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  • Neurons are separated by a synaptic cleft (approx. 20-30nm wide)
  • Presynaptic neurone refers to the neuron that released the neurotransmitter
  • Postsynaptic neurone refers to the neuron that receives the neurotransmitter
  • The enlarged section of an axon is synaptic knob
    • Large amount of mitochondria
    • Large amount of smooth endoplasmic reticulum
  • Neurotransmitters Is stored in synaptic vesicles
  • Once the neurotransmitter is released from the vesicle, it diffuses across the postsynaptic cleft


  • Single impulse along one neurone to be transmitted to a number of different neurones at a synapse, therefore allowing a single stimulus to create a number of simultaneous responses
  • A number of impulses to be combined at a synapse. This allows stimuli from different receptors to interact in order to produce a single response


  1. Action potential arrives at presynaptic neurone causing Ca2+ channels to open and therefore Ca2+ ions enter the synaptic knob
  2. Influx of Ca2+ into presynaptic neurone results in synaptic vesicles to fuse with the presynaptic membrane. This releases acetylcholine into the synaptic cleft
  3. Acetylcholine fuses with receptors on the sodium ion channel on the postsynaptic neurone resulting in sodium ion channels to open allowing Na+ to diffuse in rapidly along a concentration gradient
  4. Influx caused by the sodium ions generate a new action potential in the postsynaptic neurone
  5. Acetylcholinesterase hydrolyses acetylcholine into choline and acetyl (ethanoic acid) which diffuses back through the presynaptic neurone cleft. It also ensures that the acetylcholine does not continuously generate a new action potential in the new neurone.
  6. ATP released is used to recombine choline and acetyl to acetylcholine which is stored in the synaptic vesicles. Sodium ion channels close.
Slow Twitch and Fast Twitch Muscle Fibres

Slow Twitch and Fast Twitch Muscle Fibres

  • Skeletal muscles are made up of two types of muscle fibres
  • Different muscles have different proportions of slow and fast twitch fibres

Slow Twitch Fibres

  • Slow contraction
  • Work for a long time without fatigue
  • Good for endurance activities
  • High proportion found in muscle used for posture (i.e. back/calves)
  • Energy is released slowly through aerobic respiration in slow twitch muscle fibres
  • They have lots of mitochondria and blood vessels to supply the muscles with oxygen
  • Mitochondria are mainly found near to the edge of muscle fibres to there is a short diffusion pathway
  • Rich in myoglobin, a red coloured protein that stores oxygen (which gives muscles their red colour)

Fast Twitch Fibres

  • Quick contraction
  • Fatigues quickly
  • Good for short high intensity bursts
  • High proportions found in muscles used for fast movements (i.e. legs, eyes)
  • Energy is released quickly through anaerobic respiration using glycogen
  • Stores of PCr for quick energy release
  • Few mitochondria or blood vessels as they are not really needed as they produce their energy through glycogen – not oxygen.
  • No myoglobin (which gives them a pale white colour)
Responses in Plants

Responses in Plants

  • Tropism

    • A tropism is the response of a plant to a directional stimulus (a stimulus coming from a particular direction)
    • Plants respond to stimuli by regulating their growth
    • A positive tropism is a growth towards a stimulus
    • Negative tropism is a growth away from a stimulus


    • Phototropism is the growth of a plant in response to light
    • Shoots are positively phototropic and grow towards the light
    • Roots are negatively phototropic and grow away from light


    • Gravitropism is the growth of a plant in response to gravity
    • Shoots are negatively gravitropic and grow upwards
    • Roots are positively gravitropic and grown downwards


    • Plants respond to directional stimulus using hormonal growth factors which speed/slow growth
    • Plant growth factors are produced only in the tips of the roots and shoots where they can be moved to other parts of the plant
    • Auxin is produced in the tips of shoots and diffuses backwards to stimulate the cells just behind the tips to elongate. The cell walls because loose and stretchy so cells become longer
    • If the tips shoots are removed, no auxin will be produced and therefore shoot will stop growing

    Indoleacetic Acid (IAA)

    • Important auxin produced in tips of shoots and roots
    • Moves around plant to control tropisms
      • Moves via diffusion and active transport over short distances
      • Moves via phloem for long distances
    • Results in different parts having different concentrations of IAA
    • It is the different concentrations which result in uneven growth

