Unit 5

• Nervous system


  • Central nervous system
  • Peripheral nervous system
• Parts of the nervous system

  • Brain
  • Spinal cord
  • Sensory pathways
  • Motor pathways
  • Somatic nervous system
  • Autonomic nervous system
  • Enteric nervous system
  • Parasympathetic nervous system
  • Sympathetic nervous system
• Functional classes of neurons

  • Afferent neurons: convey information toward the CNS
  • Interneurons: connect neurons within the CNS
  • Efferent neurons: convey information away from the CNS
• Neuron


  An electrically excitable cell that transmits electrical or chemical information between other neurons or effector organs
• Common components of a neuron

  • Cell body (soma)
  • Dendrites
  • Axons
• Axon hillock


  Where the axon leaves the cell body
• Axonal transport


  The movement of materials within the axon
• Structural classes of neurons

  • Unipolar neurons
  • Pseudounipolar neurons
  • Bipolar neurons
  • Multipolar neurons
• Neuroglia


  "Nerve glue" - cells that surround soma, axon and dendrites, provide physical & metabolic support, and are capable of dividing throughout life
• Types of glial cells

  • Oligodentrocytes
  • Astrocytes
  • Microglia
  • Ependymal cells
  • Schwann cells
  • Satellite cells
• Guillain-Barré syndrome (GBS)


  Demyelinating disease of the PNS, with predisposing factors including respiratory tract, GIT infection, surgery, immunisation, viral infections. Results in myelin degeneration or destruction, leading to axonal death and molecular mimicry, causing ascending muscle weakness, flaccid paralysis, complete quadriplegia, and respiratory insufficiency.
• Nerve fibre classifications


  • Functionally (sensory or motor)
  • Structurally (number of processes)
  • Histologically (myelination)
  • Based on diameter and conduction velocity
  • Based on type of neurotransmitter released
• Myelination


  • Myelin provides a layer of insulation, inhibits the loss of current, and ion channels occur along the entire length of the axon
• Nerve fibre groups


  • Group A nerve fibres: heavy myelination, large diameter (alpha Aα; beta Aß; gamma Aγ & delta Aδ)
  • Group B nerve fibres: less myelination
  • Group C nerve fibres: no myelination and smaller diameter
• Characteristics of nerve fibre types

  • Fibre type
  • Myelin sheath
  • Diameter (µm)
  • Transmission speed (m/s)
  • Associated sensory receptors
• Nerve fibre susceptibility to various agents

  • Most susceptible
  • Intermediate
  • Least susceptible
• Synapse


  Neurons form points of contact with adjacent neurons at specialised structures, with the bulbous end of the presynaptic neuron separated from the postsynaptic neuron by a synaptic cleft. Impulses are transmitted across the synapse by chemical or electrical conduction.
• Chemical synapse


  Neurotransmission: Synthesised in the neuron, stored in small vesicles, released into synaptic cleft and bind to receptors on effector cells, then neurotransmitters are degenerated or recycled.
• Mechanisms of neurotransmitter release

  • Synthesised in the neuron
  • Stored in small vesicles
  • Released into synaptic cleft and bind to receptors on effector cells
  • Neurotransimtters degenerated or recycled
• Neurotransmitters


  Small molecules that have a N+ atom, produced in neurons and localised in axon terminals, including amino acids, peptides and amines
• Common neurotransmitters


  • Acetylcholine
  • Norepinephrine
  • Dopamine
  • Histamine
  • Serotonin
• Neurotransmitters and their effects

  • Substance
  • Location
  • Effect
  • Clinical example
• Neuromodulators


  Chemicals that modify the effects of neurotransmitters, producing slower & long-lasting changes in the membrane, enhancing or decreasing the affects of neurotransmitters
• Neurotransmitter can also act as a neuromodulator (e.g., norepinephrine)
• Alzheimer's disease


  Brain disease that is usually age-related causing a decline in intellectual function, with degeneration of cholinergic neurons in the hippocampus, amygdala & cerebral cortex, leading to declining language and perceptual abilities, confusion and memory loss. Potential contributors include genetic mechanisms and lifestyle factors.
• Alzheimer's disease treatment


  • Cholinesterase inhibitors to compensate for ACh depletion
  • Anti-amyloid beta antibodies (recently approved)
  • Treatment of secondary symptoms linked to Alzheimer's (depression and anxiety)
• Activation of postsynaptic cell

  Neurotransmitters bind to receptors on postsynaptic cell, with activated receptors that are ion channels being ionotropic receptors, and activated receptors that act via G-protein &/or second messenger being metabotropic receptors, resulting in opening or closing of postsynaptic membrane ion channels
• Fast and slow postsynaptic responses

  • Neurotransmitter + G protein-couple receptors (GPCRs) = slow response (seconds - minutes), help with neuronal growth and development, linked to long-term memory
  • Neurotransmitter + ion receptor-channel = fast response (milliseconds)
• Removal of neurotransmitters from synaptic cleft

  • Actively transported back into presynaptic axon terminal (reuptake)
  • Diffuse away form the receptor site
  • Enzymatically transformed into inactive substance (acetylcholinesterase)
• Excitatory chemical synapses


  Postsynaptic response to the neurotransmitter is a depolarisation, opening non-selective ion channels to bring membrane potential closer to threshold
• Inhibitory chemical synapses


  Postsynaptic response to the neurotransmitter is a hyperpolarisation, lowering the likelihood of exciting the postsynaptic membrane by activating receptors that open Cl- and K+ channels
• Convergence & divergence of synapses

  Convergence: multiple synapses from many presynaptic cells affect a single postsynaptic cell

  Divergence: a single presynaptic cell sends branches to affect many other postsynaptic cells
• Synaptic integration


  An electrical signal is initiated only by the combined effects of many EPSPs from temporal summation (combination of graded potentials overlapping in time) and spatial summation (combination of graded potentials from different locations)
• Long-term potentiation


  Glutamate is the key neurotransmitter, binding to two types of postsynaptic receptors (AMPA and NMDA), and is a major cellular process involved in learning and memory
• Long-term depression


  A change in the number or isoforms of postsynaptic receptors, occurring when neurotransmitter release is continued, and related to changes in the brain during clinical depression & mental illnesses
• Modification of synaptic transmission by drugs & disease

  Therapeutic, illicit & recreational drugs acting on the nervous system alter synaptic mechanisms & strength by interfering with or stimulating neurotransmitter synthesis, storage, release, or receptor activation
• Nervous system


  • Central
  • Peripheral
• Parts of the nervous system

  • Brain
  • Spinal cord
  • Sensory pathways
  • Motor pathways
  • Somatic nervous system
  • Autonomic nervous system
  • Enteric nervous system
  • Parasympathetic nervous system
  • Sympathetic nervous system
• Functional classes of neurons

  • Afferent neurons: convey information toward the CNS
  • Interneurons: connect neurons within the CNS
  • Efferent neurons: convey information away from the CNS
• Neuron


  An electrically excitable cell that transmits electrical or chemical information between other neurons or effector organs