Human Nervous System – Lecture Review

Introduction to Human Responses and the Nervous System

  • The examination guidelines group this content under “Human responses to the environment.”
  • Core idea: Humans must be able to
    • Detect changes in their internal or external surroundings (stimuli).
    • React appropriately and rapidly.
    • Coordinate multiple body parts at once (e.g.
    • Example: seeing a ball thrown toward you ➔ bringing both hands together ➔ catching it).
  • None of the above would be possible without an intact, functioning nervous system.

Key Terminology

  • Stimulus
    • Any change in the environment that can potentially elicit a response.
    • Examples: light intensity, sound waves, temperature shifts, pressure on skin.
  • Impulse
    • A nerve‐carried message generated by a receptor and transmitted along neurons.
    • Mechanistically: a rapid sequence of electro-chemical events (depolarisation/repolarisation) travelling along a neurone’s membrane.
  • Key takeaway: A stimulus (external or internal change) is different from an impulse (the electrical message produced in response to that change).

Central Nervous System (CNS)

  • Consists of two main anatomical structures:
    • Brain
    • Spinal cord
  • Both structures are delicate and cannot regenerate once neurons are destroyed ➔ underscores the importance of their multiple protective layers.

Protective Structures of the CNS

  1. Bone
    • Brain: encased by the skull (cranium).
    • Spinal cord: runs through the vertebral column (ring‐shaped vertebrae).
  2. Meninges
    • Collective term for three connective-tissue membranes that envelop both the brain and spinal cord.
    • Act as shock absorbers; retain cerebrospinal fluid (CSF) between layers.
    • Individual names (dura mater, arachnoid mater, pia mater) are not required for this syllabus, but the umbrella term meninges is essential.
  3. Cerebrospinal Fluid (CSF)
    • Occupies spaces between meningeal layers and the brain/spinal cord surface.
    • Cushions against impact, supplies nutrients, removes metabolic waste, and maintains pressure homeostasis.

Major Regions of the Brain (Structures & Functions)

The syllabus centres on four key components and their roles.

1. Cerebrum

  • Largest, most folded, upper portion of the brain; divided into left and right hemispheres.
  • Functions:
    • Voluntary actions (skeletal‐muscle control): walking, writing, speaking.
    • Sensory reception & interpretation: receives impulses from eyes, ears, taste buds, olfactory receptors, skin, etc.; converts them into conscious sensations (seeing, hearing, tasting, feeling).
    • Higher thought processes: reasoning, decision-making, problem-solving, memory, imagination, language.

2. Cerebellum

  • Located posteriorly and inferiorly (back/bottom of the brain).
  • Mnemonic: Cerebellum ➔ Balance.
  • Functions:
    • Coordinates voluntary muscular movements to ensure smooth, precise actions.
    • Maintains equilibrium and posture by adjusting skeletal‐muscle tension.
    • Fine‐tunes motor activity (e.g.
    • Riding a bicycle.
    • Standing on one leg.)

3. Medulla Oblongata

  • Narrow, elongated region that forms the transition between brain and spinal cord.
  • Functions:
    • Pathway role: conducts impulses between spinal cord and higher brain centres.
    • Autonomic (involuntary) control centre for vital reflexes:
    • Heart rate regulation.
    • Ventilation/breathing rhythm.
    • Additional reflexes (swallowing, sneezing, coughing) referenced in earlier grades.

4. Corpus Callosum

  • Thick transverse bundle of nerve fibres situated deep in the centre of the brain.
  • Functions:
    • Physically connects left and right cerebral hemispheres.
    • Enables inter-hemispheric communication (transfer of impulses), ensuring coordinated, unified cognition and action (e.g.
    • Reading aloud: left = language centres, right = visual–spatial processing ➔ need to share data).

Spinal Cord Structure and Functions

  • On cross-section:
    • Outer region: white matter (myelinated axons conveying long-distance impulses).
    • Central, butterfly-shaped region: grey matter (nerve‐cell bodies and unmyelinated neurons).
  • Spinal nerves emerge laterally at each vertebral level.
    • Each spinal nerve splits near the cord into:
    • Dorsal (posterior) root: contains sensory neurons bringing impulses into the CNS.
    • Ventral (anterior) root: contains motor neurons carrying impulses away to effectors.

Core Functions of the Spinal Cord

  1. Bidirectional Conduction Pathway
    • Carries sensory information upward to the brain.
    • Carries motor commands downward from the brain to peripheral nerves.
  2. Reflex Centre
    • Mediates rapid, automatic responses (reflex arcs) without obligatory brain involvement, providing lightning-fast protection.
    • Example: Touching a hot plate ➔ withdrawal reflex executed by spinal cord circuitry before conscious pain perception is fully registered in the cerebrum.

Functional Pathways and Reflexes (Integrated View)

  • Stimulus (change) detected by receptor ➔ generates impulse ➔ travels via sensory neurone to CNS.
  • Depending on the situation:
    • Impulse may be processed immediately by the spinal cord for a reflex response, or
    • Routed up to higher brain centres for interpretation and voluntary decision-making.
  • Motor neurones carry instructions from CNS to effectors (muscles/glands) to produce a response.

Real-World Relevance & Connections

  • Catching a ball = interplay of sensory input (vision), cerebrum (decision), cerebellum (coordination), spinal cord (signal relay), and skeletal muscles (effectors).
  • Everyday involuntary survival tasks (breathing, heart beating) are orchestrated by the medulla oblongata — emphasising why injuries to this region are life-threatening.
  • Reflex arcs illustrate an evolutionary advantage: speed > conscious deliberation in certain emergencies.

Study & Exam Tips

  • Draw/label diagrams of the brain (mark cerebrum, cerebellum, medulla oblongata, corpus callosum) and spinal cord cross-section (white vs.
    grey matter, dorsal vs.
    ventral roots).
  • Create a comparison table: voluntary vs.
    involuntary control, CNS vs.
    peripheral nervous system, sensory vs.
    motor neurones.
  • Employ mnemonics:
    • CEREBellum = Balance.
    • MEDUlla = MEDical vital centres (heart, breathing).
  • Practise tracing stimulus ➔ impulse ➔ pathway ➔ response for both reflex and voluntary actions.
  • Revisit Grade 9 content (e.g.
    breathing regulation) to strengthen vertical learning connections.
  • Remember the protective hierarchy: Bone ➔ Meninges ➔ CSF.

Ethical / Philosophical Context (Brief)

  • While not explicit in the lecture, the inability of neurons to regenerate raises questions on:
    • Neuro-ethical issues in spinal surgeries or experimental treatments.
    • Importance of safety practices (helmets, seat-belts) in preserving nervous tissue.
  • Understanding reflexes vs.
    conscious decisions also feeds into debates on free will and automated behaviour.