Nervous System

The nervous system coordinates the actions of complex organisms via the transmission of electrochemical signals

  • These signals are transmitted by a specialised network of cells called neurons

The nervous system can be divided into two main parts:

  • CNS  (Central nervous system) – Made up of the brain and spinal cord
  • PNS  (Peripheral nervous system) – Made of peripheral nerves which link the CNS to the body’s receptors and effectors

Main Components of the Human Nervous System


CNS versus PNS

The central nervous system integrates information received from peripheral nerves and coordinates bodily responses

  • The majority of this activity occurs in the brain, but certain responses can be mediated by the spinal cord (reflex actions)

The peripheral nervous system sends information to the CNS via sensory neurons and activates effectors via motor neurons 

  • The peripheral nerves for a particular region of the body feed into the spinal cord at a particular site (anatomical dermatome)
  • Consequently, damage to a particular region of the spine will affect all body parts innervated by nerves ventral to that region
  • Spinal injuries to the cervical region are most severe as more of the body is affected (C1 – C4 injuries impair normal breathing)

Organisation of the Peripheral Nervous System (Dermatome)


White Matter versus Grey Matter

The central nervous system (brain and spinal cord) can be characterised by two distinct regions – white matter and grey matter

  • White matter is composed of bundles of myelinated axons which connect the various grey matter regions together
  • Myelin acts as an insulator and hence nerve signals are transmitted at greater speed through white matter
  • Grey matter is composed of the neuronal cell bodies and dendrites, as well as unmyelinated nerve fibres
  • Grey matter functions as the regions of the brain where information is processed

Cross Section of the Human Brain

white vs grey matter

Divisions of the Nervous System

The nervous system can be broken down into several sub-divisions:

  • Firstly, the nervous system can be separated into the central nervous system (CNS) and peripheral nervous system (PNS)
  • The PNS can be divided into the sensory (afferent) pathway or the motor (efferent) pathway
  • The motor pathway can be divided according to whether the response is voluntary (somatic) or involuntary (autonomic)
  • Finally, the autonomic division can be split into sympathetic (‘fight or flight’) or parasympathetic (‘rest and digest’) responses

Hint:  How do you remember the difference between the afferent and efferent pathways when they sound the SAME?

  • SAME:  Sensory neurons = Afferent pathway  ;  Motor neurons = Efferent pathway

Schematic of the Nervous System Divisions

nerve system divisions