The Divisions of the Nervous System (AQA AS Psychology)

The central nervous system

• The human nervous system is divided into two parts:

  • The central nervous system (CNS)

  • The peripheral nervous system (PNS)

• These two systems are connected and work together so that the whole human nervous system functions effectively

• The two systems collect information from inside the body and from the surrounding environment

• Both systems process this information and then dispatch instructions to the rest of the body, facilitating an appropriate response, e.g.

  • The PNS contains a huge network of spinal and cranial nerves

    • These nerves link to the brain and spinal cord

    • Sensory receptors in the PNS process internal and external changes

    • This information is then sent to the CNS via afferent sensory nerves

• The CNS is made up of the brain and the spinal cord

  • The brain is central to maintaining life and has many functions including:

    • regulating body temperature, heart rate and breathing

    • language (production and understanding)

    • coordinating movement

    • coding sensory data via the sensory organs (e.g. the skin) and the environment

    • problem-solving and planning

  • The brain's cerebral cortex is the most sophisticated part of the brain

    • The cerebral cortex consists of the outer layer of the brain (like a shower cap that covers the brain)

    • It also carries out essential functions of the brain such as

      memory, thinking, learning, problem-solving, consciousness, sensory functions

    • The cerebral cortex is a key marker that distinguishes humans from animals

  • The spinal cord ensures that signals from the brain are transmitted to the rest of the body via the PNS

  • The brainstem connects the brain to the spinal cord

The peripheral nervous system: somatic & autonomic

• The PNS transmits messages throughout the whole body from the brain and also relays messages back to the brain

• The PNS is divided into two sections:

  • The somatic nervous system (SNS)

  • The autonomic nervous system (ANS)

The somatic nervous system

• The SNS transmits and receives messages from the senses apart from sight

  • The retina and optic nerve are connected directly to the brain

• The other senses — sound, smell, taste and touch — travel via the SNS to communicate with the brain, e.g.:

  • touch information from the skin (e.g. the texture of velvet)

  • taste information from the tongue (e.g. the flavour of ketchup)

• The sense of touch below the neck uses the SNS to communicate with the spinal cord, which then relays relevant signals to the brain

• The SNS directs the muscles to move appropriately

  • Thus, the SNS controls voluntary, conscious movements, e.g.

    • throwing a ball

    • running across a road to get out of the way of an oncoming car

• The SNS is also responsible for the reflex arc, which, although automatic (not conscious), still involves the use of muscle movement

The autonomic nervous system

• The ANS transmits and receives information from the organs, e.g.

  • heart-rate

  • breathing

  • the stress response

  • digestion

  • sexual arousal

• The ANS is involuntary; it cannot be controlled consciously

• The ANS can be further subdivided into:

  • the sympathetic nervous system (SPNS)

  • the parasympathetic nervous system (PSNS)

The sympathetic nervous system

• The SPNS is associated with the 'fight or flight' response in tandem with the endocrine system as follows:

  • The SPNS prepares the body for physical activity when the hypothalamus detects a stimulus which requires attention/action, e.g.

    • running away from a threat

    • standing and facing the threat

    • preparing to fight the threat

  • The SPNS is triggered when the body is in an 'alert' state, e.g.

    • when crossing the road

    • when a noise is heard late at night

  • Adrenaline is released from the adrenal glands to fuel any physical activity required of the body along with other physiological changes, e.g

    • accelerated heart rate

    • widened bronchial passages for increased breathing capacity

    • decreased activity of the large intestine

    • pupil dilation

    • sweating

  • The SPNS thus enables a fast, automatic response to a possible threat or dangerous situation (but it can also occur when someone is highly elated or excited)

The parasympathetic nervous system

• The PSNS is popularly known as the 'rest and digest' system:

  • The PSNS is the body at rest which (to preserve energy) is its usual state

  • The PSNS helps to conserve the body's activity levels and energy by decreasing activity which may be needed later

  • The PSNS regulates bodily functions like digestion and urination

  • The PSNS slows heart and breathing rates and lowers blood pressure as the body enters a state of relaxation

  • Relaxation enables the body to go into 'standby' (recovery mode)

    • The more time spent in a PNS state, the healthier a person is likely to be