3.6 Organisms Respond to Changes Flashcards
structure of a myelinated motor neurone
cell body, dendrons, axons, Schwann cells, myelin sheath, nodes of Ranvier
cell body
contains all the cell organelles, including the nucleus and rough endoplasmic reticulum
production of proteins and neurotransmitters
dendrons
extensions of the cell body which divide into dendrites
Transmit/carry nerve impulses towards the cell body
axons
a single long fibre
Nerve fibres carry nerve impulses away from the cell body
schwann cells
form the myelin sheath wrapped around the axon
Protect the axon, electrically insulate it, phagocytosis of cell debris + involved in nerve regeneration
myelin sheath
made up of the membranes of Schwann cells
Schwann membranes are rich in myelin (lipid)
nodes of ranvier
gaps between Schawnn cells where there is no myelin sheath
2-3 micrometers long + occur every 1-3 mm in humans
sensory neurones
Nerve impulses from a receptor to an intermediate/motor neurone
One long dendron + one axon
Cell body in the middle
motor neurones
Nerve impulses from an intermediate/relay neurone to an effector
Long axon + many short dendrites
Cell body at one end
intermediate/relay neurones
Impulses between neurones with numerous short processes
define resting potential
when the inside of the axon is negatively charged relative to the outside of the axon, when this occurs the axon is polarised
establishment of resting potential
Sodium ions are actively transported out of the axon by the sodium potassium pumps
Potassium ions are actively transported into the axon by the sodium potassium pumps
Active transport of sodium is greater than that of potassium because 3 sodium ions move out for every two potassium ions that move in
As a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm. More potassium ions are in the cytoplasm than in the tissue fluid. This creates an electrochemical gradient.
Sodium ions begin to diffuse back naturally into the axon while the potassium ions diffuse back out of the axon
Most of the gated channels for diffusion of sodium ions are closed but for potassium are open, maintaining the electrochemical gradient causing resting potential
define action potential
when the negative charge inside the membrane becomes the positive charge, depolarising the axon, occurs when transmitting a nerve impulse
establishment of action potential
At resting potential, some potassium voltage gated channels are open but the sodium voltage gated channels are closed.
The energy of the stimulus causes some of the sodium voltage gated channels in the axon membrane to open so sodium ions diffuse into the axon along their electrochemical gradient.
Sodium ions are positively charged, so this diffusion causes a reversal in the potential difference across the membrane
As more ions diffuse, more sodium ion channels open, causing a greater influx of the ions
Once the action potential of around +40 mv exists, voltage gates close on the sodium ion channels and those on potassium ion channels open
The electrical gradient which was preventing further outward movement of the potassium ions is now reversed so even more channels open, starting repolarisation of the axon due to movement of K+
This causes hyperpolarisation of the axon due to the inside being more negative than usual
Potassium ion channel gates close and the sodium potassium pump moves sodium out and potassiums in.
This reestablishes the resting potential so the axon membrane is repolarised
Passage of action potential along an unmyelinated neuron
The axon membrane is polarised because there is greater concentration of positive ions on the outside than the inside
Stimulus causes sudden influx of sodium ions into the cytoplasm so a reversal of charge on the membrane, this is the action potential and depolarises the membrane
Influx of sodium ions cause localised electrical currents which open voltage gated sodium ion channels a little further along the axon. This causes depolarisation to move along the membrane. Behind the moving depolarisation, sodium channels close and potassium ones open
The outward movement of potassium ions which occurs behind the action potential has caused the axon to become repolarised.
Repolarisation of the axon allows sodium ions to be actively transported out, returning the axon to resting potential in preparation for a new stimulus.
Passage of action potential along a myelinated neuron/saltatory conduction
When sodium channels are open, sodium ions move into the axon from the higher concentration outside to the lower inside
The concentration of sodium ions rises where the channels are open
The ions continue to diffuse but this time sideways along the neuron
They diffuse away from the increased concentration and towards a lower concentration down the neuron, nearer the next node
This causes slight depolarisation because the voltage gated channels are opened by changes in the potential difference, causing an influx of sodium ions
The continued influx causes a full depolarisation which is further along the neuron and this moves the action potential along the neuron as well
nodes of ranvier
gaps between the Schwann cells which form the myelin sheath
why is saltatory conduction faster than along unmyelinated neurons
action potential can jump from one node to another
how does the myelin sheath affect speed of action potential
Travels faster because impulses are transferred more efficiently so a response is coordinated faster and the organism is better protected
how does axon diameter affect speed of action potential
Greater the diameter the faster the speed of conductance because there is less leakage of ions from a large axon which would make membrane potentials harder to maintain
There is a reduced internal resistance
how does temperature affect speed of action potential
Higher the temperature, higher the rate of diffusion of ions, the faster the nerve impulse
Enzymes involved in respiration for active transport in the sodium potassium pump function more rapidly at higher temperatures so nerve impulse quicker when it is warmer
Too high a temperature and the enzymes/plasma membrane proteins denature so impulses fail to be conducted
Affects speed + strength of muscle contractions
explain the all or nothing principle
There is a certain level of stimulus, the threshold value, which triggers action potential. Any stimulus below the threshold value, there is no action potential and therefore no impulse is generated. This is the nothing principle.
