topic 8 Flashcards
the nervous system (NS) is split into (2)
- the central nervous system
- the peripheral nervous system
central nervous system (CNS) consists of
- brain
- spinal cord
relay nerves
the peripheral nervous system consists of
- sensory nerves. carrying sensory information from the receptors to the CNS
- motor nerves. carrying the motor commands from the CNS to the effectors
the peripheral nervous system is subdivided into
- autonomic nervous system
- somatic nervous system
autonomic nervous system is
involuntary
stimulates smooth muscle, cardiac muscle and glands
somatic nervous system is
voluntary
stimulates skeletal muscle
autonomic nervous system is subdivided into
- the sympathetic nervous system
prepares the body for fight or flight response - the parasympathetic nervous system
prepares body for rest and digest
What is the difference between a neurone and a nerve
a neurone is a single cell
a nerve is a more complex structure containing a bundle of the axons of many neurons surrounded by a protective covering.
what are the basic characteristics of a neurone
- cell body. consisting the nucleus and cell organelles within the cytoplasm.
- dendrites. very fine. conduct impulses towards the cell body
- axon. transmits impulses away from the cell body
what are the 3 types of neurone
- sensory neuron
- relay neuron
- motor neuron
motor neurones
the cell body is situated within the central nervous system (CNS) and the axon extends out conducting impulses from the CNS to effectors ( muscles or glands).
the axons of some motor neurones can be extremely long, such as those that run the full length of the leg.
the dendrites are attached to the cell body.
sensory neurones
these carry impulses from sensory cell to the CNS
the cell body is branched of the axon.
relay neurone
these are found mostly within the CNS. they can have a large number of connections with other nerve cells.
relay neurones are also known as connector neurones and interneurones
cell body is in the middle of the axon.
axon –> cell body –> axon
myelin sheath
fatty insulating area that surrounds the axon.
it is made up of shwann cells wrapped around the axon
the sheath affects how quickly nerve impulses pass along the axon.
not all animals have myelinated axons, they are not found in invertebrates and some vertebrate axons are unmyelinated.
what is a reflex arc
simple nerve pathways responsible for reflexes which is a rapid, involuntary response to a stimuli
what happens in a reflex arc
- receptors detect a stimulus and generate a nerve impulse
- sensory neurones conduct a nerve impulse to the CNS along a sensory pathway
- sensory neurone enters the spinal cord through the dorsal route.
- sensory neurone forms a synapse with a relay neurone
- relay neurone forms a synapse with a motor neurone that leaves the spinal cord through the ventral route
- motor neurone carries impulses to an effector which produces a response.
how is the brain involved with stimulus response
most nerve pathways involve numerous neurones within the central nervous system. a sensory neurone connects to a range of neurones within the CNS and passes impulses to the brain to produce a coordinated response.
even in reflex arcs there are additional connections within the CNS to ensure a coordinated response. some synapses with motor neurones will be inhibited to ensure that the desired response occurs
what is the advantage of reflex pathway?
they produce rapid response; important for protection and survival
how do the muscles of the iris respond to light
- the iris controls the size of the pupil
it contains a pair of antagonistic muscles; radial and circular muscles.
these are both controlled by the autonomic nervous system.
the radial muscles are like the spokes of a wheel and are controlled by a sympathetic reflex.
the circular muscles are controlled by a parasympathetic reflex.
one reflex dilates the pupil and the other constricts it.
for the pupil to constrict radial muscles must relax and circular muscles must contract.
for the pupil to dilate the radial muscles must contract and the circular muscles must relax
how is pupil size controlled
- high levels of light striking photoreceptors in the retina cause nerve impulses to pass along the optic nerve to a number of different sites within the CNS, including a group of coordinating cells in the midbrain.
- impulses from these cells are sent along parasympathetic motor neurones to the circular muscles of the iris, causing them to contract. as the radial muscles relax. this constricts the pupil reducing the amount of light entering the eye
what are the purpose of the pupil reflex
to prevent damage to the retina from high intensity light; in dim light it ensures maximum light reaches the retina.
potential difference
the difference in electrical voltage across the cell surface membrane
what is the resting potential of a neurone
-70 m V (millivolts)
the axon is more negative than the outside
so the membrane is said to be polarised.
what causes the potential difference in a neurone
the uneven distribution of ions across the cell surface membrane.
there is more potassium ions (K+) inside the cell then extracellular.
more sodium ions (Na+) outside (extracellular) of the cell
there is more calcium ions (Cl-) outside of the cell (extracellular)
this is achieved by the action of sodium-potassium pumps in the cell surface membrane of the axon. these carry Na+ out of the cell and K+ into the cell.
these pumps act against the concentration gradients so is active transport with energy supplied by hydrolysing ATP>
the other organic ions (e.g -ve amino acids) are large and stage within the cell, so Cl- ions move out of the cell to help balance the charge cross the cell surface membrane
how is the resting potential maintained
- Na+ / K+ pump creates concentration gradient across the membrane.
