6. Organisms Respond To Changes In Their Environment Flashcards
Define stimulus
A stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism
Give the sequence of events of the response to a stimulus
Stimulus —> receptor —> coordinator —> effector —> response
What is a taxis
A simple response whose direction is determined by the direction of a stimulus.
As a result a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one
What is meant by positive/negative taxis
Movement towards the stimulus = positive taxis
Movement away from stimulus = negative taxis
What is a kinesis
A kinesis is a form of response in which the organism does not move toward or away from a stimulus.
Instead it measures the speed at which it moves and the rate at which it changes direction
What is a tropism
A tropism is the growth of a plant in response to a directional stimulus
What are plant growth factors referring to
The hormone like substances involved in the responses of plants to external stimuli
Why do we refer to the hormone-like substances as plant growth factors over hormones ?
They exert their influence by affecting growth
They may be made by cells located throughout the plant rather than in particular organs
Unlike animal hormones, some plant growth factors affect the tissue that release them rather than acting on distant target organs
Give an example of a plant growth factor and its role
Indoleacetic acid (IAA)
- controls plant elongation
What is growth of a plant towards the light referred to as
Positive phototropism
or
Negative gravitropism
Describe the steps that explain positive phototropism
- Cells in the tip of the shoot produce IAA, which is then transported (initially evenly) down the shoot
- Light causes the movement of IAA from the light side to the shaded side of the shoot
- A greater conc of IAA builds up on the shaded side compared with the light side
- As IAA causes elongation of shoot cells, cells on the shaded side (side with greatest conc of IAA) elongate faster than the light side
- Therefore shoot tip bends towards the light
Describe the steps that explain positive gravitropism in plants
- Cells in the root tips produce IAA which is initially transported evenly to all sides of the root
- Gravity influences the movement of IAA from the upper side to the lower side of the root
- A greater conc of IAA builds up on the lower side of the root compared with the upper side
- As IAA inhibits elongation of root cells, the cells on the lower side of the root (region with greater IAA conc) elongate less than those on the upper side
- Therefore root bends downwards towards the force of gravity
Explain the proposed explanation of how IAA increases the plasticity of cells
Acid growth hypothesis
- involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall
- this causes the cell wall to become more plastic allowing the cell to elongate by expansion
What are the 2 major divisions of the nervous system and what are they made up of ?
- the central nervous system (CNS) : made up of brain & spinal cord
- the peripheral nervous system (PNS) : made up of a pair of nerves that originate from either the brain or spinal cord
What is the peripheral nervous system divided into
Sensory neurones
Motor neurones
Give the role of sensory neurones
Carry nerve impulses (electrical signals) from receptors towards the CNS
Give the role of motor neurones
Carry nerve impulses (electrical signals) away from the CNS to effectors
What are the 2 subdivisons of the motor nervous system?
- the voluntary nervous system
- the autonomic nervous system
Describe the voluntary nervous system
Carries nerve impulses to body muscles and is under voluntary (conscious) control
Describe the autonomic nervous system
Carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control (subconscious)
What is the sequence of the reflex arc
Stimulus
Receptor - generates impulses in the…
Sensory neurone - passes nerve impulses to spinal cord
Coordinator - links sensory to motor neurone in spinal cord
Motor neurone - carries nerve impulses from spinal cord to muscle
Effector - muscle is stimulated to contract
Response
Why are reflex actions important ?
- they are involuntary and therefore don’t require the brain to make decisions leaving it to carry out more complex responses = not overloaded
- protect body from harm
- effective from birth so don’t have to be learnt
- fast withdrawal reflexes as neurone pathway is short (has very few synapses)
- action is rapid due to absence of decision making process
What are the features of sensory reception as illustrated by the Pacinian corpuscule
Specific to a single type of stimulus
Produces a generator potential by acting as a transducer
How do receptors act as transducers in the nervous system
All stimuli involve a change in some form of energy.
A transducer converts the change in form of energy by the stimulus into a form that can be understood by the body
Receptors, convert (transduce) the energy of the stimulus into a nervous impulse known as a generator potential
Describe briefly the structure and function of a Pacinian corpuscule
Respond to mechanical stimuli such as pressure
Occur deep in the skin as well is in joints, ligaments and tendons
The single sensory neurone of the PC is at the centre of layers of tissue, each separated by gel
What feature of the Pacinian corpuscule enables it to transduce the mechanical energy of the stimulus into a generator potential ?