    IAA on Phototropism

    • IAA moves to more shaded parts of the shoots and roots


    • IAA concentration increases on the shaded side
    • Cells elongate and the shoot bends towards the light


    • IAA concentration increases on shaded side
    • Growth is inhibited so the root bends away from the light

    IAA on Gravitropism

    • IAA moves to the underside of shoots and roots


    • IAA concentration increases on lower side
    • Cells elongate so the shoot grows upwards


    • IAA concentration increases on the lower side
    • Growth is inhibited so the root grows downward


Resting Potential

  • There is a difference in charge between the inside and outside of the cell
  • The inside is negatively charged relative to the outside
  • There is a voltage (potential difference) across the membrane
  • Resting potential refers to this potential difference
  • The resting potential is generated by ion pumps and ion channels
  • Active transport using Sodium-Pottasium pump (3Na+ actively pumped out of the membrane, and 2K+ are actively transported into the axon)
  • Na+ diffuses back in whilst K+ diffuses back out
  • Creates a sodium ion electrochemical gradient because there is a more positive sodium ion outside and outside the cell
  • When the cell is at rest most potassium ion channels are open so that the membrane is permeable to potassium ions so more diffusion back out through the potassium ion channels.

Generator Potential

  • The cell membrane is excited when a stimulus is detected and becomes permeable which allows more ions to move in and out of the cell (which therefore alters the potential difference)
  • This change in potential difference due to a similes is the generator potential
  • A bigger stimulus excited the membrane more resulting in a bigger movement of ions and an even greater change in potential difference
  • Stimulus causes te membrane at one part of the neurone to increase in permability to Na
  • Na voltage gated channels open and na enters the axon down their electrochemical gradient by diffusion
  • This results in resiting potential

Action potential

  • If the generator potential exceeds the threshold it will trigger an action potential (an electrical impulse along a neurone)
  • It is only triggered if the generator potential reaches the threshold level.
  • Action potentials are all one size, therefore strength of stimulus is measured by frequency of action potentials
  • If the stimulus is too weak the generator potential will not reach the threshold level and so there is no action potential
  • When depolarisation reaches +30mV the Na+ ion channels close and potassium ion channels open
  • The diffuse membrane is more permeable to potassium so Kdiffuses out of the neurone down the potassium ion concentration gradient
  • This starts to get the membrane back to its resting potential

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  • Light enters the eye
  • Light hits the photoreceptor and is absorbed by light sensitive pigments
  • Light bleaches the pigments resulting in a chemical change and altering the membrane permeability to Na+
  • Generator potential is created and once it reaches a threshold a nerve impulse is sent along a bipolar neurone
  • Biplolar neurones connect photoreceptors to the optic nerve



Mainly located in peripheral parts of retina

Mainly located in fovea part of retina

Black and white


Many rods linked to same bipolar cell

One cone linked to one bipolar cell

High sensitivity to light

Low sensitivity to light

Low visual acuity

High visual acuity



  • Rods are sensitive to light as there are many rods to one bipolar neurone
    • Therefore, it is easier for 3 rod cells to reach the threshold value relative to 1
  • Cones are less sensitive than rods as they have one bipolar neurone each
    • Therefore, requires more light to reach threshold value

Visual Acuity

  • Visual acuity is the ability to tell the different points that are close together
  • Rods give a low visual acuity as many rods join to the same bipolar neurone
    • Therefore, light from two points close together cannot be differentiated
  • Cones give a high visual acuity as each cone is close together and one cone joins to one bipolar neuron
    • When light from two points hits two cones, two action potentials are given


Passage of an Action Potential

Passage of an Action Potential

Resting Potential

  • Concentration of Na+ outside axon membrane is high (relative to inside)
  • Concentration of K+ inside axon membrane is high (relative to outside)
  • Overall concentration of positive ions is however greater on the outside, making this positive compared to the inside.
  • Axon membrane is polarised

Arrival of an action potential

    • A stimulus causes a sudden influx of Na+ ions
    • A reversal of charge on the axon membrane in that action potential and the membrane become depolarised
    • Localised electrical circuits established by the influx of Na+ causes the opening of sodium voltage-gated channels to open slightly further along the axon
    • Resulting influx of Nacauses the region to become depolarised
    • Behind the newer region of depolarisation, the sodium voltage gated channels close and the potassium ones open
    • K+ begin to move from the axon along electrochemical gradient


