Biology: nervous system

• the central nervous system (CNS) consists of the brain and spinal chord
• the peripheral nervous system (PNS) consists of sensory and motor neurons
• sensory neurones receive information from receptors and take this information to the CNS. the brain processes the information, then motor neurones take the information from the brain to the effector
• what do neurones do?

  neurones - receive and facilitate nerve impulses, or action potentials, across their membrane to the next neurone.
• Information travels along neurones in the form of electrical signals called nerve impulses. A nerve impulse is scientifically known as an action potential.
• Action potentials arise from a change in the ion balance in the nerve cell which spreads rapidly from one end of the neurone to the other.
• Neurones are bundled together to form nerves and these nerves form a network all around the body. When the action potential travels along the axon, to the axon terminal, neurotransmitters (chemicals) are released across the synaptic cleft and bind to receptors on the post-synaptic membrane, regenerating the action potential on the next neuron so the nerve impulse continues.
• Glial cells provide support for the neuron by digesting dead neurons and manufacturing the components of neurons.
• Receptor – a specialised cell or group of cells that respond to changes in the surrounding environment.

  Effector – a muscle, organ or gland that is capable of responding to a nerve impulse.

  Neuron – a cell that transmits electrical impulses and is located in the nervous system.

  Action potential – the change in electrical potential along the membrane of a nerve or muscle cell. More commonly known as a nerve impulse.

  Glial cell – cells that provide support for neurons such as manufacturing neuron components and digesting dead neurons.
• Resting potential and action potential 


  There is a potential difference across the membranes of neurons, in the resting state, the interior of the cell is negative compared to the exterior.
• Resting and action potential

  Potassium ions (K+) are in higher concentration in the

  cells than outside, and sodium ions (Na+) are in higher concentration outside the cell than inside. This creates a concentration gradient which ions flow down when the appropriate voltage-gated ion channels are open.
• Once the potential difference reaches a threshold voltage the reduced voltage causes hundreds of sodium gates in the membrane to open briefly.
• After the sodium gates open, sodium ions flood into the cell and depolarise the membrane. This causes more voltage-gated ion channels to open in the adjacent membrane and a wave of depolarisation travels along the cell creating an action potential.
• When a neuron is in its resting state, its membrane is polarised, the electrical charge on the outside of the membrane is positive and the electrical charge inside the membrane is negative.
• An action potential (nerve impulse) is caused by a brief change in the voltage across a membrane due to the flow of ions into and out of a neuron. The resting potential of a neuron is around -70mV.
• A nerve impulse is initiated when a neurone is stimulated. In everyday situations a stimulus can be chemical, mechanical, thermal or electrical and a stimulus is detected by receptor cells.
• Depolarisation


  Depolarisation occurs when there is a change in the membrane potential relative to the resting potential - the inside of the membrane becomes less negative.
• An action potential is generated when a stimulus changes the permeability of the neuron’s membrane by opening specific voltage-gated channels on the axon.
• Repolarisation occurs when Na+ channels are inactivated and K+ channels open allowing K+ to rush out of the cell thus restoring the internal negativity of the neuron. The electrical resting potential is restored but repolarisation does not restore the ionic conditions. Ion redistribution is accomplished by the sodium-potassium pump which moves Na+ and K+ against the concentration gradients by active transport – two K+ are moved into the cell and three Na+ are moved out of the cell into the extracellular fluid.
• Hyperpolarisation 


  The potassium ion channels are slow too close and too many potassium ions diffuse out of the axon across the membrane. The membrane potential falls too low, past -70mV. The membrane is described as hyper-polarised.
• The time taken for the membrane to return to resting potential is known as the refractory period, during this time an action potential cannot be generated.
• The speed at which a nerve impulse travels in humans is 1–3 m/s in unmyelinated fibres and 3–120 m/s in myelinated fibres. The conduction depends on:

  Axon diameter – the larger the axon, the faster the conduction

  
  

  Myelination of neurone – the nerve impulse travels faster if the neurone is myelinated

  
  

  Number of synapses involved – the fewer synapses there are to cross, the faster the communication
• Presynaptic membrane – the axon terminal membrane of the neuron carrying the impulse to the synapse.
• Axon terminal – the axon of a neuron ends in a swelling called the axon terminal. It contains mitochondria which provide energy for synaptic transport, and synaptic vesicles which release the neurotransmitter into the synaptic cleft.
• Postsynaptic membrane – the membrane of the cell body or dendrite or the neuron carrying the impulse away from the synapse. It contains a number of channels to allow ions to flow through, and protein molecules which act as receptors for the neurotransmitter.
• Threshold level – the point at which increasing stimuli trigger the generation of an electrical impulse.
• what is the junction called where two neurones meet?

  synapse
• what is the gap called between two neurones?

  synaptic cleft
• Information crosses the synaptic cleft in the form of neurotransmitters.
• When the nerve impulse reaches the dendrites at the end of one axon, called the axon terminal, the neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on the membrane of the adjacent postsynaptic neuron.
• When the neurotransmitters bind to the receptors, it stimulates the second neuron and this journey is repeated onto the next neuron.
• Pre-synaptic membrane


  The end of the axon where the action potential arrives
• Synaptic transmission


  1. Calcium ion channels open in the presynaptic membrane

  2. Calcium ions diffuse across the pre-synaptic membrane into the axon

  3. Vesicles containing neurotransmitters move towards the membrane

  4. Vesicles fuse with the pre-synaptic membrane

  5. Neurotransmitters released into the synaptic cleft

  6. Neurotransmitters diffuse across the synaptic cleft

  7. Neurotransmitters bind to receptors on the post-synaptic cell membrane

  8. Sodium ion channels in the post-synaptic membrane open

  9. Sodium ions diffuse into the next axon

  10. Depolarisation of the post-synaptic cell membrane

  11. Action potential triggered on the post-synaptic membrane

  12. Neurotransmitters recycled and repackaged into vesicles

  13. Neurotransmitters removed from the synaptic cleft between impulses
• Calcium ion concentration increases in the axon


  Vesicles containing neurotransmitters move towards the membrane
• Neurotransmitters diffuse across the synaptic cleft

  From a high concentration to a low concentration, down a concentration gradient
• Sodium ion channels in the post-synaptic membrane open


  Sodium ions diffuse into the next axon
• Sodium ions diffuse into the next axon


  Depolarises the membrane of the post-synaptic cell
• Depolarisation of the post-synaptic cell membrane

  Triggers an action potential on the post-synaptic membrane
• Action potential triggered on the post-synaptic membrane

  The wave of depolarisation continues along the next neurone
• Between impulses

  Neurotransmitters molecules are removed from the synaptic cleft to prevent continuous stimulation of postsynaptic neurons