The nervous system is divided into two main systems (Fig. 1):
- Central nervous system (CNS)
- Peripheral nervous system (PNS)
The central nervous system consists of the brain and spinal cord. The peripheral nervous system, is the section of the nervous system that is physically located outside the brain and spinal cord – that is the nerve cells that extend from the brain and spinal cord to the body’s tissues. The longest of these cells can be over one meter long, for example those extending from the spinal cord to the toes. The toes represent here what we call the body’s peripheral tissues, i.e. body tissue that lies outside the central nervous system.
Furthermore, the peripheral nervous system is divided into sensory and motor units (Fig. 1). Sensory units consists of nerve cells that send signals to the central nervous system and motor units consists of nerve cells that send signals from the central nervous system to the body’s muscles and glands.
As Fig. 1 shows, the motor unit is further divided into the somatic nervous system, i.e. voluntary, and autonomic nervous system – which is not voluntary. The somatic nervous system thus represents the part of the nervous system under conscious control and signals from these nerves go exclusively to skeletal muscles.
The autonomic nervous system is further divided into the sympathetic and parasympathetic systems (Fig. 1). Both sympathetic and parasympathetic nerve cells transmit signals to the muscles and glands throughout the body without you being aware and regulate a wide range of physiological functions.