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2.2Unit 2 — Multicellular Organisms

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

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Section 1 of 2The 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:
    • Cerebrum: Controls conscious thought, intelligence, reasoning, memory, personality, and conscious senses.
    • Cerebellum: Coordinates muscular movement, balance, and posture.
    • Medulla: Controls involuntary vital functions including heart rate and breathing rate.
  • Types of neurons:
    1. Sensory neuron: Transmits electrical nerve impulses from sense organ receptors to the CNS.
    2. Inter neuron: Located inside the CNS; processes information and connects sensory neurons to motor neurons.
    3. Motor neuron: Transmits 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)

Section 2 of 2Hormonal control and blood glucose regulation

  • Endocrine glands release chemical messengers called hormones directly into the bloodstream.
  • 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):
    ConditionPancreas ResponseHormone ReleasedAction on LiverEffect on Blood Glucose
    Blood glucose rises (after eating)Detects high glucoseInsulinConverts excess soluble glucose into insoluble storage carbohydrate glycogenBlood glucose level falls back to normal
    Blood glucose drops (exercise / fasting)Detects low glucoseGlucagonBreaks stored glycogen back down into soluble glucose, releasing it into bloodBlood glucose level rises back to normal

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