Biology

• Nervous system


  • Provides rapid, co-ordinated responses to situations
  • Allows reaction to surroundings and co-ordination of behaviour
• Main parts of the nervous system

  • Central nervous system (brain and spinal cord)
  • Peripheral (body) system (neurons and sensory receptors)
• How nervous co-ordination works

  1. Stimulus picked up by sensory receptor

  2. Information passed along afferent neurons to central nervous system

  3. Information processed in central nervous system

  4. Instructions sent out along efferent neurons to effector organ
• Neurons


  Nerve cells that are the basic unit of the nervous system
• Neurons


  • Have cell body, dendrites, and axon
  • Are irritable and can conduct electricity
• Afferent (affector) neurons


  Carry information from receptors to central nervous system
• Efferent (effector) neurons


  Carry messages from central nervous system to body parts
• Nerves


  Bundles of neurons, can be effector, afferent, or mixed
• Action potential


  Electrical signal that travels along neurons
• Synapse


  Junction between two neurons where electrical signal is converted to chemical signal
• Neurotransmitters


  Chemicals that transmit nerve impulses across synapses
• Neuromuscular junctions


  Synapses between motor neurons and muscle fibres
• Parts of the central nervous system

  • Brain
  • Spinal cord
• Cranial nerves


  Nerves that emerge directly from the brain
• Spinal nerves


  Mixed nerves that emerge from the spinal cord
• The brain is a delicate mass of nervous tissue enclosed in the cranium and protected by the bones of the skull

  
• The spinal cord runs out from the brain down the body and is encased and protected by the vertebrae of the spine
• Cranial nerves


  12 pairs of nerves that emerge directly from the brain stem
• Spinal nerves


  Mixed nerves containing both sensory and motor neurons
• One of the most common chemical transmitters found in synapses is acetylcholine
• Botulinus toxin


  • Affects the first membrane and stops the release of acetylcholine, preventing the nervous system from working and causing death
• Strychnine and organophosphorus compounds

  • Deactivate the enzymes on the second membrane and prevent the breakdown of acetylcholine, causing rigid paralysis
• Curare


  • Stops the second membrane from responding to the chemical transmitter, causing paralysis
• An average human brain weighs 1300–1400 g (about 3 lb) yet the cerebral cortex, which controls most of our conscious thought and action, is only about 3 mm thick
• Grey matter


  Areas in the brain and spinal cord that consist of unmyelinated nerve cells
• Forebrain


  The large frontal area of the human brain
• Midbrain


  The area of the brain that deals with vision
• Hindbrain


  The area of the brain that deals with balance, orientation, and fundamental reflexes
• Relay neurons


  Neurons which are activated by other neurons
• Mental illness


  General term referring to psychological, emotional, or behavioural disorders
• Mental illness can be the result of an imbalance of the chemical transmitters in your brain
• The use of illegal drugs, which affect the chemistry of the brain, is leading to an increase in mental illness in young people
• Voluntary action


  1. Sense organ detects stimulus

  2. Information travels along afferent neurons to spinal cord

  3. Information processed in brain

  4. Efferent neurons send instructions to effectors
• Reaction time


  The length of time it takes to recognise a stimulus and react to it
• Reflex action


  Automatic, instinctive, unlearned reaction to a stimulus
• Reflex arc


  Neural path of a reflex
• Reflex action


  1. Sensory receptor detects stimulus

  2. Afferent neuron carries signal to spinal cord

  3. Relay neuron connects to efferent neuron

  4. Efferent neuron carries signal to effector
• Setting up a reflex arc

  1. Impulse travels to the spinal cord

  2. Impulse travels back to the quadriceps muscles in the thigh

  3. Quadriceps muscles contract

  4. Lower leg jerks upwards and outwards

  5. Leg drops straight back into normal position
• How reflexes work in detail

  1. Impulses from sensory receptor in skin pass along afferent neuron to spinal cord

  2. Neuron enters spinal cord through dorsal root

  3. Impulse from afferent neuron arrives at synapse with relay neuron, transmitter released

  4. Impulse travels along relay neuron

  5. Impulse reaches synapse between relay neuron and efferent neuron, transmitter released

  6. Impulses travel along efferent neuron to effector (muscle), causing contraction

  7. Efferent neuron leaves spinal cord through ventral root
• Reflex action


  Stimulus → receptor → afferent neuron → coordinator → efferent neuron → effector → response