    • Action potential (depolarisation) is propagated in the same way further along the axon
    • Outward movement of the K+ions has continued to the extent that the axon membrane behind the action potential has return to its original charged state (where the positive outside and negative inside)
    • It has repolarised
    • Repolarisation of the axon allows Na+ ions to be actively transported out
    • This returns the axon to its resting potential


Passage of an action potential along a myelinated Axon

  • Myelin forms a fatty sheath around the axon which acts as an electrical insulator (thus preventing an action potential from forming)
  • At intervals (1-3mm) there are nodes of Ranvier (breaks in the myelin insulation)
  • The localised circuits therefore arise between adjacent nodes of Ranvier and the action potential is able to ‘leap’ between the nodes through saltatory conduction
  • An action potential passes along a myelinated neurone faster due to this fashion


Pacinian Corpuscles

Pacinian Corpuscles



Mechanoreceptors – detects a mechanical stimulus (i.e. pressure)

Type of Neurone Sensory
Location Dermis (Skin)
Size ~1mm
Function Detect Mechanical Stimuli


  • Found in the deeper layers (Dermis)  of the skin primarily fingers, external genitals and soles of feet. Also in tendons, ligaments and joints. 
  • Approx 1mm diameter
  • Contains a single myelinated sensory nerve ending (the end of a neurone)
  • Sensory nerve ending is wrapped in lamellae (layers of connective tissue with viscous gel between)
  • Blood capillary which surrounds the inner wall of the capsule provides relevant nutrients for optimal cellular function (i.e. glucose, Na, water etc.) 


  • Stimulation occurs (e.g. tap on the arm)
  • Lamellae deforms and press on the sensory nerve ending
  • Sensory neurone cell membrane stretches which deforms the stretch-mediated sodium ion channels
  • Channel opens and Na+ diffuses into the sensory neurone cell creating a generator potential
  • If generator potential exceeds threshold level it will trigger depolarisation of the sensory neurone which will cause an action potential to travel along the axon
  • Action potential is taken to CNS where a response will be produced (e.g. feeling the tap on the arm) 








Cell body

· Contains nucleus and large amounts of Rough Endoplasmic Reticulum

· Associated with production of proteins + neurotransmitters


· Small extensions of the cell body

· Subdivide into smaller branched fibres (dendrites)

· Carry nerve impulses towards the cell body


· Single long fibre

· Carries nerve impulses away from cell body

Schwann Cells

· Surrounds axon

· Wraps around axon many times to form layers of membrane

· Protects axon and provides electrical insulation

· Carries out phagocytosis

· Used in nerve regeneration

Myelin Sheath

· Forms a covering to the axon

· Made up of the membranes of the Schwann cells

· Rich in lipids (myelin)

· Myelinated neurons are those that contain a myelin sheath

· Transmit nerve impulses faster (relative to unmyelinated)

Node of Ranvier

· Gaps between adjacent Schwann cells (where there is no myelin sheath)

· Gaps are 2-3μm long and occur every 1-3mm (humans)


Types of Neuron

  • Sensory Neurone:
    • Transmits nerve impulses from a receptor to an intermediate/motor neuron
    • Have one dendron which carries the impulse towards the cell body
    • One axon that carries it away from the cell body
  • Motor Neurone:
    • Transmit nerve impulses from an intermediate/sensory neuron to an effector
    • Have a long axon and many short dendrites
  • Intermediate Neurone:
    • Transmits impulses between neurones
    • Numerous short processes

Nerve Impulses

Resting Potentials

Movement of ions, such as Na+ and K+, across the axon membrane is controlled by:

  • Phospholipid bilayer of the axon plasma membrane presents sodium and potassium ions alternating across it
  • Channel proteins span the phospholipid bilayer where there is ion channels which pass through them. Some of these channels have gates on them which are able to open and close so that sodium or potassium ions can move through them via facilitated diffusion at any given time. Some of the channels however remain open all the time so that the sodium and potassium ions move unhindered through them by facilitated diffusion.
  • Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon (using a Sodium potassium pump)

As a result of the above processes the inside of the axon is negatively charged relative to the outside.