- K+ diffuse out of the cell down the K+ concentration gradient, making the outside of the membrane +ve and the inside -ve creating a potential difference
- the potential difference will pull K+ back into the cell as it diffuses down the electrical gradient
- at 70- mV potential difference, the 2 gradients counteract each other and there is no net movement of K+
why is the axon resting potential -70 mV
the 2 forces involved in the movement of potassium ions:
1. the concentration gradient generated by the Na+ / K+ pump
2. the electrical gradient due to the difference in charge on the 2 sides of the membrane resulting from K+ diffusion.
K+ ions diffuse out of the cell due to the concentration gradient.
the more K+ ions that diffuse out of the cell the larger the potential difference across the membrane.
the increased negative charge created inside the cell as a consequence attracts K+ back across the membrane into the cell.
when the potential difference across the membrane is around -70 mV, the electrical gradient exactly balances the chemical gradient.
there is no net movement of K+ and hence a steady state exists, maintaining the potential difference at -70 mV.
an electrochemical equilibrium for K+ is in place and the membrane is polarised
what happens when a nerve is stimulated
- neurones are electrically excitable cells, so the potential difference across cell membrane changes when they are conducting.
if an electrical current above a threshold level is applied to the membrane a massive change in potential difference occurs.
the potential difference across the membrane is locally reversed making inside of axon positive and outside negative this is known as depolarisation.
potential difference becomes +40 mV for 3ms before returning to resting state. in order to conduct more impulses.
this is depolarisation.
the large change in voltage across membrane is action potential
what causes an action potential
1. depolarisation
- neurone is stimulated and some depolarisation occurs
- the change in the potential difference across the membrane causes a change in the shape of the Na+ gate, opening some of the voltage dependent sodium ion channels
- as Na+ flow in, depolarisation increases, triggering more gates to open once a certain potential threshold is reached. the opening of more gates increases depolarisation further. example of positive feedback.
- a change encourages further change of the same sort and it leads to a rapid opening of all the Na+ gates.
- this means there is no way of controlling the degree of depolarisation of the membrane; action potentials are either there or they are not.
- this property is called all nor nothing
the higher conc. of sodium ions outside of the axon, so Na+ flow rapidly inwards through open voltage dependent Na+ channels. causing a build up of +ve charges inside. this reverses the polarity of the membrane. potential difference reaches +40 mV
what causes an action potential
stage 2
2. repolarisation
after about 0.5 ms, the voltage dependent Na+ channels spontaneously close and Na+ permeability of the membrane returns to its usual very low level.
voltage dependent K+ channels open due to depolarisation of the membrane. as a result K+ ions move out of the axon down the electrochemical gradient. (down conc gradient and attracted by -ve charge outside cell)
as K+ flow out of the cell, the inside of the cell once again becomes more -ve than the outside.
what causes an action potential
stage 3
3. restoring the resting potential
the membrane is now highly permeable to K+ ions and more ions move out than occurs at resting potential, making the potential difference more negative than the normal resting potential.
this is known as hyperpolarisation of the membrane. the resting potential is re-established by closing the voltage dependent K+ channels and K+ diffuse into the axon.
action potential
summarised
- depolarisation. all nor nothing. voltage dependent Na+ channels open. Na+ flow into axon depolarising the membrane
- repolarisation. voltage dependent Na+ channels close. voltage dependent K+ channels open. K+ leave the axon, repolarising the membrane
- restoring resting potential. the membrane is hyperpolarised. voltage dependent K+ channels close. K+ diffuse back into the axon to recreate the resting potential.
sodium pump action at restoring and maintaining resting potential
if a lot of action potentials occur in an neurone, the Na+ conc. inside the cell rises significantly.
the sodium potassium pumps start to function, restoring the original ion concentration across the cell membrane.
if a cell is not transmitting many action potentials these pumps will not have to be used very frequently.
