Organisms Respond to Changes in their Internal and External Environments Flashcards
define stimulus
any change in the internal or external environment
define receptors
these are cells or proteins on cell membranes which detect stimuli.
define effectors
these are cells that bring about a response to a stimulus, to produce an effect. effectors include muscle cells and cells found in glands.
what are the three main types of neurons?
sensory neurons
motor neurons
relay neurons.
what is the function of sensory neurons?
sensory neurons transmit electrical impulses from receptors to the central nervous system; the brain and spinal cord.
what is the function of motor neurons?
motor neurons transmit electrical impulses from the CNS to effectors.
what is the function of relay neurons?
relay neurons transmit electrical impulses between sensory neurons and motor neurons.
how does the nervous system send information as electrical impulses?
a stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neuron.
when an electrical impulse reaches the end of a neuron, chemicals called neurotransmitters take the information across to the next neuron, which then sends an electrical impulse.
the central nervous system processes the information and sends impulses along the motor neurons to an effector.
define reflex.
a reflex is where the body responds to a stimulus without making a conscious decision to respond.
how do simple reflexes help organisms?
they help to protect the body because they’re rapid.
define reflex arc
the pathway of neurons linking receptors to effectors in a reflex.
explain the hand-withdrawal response to heat as an example of a simple reflex arc.
thermoreceptors in the skin detect the heat stimulus.
the sensory neuron carries impulses to the relay neuron.
the relay neuron connects to the motor neuron.
the motor neuron sends impulses to the effector (your biceps muscle).
your muscle contracts to withdraw your hand and stop it from being damaged.
what happens if there is a relay neuron involved in the simple reflex?
its possible to override the reflex, e.g. like in the hand-withdrawal example where a relay neuron was involved, your brain could tell your hand to withstand the heat.
how is the nervous system communication localised, short-lived and rapid?
when an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells; so the nervous response is localised.
neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived.
electrical impulses are really fast, so the response is rapid; this allows animals to react quickly to stimuli.
how are flowering plants able to increase their chances of survival by responding to stimuli?
-they can sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
-they can sense gravity, so their roots and shoots grow in the right direction.
-climbing plants have a sense of touch, so they can find things to climb up and reach the sunlight.
define tropism
the response of a plant to a directional stimulus (a stimulus coming from a particular direction).
how do plants respond to stimuli?
by regulating their growth.
define positive tropism
growth towards the stimulus
define negative tropism
growth away from the stimulus.
define phototropism
this is the growth of a plant in response to light
how do shoots respond to phototropism?
shoots are positively phototropic and grow towards light.
how do roots respond to phototropism?
roots are negatively phototropic and grow away from light.
define gravitropism
this is the growth of a plant in response to gravity.
how do shoots respond to gravitropism?
shoots are negatively gravitropic and grow upwards.
how do roots respond to gravitropism?
roots are positively gravitropic and grow downwards.
how are responses in plants brought about?
they are brought about using specific growth factors.
what are growth factors?
these are hormone-like chemicals that speed up or slow down plant growth.
they are produced in the growing regions of the plant (shoot tips, leaves) and they move to where they’re needed in the other parts of the plant.
what are auxins?
these are a growth factor which stimulates the growth of shoots by cell elongation; this is where cell walls become loose and stretchy, so the cells get longer.
high conc. of auxins inhibit growth in roots.
what is Indoleacetic Acid (IAA)?
it is an important auxin that’s produced in the tips of shoots in flowering plants.
IAA is moved around the plant to control tropisms; it moves by diffusion and active transport over short distances, and via the phloem over long distances.
this results in different parts of the plant having different concentrations of IAA. uneven distribution of IAA means there’s uneven growth.
how does IAA act with phototropism in plants?
the IAA moves to the more shaded parts of the shoots and roots, so there’s uneven growth.
how does IAA act in shoots in phototropism?
IAA concentration increase on the shaded side; cells elongate and the shoot bends towards the light.
how does IAA act in roots in phototropism?
IAA concentration increases on the shaded side; growth is inhibited so the root bends away from the light.
how does IAA act with gravitropism in plants?
IAA moves to the underside of shoots and roots, so there’s uneven growth.
how does IAA act in shoots in gravitropism?
IAA concentration increases on the lower side; cells elongate so the shoot grows upwards.
how does IAA act in roots in gravitropism?
