2.2Unit 2 — Multicellular Organisms

Control and Communication — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 4 parts Lesson 5 of 39

📚 SQA National 5 Biology Hub / Topical Revision

0%read

Part 1 of 2The Central Nervous System (CNS) and neurons

How a nerve impulse travels along a neuron

Central nervous system and peripheral nervous system

Organisation of the human nervous system from body to neuron

Organisation of the human nervous system

Brain regions and functions (cerebrum, hypothalamus, pituitary, medulla)

Reflex arc: cat withdraws from painful stimulus (cactus)

Reflex arc in the knee: sensory, relay, motor neurone

Reflex arc: pin stimulus through spinal cord to muscle response

Flow chart of reflex pathway from stimulus to response

The Central Nervous System (CNS) and neurons

Components of CNS

The brain and spinal cord. Connected to body receptors and effectors by peripheral nerves.

Main regions of the brain

CerebrumControls conscious thought, intelligence, reasoning, memory, personality, and conscious senses.
CerebellumCoordinates muscular movement, balance, and posture.
MedullaControls involuntary vital functions including heart rate and breathing rate.

Types of neurons

Sensory neuronTransmits electrical nerve impulses from sense organ receptors to the CNS.
Inter neuron
- Located inside the CNS;
  - processes information and connects sensory neurons to motor neurons.
Motor neuronTransmits impulses from the CNS to an effector (muscle or gland).

Synapses

Microscopic gaps between adjacent neurons. Impulses cannot cross electrically; chemical neurotransmitters diffuse across the synapse to trigger an impulse in the next neuron.

Reflex arc

Rapid, automatic, involuntary response to a potentially harmful stimulus protecting the body from injury (e.g. pulling hand away from a hot surface):

Stimulus → Receptor → Sensory Neuron → Inter Neuron (Spinal Cord) → Motor Neuron → Effector (Muscle contracts)

Part 2 of 2Hormonal control and blood glucose regulation

Two-branch blood glucose regulation by insulin and glucagon

Blood glucose control: insulin and glycogen

Regulation of blood glucose by insulin and glucagon

Storage carbohydrates: glycogen, starch, cellulose

Blood Glucose Regulation — Insulin & Glucagon

Blood glucose control by insulin and glucagon feedback loops

Hormone action on target cells (pancreas, liver)

A synapse: vesicles, neurotransmitter, receptors

Comparison of Nervous and Endocrine Control — Simulation

Blood glucose control loop with insulin and glucagon

Hormone released into blood acts only on target cells with receptors

Hormone secreted by pancreatic cells acting on target cells with receptors

Skin structure including sweat glands, capillaries and sensory receptors

Hormonal control and blood glucose regulation

Endocrine glands

Endocrine glands release chemical messengers called hormones directly into the bloodstream.

Target tissues

Hormones travel via blood throughout the body but only affect specific target tissues that possess complementary receptor proteins on their cell membranes.

Regulation of blood glucose (Negative Feedback)

Condition	Pancreas Response	Hormone Released	Action on Liver	Effect on Blood Glucose
Blood glucose rises (after eating)	Detects high glucose	Insulin	Converts excess soluble glucose into insoluble storage carbohydrate glycogen	Blood glucose level falls back to normal
Blood glucose drops (exercise / fasting)	Detects low glucose	Glucagon	Breaks stored glycogen back down into soluble glucose, releasing it into blood	Blood glucose level rises back to normal

Key terms

Term	Meaning
Central nervous system	The brain and spinal cord, responsible for integrating and coordinating bodily responses.
Reflex arc	The neural pathway mediating an involuntary, instantaneous protective reflex action.
Synapse	The microscopic junction across which neurotransmitters diffuse between adjacent nerve cells.
Insulin	A pancreatic hormone stimulating the liver to convert blood glucose into glycogen.
Glucagon	A pancreatic hormone stimulating the liver to break down glycogen into glucose.
Glycogen	The insoluble storage carbohydrate stored in animal liver and muscle tissues.

Avoid these mistakes

Confusing glucagon and glycogen — glucagon is the hormone, glycogen is the storage carbohydrate. (Remember: "Gluca-GONE is when glucose is gone").

Thinking reflex actions go to the cerebrum — reflex signals pass through the spinal cord (inter neuron) so the response occurs before conscious thought.

Last updated: 29 October 2026