Organisation of the nervous system: WACE Year 12 Human Biology Unit 3
“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”
The nervous system is divided into the central nervous system (brain and spinal cord), which processes and coordinates, and the peripheral nervous system (all the nerves outside it), which carries messages in and out. The peripheral system splits into the somatic (voluntary) division and the autonomic (involuntary) division, and the autonomic into the antagonistic sympathetic (fight or flight) and parasympathetic (rest and digest) branches.
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 central nervous system is the brain and spinal cord, where information is processed. The peripheral nervous system is the network of nerves outside the brain and spinal cord that carries information to and from the central nervous system. Together they form the complete nervous system.
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.
The two autonomic branches usually have opposite effects on the same organ, so the body can fine-tune a function by shifting the balance between them. For example, the sympathetic branch speeds the heart while the parasympathetic branch slows it. This antagonistic control is a fast way to keep variables such as heart rate within their tolerance range.
Why your heart races when startled
A sudden threat is detected and processed by the brain. The hypothalamus and brainstem activate the sympathetic branch of the autonomic nervous system. Sympathetic motor impulses (and the hormone adrenaline) act on the heart (cardiac muscle), increasing heart rate and the force of contraction, and on the airways, widening them. Blood is diverted to skeletal muscle and the liver releases glucose. This prepares the body to respond. Once the threat passes, the parasympathetic branch becomes dominant, slowing the heart and restoring digestion. A marker wants you to name the branch (sympathetic), state it is part of the autonomic nervous system, and describe the antagonistic recovery by the parasympathetic branch.
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.
A common error is listing the central, peripheral and autonomic systems as three equal divisions. The autonomic system is a subdivision of the peripheral nervous system, alongside the somatic system. State the hierarchy correctly: nervous system splits into central and peripheral; the peripheral splits into somatic and autonomic; the autonomic splits into sympathetic and parasympathetic. Also avoid saying the parasympathetic branch does nothing during activity; both branches are always active, and control comes from shifting the balance.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksDescribe the organisation of the human nervous system into its central and peripheral divisions, and explain the roles of the sympathetic and parasympathetic divisions in maintaining homeostasis.Show worked answer →
A 6 mark response needs the structural divisions plus the antagonistic autonomic roles.
- Central nervous system (CNS)
- The brain and spinal cord, which process information and coordinate responses.
- Peripheral nervous system (PNS)
- All nerves outside the CNS, carrying information to and from it. It divides into the somatic system (voluntary control of skeletal muscle) and the autonomic system (involuntary control of glands, cardiac and smooth muscle).
- Sympathetic division
- Prepares the body for activity or stress, the fight or flight response, for example by increasing heart rate and breathing rate and diverting blood to muscles.
- Parasympathetic division
- Calms the body and conserves energy, rest and digest, for example slowing heart rate and stimulating digestion.
- Role in homeostasis
- The two divisions are antagonistic, so by balancing their activity the autonomic system finely adjusts variables such as heart rate back toward their set points.
Markers reward the correct CNS and PNS structures plus the antagonistic sympathetic and parasympathetic effects.
WACE 2023-style3 marksDistinguish between the somatic and autonomic divisions of the peripheral nervous system, giving one example of an effector controlled by each.Show worked answer →
A 3 mark distinguish answer needs the contrast plus one effector each.
Somatic. Carries motor signals under voluntary, conscious control to skeletal muscle, for example contracting the biceps to lift an object.
Autonomic. Carries motor signals under involuntary, automatic control to glands, cardiac muscle and smooth muscle, for example changing the rate of the heart (cardiac muscle).
Markers reward the voluntary versus involuntary distinction and a correct effector for each.