Response to stimuli

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Synapses and neurotransmitters

  • The junction between a neurone and another neurone, or a neurone and an effector cell
  • The gap is called the synaptic cleft
  • The presynaptic neurone has a swelling called the synaptic knob, this contains synaptic vesicles filled with chemicals called neurotansmitters

Effect of an action potential

  • When an action potential reaches the end of a neurone  is causes neurotransmitters to be released into the synaptic cleft
  • They diffuse across to the postsynaptic membrane and bind to specific receptors
  • This triggers an action potential
  • Neurotransmitters are removed from the cleft so the response doesn't keep happening
  • Due to receptors only being present on the postsynaptic membranes, this ensures unidirectionality
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Cholinergic synapses

These are synapses that use the neurotransmitter acetylcholine (ACh) 

Transmission of a nerve impulse across a cholinergic synapse

  • Arrival of an action potential- the arrival of an action potential stimulates the voltage-gated  calcium ion channels in the presynaptic neurone to open, calcium ions diffuse into the synaptic knob- they are pumped out afterwards via active transport
  • Fusion of the vesicles- The influx of calcium ions into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane. The vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft
  • Diffusion of ACh- ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane, causing the sodium ion channels in the postsynaptic neurone to open. The influx of sodium ions causes an action potential on the postsynaptic membrane. ACh is removed when it is broken down by the enzyme acetyl cholinesterase (AChE) into the products choline and ethanoic acid (acetyl) which diffuse back across and are recombined to form ACh using ATP. Sodium channels close
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Chemical mediators

  • A chemical messenger that acts locally, similar to communication using hormones- cells release chemicals that bind to specific receptor sites on target cells. The differences are:
    > Chemical mediators are secreted from cells all over the body not just glands
    >Their target cells are right next to where the chemical mediator is produced, menaing they only stimulate a local response
    >Produce a quicker response as they only travel a short distance


  • A chemical mediator stored in mast cells and basophils and is released in response to the body being injured or an allergen, e.g. pollen
  • Causes dilation of small arteries and arterioles and increased permeability of capillaries, leading to localised swelling, redness and itching


  • Found in cell membranes and are involved in inflammation, fever, blood pressure regulation
  • Cause increaed permeability to capillaries and dilation of arterioles as well as affecting blood pressure and neurtotransmitters and in doing so affect pain sensation
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1. Receptors

How they work

  • Specific- only detect one particular stimulus
  • Receptors in the nervous system convert the enrgy of the stimulus into the electrical energy used by neurones
  • The resting potential- The inside of the cell is negatively charged relative to the outside, meaning there is a voltage across the membrane known as the potential difference. The resting potential is generated by ion pumps and ion channels
  • The generator potential- When a stimulus is detected, the cell becomes excited and more permeable, allowing more ions to move in and out of the cell- altering the potential difference. The bigger the stimulus, the bigger the generator potential
  • The action potential- If the generator potential is reaches the threshold value, an action potential is triggered- an electrical impulse along the neurone

*see section on nervous impulses for a more detailed explanation

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2. Receptors

Pancinian Corpuscles

  • Mechanoreceptors- detect mechanical stimuli e.g. pressure and vibrations
  • Found in the skin- high concentrations in the lips, fingers and genital area
  • Contain the end of a sensory neurone- sensory nerve ending- which is wrapped in multiple layers of connective tissue called lamellae
  • How they work:
    When stimulated the lamellae are deformed and press on the sensory nerve ending.
    > This causes deformation of stretch-mediated sodium channels in the sensory neurone's cell membrane
    > The sodium channels open and sodium ions diffuse in the cell, creating a generator potential
    > If the generator potential reaches the threshold, it triggers an action potential

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Neuromuscular junctions

  • A special cholinergic synapse between a motor neurone and a muscle cell
  • Use acetyl choline (ACh), which bind to cholinergic receptors called nicotinic cholinergic receptors
  • Work in the same was as cholinergic synapses with a few differences:
    >The postsynaptic membrane has lots of folds that form clefts. These clefts store acetylecholinesterase
    >The postsynaptic membrane has more receptors than other synapses
    >When a motor neurone fires an action potential, it always triggers a response in a muscle cell
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Excitatory and inhibitory neurotransmitters