At rest there is some slow leakage of Na+ into the axon.
these sodium ions are pumped back out of the cell.
is it possible for an action potential to be triggered in a dead axon?
no. (unless ATP was added)
the polarisation of the membrane is maintained by the concentration gradients achieved by the action of energy requiring sodium potassium pumps; membrane integrity is lost in a dead axon.
how is the impulse passed along an axon
- at resting potential there is +ve charge on outside and -ve on inside of membrane with high Na+ conc on outside and high K+ concentration on inside
- when stimulated, voltage dependent Na+ channels open, Na+ flow into the axon depolarising membrane. localised electric currents are generated in the membrane. Na+ move to the adjacent polarised (resting) region causing a change in the electrical charge (potential difference) across this part of the membrane
- the change in potential difference in membrane adjacent initiates second action potential. at the site of first action potential the voltage dependent Na+ ions close and voltage dependent K+ channels open K+ ions leave axon polarising membrane becomes hyperpolarised
- a 3rd action potential is initiated by the second. in this way local electrical currents cause the nerve impulse to move along the axon. at the site of the first action potential, K+ diffuse back into the axon, restoring the resting potential.
refractory period
the new action potential cannot be generated in the same selection of membrane for about 5 milliseconds. this is known as the refractory period, which makes sure that the impulses travels in one direction along a nerve fibre.
lasts until all the voltage dependent Na+ and K+ channels have returned to their normal resting state (closed) and the resting potential is restored. the refractory period ensures that impulses only travel in one direction.
how does the refractory period ensure that an action potential will not be propagated back the way it came
a new action potential will only be generated at the leading edge of the previous one; because the membrane behind it will be recovering / incapable of transmitting an impulse; the membrane has to be repolarised and return to resting potential before another action potential can be generated.
can impulses be different sizes
no.
a stimulus must be above threshold level to generate an action potential. the all or nothing effect for action potentials means that the size of the stimulus, assuming it is above the threshold, has no effect on the size of the action potential.
the size of of the stimulus does not effect size of impulses but it does affect
- the frequency of impulses
- the number of neurones in a nerve that are conducting impulses
a frequency of firing and the firing of many neurones are usually associated with a strong stimulus.
what determines the speed of conduction
- in part determined by the diameter of the axon. the wider the diameter, the faster the impulse travels.
nerve axons of mammals ( 1- 20 um) are much narrower than others but impulses travel along them at up to 120 ms-1 .
what determines the speed of conduction of axons in mammals
presence of myelin sheath
saltatory conduction
myelin sheath acts as an electrical insulator along most of the axon, preventing any flow of ions across the membrane
gaps (nodes of ranvier) occur in the myelin sheath at regular intervals and these are the only place where depolarisation can occur.
as ions flow across the membrane at one node during depolarisation, a circuit is set up which reduces the potential difference of the membrane at the next node, triggering an action potential.
in this way, the impulse effectively jumps from one node to the next. this is much faster than a wave of depolarisation along the whole membrane. myelinated axons has a higher impulse velocity.
synapse
where 2 neurones meet.
synaptic cleft
the space between the 2 neurones
stages in synaptic transmission
- an action potential arrives at the presynaptic membrane
- the membrane depolarises. calcium (Ca2+) ions channels open. Ca2+ enters the neurone
- Ca2+ cause synaptic vesicles containing neurotransmitter to fuse wit the presynaptic membrane.
- neurotransmitter is released into the synaptic cleft (exocytosis) it takes 0.5 ms to diffuse across the synaptic cleft and reach the postsynaptic membrane.
- neurotransmitter binds with complementary receptors on the postsynaptic membrane. cation channel open. sodium ions Na+ flow through the channels.
- the membrane depolarises and initiates an action potential.
extent of depolarisation depends on amount of neurotransmitter reaching the postsynaptic membrane. depends on frequency of impulses reaching the presynaptic membrane and the number of functioning receptors in the postsynaptic membrane. - when released from the receptor the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down) or it can diffuse away and be broken down.
what is the role of synapses in nerve pathway
control and coordination
- control of nerve pathways, allowing flexibility of response
- integration of information from different neurones allowing a coordinated response.
the postsynaptic cell is likely to be receiving input from many synapses at the same time.
what are the main factors that affect the likelihood that the postsynaptic membrane will depolarise
- the type of synapse
- the number of impulses received.
what are the 2 types of synapses
- excitatory synapses.
