Nervous System and Brain

What you'll learn

This revision guide covers the nervous system's role in coordination and control, focusing on how the body detects stimuli and produces rapid responses. You'll learn about the structure and function of neurones, how electrical impulses transmit information, and the organisation of the nervous system including the brain and spinal cord. This topic is essential for understanding homeostasis and response in living organisms.

Key terms and definitions

Stimulus — a change in the environment (internal or external) that is detected by receptors and may lead to a response.

Receptor — a specialised cell or group of cells that detects a specific stimulus and converts it into an electrical impulse.

Effector — a muscle or gland that carries out a response to a stimulus.

Neurone — a specialised nerve cell that transmits electrical impulses rapidly around the body.

Synapse — the microscopic gap between two neurones where chemical transmission of impulses occurs.

Reflex arc — the pathway taken by nerve impulses in a reflex action, involving receptor, sensory neurone, relay neurone, motor neurone and effector.

Central nervous system (CNS) — the brain and spinal cord, which coordinate and control responses to stimuli.

Motor neurone — a neurone that carries electrical impulses from the CNS to effectors (muscles or glands).

Core concepts

Structure and function of the nervous system

The nervous system enables organisms to detect changes and respond appropriately. It consists of two main parts:

The central nervous system (CNS):

• Brain — coordinates complex behaviours and processes information from receptors
• Spinal cord — connects the brain to the rest of the body and coordinates simple reflex actions

The peripheral nervous system:

• Sensory neurones — carry impulses from receptors to the CNS
• Motor neurones — carry impulses from the CNS to effectors
• All nerves outside the brain and spinal cord

The basic pathway for nervous coordination follows this sequence:

Stimulus → Receptor → Coordinator (CNS) → Effector → Response

This allows rapid responses to environmental changes, essential for survival.

Types of receptors

Different receptors detect different types of stimuli. You must know the main receptor types and their locations:

Light receptors (photoreceptors):

• Located in the retina of the eye
• Detect light intensity and colour
• Enable vision

Sound receptors (mechanoreceptors):

• Located in the cochlea of the inner ear
• Detect vibrations and sound waves
• Enable hearing

Chemical receptors (chemoreceptors):

• Taste receptors on the tongue detect chemicals in food
• Smell receptors in the nose detect airborne chemicals
• Enable taste and smell

Touch, pressure and temperature receptors:

• Located in the skin
• Detect mechanical pressure, temperature changes and pain
• Enable touch sensation and temperature regulation

Position receptors:

• Located in muscles, tendons and the inner ear
• Detect body position and balance
• Enable coordination and balance

Structure and function of neurones

Neurones are specialised cells adapted for rapid transmission of electrical impulses. There are three types you must know:

Sensory neurones:

• Carry impulses from receptors to the CNS
• Cell body located in the middle of the axon
• Long dendron carries impulses from receptors
• Shorter axon carries impulses to CNS

Relay neurones (also called connector or intermediate neurones):

• Found within the CNS (brain and spinal cord)
• Connect sensory and motor neurones
• Have many short dendrites and a short axon
• Enable complex processing of information

Motor neurones:

• Carry impulses from CNS to effectors
• Cell body at one end with dendrites extending from it
• Long axon carries impulses to effector
• Axon ends at effector organ (muscle or gland)

Adaptations of neurones:

• Long axons/dendrons — transmit impulses over long distances
• Branched dendrites — connect with many other neurones
• Myelin sheath — insulating fatty layer that speeds up impulse transmission (not required for all exam boards but useful to know)
• Many mitochondria in cell body — provide energy for impulse transmission

How synapses work

Synapses are junctions between neurones where chemical transmission occurs. Understanding synapse function is essential for GCSE:

Structure:

• Microscopic gap (20-30 nanometres) between neurones
• Pre-synaptic neurone (before the synapse)
• Post-synaptic neurone (after the synapse)
• Synaptic vesicles containing neurotransmitter chemicals

Process of transmission across a synapse:

1. Electrical impulse arrives at the end of the pre-synaptic neurone
2. This triggers release of neurotransmitter chemicals from vesicles
3. Neurotransmitters diffuse across the synaptic gap
4. Neurotransmitters bind to receptor molecules on the post-synaptic membrane
5. This triggers a new electrical impulse in the post-synaptic neurone
6. Neurotransmitters are then broken down or reabsorbed to prevent continuous stimulation

Why synapses are important:

• Ensure impulses travel in one direction only (neurotransmitters only released from pre-synaptic side)
• Allow one neurone to connect with many others, enabling complex neural pathways
• Enable filtering of low-level stimuli (multiple weak signals needed to trigger response)

Reflex actions and the reflex arc

Reflexes are rapid, automatic responses that don't involve conscious thought. They protect the body from harm.

Characteristics of reflex actions:

• Rapid — occur in milliseconds
• Automatic — don't require conscious decision-making
• Protective — prevent injury or damage
• Involuntary — cannot be consciously controlled

The reflex arc pathway:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

Example: Hand withdrawal reflex (touching something hot)

1. Heat stimulus detected by temperature receptors in skin
2. Sensory neurone carries impulse to spinal cord
3. Relay neurone in spinal cord passes impulse to motor neurone
4. Motor neurone carries impulse to arm muscles (effector)
5. Muscles contract, withdrawing hand from heat source

The reflex arc bypasses the brain initially, allowing faster responses. Information is sent to the brain afterwards so you become aware of what happened.