Depolarise the postsynaptic membrane (the potential difference across the membrane becomes more positive) making it fire an action potential if the threshold value is reached  e.g. Acetylcholine


Hyperpolarise the postsynaptic membrane (the potential difference across the membrane becomes more negative), preventing it from firing an action potential e.g.GABA- causes potassium ion channels to opeon the postsynaptic membrane, hyperpolarising the neurone

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2. Control of heart rate

High blood pressure

Baroreceptores detect high blood pressure and send impulses along the sensory neurones to the medulla oblongata, which sends impulses along the parasypathetic neurones. These secrete acetylcholing, which binds to receptors on the SAN and causes  the hear rate to slow down in order to reduce blood pressure.

Low blood pressure

Baroreceptores detect low blood pressure and send impulses along the sensory neurones to the medulla oblongata, which sends impulses along the sympathetic neurones. These secrete noradrenaline which binds to the receptors on the SAN and causes the heart rate to increase, increasing the blood pressure.

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3. Control of heart rate

High blood O2, low CO2, high blood pH

Chemoreceptors detect chemical changes in the blood and send impulses along sensory neurones to the medulla oblongata, which sends impulses along parasympathetic neurones. These secrete acetylcholine which binds to the receptors on the SAN, causing the heart rate to decrease and return levels back to normal.

Low blood O2, low CO2, high blood pH

Chemoreceptors detect the chemical changes in the blood and send impulses along sensory neurones to the medulla oblongata, which sends impulses along sympathetic neurones. These secrete noradrenaline which bind to the recptors on the SAN, causing the heart rate to increase ad return levels back to normal.

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1. Control of heart rate

  • The autonomic system of the peripheral nervous system is involved in the control of heart rate 
  • Split into the parasympathetic nervous system (rest and digest)- cardio acceleratory centre and the sympathetic nervous syestem (fight or flight)- cardioinhibitory centre

Communication between the heart an brain

  • The rate at which the SAN generates an electrical impulse is unconsciously controled by the medulla oblongata in the brain
  • Animals need to alter their heart rate to respond to internal stimuli which is detected by pressure and chemical receptors:
    >Pressure receptors- these are called Baroreceptors and are found in the aorta and carotid arteries. They are stimulated by high and low pressure
    >Chemical receptors- these are called Chemoreceptors are found in the aorta, carotid arteries and medulla oblongata. They moniter the oxygen level, CO2 levels and pH of the blood (which are indicators of the oxygen level)
  • Electrical impulses from receptors are sent t the medulla oblongata along sensory neurones. The medulla processes the information and sends impulses to the SAN along the sympathetic or parasympathetic neurones
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Drugs at synapses

  • Same shape as the neurotransmitter- the drug mimics the action of the neurotransmitters at the receptor meaning more receptors are activated. These are called agonists and an example is nicotine which mimics acetylcholine
  • Block receptors- this means the receptor cannot be activated by the neurotransmitter meaning fewer neurotransmitters are cativated. These are called antagonists and an example is curare which blocks the nicotinic cholinergic receptors at neuromuscular junctions meaning the muscle cell cannot be stimulated resulting in paralysis
  • Inhibit enzymes- the drug inhibits the enzyme that breaks down the neurotransmitter meaning there are more neurotransmitters in the synaptic cleft to bind to receptors, e.g. nerve gas which stops acetylcholine being broken down leading to loss of muscle control
  • Stimulate the release of neurotransmitter- this means more receptors are activated, e.g. amphetamines which force the neurotransmitter dopamine into the synaptic cleft resulting in increased alertness
  • Inhibit the release of neurotransmitters- this means fewer receptors are activated, e.g. opiods block calcium ion channels meaning fewer vesicles fuse to the membrane and release the neurotransmitter
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Summation at synapses

if a stimulus is weak, only a small amount of neurotransmitter will be released, resulting in the threshold not being reached. Summation is therefore used to add together the effect of neurotransmitters realeased from multiple neurones.

Spatial summation

Two or more neurones release their neurotransmitters at the same time onto the same postsynaptic neurone. The sum of these neurotransmitters is enough to reach the threshold value and and trigger an action potential in the postsynaptic neurone.

Temporal summation

Two or more nerve impulses arrive in quick succession from the same presynaptic neurone. This makes an action potential more likely because more transmitter is released onto the synaptic cleft, reaching the threshold value.