The sensory neurone ending at the centre of the pc has a special stretch-mediated sodium channel in its plasma membrane.
Permeability to sodium changes when they are deformed (stretched)
Give the step by step functioning of the Pacinian Corpuscule
- in resting state, the stretch-mediated sodium channels are too narrow to allow Na+ ions to pass along them. (Neurone in the pc has a resting potential)
- when pressure is applied to the pc it becomes deformed and the membrane around its neurone becomes stretched
- stretching widens the channels and so Na+ ions diffuse into the neurone
- influx of Na+ ions changes the membrane potential (becomes depolarised) thereby producing a generator potential
- generator potential in turn creates an action potential
Both rod and cone cells act as…
Transducers by converting light energy to the electrical energy of a nerve impulse
Why can rod cells only lead to images in black and white?
Because they cannot distinguish between different wavelengths of light
Explain how rod cells allow us to see in low light intensities ?
As a number of rod cells are connected to a single bipolar cell there is a much greater chance that the threshold value for a generator potential to be produced is exceeded compared to if a single rod cell was connected to each bipolar cell.
What needs to occur for rod cells to respond to low-intensity light ?
In order to create a generator potential the pigment in the rod cells (rhodopsin) must be broken down.
There is enough energy from low intensity light for this breakdown
Why do rod cells give a low visual acuity?
As a result of many rod cells linking to a single bipolar cell, the light received by the rod cells sharing the same neurone will only generate a single impulse travelling to the brain regardless of how many neurones are stimulated
Means that the brain cannot distinguish between the separate sources of light that stimulated them
Explain how cone cells give good visual acuity
Each cone cell has its own connection to a single bipolar cell which means that if 2 agacent cone cells are stimulated, the brain receives 2 separate impulses.
Therefore the brain can distinguish between the 2 separate sources of light that stimulated the 2 cone cells
Why do cone cells respond only to high intensity light ?
Each cone cell is connected to their own serpente bipolar cell connected to a sensory neurone in the optic nerve.
Means that the stimulation of a number of cone cells cannot be combined to help exceed threshold value and so create generator potential
Also the pigment in cone cells, iodopsin, requires a higher light intensity for its breakdown
There are 3 types of cone cell - what feature of each type makes them sensitive to a specific range of wavelengths ?
Each contain a specific type of iodopsin requiring different levels of light intensities for its breakdown to produce generator potentials
Why is the distribution of rod and cone cells uneven?
Light is focused by the lens onto the fovea (part of retina opposite pupil)
Therefore the fovea receives the highest intensity of light = cone cells found here
Concentration of cone cells diminishes further from the fovea
At the peripheries of the retina, where light intensity is at its lowest, rod cells are found
Rod cells vs cone cells
Rod:
- rod shaped
- greater number
- distributed nearer periphery of retina
- poor visual acuity
- sensitive to low light intensities
- one type only
Cone cells opposite
What does the autonomic system control ?
Controls the involuntary (subconscious) activities of internal muscles and glands
What are the 2 divisions of the autonomic nervous system?
What are their roles?
sympathetic nervous system
- stimulates effectors and so speeds up any activity
parasympathetic nervous system
- inhibits effectors and so slows down any activity
- concerned with conserving energy and replenishing the body’s reserves
If we say the sympathetic and parasympathetic nervous systems are antagonistic what do we mean?
(Normally) they oppose eachother
The activities of internal glands and muscles are regulated by a balance of the 2 systems
Describe the role of the nervous system
- The nervous system uses nerve cells to pass electrical impulses along their length.
- They stimulate target cells by secreting neurotransmitters directly onto them
- Results in rapid communication
- Responses produced are often short lived and localised
Describe the hormonal system
- produces chemicals (hormones) that are transported into the blood plasma to target cells
- the receptors of the target cells detect change in the concentration of hormones and this stimulates them
Describe the difference between the hormonal and nervous systems
- hormonal system is a slower, less specific form of communication
- responses of the hormonal system are often long lasting and wide spread whereas those of the nervous system are often short lived and localised
What are dendrons
Extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses towards the cell body
What are shwann cells ?