Any stimulus above the threshold will generate an action potential so a nerve impulse can travel.
how to detect the size of the stimulus
The number of impulses passing in a given time. The larger the stimulus, the more impulses that are generated in a given time
By having different neurons with different threshold values. The number and type of neurons used to pass impulses to the brain determines the size of the stimulus.
define refractory period
Occurs after action potential when inward movement of sodium ions is prevented because sodium ion voltage gated channels are closed so it is impossible for a further action potential to be generated.
purposes of the refractory period
Ensures action potentials are propagated in one direction only
Produces discrete impulses so action potentials are separated from one another
Limits the number of action potentials that can pass along the axon in a given time so it limits the strength of stimulus that can be detected
Features of sensory reception common to all receptors
Is specific to a single type of stimulus (will not respond to any other kind of stimulus)
Produces generator potential (converting the energy of stimulus into a nervous impulse) by acting as a transducer
structure + description of pacinian corpuscle
Sensory receptors found in the skin, joints, ligaments + tendons
A single sensory neuron is in the centre of layers of connective tissue
Oval shaped
detects changes in pressure (mechanical stimuli)
how does a pacinian corpuscle produce a generator potential
In resting, stretch mediated sodium channels are too narrow for sodium ions to pass along them so the neuron has a resting potential (PD is negative)
Pressure is applied causing the corpuscle to deform which stretches the membrane around the neuron
The stretching widens the sodium ion channels so sodium ions can diffuse into the neuron down their concentration gradient via facilitated diffusion
An influx of sodium ions depolarises the membrane and produces a generator potential
explain the low intensity response of rod cells
rhodopsin can break down even with low energy to create a generator potential
Stimulation of multiple rod cells can be combined to exceed threshold value + stimulate a generator potential
explain the visual acuity of rod cells
Low visual acuity because the brain cannot distinguish between separate sources of light due to multiple rod cells linking to one bipolar cell and therefore only being able to send a single nervous impulse
location of the rod cells
At the peripheries, rod cells are in abundance because light intensity is lowest
explain the high intensity response of cone cells
Each cone cell has their own separate bipolar cells so the stimulation of multiple cone cells cannot be combined to exceed the threshold value and thus produce a generator potential
Iodopsin requires a higher intensity for its break-down and create a generator potential
explain the visual acuity of cone cells
High visual acuity because each cone cell is connected to its own bipolar cell/sensory neuron, meaning two generator potentials are produced and two nerve impulses can be sent to the brain
location of the cone cells
Fovea receives the highest intensity of light so cone cells are present
define visual acuity
the clarity of vision, determined by the individual’s ability to recognise small details with precision
colour seen by rod and cone cells
rod - Cannot distinguish different wavelengths of light so black + white only
cone - Each cone cell is sensitive to a different specific range of wavelengths so can see in full colour
features of the presynaptic bulb
A number of voltage gated ion channels which allow a rapid influx of calcium 2+ ions into the cytoplasm
Many smooth endoplasmic reticulum which provide an intra-cellular store of calcium 2+ for release
Many mitochondria which generate the ATP that is used to actively transport calcium 2+ ions out of the cell
Synaptic vesicles store acetylcholine
features of the presynaptic bulb
A number of voltage gated ion channels which allow a rapid influx of calcium 2+ ions into the cytoplasm
Many smooth endoplasmic reticulum which provide an intra-cellular store of calcium 2+ for release
Many mitochondria which generate the ATP that is used to actively transport calcium 2+ ions out of the cell
Synaptic vesicles store acetylcholine
what does the post synaptic membrane contain
specialised sodium ion channels which has specific receptor proteins to receive the acetylcholine
function of synapses
Different stimuli contribute to a single response by the number of impulses from them being combined at the synapse.