IAA concentration increases on the lower side; growth is inhibited so the root grows downwards.
what are the two simple responses that keep simple organisms in a favourable environment?
tactic responses (taxes)
kinetic responses (kineses)
define taxes
the organisms move towards or away from a directional stimulus e.g light.
explain an example of taxes using woodlice
woodlice show a tactic response to light (phototaxis). they move away from a light source. this helps them survive as it keeps them concealed under stones during the day (safe from predators) and keeps them in damp conditions (which reduces water loss).
define kineses
the organisms’ movement is affected by a non-directional stimulus, e.g humidity.
explain an example of kineses using woodlice
woodlice show a kinetic response to humidity. in high humidity they move slowly and turn less often, so that they stay where they are. as the air gets drier, they move faster and turn more often, so that they move into a new area.
(may need editing with another example).
why are receptors described as specific?
they only detect one particular stimulus.
what are the different types of receptors?
cells and proteins on cell surface membranes
how do receptor cells that communicate information via the nervous system, work?
what are pacinian corpuscles?
they are mechanoreceptors; they detect mechanical stimuli, e.g pressure and vibrations.
they are found in your skin.
what do pacinian corpuscles contain?
they contain the end of a sensory neurone, imaginatively called a sensory nerve eneding. the sensory nerve ending is wrapped in lots of layers of connective tissue called lamallae.
what happens when a pacinian corpuscle is stimulated?
- the lamellae are deformed and this presses on the sensory nerve ending.
- this causes the sensory neurone’s membrane to stretch, deforming the stretch-mediated sodium ion channels. the channels open and sodium ions diffuse into the cell, creating a generator potential.
- if the generator potential reaches the threshold, it triggers an action potential.
how do photoreceptors detect light?
- light enters the eye through the pupil. the amount of light that enters is controlled by the muscles of the iris.
- light rays are focused by the lens onto the retina, which lines the inside of the eye. the fovea is an area of the retina where there are lots of photoreceptors. the retina therefore contains photoreceptor cells — these detect light.
what is a blind spot?
- nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones.
- where the optic nerve leaves the eye is called the blind spot. there are no photoreceptor cells and so it’s not sensitive to light.
how do photoreceptors convert light into an electrical impulse?
- light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments.
- light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.
- a generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
- bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
what are the two types of photoreceptors?
rods
cones
where are rods found in the eye?
mainly found in the peripheral parts of the retina
where are cones found in the eye?
they are found packed together in the fovea.
what is the differences of optical pigments between rods and cones?
rods only give information in black and white (monochromatic vision), but cones give information in colour (trichromatic vision).
what are the three different optical pigment cones?
red-sensitive
green-sensitive
blue-sensitive
explain sensitivity of light in rods.
rods are very sensitive to light (they work well in dim light). this is because many rods join one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
explain sensitivity of light in cones.
cones are less sensitive than rods (they work best in bright light). this is because one cone joins one neurone, so it takes more light to reach the threshold and trigger an action potential.
define visual acuity
the ability to tell apart points that are close together.
explain the visual acuity of rods.
rods give low visual acuity because many rods join the same neurone, which means light from two points together can’t be told apart.
explain the visual acuity of cones.
cones give high visual acuity because cones are close together and one cone joins one neurone.
when light from two points hits two cones, two action potentials (one from each cone) go to the brain — so we can distinguish two points that are close together as two separate points.
explain the cardiac heart muscle as ‘myogenic’
it can contract and relax without receiving signals from nerves. this pattern of contractions controls the regular heartbeat.
what type of muscle is cardiac muscle?
cardiac muscle is ‘myogenic’ — it can contract and relax without receiving signals from nerves.
where does the process of heartbeat initiation start?
the process starts in the sinoatrial node (SAN), which is in the wall of the right atrium.
what is the function of the sinoatrial node (SAN)?
the SAN sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.
what happens when the SAN sends electrical activity to the atrial walls?
this causes the right and left atria to contract at the same time.
what prevents the direct passage of electrical activity from the atria to the ventricles?
a band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.
where do the waves of electrical activity transferred from the SAN go to?
These waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN).
what is the role of the AVN?
the AVN is responsible for passing the waves of electrical activity on to the bundle of His, with a slight delay to ensure the atria have emptied before the ventricles contract.
what is the bundle of His?
the bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex of the heart.
what are the Purkyne tissue?
the Purkyne tissue are finer muscle fibres in the right and left ventricle walls that carry the waves of electrical activity into the muscular walls of the ventricles.
what is the result of the Purkyne tissue carrying electrical activity?
this causes the right and left ventricles to contract simultaneously, from the bottom up.
what does the SAN generate?
it generates electrical impulses that cause the cardiac muscles to contract.
how does controlling the heart rate involve the brain and autonomic nervous system?