Surround the axon, protecting it and providing electrical insulation
Also carry out phagocytosis and play a part in nerve regeneration
What is the axon ?
A single long fibre that carries nerve impulses away from the cell body
What is the myelin sheath ?
Forms a covering to the axon and is made up of the membranes of shwann cells
These membranes are rich in a lipid called myelin
What are nodes of ranvier
Constrictions between adjacent shwann cells where there is no myelin sheath
Occur every 1-3mm in humans
Describe sensory neurones
Transmit nerve impulses from a receptor to a intermediate/motor neurone.
They have one, often very long, dendron that carries nerve impulse towards the cell body and the axon carries it away
Describe motor neurones
Transmit nerve impulses from a relay/intermediate neurone to an effector such as a gland or muscle
Have a long axon and many short dendrites
Describe intermediate/relay neurones
Transmit impulses between neurones eg. from sensory to motor neurones
How can a nerve impulse be described ?
A self propagating wave of electrical activity that travels along the axon membrane
It is a temporary reversal of electrical potential difference across the axon membrane
What are the ways in which the movement of ions across the axon membrane is controlled ?
- phospholipid bilayer of axon plasma membrane prevents Na+ and K+ diffusing across
- channel proteins span the phospholipid bilayer
- carrier proteins acting as sodium potassium pumps
What is the resting potential
The inside of the axon is negatively charged relative to the outside - the axon is said to be polarised
Give the steps to establishing the potential difference (resting potential)
- higher concentration of K+ ions inside and higher concentration of Na+ ions outside the neurone
- membrane is more permeable to K+ ions leaving than Na+ ions entering
- Na+ ions are actively transported out and K+ ions in
What is the action potential ?
When a stimulus of sufficient size is detected by a receptor in the nervous system its energy causes a temporary reversal of the charges either side of this part of the axon membrane.
This part of the axon membrane is said to become depolarised
Why does depolarisation occur
Because the channels in the axon membrane change shape and hence open or close depending on the voltage across the membrane
Describe the step by step process of the action potential
- the energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and therefore Na+ ions diffuse along the electrochemical gradient into the axon?
- being positively charged, they trigger a reversal in the potential difference across the membrane
- as the Na+ ions diffuse into the axon more sodium channels open causing an even greater influx of Na+ ions by diffusion - once the action potential of around +40mV is established (preventing further influx of Na+) the voltage gates on the K+ ion channels begin to open
- as a result the electrical gradient that was preventing further outward movement of K+ ions is now reversed causing more K+ ion channels to open so more K+ diffuses out, starting the depolarisation of the axon
- outward diffusion of K+ causes a temporary overshoot of the electrical gradient with the inside of the axon being more negative than usual.
- gates on K+ ion channels close - resting potential can be re-established
Describe and explain the passage of an action potential along a myelinated axon
Fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming.
Action potentials can occur at the nodes of Ranvier, jumping from node to node
Therefore action potential passes along a myelinated neurone faster than an unmyleinated neurone
What does the process of salatory conduction describe
The action potentials jumping from node to node
What are the 3 factors affecting the speed at which the action potential travels
- the myelin sheath
- the diameter of the axon
- temperature
How does the diameter of the axon affect the speed at which action potential travels
Greater the diameter of the axon, the faster the speed of the conductance.
Due to less leakage of ions from a large axon (leakage makes membrane potentials harder to maintain)
How does temperature affect the speed at which action potentials travel
Higher the temp, the faster the rate
of diffusion of ions, faster the nerve impulse
Temp affects the speed and strength of muscle contractions
Enzymes controlling sodium-potassium pump function more rapidly at higher temps
What is the all-or-nothing principle
- Nerve impulses are described as all-or-nothing responses
- Below threshold value no action potential, any stimulus above threshold potential produces action potential of more or less same size
- Strength of stimulus cannot be detected by size of action potential
How can organism perceive the size of a stimulus if size of action potentials are more or less the same?
