The nervous system, monosynaptic and polysynaptic reflexes, the brain regions of voluntary movement, and the limbic system and prefrontal cortex in emotion (QCE Psychology Unit 3)

What this dot point is asking

Unit 3 Topic 1 (Brain function) in the 2025 syllabus opens with four dot points, all examinable in the external assessment:

1. describe the structure of the nervous system: the central (brain and spinal cord) and peripheral (somatic and autonomic) nervous systems;
2. describe the role of the spinal cord, with reference to monosynaptic and polysynaptic spinal reflexes;
3. explain the interaction of the primary motor cortex, cerebellum and basal ganglia in coordinating voluntary movement;
4. explain the importance of the limbic system and the prefrontal cortex for the experience of emotion.

Localisation of function and language are on our localisation page, and neurotransmission is on our neurotransmission page.

The answer

Structure of the nervous system

• Central nervous system (CNS): the brain and the spinal cord. It receives and processes information and initiates responses.
• Peripheral nervous system (PNS): all the nerves outside the CNS, which carry information between the CNS and the rest of the body. It has two divisions:
  • the somatic nervous system, which carries sensory information from receptors to the CNS and motor commands from the CNS to skeletal muscles, allowing voluntary movement;
  • the autonomic nervous system, which controls involuntary functions of internal organs and glands (heart rate, breathing, digestion). Its sympathetic division arouses the body (fight or flight), and its parasympathetic division calms it and conserves energy.

The spinal cord and spinal reflexes

The spinal cord has two roles. It is the main pathway between the brain and the body: sensory information travels up it and motor commands travel down it. It can also produce spinal reflexes: fast, automatic, involuntary responses to a stimulus that are organised within the spinal cord, without waiting for the brain.

• Monosynaptic reflex: there is a single synapse, directly between the sensory neuron and the motor neuron. The knee-jerk (patellar) reflex is the classic example: a tap below the kneecap stretches the thigh muscle, the sensory neuron signals the spinal cord, and the motor neuron immediately contracts the muscle, kicking the leg forward. With only one synapse it is extremely fast.
• Polysynaptic reflex: one or more interneurons sit between the sensory and motor neurons, so there are several synapses. The withdrawal reflex, pulling your hand away from something hot, is polysynaptic. Interneurons allow a more coordinated response involving several muscles (and, when you step on something sharp, adjusting the other leg to keep balance).

In both, the brain is informed after the response has started, which is why you feel the pain a moment after pulling your hand away. The speed protects the body from harm.

Voluntary movement: primary motor cortex, basal ganglia and cerebellum

Voluntary movement depends on several regions working together in loops:

• Primary motor cortex (at the back of the frontal lobe): sends the commands that travel down the spinal cord to the skeletal muscles. Different parts control different body areas, and each hemisphere controls the opposite side of the body.
• Basal ganglia (deep structures in each hemisphere): help select and initiate the intended movement and suppress unwanted ones, and help regulate how forceful a movement is. They depend on dopamine from the substantia nigra, which is why their dysfunction in Parkinson's disease makes movements hard to start.
• Cerebellum (at the back of the brain, below the cerebral hemispheres): coordinates the timing, precision and smoothness of movement and maintains balance and posture. It compares the intended movement with sensory feedback about the actual movement and corrects errors, and it is central to learning motor skills.

The interaction: the basal ganglia help choose and start the movement, the primary motor cortex executes it, and the cerebellum fine-tunes it in real time, with information passing back and forth between them. Damage to any one produces a characteristic problem: weakness or paralysis on the opposite side (motor cortex), difficulty starting or stopping movements (basal ganglia), or clumsy, poorly timed movement and poor balance (cerebellum).

Emotion: the limbic system and the prefrontal cortex

The limbic system is a group of interconnected structures deep in the brain that is central to emotion, motivation and memory:

• the amygdala rapidly detects emotionally significant stimuli, especially threats, generates fear, and is involved in emotional learning;
• the hypothalamus produces the physiological side of emotion by activating the autonomic nervous system and the release of hormones;
• the hippocampus links emotional experiences with memory and context.

The prefrontal cortex (the front of the frontal lobe) is important for the conscious experience, interpretation and regulation of emotion. It appraises the situation, weighs up consequences and can dampen the amygdala's response, allowing emotional reactions to be controlled and expressed appropriately. Damage to the prefrontal cortex is associated with poor emotional control and changes in personality; the classic historical case of Phineas Gage, whose frontal lobe was damaged by an iron rod in 1848 and whose personality reportedly changed markedly afterwards, is often used to illustrate this.

Emotion therefore involves a fast, automatic limbic response and a slower, reflective prefrontal response working together.