Nervous System 2

• Nervous system


  Conducts information in a directed way through the body via electrical and/or chemical signals and cells specialised for these functions
• Nervous system


  • Specialised for fast and targeted information conduction
• Sensory organs/structures


  Take in and get information then translate this information into signal recognisable by the nervous system
• Components of the nervous system

  • Nerve Cells (neurons)
  • Glial Cells (gila)
• Humans have more than 10^11 neurons and these have 1000s of connections
• Regions of a neuron

  • Cell Body
  • Dendrites
  • Axons (1+)
  • Axon Terminals
• Action potentials (APs)


  The key characteristic of neurons is the electrical conduction of information by action potentials
• There are some neural cells that do not generate APs like photoreceptors
• Types of neurons


  • Afferent Neurons (carry sensory information into the nervous system)
  • Efferent Neurons (carry compounds to physiological and behavioural effectors)
  • Interneurons (integrate and store information and communicate between afferent and efferent neurons)
• Neural networks can be simple or extremely complex
• Membrane potential (Em)


  The difference in charge inside and outside the cell, with the inside of the cell negative at rest
• In excitable cells (like neurons) rapid and transient shifts in Em occur - Action potentials (APs) or nerve impulses are an example of this
• Stages of a neuron going through electrical changes

  1. Neurotransmitter binds to the cell

  2. Opposite can happen - some transmitters like GABA and glycine can make the cell more negative returning the action potential
• There is a very rapid rise and fall in the electrical difference in an action potential - with it happening in around 2 milliseconds
• Threshold for an action potential

  If the entry of sodium is enough to depolarise the membrane to the threshold potential, more voltage gated channels open
• Generation of an action potential

  1. Voltage-gated ion channels open

  2. Rapid entry of lots of sodium

  3. Inside of the cell becomes so positive the inactivation channels will close, blocking the sodium from entering the cell but the potassium channels are left open meaning ions can leave the cell but no enter it - repolarising the membrane
• Invertebrate nervous structures


  • Speed of conduction directly proportional to the diameter of the fibre - the larger this is, the faster the conduction = giant fibres
• Vertebrate nervous structures


  • Nerves are sheathed in myelin (insulation) resulting in saltatory conduction (jumping) and speeds up to 100m.s-1 without a fibre diameter increase
• Synapse


  Neurons communicate with each other and with other cells at synapses
• Common neurotransmitters


  • Acetylcholine, dopamine, serotonin, His, Gly, Glucose, Asp, Adr/Nor Adr, Gamma-aminobutyric acid (GABA)
• Electrical synapse


  Potential change spreads directly from the pre- to post-synaptic cell via gap junctions
• Excitatory and inhibitory synapses

  Excitatory synapse causes depolarisation, inhibitory synapse causes hyperpolarisation of the post-synaptic cell
• Spatial summation


  Adds up the simultaneous influences of simultaneous synapses at different sites on the post-synaptic cells
• Temporal summation


  Adds up post-synaptic potentials generated at the same site in a rapid structure
• Most neurons communicate with several others, not just 1-1
• Sensory receptors


  Convert physical and chemical stimuli into neural signals that are then transmitted to the central nervous system for processing and interpretation
• Conversion of external stimuli into neural signals

  1. Sensory receptor protein causes a change in an ion channel protein

  2. External stimuli is coupled with a change in membrane potential
• Receptor potential


  A change in resting membrane potential of a sensory receptor cell in response to a stimulus
• Ways receptor potentials can generate APs


  • The receptor potential may generate APs within the receptor cell itself
  • The receptor potential may trigger the release of a neurotransmitter that induces post-synaptic neuron to generate APs
• Sensation depends upon which neurons receive APs from sensory cells
• All sensory systems process information in the form of APs - sensations we perceive differ because of the APs from different kinds of sensory cells arrive at different places on the central nervous system
• Stronger stimuli causes more APs conveying the intensity of the sense
• Adaptation


  Sensory cells give gradually diminishing response to maintain or repeated stimulation
• Chemoreceptors


  Receptor proteins that bind specific molecules (ligands)
• Chemoreceptors


  • Responsible for smell and taste, levels of CO2 in the blood
• Mating in Silkworm moths is coordinated by a pheromone called bombykol
• Olfaction in the Mosquito


  Odorants are bound by ODP within microscopic hairs (sensilliae) and transported to receptors which generates a receptor potential
• Ades aegypti females mosquitoes (Africa) show a strong preference for humans, while Ades aegypti formosus in Asia bites non-human animals mainly
• Preference for humans is tightly linked to increases in expression and ligand-sensitivity of odorant receptor Or4
• Mechanoreceptors


  Receptors that respond to mechanical forces, functions include interpreting skin sensations, sensing blood pressure, hearing and maintenance of balance