- by the number of impulses passing in a given time (larger stimulus, more impulses generated in given time)
- by having different neurones with different threshold values. Brain interprets number and type of neurones that pass impulses as a result of given stimulus = determines size
What is the refractory period
There is a period after the action potential has been generated in any region of an axon, where inward movement of sodium ions is prevented because sodium voltage-gated channels are closed.
During this time no further action potential can be generated
What are the 3 purposes served by the refractory period ?
- Ensures that the action potentials are propagated in one direction only
- It produces discrete impulses
- It limits the number of action potentials
What is the synapse
The point where one neurone communicates with another or effector
What do synapses transmit
Information by means of chemicals known as neurotransmitters
What do synapses transmit
Information by means of chemicals known as neurotransmitters
What is the synaptic cleft
The gap that separates 2 neurones (20-30nm wide)
The neurone that releases the neurotransmitter is called the …
Presynaptic neurone
What is the synaptic knob and what does it contain ?
Where the axon of the presynpatic neurone ends
Possesses many mitochondria and large amounts of Endoplasmic reticulum
Where are neurotransmitters stored
Synaptic vesicles
Explain uniderctionarlity (feature of a synapse)
Synapses can only pass information in one direction - from the presynaptic neurone to post synaptic neurone
Explain summation (feature of a synapse)
Low-frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone.
They can however do so in a process called summation (entails of a rapid build-up of a neurotransmitter in the synapse) by one of 2 methods
What is spatial summation
- in which a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone, therefore triggering a new action potential
What is temporal summation
Where a single presynaptic neurone relates neurotransmitter many times over a very short period.
If the concentration of the neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered
How do inhibitory synapses operate (step by step)
- the presynaptic neurone releases a type of neurotransmitter that binds to the Cl- ion protein channels on the postsynaptic neurone
- causes the Cl- protein channels to open
- Cl- ions move into the postsynaptic neurone by facilitated diffusion
- the binding of the neurotransmitter causes the opening of nearby K+ protein channels
- K+ ions move out of the postsynaptic neurone into the synapse
- the combined effect of the movement of these ions causes the inside of the postsynaptic neurone to be more negative and the outside more positive
- membrane potential increases (called hyperpolarisation) - makes it less likely that a new action potential will be created because a larger influx of Na+ ions is needed to produce one
Synapses act as junctions for the transmission of information, allowing:
- a single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse. This allows a single stimulus to create a number of simultaneous responses
- a number of impulses to be combined at a synapse. Allows nerve impulses from receptors reacting to different stimuli to contribute to a single response
What is a cholinergic synapse ?
A synapse in which the neurotransmitter is a chemical called acetylcholine
Describe the step by step process of transmission across a cholinergic synapse
- arrival of an action potential at the end of the presynaptic neurone causes Ca2+ ion protein channels to open & Ca2+ ions to enter synaptic knob by facilitated diffusion
- the influx of Ca2+ ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
- acetylcholine binds to receptor sites on Na+ ion protein channels in the membrane of the postsynaptic neurone, causing Na+ ion channels to open allowing Na+ ions to diffuse along the conc gradient
- influx in Na+ ions causes new action potential in postsynaptic neurone
- acetylcholine is hydrolysed, reforming choline & acetyl which diffuse back across syntactic cleft into presynaptic neurone
- ATP released by mitochondria is used to recombine the above into acetylcholine which is stored in syntactic vesicles for future use. Na+ ion channels close
What are the 3 types of muscle
- Cardiac muscle
- Smooth muscle
- Skeletal muscle (only type that acts under voluntary, conscious control)
What is sacroplasm?
The nuclei and cytoplasm shared by muscle fibres
Within the sacroplasm is a large concentration of Endoplasmic reticulum and mitochondria
What is each muscle fibre made up of
Microfibrils
Microfibrils are made up mainly of which 2 types of protein filament ?
Describe the structure of both
- actin : thinner & consists of 2 strands twisted round eachother
- myosin : thicker & consists of long rod-shaped tails with bulbous heads that project to the sides
Explain why microfibrils appear striped?
Due to their alternating light-coloured and dark-coloured bands.
The light bands (I bands) appear lighter because the thick & thin filaments do not overlap in this region
The dark bands (A bands) appear darker because the thick & thin filaments do overlap in this region
What is a sacromere ?