- the rate at which the SAN fires is unconsciously controlled by a apart of the brain called the medulla oblongata.
- animals need to alter their heart rate to respond to internal stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen.
- stimuli are detected by pressure receptors and chemical receptors:
- there are pressure receptors called baroreceptors in the aorta and the carotid arteries (major arteries in the neck). they’re stimulated by high and low blood pressure.
- there are chemical receptors called chemoreceptors in the aorta, the carotid arteries and in the medulla. they monitor the oxygen level in the blood and also carbon dioxide and pH.
- electrical impulses from receptors are sent to the medulla along sensory neurones. the medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones.
what are all the stimuli that are detected by receptors which cause the heart rate to speed up or slow down?
high blood pressure
low blood pressure
high blood oxygen, low carbon dioxide or high pH levels
low blood oxygen, high carbon dioxide or low pH levels
explain how the heart rate responds to high blood pressure
stimulus= high blood pressure
receptor= baroreceptors detect high blood pressure.
neurone and transmitter= impulses are sent to the medulla, which sends impulses along parasympathetic neurones. these secrete acetylcholine (a neurotransmitter), which binds to receptors on the SAN.
effector= cardiac muscles
response= heart rate slows down to reduce blood pressure back to normal
explain how the heart rate responds to low blood pressure
stimulus= low blood pressure
receptors= baroreceptors detect low blood pressure
neurone and transmitter= impulses are sent to the medulla, which sends impulses along sympathetic neurones. these secrete noradrenaline (a neurotransmitter), which binds to receptors on the SAN.
effector= cardiac muscles
response= heart rate speeds up to increase blood pressure back to normal.
explain how the heart rate responds to high blood oxygen, low carbon dioxide or high pH levels
stimulus= high blood oxygen, low carbon dioxide or high pH levels
receptors= chemoreceptors detect chemical changes in the blood
neurone and transmitter= impulses are sent to the medulla, which sends impulses along parasympathetic neurones. these secrete acetylcholine (a neurotransmitter), which binds to receptors on the SAN.
effector= cardiac muscles
response= heart rate decreases to return oxygen, carbon dioxide and pH levels back to normal
explain how the heart rate responds to low blood oxygen, high carbon dioxide or low pH levels
stimulus= low blood oxygen, high carbon dioxide or low pH levels
receptors= chemoreceptors detect chemical changes in the blood
neurone and transmitter= impulses are sent to the medulla, which sends impulses along sympathetic neurones. these secrete noradrenaline (a neurotransmitter), which binds to receptors on the SAN.
effector= cardiac muscles
response= heart rate increases to return oxygen, carbon dioxide and pH levels back to normal.
what is the charge of a neurone’s membrane at rest?
the outside of the membrane is positively charged compared to the inside.
this is because there are more positive ions outside the cell than inside.
what does the term ‘the membrane is polarised’ mean?
there’s a difference in charge (potential difference or voltage) across it.
what is the resting potential of a neurone’s membrane when it’s at rest?
the resting potential is about -70 mV (millivolts)
what creates and maintains the resting potential in a neurone’s membrane?
the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels.
what do sodium-potassium pumps do?
they use active transport to move three sodium ions (Na+) out of the neurone for every two potassium ions (K+) moved in.
ATP is needed to do this.
what is the role of potassium ion channels?
these channels allow facilitated diffusion of potassium ions (K+) out of the neurone, down their concentration gradient.
what is the process of creating and maintaining the resting potential of a neurone’s membrane, using sodium-potassium pumps and potassium ion channels?
- the sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in. this creates a sodium ion electrochemical gradient (a conc. gradient of ions) because there are more positive sodium ions outside the cell than inside.
- the sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.
- this makes the outside of the cell positively charged compared to the inside.
what happens to neurone cell membranes when they are stimulated?
neurone cell membranes become depolarised when they are stimulated.
a stimulus triggers sodium ion channels to open.
what is action potential?
when a sufficient stimulus triggers a rapid change in potential difference
what are the 5 sequence of events in the process of action potential?
stimulus
depolarisation
repolarisation
hyperpolarisation
resting potential
explain ‘stimulus’ in the process of action potential
stimulus — this excites the neurone cell membrane, causing the sodium ion channels to open. the membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. this makes the inside of the neurone less negative.
explain ‘depolarisation’ in the process of action potential
depolarisation — if the potential difference reaches the threshold (around -55mV), more sodium ion channels open. more sodium ions diffuse rapidly into the neurone.
explain ‘repolarisation’ in the process of action potential
repolarisation — at a potential difference of around +30mV the sodium ion channels close and potassium ion channels open. the membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. this starts to get the membrane back to its resting potential.
