Neuronal Communication

• Nervous Communication

  • A stimulus is detected by receptor cells.
  • A nerve impulse is then passed along a sensory neurone to the CNS.
  • The CNS coordinates a response by sending impulses along motor neurones to effectors.
• Features of Sensory Receptors

  • Specific to a single type of stimulus.
  • They convert a stimulus into a nerve impulse, so they act as a transducer.
• Mechanoreceptors

  • Stimulus - Pressure and Movement
  • Example - Pacinian Corpuscle
  • Organ Example - Skin
• Chemoreceptors

  • Stimulus - Chemicals
  • Example - Olfactory Receptor
  • Organ Example - Nose
• Thermoreceptor

  • Stimulus - Temperature Change
  • Example - End-Bulbs of Krause
  • Organ Example - Tongue
• Photoreceptors

  • Stimulus - Light
  • Example - Rod Cell
  • Organ Example - Eye
• Pacinian Corpuscle

  • Sensory receptors that detect mechanical pressure.
  • They are found deep in the skin, are especially numerous in the fingers and soles of the feet.
  • The sensory nerve ending is wrapped in lots of layers of connective tissue, called lamellae.
• Neurones

  • The role neurones is to transmit electrical impulses rapidly around the body so that the organism can respond to changes in its internal and external environment.
• Neurone structure - Myelin Sheath

  • Forms from Schwann cells, which wrap around the axon several times as they grow.
  • This acts as a insulatory layer, made up of fatty material from the schwann cell membranes.
  • The junctions between adjacent Schwann cells are called nodes of Ranvier.
• Axon

  • A single, elongated nerve fibre.
  • Transmits nerve impulses away from the cell body.
• Dendrons

  • Short extensions from the cell body.
  • They divide into smaller branches called dendrites.
  • Transmit impulses towards the cell body.
• Cell Body

  • Contains the nucleus surrounded by the cytoplasm.
  • The cytoplasm contains large amounts of endoplasmic reticul and mitochondria. These are involved in the production of neurotransmitters.
• Sensory Neurone

  • Short dendrites and one long dendron carry nerve impulses from receptor cells to the cell body.
  • One short axon that carries nerve impulses from the cell body to the CNS.
• Relay Neurone

  • Many short dendrites that carry nerve impulses from sensory neurones to the cell body.
  • One axon that carries nerve impulses from the cell body to motor neurones.
• Motor Neurone

  • Many short dendrites that carry nerve impulses from the CNS to the cell body.
  • One long axon that carries nerve impulses from the cell body to effector cells.
• The Resting Potential

  • All cells have a difference in electrical charge across the plasma membrane. This is called potential difference, measured in millivolts.
  • The resting state of an axon is called the resting potential. In this state potential difference across the axon is -70mV.
  • The outside of the membrane is positively charged compared to the inside. There are more positive ions outside the cell than inside.
• Movement of Sodium and Potassium ions.

  • Resting potential is created and maintained by Sodium-Potassium pumps and Potassium ion channels in a neurone's membrane.
  • Sodium-Potassium pumps use active transport to move 3 Sodium ions (Na+) out of the neurone for every 2 Potassium ions (K+) moved in.
  • Potassium ion channels allow facilitated diffusion of Potassium ions (K+) out of the neurone, down the concentration gradient.
• Movement of Potassium and Sodium Ions.

  1. 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, creating a Sodium ion electrochemical gradient, as there are more positive Sodium ions outside the cell than inside.
  2. The Sodium-Potassium pumps also move Potassium ions into the neurone.
  3. When the cell is at rest, most K+ ion channels are open, meaning the membrane is permeable to K+ ions, so some diffuse back out.
• The Action Potential.