The resting potential ranges from 50mW to 90mW with in humans averaging 65mW. So in this state it is polarised.

For the neurone to become polarised:

  • Sodium ions are actively transported out of the axon by sodium potassium pumps
  • Potassium ions are actively transported into the axon by sodium potassium pumps
  • Active transport of sodium ions is greater than that of potassium ions.

3Na+ move out for every 2K+ that move in.

  • Although both sodium and potassium have a +1 charge, the outward movement of sodium ions is greater than the inward movement of potassium ions. Ultimately there are more sodium ions in the tissue fluid that the axon thus creating an electrochemical gradient
  • The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon
  • However most of the potassium gates remain open whereas the sodium channels are closed

Action Potential

  • When a stimulus reaches a threshold, the energy causes a temporary reversal of the charges either side of the axon membrane.
  • If the stimulus is great enough it will cause the -65mW inside the membrane to become +40mw. This is the action potential and in this condition this part of the axon membrane is said to be depolarised.
  • This depolarisation occurs as the voltage-gated channels in the axon membrane change shape and hence open or close depending on the voltage across the membrane

For the neurone to become polarised:

  • At resting potential some potassium voltage-gated channels are open (mainly permanently open gates) but the sodium voltage gated channels remain closed.
  • The energy of the stimulus result in some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient.
  • As Sodium ions diffuse into the axon, so more sodium channels open causing an even greater influx of sodium ions by diffusion.
  • Once the action potential of around +40mV has been established the voltage gates on the sodium ion channels close (and therefore preventing a further influx of sodium ions) but the potassium voltage gates begin to open
  • Some of the potassium voltage gates being open the electrical gradient that was preventing further outward movement of potassium ions is now revered, resulting in more potassium ion channels to open. This starts the repolarisation of the axon.
  • The outward diffusion of these potassium ions cause a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual, hyperpolarisation. The gates on the potassium ion channels now close and the activities of the sodium potassium pumps once again cause sodium ions to be pumped out and potassium ions in.
  • The neuron repolarises at -65mW.
Muscle Structure

Muscle Structure

Types of Muscle

  • Smooth
    • Does not require conscious
    • Found in walls of internal organs (excluding heart)
    • g. Intestines, blood vessels
  • Cardiac
    • Does not require conscious
    • Found only in the heart
  • Skeletal (striated/striped/voluntary)
    • Requires conscious
    • Controls mobility
    • g. biceps, triceps

Skeletal Muscle

  • Skeletal muscle is attached to bones via tendons
  • Ligaments attach bones to another bone
  • Pairs of skeletal muscles work antagonistically at the joints
  • The bones act as levers, allowing the muscles to pull against them
  • Skeletal muscle is made up of bundles of muscle fibres
    • Cell membranes of muscle fibre cells is the sarcolemma
  • Transverse Tubules are the folds of the sarcolemma inwards across the muscle fibre and into the sarcoplasm (cytoplasm)
    • Transverse tubules help to spread electrical impulses through the sarcoplasm so all parts of the muscle fibre is reached equally
  • Sarcoplasmic reticulum is a network of internal membranes which through the sarcoplasm
    • Sarcoplasmic reticulum stores and releases Ca2+
  • There is high number of mitochondria to provide adequate ATP
  • Muscle fibres are multinucleate (many nuclei)
  • They have lots of myofibrils
    • Long cylindrical organelles which are made up of proteins which are specialised for muscle contraction


  • Contain bundles of thick and thin myofilaments which can slide past each other
    • Thick myofilament is Myosin (dark band)
    • Thin myofilament is Actin (light band)

A band) some overlapping thick and thin filaments

I band) Only contains (thin) actin filament

H Zone) Only contains myosin filaments

Z line) End of each sarcomere

M line) Middle of the myosin filament

Sliding Filament Theory

  • Muscle contraction occurs due to sliding filaments
  • Myosin and actin filaments slide over one another to make the sarcomere contract
  • Myofilaments do no contract.
  • The larger amount of simultaneous sarcomere contractions results in the muscle fibres and myofibrils contracting
  • Sarcomeres return to original length after on relax