The human nervous system: central and peripheral nervous systems

What you'll learn

This revision guide covers the structure and function of the human nervous system, focusing on the central nervous system (CNS) and peripheral nervous system (PNS). You will learn how these systems coordinate responses to stimuli, the role of different neurons, and how reflex actions protect the body. This topic is essential for understanding coordination and response in humans, a core component of the CSEC Human and Social Biology syllabus.

Key terms and definitions

Nervous system — the body's communication network consisting of specialised cells that transmit electrical impulses to coordinate body functions and responses to stimuli.

Central nervous system (CNS) — the brain and spinal cord; the body's processing centre that receives information, interprets it, and coordinates appropriate responses.

Peripheral nervous system (PNS) — all the nerves outside the CNS that connect the brain and spinal cord to the rest of the body, carrying information to and from the CNS.

Neuron — a specialised nerve cell that transmits electrical impulses (nerve impulses) throughout the body.

Synapse — the microscopic gap between two neurons where chemical transmission of nerve impulses occurs using neurotransmitters.

Reflex action — a rapid, automatic, involuntary response to a stimulus that does not involve conscious thought, providing protection from harm.

Reflex arc — the pathway taken by nerve impulses during a reflex action, from receptor to effector.

Effector — a muscle or gland that responds to nerve impulses by producing a response such as movement or secretion.

Core concepts

Structure of the nervous system

The human nervous system divides into two main parts:

Central Nervous System (CNS):

• Brain: the control centre located in the skull, responsible for conscious thought, memory, emotions, and interpreting sensory information
• Spinal cord: a cylindrical bundle of nerve tissue extending from the brain through the vertebral column, coordinating reflexes and transmitting impulses between the brain and body

Peripheral Nervous System (PNS):

• Sensory neurons: carry impulses from receptors (sense organs) to the CNS
• Motor neurons: carry impulses from the CNS to effectors (muscles and glands)
• Connects all parts of the body to the CNS through a network of nerves

The PNS further subdivides into:

• Somatic nervous system: controls voluntary movements of skeletal muscles
• Autonomic nervous system: controls involuntary actions such as heart rate, digestion, and breathing

Types of neurons and their functions

Three types of neurons work together to transmit information:

Sensory neurons:

• Carry impulses from receptors to the CNS
• Cell body located in the middle of the neuron
• Long dendrites receive information from receptors
• Found in sense organs like the eye, ear, skin, tongue, and nose
• Example: sensory neurons in the skin detect heat when touching a hot pot handle during cooking

Motor neurons:

• Carry impulses from the CNS to effectors
• Cell body located at one end near the CNS
• Long axon extends to muscles or glands
• Stimulate muscle contraction or gland secretion
• Example: motor neurons stimulate leg muscles to contract when jumping to catch a cricket ball

Relay neurons (intermediate neurons):

• Found only within the CNS (brain and spinal cord)
• Connect sensory neurons to motor neurons
• Process information and coordinate appropriate responses
• Multiple short dendrites and a short axon
• Enable complex processing in the brain

Structure of a typical neuron

All neurons share common structural features:

• Cell body: contains the nucleus and cytoplasm with organelles
• Dendrites: short branched extensions that receive impulses from other neurons or receptors
• Axon: a long fiber that transmits impulses away from the cell body
• Myelin sheath: a fatty insulating layer around some axons that speeds up impulse transmission
• Nerve endings: branched terminals at the axon end that connect to other neurons or effectors

The myelin sheath is crucial for rapid impulse transmission. In diseases like multiple sclerosis, damage to the myelin sheath slows nerve transmission, causing coordination problems.

How nerve impulses are transmitted

Nerve impulses travel as electrical signals along neurons:

Along a single neuron:

• A stimulus triggers a change in electrical charge at the receptor
• This creates an action potential (electrical impulse) that travels along the neuron
• The impulse moves in one direction: dendrite → cell body → axon → nerve endings
• Speed of transmission depends on the presence of myelin sheath and axon diameter

Across synapses:

• Electrical impulses cannot jump across the synaptic gap
• When an impulse reaches the nerve ending, it triggers release of chemical neurotransmitters
• Neurotransmitters diffuse across the synapse
• They bind to receptors on the next neuron's dendrite
• This stimulates a new electrical impulse in the next neuron
• Neurotransmitters are then broken down to prevent continuous stimulation

This chemical transmission at synapses ensures impulses travel in one direction only and allows the nervous system to control which signals are transmitted.

Reflex actions and reflex arcs

Reflex actions protect the body from harm through rapid, automatic responses. Common examples include:

• Withdrawing your hand from a hot surface
• Blinking when an object approaches the eye
• The knee-jerk reflex when the patellar tendon is tapped
• Sneezing when nasal passages are irritated by pepper dust

A Caribbean student playing football might experience the pupil reflex when suddenly moving from shaded areas under trees into bright tropical sunlight—pupils automatically constrict to protect the retina.

The reflex arc pathway:

1. Stimulus — a change in the environment (e.g., touching a hot surface)
2. Receptor — detects the stimulus (e.g., heat receptors in skin)
3. Sensory neuron — carries impulse to the spinal cord
4. Relay neuron — processes the signal in the spinal cord's grey matter
5. Motor neuron — carries impulse from spinal cord to effector
6. Effector — responds (e.g., arm muscles contract to withdraw hand)
7. Response — the action taken (e.g., hand moves away from heat)

Key features of reflex actions:

• Rapid: bypass the brain for speed, processed in the spinal cord
• Automatic: occur without conscious thought
• Protective: prevent or minimize tissue damage
• Involuntary: cannot be consciously stopped once initiated

The brain receives information about reflex actions after they occur, which is why you feel pain slightly after withdrawing your hand from a hot surface.