How does muscle contraction affect it ?
Distance between adjacent Z lines (found at the centre of each I band)
When the muscles contract the sarcomeres shorten and the pattern of light & dark bands changes
What is tropomyosin
A protein found in the muscle that forms a fibrous fibre around the actin filament
What are the 2 types of muscle fibre & what are their roles ?
- slow-twitch fibres : contract slower & less powerfully, adapted for endurance work
- fast-twitch fibres : contract faster & more powerfully but only for a short period, adapted for intense exercise
How are slow-twitch fibres adapted for their function
Adapted for aerobic respiration …
- large store of myoglobin (stores oxygen)
- rich supply of blood vessels to deliver oxygen and glucose
- numerous mitochondria to produce ATP
How are fast-twitch fibres adapted for their role
- thicker and more numerous myosin filaments
- high conc of glycogen
- high conc of enzymes involved in anaerobic respiration which provide ATP rapidly
- store of phosphocreatine (molecule which can generate ATP from ADP rapidly in anaerobic conditions)
What is a neuromuscular junction
Point where a motor neurone meets a skeletal muscle fibre
What does myogenic mean
Contraction of the heart is initiated from within the muscle itself rather than by nervous impulses from outside
Where is the sinoatrial node (SAN) located and what is its purpose ?
Within the wall of the right atrium
Initial stimulus for contraction originates here
The SAN has a basic rhythm of stimulation that determines the beat of the heart (often referred to as the pacemaker)
Give the sequence of events that controls the basic heart rate
- wave of electrical excitation spreads out from the SAN across both atria, causing them to contract
- the atrioventricular septum (layer of non conductive tissue) prevents the wave crossing to the ventricles
- wave enters the atrioventricular node (AVN) which lies between the atria
- The AVN, after a short delay, conveys a wave of electrical excitation between the ventricles along series of specialised muscle fibres called Purkyne tissue (collectively, the bundle of His)
- bundle of His conducts the wave through through the atrioventricular septum to the base of the ventricles where the bundle branches into smaller fibres of Purkyne tissue
- wave is released from Purkyne tissue causing the ventricles to contract quickly simultaneously
Why is it essential that the heart rate can be altered ?
To meet varying demands for oxygen
During exercise for example, the heart rate may need to more than double
Changes made to the heart rate are controlled by which region of the brain
The medulla oblongata
What are the two centres of the medulla oblongata that are concerned with heart rate
- a centre that increases heart rate linked to the SAN by the sympathetic nervous system
- a centre that decreases the heart rate linked to the SAN by the parasympathetic nervous system
Where are chemoreceptors found and what are they sensitive to ?
In the wall of carotid arteries and in the aorta
Sensitive to changes in the pH of the blood caused by changes in co2 concentration
Describe how the process of control by chemoreceptors works
- when blood has high conc of co2, pH is lowered
- chemoreceptors detect and increase frequency of nervous impulses to the medulla oblongata that increase heart rate
- increases the frequency of impulses via sympathetic nervous system to the SAN, increasing the production of electrical waves by SAN which increases heart rate
- increased blood flow this causes leads to more co2 being removed by lungs and so conc in blood returns to normal
- pH rises & chemoreceptors reduce frequency of nerve impulses to medulla oblongata
- medulla reduces frequency of impulses to SAN, leading to a reduction in heart rate
How do pressure receptors operate to control heart rate
When blood pressure is higher than normal, pressure receptors transmit more nerve impulses to the centre in the medulla oblongata that decrease heart rate.
