Nervous System

• the nervous system is a system of organs working together to detect and respond to stimuli
• the central nervous system includes the brain and spinal cord and sends information from the sense organs to reflexes to carry out tasks
• peripheral nervous system includes all nervous tissue which carry out a response outside the CNS including cranial nerves and spinal nerves
• neurons are nerve cells that act as a wire that transmits electrical impulses all over the body
• sensory neuron transports messages from receptors in the sense organs to the CNS, it is a part of the afferent division
• interneuron is located in the CNS and is the link between the sensory and motor neuron
• motor neuron transports messages from CNS to effectors (muscles/glands) and is a part of the efferent division
• the myelin sheath is made up of Schwann cells that wrap around the axon like insulation on wires
• unipolar neuron has one axon only and is not in humans

  
• Bipolar neuron has one axon and one dendrite (sense receptors)

  
• pseudounipolar neuron has a cell body that extends from one axon and separates into two extensions, such as a sensory neuron

  
• Multipolar neuron has one axon and multiple dendrites from the cell body , commonly an interneuron or motor neuron

  
• Dorsal root is the front of a vertebrae such as a sensory neuron
• Ventral root is the back of a vertebrae such as a motor neuron
• a nerve impulse is a message that travels along a nerve fibre, transported very quickly through electrochemical change
• Intracellular fluid

  Negatively charged due to large organic negative ions
• Resting membrane potential is measured at -70mV in a non-stimulated nerve fibre
• Depolarisation at Nodes of Ranvier


  Occurs on the axon
• Restoration of original ion concentration


  Sodium potassium pump restores the original ion concentration by pumping 3 sodium out of cell and 2 potassium back into cell
• After an ion channel has opened and transported ions, the axon needs to rest before it can open again. This is called a refractory period and lasts for about 2ms. The nerve fibre cannot be stimulated during this rest period
• Repolarisation


  Sodium gates close on the outside, potassium channels/gates open from the inside allowing potassium ions to diffuse out into the extracellular fluid
• Neuron is polarised


  Extracellular fluid maintained positively charged, intracellular fluid negatively charged
• Depolarisation of axon membrane


  1. Begins when a strong enough stimulus is applied to the axon and reaches the threshold of at least -55mV

  2. Stimulus allows the membrane to become more permeable to Sodium ions
• Active transport


  Occurs when ions move across the membrane
• Return to resting state


  Membrane returns to an unstimulated state
• Hyperpolarisation


  Occurs due to an overshoot of Potassium ions
• The nerve fibre returns to a resting membrane potential of -70mV with the axon membrane unstimulated
• all or none response is a nerve impulse being transmitted at full strength if the threshold of -55mV is met, regardless of the strength of the stimulus
• saltatory conduction is the conduction of a nerve impulse along a myelinated neuron as the impulse jumps from each node of Ranvier to the next, at a speed of 140m/s
• differences in intensity of nerve impulses are regardless of the strength of the stimulus, but instead depend on the number of nerve fibres depolarised and the frequency of nerve impulses
• Transmission across a synapse

  1. An action potential arrives at the axon terminal (pre-synaptic terminal)

  2. Depolarisation of the pre-synaptic terminal causes voltage gated calcium channels to open and calcium ions to diffuse into the pre-synaptic terminal

  3. Calcium in the cell causes the vesicles to release neurotransmitters into the synaptic cleft through exocytosis

  4. Neurotransmitters diffuse onto receptor sites on the post-synaptic terminal's sodium gated channels of acetylcholine receptors

  5. Sodium channels open and the sodium will diffuse from the synaptic cleft into the post-synaptic terminal causing depolarisation of the post-synaptic terminal

  6. The neurotransmitters are broken down by an enzyme in the post-synaptic terminal
• neurotransmitters are chemicals that transmit signals from a neuron to a target cell across a synapse which will either stimulate or depress the transmission
• examples of neurotransmitters:

  • synthetic or natural
  • dopamine
  • adrenaline/noradrenaline
  • natural only
  • acetylcholine
• acetylcholine is responsible for stimulating muscles by activating motor neurons that control skeletal muscle - decreased levels is associated with Alzheimer's disease
• dopamine controls voluntary movement of the body - decreased levels is associated with Parkinson's disease
• adrenaline/noradrenaline is an excitatory neurotransmitter that increases heart rate and blood pressure
• receptors detect a change in the body's internal or external environment and group together to form a sense organ
• thermoreceptors are situated in the skin or hypothalamus and respond to hot or cold temperatures (not both)

  the skin informs the hypothalamus of changes in external temperature

  peripheral thermoreceptors are located in the skin where nerve endings are sensitive
• osmoreceptors stimulate the hypothalamus and respond to changes in water or osmotic pressure in the body so body water content is maintained
• chemoreceptors are situated in the nose, mouth, internal blood vessels, respiratory organs, and the heart, responding to chemicals and odours such as the composition of body fluids, blood pH, and concentrations of oxygen and carbon dioxide