(the sodium channels have to close or the membrane will remain depolarised).
explain ‘hyperpolarisation’ in the process of action potential
hyperpolarisation — potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. the potential difference becomes more negative than the resting potential (less than -70mV).
explain ‘resting potential’ in the process of action potential
resting potential — the ion channels are reset. the sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane’s excited by another stimulus.
explain the refractory period
the period of recovery.
after an action potential, the neurone cell membrane can’t be excited again straight away. this is because the ion channels are recovering and they can’t be made to open — sodium ion channels are closed during repolarisation and potassium ion channels are closed during hyperpolarisation.
what happens when an action potential occurs?
when an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
this causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
this causes a wave of depolarisation to travel along the neurone.
the wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.
what is the role of the refractory period?
during the refractory period, ion channels are recovering and can’t be opened.
so the refractory period acts as a time delay between the one action potential and the next.
this means that:
- action potentials don’t overlap, but pass along as discrete (separate) impulses.
- there’s a limit to the frequency at which the nerve impulses can be transmitted.
- action potentials are unidirectional (they only travel in one direction).
how do action potentials have an ‘all-or-nothing nature’?
once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
if the threshold isn’t reached, an action potential won’t fire. this is the all-or-nothing nature of action potentials.
a bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.
what are the three factors that affect the speed of conduction of action potentials?
myelination
axon diameter
temperature
explain how myelination affects the speed of conduction of action potentials
- some neurones are myelinated — they have a myelinated sheath.
- the myelin sheath is an electrical insulator.
- in the peripheral nervous system, the sheath is made of a type of cell called a Schwann cell.
- between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier (where sodium ions can get through the membrane).
- the neurone’s cytoplasm conducts enough electrical charge to depolarise the net node, so the impulse ‘jumps’ from node to node.
- this is called saltatory conduction and it’s really fast.
- in a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane (and so depolarisation occurs along the whole length of the axon membrane).
- this is slower than saltatory conduction (but still pretty quick).
explain how the axon diameter affects the speed of conduction of action potentials
action potentials are conducted quicker along axons with bigger diameters beause there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon. with less resistance, depolarisation reaches other parts of the neurone cell membrane quicker.
explain how temperature affects the speed of conduction of action potentials
the speed of conduction increases as the temperature increases too, because ions diffuse faster. the speed only increases up to 40 degrees celsius though — after that the proteins begin to denature and the speed decreases.
define synapse
a synapse is a junction between a neurone and another neurone or between a neurone and an effector cell, e.g. a muscle or gland cell.
what is the synaptic cleft?
the tiny gap between the cells at a synapse.
what is the synaptic knob?
the swelling of the presynaptic neurone.
it contains synaptic vesicles filled with chemicals called neurotransmitters.
what occurs when an action potential reaches the end of a neurone?
it causes neurotransmitters to be released into the synaptic cleft. they diffuse across to the postsynaptic membrane and bind to specific receptors.
what happens when the neurotransmitters bind to the receptors?
they might trigger an action potential in a neurone, cause muscle contraction in a muscle cell, or cause a hormone to be secreted from a gland cell.
why do synapses make sure the impulses are unidirectional?
because the receptors are only on the postsynaptic membranes and so the impulse must only travel in one direction.
why are neurotransmitters removed from the cleft?
so the response doesn’t keep happening.
they’re taken back into the presynaptic neurone or they’re broken down by enzymes and the products are taken back into the neurone.
what are the many different neurotransmitters used for synaptic transmission?
acetylcholine ACh) and noradrenaline
what are cholinergic synapses?
synapses that use acetylcholine
how is a nerve impulse transmitted across a cholinergic synapse?
1) an action potential arrives at the synaptic knob of the presynaptic neurone.
2) the action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open.
3) calcium ions diffuse into the synaptic knob (they’re pumped out afterwards by active transport).
4) the influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane. they then fuse with the presynaptic membrane.
5) the vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft — this is called exocytosis.
6) ACh diffuses across the synpatic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
7) this causes sodium ion channels in the postsynaptic neurone to open.
8) the influx of sodium ions into the postsynaptic membrane causes depolarisation. an action potential on the postsynaptic membrane is generated if the threshold is reached.
