure | many short dendrites
one axon with many axon terminals
unmyelinated |
| axons | an extension that transmits nerve impulses away from the cell body |
| dendrites/dendrons | an extension from a neuron that carries nerve impulses to the cell body |
| define effectors | a muscle or gland that produces a response to a stimulus |
| what is an action potential? | a large rapid change in potential difference across a membrane |
| define potential difference | the voltage across a membrane |
| what is a resting potential? | the potential difference across the membrane when the neurone is at rest
the outside is more positively charged than the inside so the membrane is polarised
resting potential is at about -70mV |
| define stimulus | a change in internal or external conditions of an organisms environment which brings about a response |
| CNS | central nervous system
made of the brain and spinal cord |
| myelination | a layer of fatty substance produced by Schwan cells to form the myelin sheath
acts as an electric insulator around axons and dendrites |
| nodes of Ranvier | the gaps between the myelin sheaths |
| Schwan cells | cells that form the myelin sheath around nerve cells |
| saltatory conduction | the mechanism by which speed of the action potential increases by jumping between the nodes of Ranvier |
| structure of the eye | iris, pupil, fovea, retina, optic nerve, rod cells and cone cells, circular and radial muscles, blind spot |
| function of the iris | coloured part of the eye
controls the amount of light that enters the eye using the circular and radial muscles |
| function of the pupil | also controls the amount of light that enters the eye
it involuntarily constricts in bright light and widens in dim light |
| function of the fovea | where most cone cells are found |
| function of the retina | where the photoreceptor cells - cone and rod cells - are found
creates electrical impulses that are sent to the optic nerve |
| what does the optic nerve do? | carries nerve impulses from the retina to the brain to interpret as images |
| rod cells | dominant in dim light
1 type
found only in the retina
can detect all visible wavelengths
low visual acuity
most abundant photoreceptor
has rhodopsin
senses only black and white and movement |
| cone cells | dominant in bright light
3 types
concentrated at the fovea
high visual acuity
has iodopsin
senses colour |
| blind spot | this is where all the axons of the neurones collect together and leave the eye forming the optic nerve
there are no photoreceptors here |
| circular and radial muscles | circular muscles contract to constrict the pupil and make it smaller
radial muscles contract to dilate the pupil and make it bigger |
| how does the pupil constrict? | occurs in the presence of bright light
photoreceptors in the retina detect the light stimulus
an action potential is generated in the bipolar neurones then the optic nerve
the action potential travels along the sensory neurone to the brain
the brain transmitts an action potential along the oculomotor neurones to the circular muscles in the iris
they contract and the pupil constricts, getting smaller |
| how does the pupil dilate? | occurs in dim light
photoreceptors in the retina detect the light stimulus
an action potential is generated in the bipolar neurone then the optic nerve
the action potential travels along the sensory neurones to the brain
the brain transmitts an action potential along the oculomotor neurone to the radial muscles in the iris
they contract and the pupil dilates, getting wider |
| 5 steps for generating an action potential | resting potential
threshold met
depolarization
repolarization
hyperpolarization |
| how it the resting potential established? | the sodium/potassium pump actively transports 3Na+ out of the axon for every 2K+ pumped in to the axon using ATP
K+ diffuses back out of the cell down the chemical gradient through protein channels
the K+ diffusion out of the cell is opposed by the electro gradient in the opposite direction - the K+ moving into the cell - so a resting potential of -70mV is achieved
the membrane becomes less permeable to Na+ so less diffuses back into the cell |
| what happens when the threshold is met? | the point when an action potential can be generated |
| what happens during depolarisation? | the membrane is polarised at rest
neurotransmitters arrive at the post synaptic membrane
some voltage dependent Na+ channels open
the membrane depolarises slightly to the threshold value of -50mV
more voltage dependent Na+ channels open and Na+ floods in along their electrochemical concentration gradient
the membrane reaches a potential difference of +40mV
the membrane is depolarised as the inside is more positive than the outside |
| what happens during repolarisation? | once the potential difference of the membrane has reached 40mV the Na+ channels close and the voltage dependent K+ channels open
the membrane is now impermeable to Na+
K+ diffuse out of the axon down the concentration and electrochemical gradient
The inside now becomes more negative than the outside so the membrane is polarised again |
| what happens during the refractory periods? | absolute refractory period and relative refractory period
periods of time where another action potential cannot take place
in the absolute RP no action potential can be generated at all because the Na+ voltage dependent channels are closed so the membrane cannot be depolarised by the entry of Na+
in the relative RP there is a really low chance that an action potential can be generated |
| what are the names of the receptor cells in the eye? | photoreceptors rod and cone cells |
| structure of synapses | presynaptic neurone
synaptic cleft
neurotransmitters
voltage dependent Ca2+, Na+ and K+ channels |
| function of synapses | the gap between two neurones or between a neurone and a muscle or gland
controls the nerve pathways allowing flexibility of response
integrates information from different neurones allowing a coordinate response |
| what are neurotransmitters? | chemicals which diffuse across the synaptic cleft to stimulate other neurones or effectors
they are made in the RER and Golgi like proteins
examples include acetylcholine, glutamate or adrenaline |
| the role of acetylcholine | the neurotransmitter used in the parasympathetic nervous system |
| how does a synapse transmit an impulse? | the action potential arrives
the membrane the depolarises
Ca2+ channels open and they enter the neurone
the presence of Ca2+ causes the synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane
the neurotransmitter are released by exocytosis into the synaptic cleft
the neurotransmitters bind with the receptors on the postsynaptic membrane opening the cation channels (Na+)
the postsynaptic membrane can now depolarise and initiate an action potential
once released from the receptors they are broken down by enzymes found in the synaptic cleft and diffuse away or be taken in by the presynaptic neurone |