Voluntary actions versus reflex actions

Understanding the difference is crucial for exam success:

Voluntary actions:

• Involve conscious decision-making in the brain
• Slower than reflexes
• Can be controlled and stopped
• Pathway: receptor → sensory neuron → brain → motor neuron → effector
• Example: deciding to pick up a mango from a tree

Reflex actions:

• Do not involve conscious thought
• Much faster response time
• Cannot be consciously prevented
• Pathway: receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector
• Example: automatically withdrawing hand from a thorn on a tree branch

Both voluntary and reflex actions may use the same receptors and effectors, but the processing pathway differs.

Worked examples

Example 1: Identifying components of a reflex arc

Question: A person accidentally steps on a sharp object with their bare foot and immediately lifts their foot off the ground.

(a) Name the stimulus in this situation. [1 mark]

(b) Identify the receptor involved. [1 mark]

(c) Explain why this response occurs before the person feels pain. [2 marks]

Mark scheme answers:

(a) The sharp object / pressure / pain [1 mark]

(b) Pain receptors / pressure receptors in the skin of the foot [1 mark]

(c) The response is a reflex action that is processed in the spinal cord [1 mark], not in the brain, so the foot is withdrawn before the impulse reaches the brain where pain is consciously felt [1 mark].

Example 2: Describing nerve impulse transmission

Question: Describe how a nerve impulse is transmitted from one neuron to another across a synapse. [4 marks]

Mark scheme answer:

When the electrical impulse reaches the end of the first neuron [1 mark], it triggers the release of chemical neurotransmitters [1 mark]. These chemicals diffuse across the synaptic gap [1 mark] and bind to receptors on the membrane of the next neuron, stimulating a new electrical impulse [1 mark].

Examiner note: This question uses the command word "describe," requiring a sequential account of the process. Four marking points means you need four distinct pieces of information.

Example 3: Comparing nervous system divisions

Question: A cricket player in Barbados sees the ball approaching (stimulus A) and decides to play a defensive stroke. As he moves, a bee stings his arm (stimulus B) and he immediately pulls his arm back.

(a) State which division of the nervous system processes stimulus A. [1 mark]

(b) Explain why the response to stimulus B is faster than the response to stimulus A. [3 marks]

Mark scheme answers:

(a) Central nervous system / CNS / brain [1 mark]

(b) Stimulus B produces a reflex action [1 mark] which is processed in the spinal cord rather than the brain [1 mark], so the pathway is shorter and the response is automatic without conscious thought [1 mark].

Common mistakes and how to avoid them

• Confusing neurons with nerves: A nerve is a bundle of many neurons held together by connective tissue. A single neuron is one nerve cell. Always use the correct term in your answers.

• Forgetting the relay neuron in reflex arcs: Many students only mention sensory and motor neurons. Remember that relay neurons in the spinal cord or brain connect these two and are essential for coordinating responses.

• Stating that reflexes involve the brain: Reflex actions are processed in the spinal cord. The brain receives information afterwards but does not control the reflex response. This is why reflexes are so rapid.

• Confusing electrical and chemical transmission: Impulses travel electrically along neurons but chemically across synapses. Be specific about which type of transmission you're describing.

• Mixing up CNS and PNS components: The brain and spinal cord form the CNS only. All other nerves throughout the body belong to the PNS. Sensory and motor neurons are part of the PNS, not the CNS.

• Not identifying specific receptors: Saying "sense organs" or "receptors" is too vague. Specify the type: pain receptors, heat receptors, light receptors (in the eye), etc., depending on the stimulus mentioned in the question.

Exam technique for "The human nervous system: central and peripheral nervous systems"

• Learn the reflex arc sequence thoroughly: Many exam questions ask you to describe or explain reflex actions. Practice writing the seven-step sequence (stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector → response) until you can recall it perfectly.

• Use correct anatomical terminology: "Nerve impulse" not "signal"; "synapse" not "gap"; "neurotransmitter" not "chemical." Examiners expect precise scientific vocabulary at CSEC level, and using correct terms secures marks.

• For "explain" questions, give reasons: Don't just describe what happens—say why it happens. For example, if asked to explain why reflexes are rapid, mention that they bypass the brain and involve shorter pathways through the spinal cord.

• Draw and label diagrams carefully: When asked to draw a reflex arc or neuron structure, use a ruler for straight lines, add clear labels with label lines, and ensure all required components are shown. Diagrams typically carry 3-4 marks, so accurate detail matters.

Quick revision summary

The nervous system coordinates body responses through the CNS (brain and spinal cord) and PNS (all other nerves). Three neuron types—sensory, relay, and motor—transmit electrical impulses along their length and use chemical neurotransmitters to cross synapses. Reflex actions provide rapid, automatic, protective responses processed in the spinal cord without conscious thought. The reflex arc pathway involves seven steps from stimulus to response. Understanding the differences between voluntary actions (processed in brain) and reflex actions (processed in spinal cord) is essential for exam success.