Other examples of reflex actions:

• Pupil reflex — pupils constrict in bright light, dilate in dim light
• Knee-jerk reflex — leg kicks forward when tendon below kneecap is tapped
• Blinking reflex — eyelids close when object approaches eye

Structure and function of the brain

The brain is a complex organ that controls and coordinates body functions. You need to know the main regions and their functions:

Cerebrum (cerebral hemispheres):

• Largest part of the brain
• Divided into two hemispheres
• Controls consciousness, intelligence, memory and language
• Processes information from senses
• Initiates voluntary movement

Cerebellum:

• Located at the back of the brain, below the cerebrum
• Coordinates muscle movement and balance
• Controls posture and fine motor skills
• Processes information from muscles and the inner ear

Medulla oblongata:

• Located at base of brain, connects to spinal cord
• Controls unconscious activities (autonomic functions)
• Regulates breathing rate, heart rate and blood pressure
• Controls reflexes like swallowing and coughing

Hypothalamus:

• Small region below the cerebrum
• Monitors and controls body temperature
• Links nervous and endocrine systems
• Controls water balance and appetite

Studying brain function:

Scientists investigate brain function through various methods:

• MRI and CT scans — create detailed images showing brain structure and identify damaged areas
• PET scans — show which brain areas are active during specific tasks by tracking blood flow
• Studying patients with brain damage — observe which functions are affected when specific areas are damaged
• Electrical stimulation — stimulate brain regions during surgery to map their functions

Treating brain damage is difficult because:

• The brain is extremely complex with billions of interconnected neurones
• Different areas control different functions
• Damage is often permanent as neurones cannot be easily replaced
• Access to the brain requires delicate surgery with high risks

Worked examples

Example 1: Describing neurone structure (3 marks)

Question: Describe how motor neurones are adapted for their function.

Answer:

• Motor neurones have a long axon to carry impulses over long distances from the CNS to effectors (1 mark)
• They have many dendrites to receive impulses from many relay neurones in the CNS (1 mark)
• The cell body contains many mitochondria to provide energy for impulse transmission (1 mark)

Examiner note: Each adaptation must be linked to its function to gain the mark. Simply listing structures without explaining their purpose would not earn full marks.

Example 2: Explaining synapse function (5 marks)

Question: Explain how an electrical impulse is transmitted across a synapse.

Answer:

• An electrical impulse arrives at the end of the pre-synaptic neurone (1 mark)
• This causes vesicles to release neurotransmitter chemicals (1 mark)
• Neurotransmitters diffuse across the synaptic gap (1 mark)
• Neurotransmitters bind to receptor molecules on the post-synaptic membrane (1 mark)
• This triggers a new electrical impulse in the post-synaptic neurone (1 mark)

Examiner note: The sequence of events must be in the correct order. Using precise terminology like "pre-synaptic" and "post-synaptic" demonstrates good understanding.

Example 3: Describing a reflex arc (6 marks)

Question: A person accidentally touches a sharp pin. Describe the reflex arc that causes them to pull their finger away.

Answer:

• Pain receptors in the skin detect the sharp stimulus (1 mark)
• Sensory neurone carries impulse from finger to the spinal cord/CNS (1 mark)
• Relay neurone in the spinal cord connects sensory and motor neurones (1 mark)
• Motor neurone carries impulse from spinal cord to arm muscle (1 mark)
• Muscle (effector) contracts (1 mark)
• Hand/finger is withdrawn from the pin (1 mark)

Examiner note: You must follow the complete pathway and name all components. Stating "neurone" without specifying sensory/relay/motor would lose marks.

Common mistakes and how to avoid them

• Confusing sensory and motor neurones — Remember: sensory neurones carry impulses TO the CNS (think "sensory sends to CNS"), while motor neurones carry impulses FROM the CNS to muscles/glands.

• Thinking electrical impulses cross synapses — Transmission across synapses is always chemical. Electrical impulses trigger neurotransmitter release, but do not jump the gap themselves.

• Mixing up cerebrum and cerebellum — The cerebrum is the large thinking/conscious part; the cerebellum is smaller and controls coordination and balance. Remember: cerebellum = balance.

• Forgetting that relay neurones are only in the CNS — Sensory and motor neurones extend outside the CNS, but relay neurones are only found in the brain and spinal cord.

• Not distinguishing between receptor and effector — Receptors detect stimuli (input), effectors produce responses (output). Never call a muscle a receptor or an eye an effector.

• Stating the brain controls reflexes — Simple reflexes are coordinated by the spinal cord and bypass the brain initially for speed, though the brain receives information afterwards.

Exam technique for "Nervous System and Brain"

• "Describe" questions require you to state what happens in sequence. For reflex arcs, work through each component methodically: receptor → sensory neurone → relay neurone → motor neurone → effector → response. Each step typically earns one mark.

• "Explain" questions need you to give reasons why something happens or how structures relate to function. Link each feature to its purpose: "The motor neurone has a long axon so that impulses can travel long distances to effectors."

• Diagrams of reflex arcs — Always label with arrows showing direction of impulse travel. Include all five components (receptor, sensory, relay, motor, effector) and specify the example (e.g., "pain receptor in skin" not just "receptor").

• Extended response questions on nervous system coordination may require 6-mark answers. Structure your response logically: stimulus detection → impulse transmission → coordination → response. Use technical terminology (synapse, neurotransmitter, CNS) and link structure to function throughout.

Quick revision summary

The nervous system enables rapid responses to stimuli through electrical impulses carried by neurones. Receptors detect specific stimuli and generate impulses transmitted by sensory neurones to the CNS. The brain and spinal cord process information and coordinate responses. Motor neurones carry impulses to effectors (muscles or glands) which produce responses. Synapses enable chemical transmission between neurones, ensuring one-way impulse flow. Reflex arcs provide rapid, automatic protective responses coordinated by the spinal cord, bypassing conscious thought for speed.