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Hormonal system vs Nervous system

Hormonal System

  • Hormones carried in the blood
  • Slow response
  • Widespread effect
  • Long-lasting effect
  • Either reversible or non-reversible

Nervous System

  • Electrical impulse carried by neurones
  • Fast response
  • Localised effect
  • Short-lived effect
  • Reversible


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Simple Responses

  • Stimuli- a change in the internal or external environment e.g. temperature, light, pressure

Taxis response (tactic)

  • Directional movement in response to a stimulus
  • The direction of the stimulus affects the response
  • E.g. Woodlice move away from a light. Keeps them concealed under stone during the day which increases survival chances

Kinesis response (kinetic)

  • Non-directional (random) movement in response to a stimulus
  • The intensity of the stimulus affects the response- the higher the intentensity the increased frequency of directional changes taken and movement made
  • E.g. woodlice respond to high humidity with few turns and move slowly but in low humidity they move faster and turn more often in order to reduce water loss by moving away from the stimulus
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5. The Nervous Impulse

Speed of conduction

  • Myelination- The axon of a neurone surrounded by a myelin sheath, this is an electrical insulator made of schwaan cells. Betweent the schwaan cells are patches of bare membrane named the nodes of Ranvier, sodium ion channels are concentrated at these nodes.
    Saltatory conduction: Depolarisation only happens at the nodes of Ranvier, the neurone's cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node, increasing the speed of transmission= saltatory conduction
  • Axon diameter- Action potentials are conducted quicker along axons with bigger diameters because there is 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
  • Temperature-The speed of conduction icreases with temperature, as ions diffuse faster. Also enzymes function more rapidly at higher temperatures up to a point and the sodium potassium pump is controlled by enzymes
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2. Responses in plants


  • A type of growth factor (chemicals that speed up or slow down plant growth) that are used by the plant to respond to the stimuli
  • Produced in the tips of shoots and diffuse backwards to stimulate the cell just behind the tips to elongate- this is where the cell walls become looseand stretchy causing the cell to get longer
  • If the tip of the shoot is removed, auxin will no longer be produced
  • Stimulates growth in shoots and inhibits growth in roots
  • Indoleacetic acid (IAA)- A particular type of auxin moved around the plant to control tropisms by diffusion and active transport for short distances and the phloem for long distances, this results in different parts of the plant having different concentrations of IAA causing uneven growth of the plant:
    >Phototropism: IAA moves to the shaded parts of roots and and shoots, in roots this inhibits growth on the shaded side causing the root to bend away from the light. In shoots this stimulates cell elongation on the shaded side causing the shoot to bend towards the light
    >Geotropism: IAA moves to the underside of roots and shoots, in roots this causes inhibits growth on the underside causing the root to bend downwards, in shoots this causes cell elongation on the underside causing the shoot to bend upwards
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4. The Nervous Impulse

Waves of depolarisation

  • 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 sodum ions diffuse into that part
  • This causes a wave of depolarisatin 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

All-or-nothing principle

  • 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 will not be stimulated
  • A bigger stimulus will not cause a bigger action potential, but it will increase the frequency at which action potentials are stimulated
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1. The Nervous Impulse

The resting membrane potential

  • The potential when the neurone isn't being stimulated
  • The outside of membrane is positively charged compared to the inside- the membrane is polarised
  • The voltage across the membrane is -70mV
  • Sodium-potassium pumps- The pump actively transports three sodium ions out of the neurone for every two potassium ions moved in, this creates and electrochemical gradient
  • Potassium ion channels- Potassium ion channels are open whereas the sodium ion channels are closed. This means the membrane is 100X more permeable to potassium ions diffusing out of the membrane via facilitated diffusion, compared to sodium ions
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2. The Nervous Impulse

The formation of action potentials

  • Depolarisation- The arrival of a stimulus excites the cell membrane. This causes the voltage gated sodium channels to open, allowing sodium ions to diffuse into the cell down the electrochemical gradient. This stimulates more voltage gated sodium channels too open causing more sodium ions to diffuse in. The potential difference across the membrane is now +40mV- the charge across the membrane has changed to positive on the inside and negative on the outside
  • Repolarisation- The volatge gated sodium channels close and the voltage gated potassium channels open. The membrane is more permeable to potassium allowing the potassium ions to diffuse out of the neurone down the concentration gradient. This starts to get the membrane back to its resting potential- positive on the ouside compared to the inside
  • Hyperpolarisation- The voltage gated potassium channels are slow to close resulting in a brief period where the potential difference across the membrane is less than the resting potential-
  • Resting potential- The sodium potassium pump is re established returning the neurone back to its resting potential
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1. Responses in plants