Centre sends impulses via the parasympathetic nervous system to the SAN which leads to a decrease in heart rate
Opposite for when blood pressure is lower than normal
What are the similarities between a neuromuscular and cholinergic synapse
- have neurotransmitters that are transported via diffusion
- have receptors that upon binding with the neurotransmitter cause an influx of Na+ ions
- use a sodium potassium pump to repolarise the axon
- uses enzymes to breakdown the neurotransmitter
What are the difference between a neuromuscular and cholinergic synapse
Neuromuscular :
Only excitatory (not inhibitory also)
Only links neurones to muscles (not to neurones also)
Only motor neurones are involved (rather than all types)
Action potential ends here (can’t produce new one along another neurone)
Acetylcholine binds to receptors on membrane of muscle fibre (rather than on membrane of post-synaptic neurone)
Explain why speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon
Myelination provides electrical insulation
Depolarisation only at nodes in myelinated axon but in a non myelinated axon depolarisation occurs across entire length
Why are skeletal muscles described to occur and act in antagonistic pairs
The skeletal muscle will move part of the skeleton (eg. a limb) in one direction but the same muscle cannot move it in the opposite direction
(Muscles can only pull, not push)
Therefore to move the limb in the opposite direction a second muscle is required that works antagonistically to the first one
In doing so it stretches its partner muscle returning it to its original state ready to contract again
When a muscle contracts what are the 3 changes to that occur to the sacromere?
- the I-band becomes narrower
- the Z-lines move closer together (sacromere shortens)
- the H-zone becomes narrower
What evidence is there to support the sliding filament mechanism?
- myofibrils appear darker in colour where the actin and myosin filaments overlap so when muscle is contracted there is more overlap
- A band remains the same width and as the width of this band is determined by the length of myosin filaments, it discounts the theory that muscle contraction is due to the filaments themselves shortening
What are the 3 main proteins involved in the sliding filament mechanism
- myosin
- actin
-tropomyosin
Describe the structure of the protein, myosin
Made up of 2 types of protein :
- a fibrous protein arranged into a filament made up of several hundred molecules (the tail)
- a globular protein formed into two bulbous structures at one end (the head)
Describe the structure of the the protein, actin
A globular protein whose molecules are arranged into long chain that are twisted around one another to form a helical strand
Describe the structure of the protein, tropomyosin
Forms long thin threads that are wound around actin filaments
What is ‘sliding’ in the sliding filament mechanism
Process involves actin and myosin filaments sliding past one another
What are the 3 steps in the sliding filament mechanism of muscle contraction
- muscle stimulation
- muscle contraction
- muscle relaxation
Give the steps in the 1st stage of the sliding filament mechanism of muscle contraction : muscle stimulation
- an action potential reaches many neuromuscular junctions simultaneously, causing the Ca2+ protein channels to open and Ca2+ ions to diffuse into the synaptic knob
- the Ca2+ ions cause the synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
- acetylcholine diffuses across the syntactic cleft and binds to reception on the muscle csm, causing it to depolarise
Give the steps in the 2nd step of the sliding filament mechanism of muscle contraction: muscle contraction
- the action pot travels deep into the fibre through T-tubules
- the tubules are in contact with the ER of the muscle which has actively transported Ca2+ ions leaving low conc in the cytoplasm
- the action pot opens the Ca2+ ion channels on the ER and so Ca2+ ions diffuse down a conc gradient into the muscle cytoplasm
- causes the tropomyosin molecules that were blocking binding sites on the actin filament to pull away
- ADP molecules attached to the myosin heads mean they are in a state to bind to the actin filament and form a cross-bridge
- once attached the myosin heads change angle, pulling the actin filament and releasing x1 ADP
- an ATP molecule attached to each myosin head causing it to become detached from the actin filament
- the Ca2+ ions activate ATPase which hydrolysed ATP -> ADP providing energy for the myosin head to return to its original position
- the myosin head reattaches itself further along the actin filament and the process repeats itself so long as the Ca2+ ion conc in the myofibril remains high
- myosin molecules are joined tail-tail in 2 oppositely facing sets so move in opposite directions meaning the actin filaments they are attached to also move in opposite directions
- this movement pulls them towards eachother shortening the distance between ajacent Z-lines (shortens it)
What are T-tubules
They are extensions of the csm and branch throughout the cytoplasm of the muscle (sacroplasm)
Give the steps in the 3rd stage of the sliding filament mechanism for muscle contraction: muscle relaxation
- when nervous stimulation ceases, Ca2+ ions are actively transported back into the ER using energy from the hydrolysis of ATP
- the reabsorption of Ca2+ ions allows tropomyosin to block the actin filaments again
- myosin heads are now unable to bind to the actin filaments and contraction ceases ie. Muscle relaxes
