The Nervous System and Neurons

• The divisions of the nervous system;

  • this is a network of cells in the human body and is our main internal communication system

  it is divided into two sub-systems;

  • central nervous system (CNS)
  • peripheral nervous system (PNS)
• The CNS;

  • the CNS is made up of the brain and the spinal cord
  • its function is to control behaviour and regulate the body's physiological process such as breathing, digestion and heartbeat
• The brain;

  • this is the centre of all consciousness and deals with higher order thinking skills (such as problem solving, decision making and language)
• The spinal cord;

  • the spinal cord is a collection of nerve cells that are attached to the brain and run the length of the spinal column
  • it relays information from the brain to the body
  • it also deals with some reflex responses without the brain
• The PNS;

  • the peripheral nervous system transmits millions of neurons to and from the CNS

  it has two sub divisions:

  • autonomic nervous system
  • somatic nervous system
• Autonomic nervous system;

  • the autonomic nervous system transmits and receives messages from the organs
  • it is made of the sympathetic (bodily arousal) and parasympathetic branch (bodily calm) of the nervous system
  • this is involuntary
• Somatic nervous system;

  • the somatic nervous system transmits and receives messages from the senses and controls muscle movement
  • this is under conscious control
  • information taken from the senses to the brain and information taken from the brain to the muscles (voluntary movement)
• The structure of a neuron;

  • the cells of the nervous system are called neurons (they all share the same basic structure)
  • it consists of a cell body (soma) that includes a nucleus (contains genetic material of the cell)
  • dendrites are the branch like structures that come away from the cell body. They carry nerve impulses from neighbouring neurons towards the cell body
  • the axon carries the impulses away from the cell body down the length of the neuron
• The structure of a neuron;

  • the axon is covered by the myelin sheath that protects the axon and speeds up electrical transmission
  • the myelin sheath is broken up into chunks so there are gaps between it. This makes the electrical transmission even faster as it forces the impulse to jump across gaps along the axon. These gaps are called the nodes of Ranvier
  • at the end of the axon there are axon terminal buttons that communicate with the next neuron in the chain across a gap called the synapse
• The function of neurons;

  • neurons are nerve cells that allow the nervous system to communicate by sending information using chemical and electrical signals
  • neurons carry message from one part of the body to another

  there are 3 types of neurons that all have slightly different structures and functions

  • sensory neurons
  • relay neurons
  • motor neurons
• Sensory neuron;

  • these send signals from your sense organs to the brain, they send messages from the peripheral nervous system (PNS) to the central nervous system (CNS)
  • they have short axons but long dendrites
  • for example, sensory neurons will register pain
• Relay neuron;

  • these connect the sensory neurons to the motor or other relay neurons
  • they have short axons and short dendrites
  • for example, this would involve the analysis of the sensation and decide how to respond to this pain
• Motor neuron;

  • these connect the central nervous system (CNS)/ the brain to effectors such as muscles and glands
  • they have long axons and short dendrites
  • for example, these neurons tell the person's muscles to move away from the pain
• Synaptic transmission;

  • signals cross between neurons at the synapse
  • this is called synaptic transmission; it refers to how the nervous system transmits information across a synaptic cleft (the physical gap between nerve cells) from one neuron to another
  • this is accomplished by the body's production of chemicals called neurotransmitters
• Neurotransmitters, excitation and inhibition;

  • neurotransmitters are chemicals that diffuse across the synapse to the next neuron
  • they carry signals across the synaptic cleft to the receptor site
  • neurotransmitters can either be excitatory or inhibitory
  • for example, serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming negatively charged and less likely to fire
  • dopamine causes excitation of the receiving neuron, resulting in a positive charge and making it more likely to fire
• Process of synaptic transmission;

  • an electrical impulse known as action potential, triggers vesicles containing neurotransmitters in the pre-synaptic neuron to move to the pre-synaptic membrane
  • the vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft
  • neurotransmitters diffuse across the synaptic cleft and bind to receptor sites on the post-synaptic cell membrane . Any that don't bind are lost by diffusion, broken down by enzymes, or recycled (reuptake)
• Process of synaptic transmission;

  • once enough receptors have neurotransmitters bound to them, the signal is transmitted
  • stimulation of postsynaptic receptors by neurotransmitters result in either excitation or inhibition of the postsynaptic membrane
  • during this process, summation occurs. This is the net effect of the excitatory and inhibitory potentials (adding up the positive and negative charges)
• Summation;

  • whichever type of neurotransmitter is attached the most; it leads to that effect
  • for example, if there is more inhibitory neurotransmitters then it leads to a negative charge in the post synaptic neuron and the message is less likely to fire
  • if there is more excitatory neurotransmitters, then it leads to a positive charge in the post synaptic neuron and the message is more likely to fire