PCBI101: NERVOUS (2)

• Neural pathway


  The route that a signal takes through the nervous system
• Types of neural pathways

  • Converging
  • Diverging
  • Reverberating
• Converging neural pathway


  • Several neurons pass signals to one particular neuron
  • Allows for increased sensitivity as several weak signals can come together to generate an impulse
• Example of converging neural pathway


  • Seeing in low levels of light - Converging neural pathways from rods in the retina allow detection of movement and distinguish objects in low light
• Diverging neural pathway


  • One signal from the nervous system is divided up into two or more neurons
  • Allows for more coordination between muscles and movement
• Example of diverging neural pathway


  • Fine motor control, e.g. balancing rocks, sewing, playing musical instruments
• Reverberating neural pathway


  • A neuron at the end of the pathway is linked back to a neuron at the start
  • Allows for continuous operation of that pathway by the signal looping back and re-triggering itself
• Example of reverberating neural pathway


  • Breathing - a continuous rhythmic process involving stimulating the reverberating pathway to keep the intercostal muscles and diaphragm operational
• The most common exam question on neural pathways is about identifying reverberating pathways from a diagram
• The next key area to be covered is the cerebral cortex (also known as the cerebrum)
• Spinal cord


  Communication gateway between the brain and spinal nerves, which innervate the trunk and limbs
• Spinal cord


  • Long, thin tube of nervous tissue
  • Enclosed in 3 membranes of the meninges
  • Protected within the bones of the vertebral column
• Spinal nerves


  31 pairs that arise from the cord and emerge from the vertebrae
• Spinal cord extent


  Extends from the brainstem to the level of upper lumbar vertebrae
• Cauda equina


  Nerve roots that descend within the spinal canal before exiting in the lower lumbar and sacral regions
• Spinal cord cross section


  • Contains a butterfly-shaped central core of gray matter
  • Surrounded by white matter
• Gray matter


  Contains cell bodies and dendrites of neurons, where neurons synapse and transmit information
• White matter


  Made of bundles of axons, conducts information up and down the cord
• Spinal tracts


  Bundles of axons organised into specific groups with specific functions, acting as high-speed cables
• Spinal tracts


  • Occur on both sides of the cord
  • Ascending tracts conduct sensory information up to the brain
  • Descending tracts convey motor instructions down the cord
• Contralateral transmission


  Tracts that cross over to the other side of the cord before reaching the brain, conveying sensory information from one side of the body to the other side of the brain
• Ipsilateral transmission

  Tracts that stay on the same side all the way, conducting information to the same side of the brain
• Spinal nerves


  Mixed nerves containing both sensory and motor fibers
• Sensory pathway


  1. First-order neurons detect stimuli and transmit signals to the spinal cord

  2. First-order neurons synapse with second-order neurons in the cord, which ascend a specific tract to the brainstem or thalamus

  3. Third-order neurons conduct the information the rest of the way to the sensory cortex
• Motor pathway


  1. Upper motor neuron starts in the motor cortex or brainstem

  2. Lower motor neuron continues from the brainstem or spinal cord, conducting motor instructions down a specific descending tract
• Somatic reflexes


  Fast, involuntary responses of skeletal muscles, not requiring input from the brain
• Somatic reflex arc


  1. Somatic receptor

  2. Sensory neuron

  3. Interneuron

  4. Motor neuron

  5. Effector muscle
• Some reflexes are more complex, requiring multiple pathways and central coordination from the brain
• Axons are covered by myelin sheath, which insulates them from electrical current.
• Pain


  A very important defense mechanism that warns the body about potential or actual injuries or diseases, so that protective actions can be taken
• Pain sensation


  1. Noxious signals send impulses to the spinal cord

  2. Spinal cord relays information to the brain

  3. Brain interprets information as pain, localizes it, and sends back instructions for the body to react
• Pain receptors (nociceptors)


  • Present in the skin, superficial tissues and virtually all organs, except for the brain
  • Nerve endings of "first-order neurons" in the pain pathway
• First-order neurons


  • Can be myelinated (A type) or unmyelinated (C type)
  • Myelinated A fibers conduct at fast speeds and are responsible for the initial sharp pain
  • Unmyelinated C fibers conduct at slower speeds and are responsible for a longer-lasting, dull, diffusing pain
• Pain signal transmission


  1. First-order neurons travel by way of spinal nerves to the spinal cord

  2. Synapse with second-order neurons in the dorsal horn

  3. Second-order neurons cross over to the other side of the cord

  4. Ascend to the brain
• Major pathways for pain signals from spinal cord to brain


  • Spinothalamic tract
  • Spinoreticular tract
• Spinothalamic tract


  • Second-order neurons travel up to the thalamus where they synapse with third-order neurons
  • Third-order neurons project to the somatosensory cortex
  • Involved in localization of pain
• Spinoreticular tract


  • Second-order neurons ascend to the reticular formation of the brainstem, before running up to the thalamus, hypothalamus, and the cortex
  • Responsible for the emotional aspect of pain
• Pain from the face


  First-order neurons travel mainly via the trigeminal nerve to the brainstem, where they synapse with second-order neurons, which ascend to the thalamus
• Somatic pain


  Pain from the skin, muscles and joints
• Visceral pain


  • Pain from internal organs
  • Often perceived at a different location due to the convergence of pain pathways at the spinal cord level