9) ACh is removed from the synaptic cleft so the response doesn’t keep happening. it’s broken down by an enzyme called acetylcholinesterase (AChE) and the products are re-absorbed by the presynaptic neurone and used to make more ACh.
what are excitatory neurotransmitters (+)? give an example
excitatory neurotransmitters depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.
e.g. acetylcholine is an excitatory neurotransmitter at cholinergic synapses in the CNS — it binds to cholinergic receptors to cause an action potential in the postsynaptic membrane — and at neuromuscular junctions.
what are inhibitory neurotransmitters (-)? give an example
inhibitory neurotransmitters hyperpolarise the postsynaptic membrane (makes the potential difference more negative), preventing it from firing an action potential.
e.g. acetylcholine is an inhibitory neurotransmitter at cholinergic synapses in the heart. when it binds to receptors here, it can cause potassium ion channels to open on the postsynaptic membrane, hyperpolarising it.
what happens if a stimulus is weak?
if a stimulus is weak, only a small amount of neurotransmitter will be release from a neurone into the synaptic cleft. this might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.
what is summation?
summation is where the effect of neurotransmitters released from many neurones (or one neurone that’s stimulated a lot in a short period of time) are added together.
what are the two types of summation?
spatial summation
temporal summation
both types of summation means synapses accurately process information, finely tuning the response.
explain spatial summation
sometimes many neurones connect to one neurone.
the small amount of neurotransmitters released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential.
if some neurones release an inhibitory neurotransmitter then the total effect of the neurotransmitters might be no action potential.
more inhibitory neurotransmitters (-) are released than excitatory neurotransmitters (+) = no action potential
explain temporal summation
temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone. this makes an action potential more likely because more neurotransmitter is released into the synaptic cleft.
what is a neuromuscular junction?
it is a synapse between a motor neurone and a muscle cell.
neuromuscular junctions use the neurotransmitter acetylcholine (ACh), which binds to cholinergic receptors called nicotinic cholinergic receptors.
what are the differences in a neuromuscular junction compared to a cholinergic synapse?
the postsynaptic membrane has a lot of folds that form clefts. these clefts store the enzyme that breaks down ACh.
the postsynaptic membrane has more receptors than other synapses.
ACh is always excitatory at a neuromuscular junction. so when a motor neurone fires an action potential, it normally triggers a response in a muscle cell.
what are the ways in how drugs can affect synaptic transmission?
1) some drugs are the same shape as neurotransmitters so they mimic their action at receptors (these drugs are called agonists). this means more receptors are activated.
e.g nicotine mimics acetylcholine so binds to nicotinic cholinergic receptors in the brain.
2) some drugs block receptors so they can’t be activated by neurotransmitters (these drugs are called antagonists). this means fewer receptors (if any) can be activated.
e.g curare blocks the effects of acetylcholine by blocking nicotinic cholinergic receptors at neuromuscular junctions, so muscle cells can’t be stimulated. this results in the muscle being paralysed.
3) some drugs inhibit the enzyme that breaks down neurotransmitters (they stop it from working). this means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer.
e.g. nerve gases stop acetylcholine from being broken down in the synaptic cleft. this can lead to loss of muscle control.
4) some drugs stimulate the release of neurotransmitter from the presynaptic neurone so more receptors are activated, e.g. amphetamines.
5) some drugs inhibit the release of neurotransmitters from the presynaptic neurone do fewer receptors are activated, e.g. alcohol.
what are skeletal muscles?
a skeletal muscle (also called a voluntary muscle) is the type of muscle you use to move, e.g. the biceps and triceps move the lower arm.
skeletal muscles are attached to other bones by tendons.
pairs of skeletal muscles contract and relax to move bones at a joint. the bones of the skeleton are rigid so they act as levers, giving the muscles something to pull against.
what are antagonistic pairs?
muscles that work together to move a bone.
the contracting muscle is called the agonist and the relaxing muscle is called the antagonist.
what are ligaments?
ligaments attach bones to other bones to hold them together.
what do muscles act as in the body?
they act as effectors and are stimulated to contract by neurones.
explain the structure of a skeletal muscle
– skeletal muscle is made up of large bundles of long cells, called muscle fibres.
– the cell membrane of muscle fibre cells is called the sarcolemma.
– bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (a muscle cell’s cytoplasm). these folds are called transverse (T) tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
– a network of internal membranes called the sarcoplasm reticulum runs through the sarcoplasm. the sarcoplasm reticulum stores and releases calcium ions that are needed for muscle contraction.
– muscle fibres have lots of mitochondria to provide the ATP that’s needed for muscle contraction.
– muscle fibres are multinucleate (contain many nuclei).
– muscle fibres have lots of long cylindrical organelles called myofibrils. they’re made up of proteins and are highly specialised for contraction.
