Nervous system

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The nervous system consists of its main functional unit – the neuron –, which makes up several systems, including the central and peripheral nervous systems.

Where neurons contact, the point of contact is called a synapse (clasp together), and where there are swellings of functional groups of neurons, they are called ganglia (swelling or knot).

One type of control is involuntary, and happens via the autonomic nervous system, while the other type is voluntary, and happens via the somatic nervous system.

The nervous system can be said to start in the brain, which then exits as the spinal cord, wrapped in meninges and cerebrospinal fluid (CSF).

Spinal Nerve, Reflex Arc, and Dermatome

Define the term spinal nerve, and list its parts and functional components

  • A bundle consisting of the dorsal and ventral roots
  • splits into dorsal and ventral rami (each contains sensory and motor nerves)
  • Components:
    1. Somatic afferent (somato-sensory)
    2. Somatic efferent (somato-motor)
    3. Sympathetic visceral efferent (viscero-motor)
      • Note: Sympathetic visceral afferent are a minor component.

Describe a simple reflex arc

The reflex arc pathway

Sensory fibres pass either directly to the ventral grey horn, or synapse with one or more connector neurons (inter-neurons) which themselves pass to the ventral grey horn.

Describe the clinical significance of dermatomes

  • Allow observation of spinal nerve branches
  • Innervated by cutaneous branches of spinal nerves.

Review terminology for groupings of neurons and axonal fibers

In Peripheral system In central System
A group of neuronal cell bodies Ganglion Nucleus (except basal ganglia)
A bundle of axonal fibres Nerve Tract or Fasciculus Column or Funiculus Lemniscus

Describe and sketch the stretch reflex and related events

Monosynaptic reflex arc

  • The basic pathway involves only one synapse in the ventral horn, therefore monosynaptic
  • e.g., patellar (knee-jerk) reflex
  • aka deep tendon or stretch reflex
  • begins with the muscle spindle receptors (intrafusal fibres) within the tendon being stretched upon tapping
  • This stretching initiates a signal which travels in afferent (sensory) fibres into the grey matter of the spinal cord (cell bodies of the afferent fibres reside in the dorsal root ganglion)
  • In the spinal cord, afferent fibres travel through the dorsal horn to the ventral horn where they synapse with the alpha motor neurons
  • Efferent neurons (motor) leave the spinal cord via the ventral root
  • signal the contraction of the striated muscle fibres (extrafusal fibres)

Modulation of the monosynaptic reflex arc

  • In addition to sending signals to activate extension of the limb, alpha motor neurons send axons to innervate interneurons within the ventral horn
  • Interneurons are inhibitory in function
  • Interneurons send short axonal fibres to modulate the activated alpha motor neurons via inhibitory neurotransmitters
  • This results in a smooth reflex action achieved by a balance between excitatory and inhibitory effects on the motor neuron

Polysynaptic inhibition of antagonistic muscle group

  • For a reflex to be successful, there must be simultaneous inhibition in the contraction of the antagonistic muscle group
  • contraction of the synergist (e.g., quadriceps) activates the golgi tendon organ receptor in the collagen fibres of the quadriceps tendon
  • The receptor sends signals via afferent fibres into the dorsal horn of the spinal cord
  • These synapse with an interneuron which releases inhibitory neurotransmitter to modulate the alpha motor neurons of the antagonists

Describe and sketch the reflexes of cutaneous origin

Flexor/Extensor reflex of cutaneous origin

  • Example of a single-level reflex of cutaneous origin
  • E.g., withdrawal of limb from an external noxious stimulus
  • Sequence of events:
    • Contact with a noxious stimulus (e.g., stepping on a nail)
    • Sensory receptors convey pain signal to the dorsal horn via afferent fibers
    • Signal is relayed to both excitatory and inhibitory interneurons on both sides of the spinal cord
    • Excitatory interneurons synapse simultaneously with:
      1. ipsilateral motor neurons (ipsilateral flexion; withdrawal)
      2. contralateral motor neurons (contralateral extension; balance)
    • Inhibitory interneurons:
      1. inhibit ipsilateral extensors (permit withdrawal)
      2. inhibit contralateral flexors (aid balance)


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