- inhibitory synapses
excitatory synapses
make the postsynaptic membrane more permeable to sodium ions.
as single excitatory synapse typically does not depolarise the membrane enough to produce an action potential, but several impulses arriving within a short time produce sufficient depolarisation via the release of neurotransmitter to produce an action potential in the postsynaptic cell.
summation
the overall effect of impulses on the postsynaptic membrane
what are the 2 types of summation
- spatial summation
- temporal summation
spatial summation
here the impulses are from different synapses, usually from different neurones. the number of different sensory cells stimulated can be reflected in the control of the response.
temporal summation
in this case several impulses arrive at a synapse having travelled along a single neurone one after the other. their combined release of neurotransmitter generates an action potential in the postsynaptic membrane.
inhibitory synapses
make it less likely that an action potential will result in the postsynaptic cell.
the neurotransmitter from these synapses opens channels for chloride ions and potassium ions in the postsynaptic membrane, and these ions will then move through the channels down their diffusion gradients.
chloride ions will move into the cell carrying a -ve charge and K+ ions will move out carrying a +ve.
greater potential difference across the membrane as the insides become more negative than usual (about -90 mV)
this is called hyperpolarisation.
this makes subsequent depolarisation less likely. more excitatory synapses will be required to depolarise the membrane.
how do hormone bring about a change in the activity of its target cell
binds to receptors on target cell surface or within target cell; directly or indirectly via a second messenger molecule, the hormone affects gene expression.
what is the differences between nervous control and hormonal control (5)
- NS - electrical transmission by nerve impulses and chemical transmission at synapses
- HS - chemical transmission through blood
- NS- faster acting
- HS- slower acting
- NS- usually associated with short term changes
- HS- can control long term changes
- NS- action potential carried by neurones with connections to specific cells
- HS- blood caries the hormones to all cells, but only target cells are able to respond.
- NS- response is often very local, such as a specific muscle cell or gland
- HS- response may be widespread, such as growth and development.
what are the chemicals that control development and response to environment in plants
plant growth substances
they are chemicals produced in the plant in very low concentrations and transported to where they cause a response
early experiments on auxin
Darwin and son Francis completed experiments on phototropism (bending of plants towards a light source) which are considered to be some of the earliest work on the effects of auxin.
their experiments showed that an oat coleoptile with its tip cut off stops bending towards the light.
replacing the tip starts to growth towards the light again.
they concluded that some influence was transmitted from the shoot top to the lower parts of the seedlings, causing them to bend.
later experiments by researchers Boysen Jensen and Went.
Boysen Jensen and went research into phototropism and auxin
showed that a chemical made in the tip passed down the coleoptile.
demonstrated by removing the tip, placing it on a small block of agar jelly and putting the agar on top of the cut end. the coleoptile started to grow again; a chemical produced by the tip had diffused down through the agar jelly.
went provided further evidence by placing the agar blocks on one side of the cut coleoptile tip in the dark; this caused the coleoptile to curve away from the side receiving the chemical messenger from the agar.
the chemical identified as Auxin one its major functions is to stimulate growth.
the growth response is a result of cell elongation.
why does the coleoptile curve towards the light when the tip is in place
measured amount of chemical being produced on the shaded an unshaded side of the shoot and found that the total amount produced did not change compared with a shoot illuminated from all sides.
instead more auxin has passed down the shaded side.
the increased conc. of auxin on the shaded side increased cell elongation; the reduced concentration on the illuminated side inhibited cell elongation. as a result the shoot grew towards the light.
Cholodny-Went model
explained growth curvatures as resulting from the unequal distribution of axin due to lateral transport of auxin.
critised for small sample sizes and the difficulty of measuring the very small concentrations involved.
research still investigate. new techniques used to study tropisms include the use of genetically modified plants that produce fluorescent proteins in the presence of auxin, making it possible to visualise the location of the auxin
where are auxin used in phototropism
indoleacetic acid (IAA)
where are auxins such as IAA synthesised
synthesised in actively growing root and shoot tips (meristems) and in developing leaves, seeds and fruits. they are transported from where they are synthesised to sites of action where they bring about a range of responses.
they are transported long distances in the phloem and shorter distances between cells via specific carrier proteins in the cell membrane.
auxin’s affect on cells
auxin bind to protein receptors in the target cells.
this activates intracellular second messenger signal molecules, which activate transcription factors.
these control the transcription of auxin related genes, and the proteins produced bring about metabolic changes that result in a range of responses through changes in cell expansion, division and differentiation.
the auxin’s functions in a similar way to animal hormones (topic 7)