A single stimulus creates a number of simultaneous responses because it can initiate new impulses in a number of different neurons at a synapse.
describe the cascade of events at a cholinergic synapse
An action potential is conducted along the length of the presynaptic neuron and arrives at the presynaptic bulb which causes a potential difference change and the voltage-gated calcium ion channels to open
The influx of calcium ions into the presynaptic neuron bind to the proteins that connect the synaptic vesicles to the presynaptic membrane causing the vesicles to fuse with the presynaptic membrane and release the acetylcholine into the synaptic cleft by exocytosis
Acetylcholine diffuses across the synaptic cleft and binds to the receptor sites on the sodium ion protein channels in the membrane of the postsynaptic neuron which causes the sodium ion protein channels to open so sodium ions diffuse rapidly into the postsynaptic membrane along a concentration gradient
This influx generates a new action potential in the postsynaptic in the postsynaptic membrane because the change in potential difference causes the channels to open causing depolarisation to peak at threshold value so action potential is transmitted
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which diffuse back across the synaptic cleft into the presynaptic neuron {= recycling). In addition to recycling the choline and acetyl, the rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the postsynaptic neurone, and so leads to discrete transfer of information across synapses
ATP released from mitochondria recombines choline and ethanoic acid into acetylcholine which is stored on the synaptic vesicles for future use, sodium ion protein channels close in the absence of acetylcholine in the receptor sites
importance of a functioning cholinesterase
Acetylcholine in the receptor sites is inhibited and broken down into acetate + choline
If it does not inhibit this, a continuous nerve impulse will be generated because of further action potentials
Describe the events that will occur in the synaptic cleft and the result in the postsynaptic neuron when nicotine (similar to acetylcholine) is present in the cholinergic synapse:
Nicotine is a stimulant and binds to receptor sites on sodium ion channels on the postsynaptic membrane
Causes a GP to establish in the generator region which combines to take the membrane potential above the threshold value
Leading to an action potential being transmitted by the postsynaptic neuron
Describe how the effect of MDMA in ecstasy on synaptic transmissions explains the categorisation of this drug as a stimulant
MDMA binds to SERT so there is no reuptake of serotonin into the presynaptic terminal
This means serotonins remains in the synaptic cleft so it can bind to the SERT receptors on the postsynaptic membrane
Higher levels of serotonin which is linked to happiness and wellbeing hence stimulant
similarities in structure between NMJ and CS
Action potentials arrive at the presynaptic neuron bulb in a synapse and the end of the axon of the motor neuron
Same series of events
A chemical signal diffuses across both
acetylcholine
differences in structure between NMJ and CS
Chemical signal diffuses across the cleft between two neurons but just across one NMJ
Arrives at the neuron after the synaptic cleft but just the skeletal fibres after the NMJ
Acetylcholine binds to the receptors on the sarcolemma in NMJ but bind to specific receptor sites on the postsynaptic membrane
what is summation and why is it needed
the rapid build-up of neurotransmitter impulses in order to exceed threshold potential to increase the membrane depolarisation
The inputs have to arrive closely enough in time so the influence of the earliest arriving input has not yet diminished
why is summation necessary to transmit an action potential
because small depolarisations means the membrane potential does not exceed threshold so sodium voltage-gated channels remain shut. There is therefore no influx of sodium ions to depolarise the neuron to threshold and no AP is transmitted
what do high frequency action potentials above threshold lead to
the quick release of acetylcholine across the synaptic cleft in a short period of time and triggers an action potential in the postsynaptic neuron
temporal summation
Results from action potentials being transmitted from the same presynaptic neuron
If concentration exceeds threshold value, a new action potential is triggered
spatial summation
Action potentials arrive from several different presynaptic neurons which can trigger a new action potential
describe inhibition by inhibitory synapses
Make it less likely that a new action potential will be created on the postsynaptic neuron
A single IPSP can prevent an action potential being produced in the postsynaptic membrane from the summation of several EPSPs
explain the process of inhibition by inhibitory synapses
The presynaptic neuron releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neuron.
The neurotransmitter causes the chloride ion protein channels to open.
Chloride ions (CI-) move into the postsynaptic neuron by facilitated diffusion.
The binding of the neurotransmitter causes the opening of nearby potassium (K*) protein channels.
Potassium ions move out of the postsynaptic neuron into the synapse.
The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive.
The membrane potential increases to as much as -80 mV compared with the usual —65 mV at resting potential.
This hyperpolarization makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one.
what is unidirectionality
Synapses can only pass information in one direction from the presynaptic neuron to the postsynaptic neuron
What does the SAN do in the heart?
Conducts a wave of electrical excitation that spreads across the walls of the right and left atria, causing them to contract.
What happens after the wave of excitation reaches the AVN?
It conveys the wave between the ventricles along the Bundle of His.
What is the role of the Bundle of His?
Conducts the wave through the AV septum to the base of the ventricles.
Where does the Bundle of His branch into smaller fibres?
At the base of the ventricles into smaller fibres of Purkyne tissue.