  • A response of a plant to a directional stimulus, plants respond to stimuli by regulating their growth
  • A positive tropism is growth towards the stimulus whereas negative tropism is growth away from the stimulus
  • Phototropism- the growth of a plant in response to light, shoots are positively phototropic and roots are negatively phototropic
  • Geotropism- the growth of a plant in response to gravity, shoots are negatively geotropic and roots are positively geotropic
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4. Receptors

Rod Cells

  • Found in the peripheral parts of the retina
  • Only give information in black and white
  • Function in dim light
  • Sensitivity- Very sensitive to light because three rod cells join to one neurone, so many weak poyentials combine to reach the threshold and trigger an action potential 
  • Visual acuity- Low visual acuity because three rod cells join to the same neurone. This means that light from two objects close together cannot be told apart

Cone Cells

  • Found packed together in the foevea
  • Gives information in colour, three types- red,green and blue, each stimulated different amounts
  • Sensitivity- Not very sensitivity because one cone joins to one neurone so more light is needed to reach the threshold
  • Visual acuity- High visual acuity because cones are close together and there is one cone per neurone meaning two seperate action potentials are formed
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1. The nervous system

  • Receptors- Detect stimuli- cells, proteins on the cell surface membranes
  • Effectors- Cells that bring about a response to a stimulus to produce an effect- muscle cells, cells found in glands


  • Sensory neurones- Transmit electrical impulses from receptors to the CNS
  • Motor neurones- Transmit electrical impulses from the CNS to effectors
  • Relay neurones/ Intermediate neurones- Transmit electrical impulses between sensory neurones and motor neurones

Nervous communication

  • A stmiulus is detected at the receptor cells and an electrical impulse is sen along a sensory neurone
  • When the impulse reaches the end of the neurone neurotransmitters take the impulse across the synapse to the next neurone
  • The CNS processes the information and sends impulses along the motor neurone to the effector
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2. The nervous system

The reflex arc

  • A simple reflex is a rapid, involuntary response to a stimulus
  • The pathway of communication goes through the spinal cord but not through conscious parts of the brain, so the response happens automatically
  • These are protective- they help organisms to avoid damage to the body because the response happens so quickly
  • Stimulus --> Receptors --> CNS --> Effectors --> Response

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The hormonal system

  • Glands- A group of cells that are specialised to secrete a useful substance, such as a hormone. They can be stimulated by a change in concentration of a specific substance or electrical impulses
  • Hormones- Chemical messengers, many are proteins or peptides

Hormonal communication

  • Hormones diffuse directly into the blood and carried around the body by the circulatory system
  • They diffuse out of the blood all over the body but each hormone will only bind to specific receptors for that hormone found on the membreanes of some cells= target cells
  • Stimulus --> Receptors --> Hormone --> Effectors --> Response
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3. Receptors


  • Found in the retina (particularly the fovea area) and detect light- light enters the eye through the pupil, the amount of light is controlled by the muscles of the iris
  • 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 not photoreceptor cells
  • How they work:
    > Light enters the eye, hits the photoreceptors and is absorbed by the light-sensitive pigments
    > Light bleaches the pigments, causing a chemical change and altering the membrane, perticularly the permeability to sodium
    > 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) 
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3. The Nervous Impulse

The refractory period

  • After an action potential, the neurone cannot be stimulated again immediately after
  • The voltage gated sodium channels are closed during repolarisation and the voltage gated pottasium channels are closed during hyperpolarisation
  • Absolute refractory period- The sodium channels are closed, preventing the movement of sodium ions, no matter how strong the stimulus, no action potentials can be stimulated
  • Relative refractory period- The potassium channel open, even though the membrane is not fully repolarised an action potential can occur if the stimulus is above the threshold value
  • Importance of the refractory period:
    Ensures unidirectionality
    >Produces discrete impulses
    >Limits the number of action potentials
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