  • Occurs when the potential difference across an axon is temporarily reversed.
  • The potential difference changes from -70mv to around +35mv.
  • The membrane is said to be depolarised.
• Depolarisation

  • Caused by Sodium gates opening.
• Repolarisation

  • Sodium Gates close, Whilst Potassium gates open.
• Hyperpolarisation

  • Potassium gates close
• Resting potential

  • Sodium-Potassium pumps returns to resting potential.
• Receptors

  • Specialised cells that can detect changes in the body's internal and external environment.
  • Most receptor cells are only sensitive to one type of Stimulus.
  • Receptors convert the energy of the Stimulus into the start of a nerve impulse - known as generator potential.
• Stimulation of a Pacinian Corpuscle

  • Pressure on the skin is transmitted to the Corpuscle in the dermis.
  • The shape of the corpuscle is changed.
  • This causes Sodium ion channels in the neurone membrane to open, sodium ions diffuse into neurones down the concentration gradient.
  • This depolarises the membrane, and produces a generator potential.
  • The greater the pressure, the more Sodium channels open, causing a greater generator potential.
  • If the threshold of that neurone is reached, an action potential develops, and is transmitted along the sensory Neurone.
• Initiation of Impulses

  • Despite the variety of stimuli that excite and stimulate impulses in a neurone, all impulses are identical, no matter the stimuli that caused them.
• Size of Impulse

  • The strength of a stimulus must reach a certain threshold level in order for an impulse to be evoked.
  • Once that happens, a full-sized action potential will always occur.
  • The size of the action potential will always be independent of the stimulus size.
  • This is called the all or nothing law.
• Frequency of Impulses

  • As all impulses are the same, information about the Stimulus is coded by the impulse frequency.
  • The shorter the delay between action potentials, the more intense the stimulus.
• Time between Impulses

  • Each impulse in a nuerone must be followed by a short period of recovery.
  • This is so the Ionic balance can return to normal, this is known as a refractory period.
  • It's composed of 2 parts: Absolutely refractory period and Relative Refractory period.
  • It has 3 consequences.
• Time between Impulses - Absolute Refractory Period

  • The axon is completely incapable of impulse transmission .
  • This Lasts around 1ms.
• Time between Impulses - Relative Refractory Period

  • The axon can transmit an impulse if stimuli is stronger than usual.
  • This lasts around 5-10ms.
• Time between Impulses - 3 Consequences

  1. Action potential remain separate, so do not merge or overlap.
  2. Speed of transmission is restricted to a maximum of 500-1000 per second.
  3. Impulses travel in one direction because the previously active region is now in its Refractory Period.
• Transmission Speed

  • In non-myelinateted fibres, the thicker the diameter of the axon, the faster the speed of transmission, as resistance is less.
  • Complex animals demand fast responses, so impulses must travel faster than 100m/s. This speed can't be achieved by just a wider axon, so Myelin Sheath comes in. Composed of fatty material with a high electric resistance, Myelin acts as an electrical insulator. Its speeds up transmission as impulses leap from node to node, as depolarisation only occurs where the Myelin is absent. Ions flow in and out the axon, depolarising the next node.
• Speed of Nerve Impulse and Axon Diameter

  • The greater the diameter of the axon, the faster the impulse travels.
  • Axons with a small diameter have a larger surface area : volume ratio than axons with a large diameter, this causes a larger amount of ions to leak out of the axon, making it more difficult for an action potential to propagate.
• Speed of Nerve Impulse and Temperature

  • The higher the temperature, the faster the speed of the impulse.
  • Temperature affects the rate of diffusion of ions across the axon.
• Synapses

  • Nerve impulses can be transmitted from one neurone to another.
  • This transfer occurs at a junction, called a synapse.
• Synaptic Transmission - Step 1

  • An action potential arrives at the synaptic knob.
  • This causes calcium ion to open in the presynaptic membrane, allowing calcium ions to enter the synaptic knob from the extracellular fluid.
• Synaptic Transmission - Step 2

  • Calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release the acetylcholine into the synaptic cleft.
• Synaptic Transmission - Step 3

  • The acetylcholine diffuses across the synaptic cleft and binds to specific receptor sites in the membrane.
• Synaptic Transmission - Step 4

  • Sodium ion channels in the postsynaptic membrane open.
  • Sodium ions diffuse into the postsynaptic neurone, which produces a small reduction in the membrane potential called an excitatory postsynaptic potential (EPSP).