How is the nervous system divided up so that some responses are voluntary and others run automatically?
Describe the organisation of the nervous system into central and peripheral divisions, and the somatic and autonomic (sympathetic and parasympathetic) divisions and their roles in homeostasis
A focused answer to the WACE Year 12 Human Biology Unit 3 dot point on how the nervous system is organised. The central and peripheral divisions, the somatic and autonomic systems, and the antagonistic sympathetic and parasympathetic branches in homeostasis.
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What this dot point is asking
WACE wants you to map out the nervous system as a labelled hierarchy and to explain what each part does, especially the parts that run automatically and keep the internal environment stable. This page deals with the organisation; the action potential and the synapse are covered separately. Knowing the divisions cleanly is worth easy marks and stops you confusing voluntary control with automatic homeostatic control.
The central nervous system
The central nervous system (CNS) is the brain and the spinal cord. It is the control and integration centre: it receives sensory information, processes and interprets it, makes decisions, and sends out instructions. The hypothalamus, a small region of the brain, is the key homeostatic control centre, setting and defending the set points for temperature, water balance and other variables. The spinal cord carries information between the body and the brain and also coordinates fast reflexes without involving the brain.
The peripheral nervous system
The peripheral nervous system (PNS) is all the nervous tissue outside the brain and spinal cord, made up of the nerves that connect the CNS to receptors and effectors. Sensory (afferent) nerves carry impulses toward the CNS; motor (efferent) nerves carry impulses away from the CNS to effectors. The peripheral system is divided by function into the somatic and autonomic divisions.
The somatic nervous system
The somatic division controls voluntary, conscious actions, mainly the contraction of skeletal muscle, such as deciding to pick up a pen. It also carries the sensory information that lets you feel touch, pain and temperature. The somatic reflexes (like the withdrawal reflex) are automatic but still act through somatic motor neurons to skeletal muscle.
The autonomic nervous system
The autonomic division controls involuntary, automatic functions, the ones you do not consciously think about, such as heart rate, breathing rate, digestion, and gland secretion. Because it acts on smooth muscle, cardiac muscle and glands without conscious effort, the autonomic nervous system is central to homeostasis. It has two branches that usually work in opposition.
The sympathetic branch prepares the body for action, the fight or flight response. It increases heart rate and breathing rate, dilates the pupils, diverts blood to skeletal muscles and releases glucose for energy, while slowing digestion. It dominates during stress, danger or exercise.
The parasympathetic branch restores the body to a calm, energy-conserving state, the rest and digest response. It slows heart rate and breathing, constricts the pupils, and promotes digestion. It dominates during rest.
How this maps to the exam
Expect questions that ask you to draw or complete a flow chart of the nervous system divisions, classify a named function as somatic or autonomic, or describe and contrast the sympathetic and parasympathetic effects on a given organ. A favourite is to give a scenario (exercise, fright, eating a meal) and ask which branch dominates and what it does. Always anchor your answer in homeostasis where the question allows.