The Human Nervous System

• • The human nervous system consists of:
  • Central nervous system (CNS) – the brain and spinal cord
  • Peripheral nervous system (PNS) – all of the nerves in the body
  • The nervous system enables humans to react to their surroundings and to coordinate their behaviour
  • Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones
  • A bundle of neurones is known as a nerve
• Adaptations of the nervous system

  • Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites
  • Some human neurones have axons over a metre in length
  • This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another
  • The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along
• Structure of the Nervous System

  • Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS)
  • The CNS is the brain and spinal cord
  • The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones
  • The pathway through the nervous system is:

  stimulus → receptor → coordinator → effector → response
• Reflex Arc

  • An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction
  • Awareness of a response having happened occurs after the response has been carried out
  • Responses are therefore automatic and rapid – this helps to minimise damage to the body
• 1. The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin
  2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
  3. An electrical impulse is passed to a relay neurone in the spinal cord
  4. A relay neurone synapses with a motor neurone
  5. A motor neurone carries an impulse to a muscle in the leg (the effector)
  6. The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone
• The neurones of the reflex arc

  • There are three main types of neurone in a reflex arc: sensory, relay and motor
  • Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
  • Relay neurones are found inside the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)
• • Sensory neurones are long and have a cell body branching off the middle of the axon
  • Relay neurones are short and have a small cell body at one end with many dendrites branching off it
  • Motor neurones are long and have a large cell body at one end with long dendrites branching off it
• Synapses

  • Neurones never touch each other, they are separated by junctions (gaps) called synapses
  • Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them
  • In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones
  • Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient)
• Synapses 1

  • The electrical impulse travels along the first axon
  • When an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles
  • The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane)
  • This stimulates the second neurone to generate an electrical impulse that travels down the second axon
• Synapses 2

  • The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent
  • Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions
  • As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg this is where heroin works
• Exam Tip

  • Neurotransmitters move by diffusion – remember this requires a concentration gradient and is a passive process
  • Receptors that are complementary in shape to neurotransmitters are located on the postsynaptic neurone
  • Drugs (such as heroin, ecstasy and cocaine) can bind to neurotransmitter receptors, triggering impulses in different regions of the brain
  • Frequent drug-use can lead to overstimulation of neurones, leading to loss of function or overstimulation of different regions of the brain and body