The Nervous system III

• Nervous systems conduct information in a directed way through the body via electrical and/or chemical signals
• Nervous systems are specialisations for fast and targeted information conduction
• Sensory organs receive information and translate it into a signal recognisable to the nervous system (e.g., light, mechanical, taste, odour, sound)
• Components of nervous systems

  • Nerve cells (neurons)
  • Glial cells (glia)
• Human nervous system has more than 10^11 neurons with 1000-times more connections
• Properties of individual nerve cells are understood better than most other cell types
• Neurons have 4 main regions: Cell body, Dendrites, Axons, Axon terminals. They conduct information by action potentials (APs)
• Neural networks process information as neurons are arranged into them. Afferent neurons carry sensory information into the NS, Efferent neurons carry commands to effectors, and Interneurons integrate and store information
• All cells have a membrane potential (Em) which can undergo rapid and transient shifts in excitable cells resulting in action potentials (APs) or nerve impulses
• In invertebrates, the speed of conduction is directly proportional to the diameter of the fibre, leading to the development of giant fibres. Vertebrates have myelin sheaths resulting in saltatory conduction and high speeds without a fibre diameter increase
• Neurons communicate at synapses, with the most common type being chemical synapses. Common neurotransmitters include Acetylcholine, dopamine, serotonin, His, Gly, Glu, Asp, Adr/Nor Adr, Gamma-aminobutyr
• Myelin sheath results in saltatory conduction and speeds up to 100 m/s without a fibre diameter increase
• Synapse

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

  • Acetylcholine
  • dopamine
  • serotonin
  • His
  • Gly
  • Glu
  • Asp
  • Adr/Nor Adr
  • Gamma-aminobutyric acid (GABA)
• In electrical synapses, the potential change spreads directly from pre- to post-synaptic cell (via gap junctions)
• Synaptic activity can lead to a positive or negative change in Em and this is called a graded potential
• A graded potential is transformed into an AP if depolarization exceeds threshold at axon hillock
• The neuromuscular junction is a model of a chemical synapse
• Synapses between neurons can be excitatory or inhibitory
• A synapse that causes hyperpolarisation of the post-synaptic cell is inhibitory (usually due to Cl- entry, but also by K+ exit)
• The post-synaptic cell sums excitatory and inhibitory input over space and over time
• Spatial summation adds up the simultaneous influences of simultaneous synapses at different sites on the post-synaptic cell
• Temporal summation adds up post-synaptic potentials generated at the same site in a rapid sequence
• Sensory receptor cells convert physical and chemical stimuli into neural signals
• Signals are transmitted to the CNS for processing and interpretation
• Signal transduction involves changes in membrane potential
• Receptor potential

  A change in resting Em of a sensory receptor cell in response to a stimulus
• Receptor potentials must generate APs
• Sensation depends upon which neurons receive APs from sensory cells
• All sensory systems process information in the form of APs
• Sensations differ because the APs from different kinds of sensory cells arrive at different places in the CNS
• Intensity of sensation is coded by the frequency of APs
• Many receptors adapt to repeated sensory stimulation
• Some sensory cells give gradually diminishing responses to maintained or repeated stimulation
• Adaptation is known as diminishing responses to maintained or repeated stimulation
• Chemoreceptors are receptor proteins that bind specific molecules (ligands)
• Chemoreceptors are responsible for smell and taste, levels of CO2 in the blood
• Chemoreceptors can trigger responses including feeding, mating, fighting, and recognising individuals
• Mating in silkworm moths is coordinated by a pheromone called bombykol
• Human pheromones exist