The Nervous System


The three basic functions(types of neurones):

  1. Sensory – Perceives changes in the internal and external environments
  2. Integrative – Analysis of stimuli that results in decisions
  3. Motor – Initiates voluntar muscular contraction or reflexes

Divisions of the nervous system (N.S.):

  • CNS – Central Nervous System
    • Encephalon ( =brain), in the cranium
    • Medulla Spinalis (=spinal cord), on the dorsum. It is enclosed by the vertebral column.
  • PNS – Peripheral N.S.
  • ANS – Autonomic Nervous System includes Paraganglia(E.g. glomus caroticum and glomus aorticum) and Enteric Nervous System.

Compartments(Histology) of the N.S.:

  • Somatic – Receives info. coming from stimuli, processes, and sends action potentials to effectors (skeletal muscles).
  • Vegetative (Visceral or Autonomic) – Info. is received from the internal environment (e.g. kidney), processed, and action potentials are sent to effectors (smooth muscles).

Functional Circuits:

  • Sensory (Afferent) nerves – send impulses to CNS
  • Motor (Efferent) nerves – send impulses to effectors

Every functional system has four principles.

  1. Sensory, Motor and Integrative neurones are all interconected by Relay Nuclei. These are important processing centres where info. is modified accordingly by all the inputs of the Relay nucleus.
    1. Local interneurons lie in the relay nucleus, mediating and inhibiting synaptic interactions
    2. Projection interneurons transmit the output of the nucleus. Their axons leave the relay nucleus and interconnect with other areas of the CNS.
  2. A neuronal system is composed of several distinct pathways. For instance – the brain uses a pathway to see colour, but another pathway to process movement. Both pathways include the eye.
  3. Each pathway is topographically organised, i.e. every pathway can be traced to a certain region of the CNS (Vision – Occipital Lobe).
  4. Most pathways cross the body’s midline. An amazing yet unexplained feature of the brain, neurones from the hemispheres cross eachother in the region called ‘Corpus Callosum’ to innervate opposite sides. The left hemisphere is therefore responsible for the movement of the right side of the body.
    1. Crossings in the spinal cord and brain are called decussations.
    2. Crossings that only contain decussations are termed commisures.