What causes the ventricles to contract quickly?
The release of the wave of excitation from Purkyne tissue.
What is the function of the sympathetic nervous system?
Stimulates effectors to increase activity and heightens awareness.
What does the parasympathetic nervous system do?
Inhibits effectors to decrease activity, conserving energy and replenishing reserves.
What are the two centres in the medulla oblongata related to heart rate control?
A centre that increases heart rate and a centre that decreases heart rate.
How is the centre that increases heart rate linked to the SAN?
By the sympathetic nervous system.
How is the centre that decreases heart rate linked to the SAN?
By the parasympathetic nervous system.
Where are chemoreceptors located?
In the wall of the carotid arteries and aorta.
What do chemoreceptors detect?
Changes in pH resulting from changes in carbon dioxide concentration.
What happens to the blood pH during the early stages of movement?
It is lowered due to increased concentration of protons.
What do chemoreceptors do when they detect a decrease in blood pH?
Increase the frequency of nervous impulses to the medulla oblongata that increases heart rate.
What is the result of increased heart rate due to chemoreceptor activity?
Increased blood flow that removes more carbon dioxide by the lungs.
What do baroreceptors detect?
Pressure changes in the walls of the carotid arteries and aorta.
What occurs when baroreceptors detect an increase in blood pressure?
Transmit more nerve impulses to the centre in the medulla oblongata that decreases heart rate.
What is the effect of decreased blood pressure detected by baroreceptors?
Transmit more nerve impulses to the centre that increases heart rate.
What is the effector that alters the frequency of the wave of excitation?
The SAN.
What type of system is used to ensure rapid responses in heart rate control?
The neuronal system.
Fill in the blank: The wave of excitation in the heart is initiated by the _______.
SAN
True or False: The sympathetic nervous system conserves energy.
False
True or False: Baroreceptors send more impulses when blood pressure decreases.
True
explain taxis
Moving towards the stimulus means positive taxis
Moving away the stimulus means negative taxis
As a result of taxis, a motile organism responds directly to environmental changes by moving its whole body towards a favourable stimulus or away from an unfavourable one thus increasing their chances of survival
kinesis
If an organism crosses a sharp division between a favourable and unfavourable environment the rate of turning increases. This improves chances of a quick return to a favourable environment.
If it moves a considerable distance into the unfavourable environment, the rate of turning will slowly decrease and so it moves in long straight lines and turns very sharply. This tends to bring the organism into a new region with favourable conditions
what does kinesis ensure
they spend as little time as possible in an unfavorable environment and more time in the favourable conditions, increasing their chances of survival
tropism to capture more light
Positive phototropism and negative gravitropism to capture as much light for photosynthesis so leaves are in the most favourable conditions
tropism to absorb more water and ions
Negative phototropism and positive gravitropism of roots to grow further into soil and better absorb water and mineral ions
benefit of tropisms
Able to obtain most beneficial conditions eg more water or light for photosynthesis which increases their chances of survival so more likely to reproduce and pass on alleles to their offspring
why should plant growth factors not be described as hormones
They affect the plant by impacting growth and might be made by cells located throughout the plant instead of particular organs. They also affect the tissues that release them instead of acting on a distant organ target.
positive phototropism
Cells in the tip of the shoot produce IAA which is transported down the shoot
Initially transported evenly throughout all regions as it is transported from the shoot
Light causes IAA to move to the shaded side resulting in a greater concentration of IAA on the shaded side of the root than the light side
IAA causes elongation of shoot cells so the cells on the shaded side elongate more due to a greater concentration
This causes the shoot tip to bend towards the light
negative phototropism
IAA inhibits cell elongation in roots so elongation of cells is greater on the light side than on the shaded side. This causes them to bend away from light
positive gravitropism
Cells in the tip of the root produce IAA which is transported along the root
IAA is initially transported to all sides of the root but gravity influences movement and favours it on the upper side
A greater concentration of IAA occurs on the lower side of the root than the upper side
IAA inhibits elongation of root cells and with a greater concentration on the lower side, the cells on the lower elongate less
The greater elongation of cells on the upper side causes the root to bend downwards towards the force of gravity
negative gravitropism
In shoots, greater concentration of IAA on the lower side increases the cell elongation and this side elongates more than the upper side. This causes the shoot to grow upwards away from the force of gravity
what are the two roles of IAA in elongation growth
Increases plasticity of cell walls on young cells which still have the ability to elongate. As cells mature they develop greater rigidity so older cells will not be able to respond to IAA. This proposed explanation is known as the acid growth hypothesis.
Acid growth hypothesis involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall, making the cell wall more plastic and allowing the cell to elongate by expansion.
