Unit 3 Human Perspectives: Nerve Impulse Transmission and Coordination

Science Inquiry Skills and Investigation Guidelines

• Identify, Research, and Construct: Science inquiry begins with identifying and researching questions for investigation, proposing hypotheses, and predicting possible outcomes.
• Design and Procedure: Investigations must include detailed procedures, required materials, and the type and amount of data (primary and/or secondary) to be collected.
• Ethics and Safety: Researchers must conduct risk assessments and consider research ethics, specifically including animal ethics.
• Data Collection and Analysis: Investigations related to homeostasis and disease transmission models must be conducted safely and methodically to ensure valid and reliable data. Data representation involves using mean, median, range, and probability.
• Uncertainty and Limitations: Scientific discussion must address how measurement error, instrumental accuracy, the nature of the procedure, and the sample size influence data uncertainty.
• Evaluation and Communication: Scientific texts and models must be interpreted and evaluated based on evidence quality. Findings should be communicated to specific audiences using appropriate nomenclature, diagrams, flow charts, and mathematical representations.

Structure and Function of Nerve Cells (Neurons)

• The Nervous System: Along with the endocrine system, the nervous system coordinates voluntary and involuntary actions. It receives and processes information from sense organs to trigger appropriate responses.
• Basic Unit: The neuron (nerve cell) is the basic structural and functional unit of the nervous system, specialized for rapid communication.
• Cell Body: The part of the neuron containing the nucleus and organelles (mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus). It controls cell functioning.
• Dendrites: Short extensions of the cell body's cytoplasm. They are usually highly branched and carry nerve impulses into the cell body.
• Axon: A single, long extension of cytoplasm that carries nerve impulses away from the cell body. Axons vary in length from a few millimetres in the brain to approximately $1\,m$ from the spinal cord to the foot.
• Axon Terminals: The small branches at the end of an axon that terminate at the synapse.
• Myelin Sheath: A layer of fatty material covering most axons.
  • Schwann Cells: Peripheral nervous system cells that wrap around the axon to form the myelin sheath.
  • Oligodendrocytes: Cells that produce myelin in the brain and spinal cord.
  • Neurilemma: The outermost coil of a Schwann cell, assisting in the repair of injured nerve fibres.
  • Nodes of Ranvier: Gaps in the myelin sheath occurring at intervals along the axon.
• Functions of Myelin:
  • Acts as an electrical insulator.
  • Protects the axon from physical damage.
  • Speeds up the movement of nerve impulses along the axon.
• Grey vs. White Matter:
  • White Matter: Areas containing myelinated fibres.
  • Grey Matter: Areas consisting of cell bodies and unmyelinated fibres.

Classification of Neurons

• Functional Classification:
  • Sensory (Afferent/Receptor) Neurons: Carry messages from sense organs or skin to the central nervous system (CNS).
  • Motor (Efferent/Effector) Neurons: Carry messages from the CNS to effectors (muscles and glands).
  • Interneurons (Association/Connector/Relay Neurons): Located within the CNS and serve as a link between sensory and motor neurons.
• Structural Classification:
  • Multipolar Neurons: One axon and multiple dendrites (most common; includes motor neurons and interneurons).
  • Bipolar Neurons: One axon and one dendrite (found in the eye, ear, and nose).
  • Unipolar Neurons: Just one extension (an axon); found in insects, not humans.
  • Pseudounipolar Neurons: A single axon that divides into two extensions; the cell body lies to one side. Most human sensory neurons are pseudounipolar.

Anatomy of a Nerve

• Nerve Fibre: Any long extension of a neuron (usually refers to the axon).
• Nerve: A bundle of nerve fibres held together by connective tissue outside the brain and spinal cord.
• Grouping: Nerve fibres are organized into bundles, and multiple bundles join to form a single nerve.

The Nature of Nerve Impulses and Membrane Potential

• Nerve Impulse: An electrochemical change (electrical voltage change brought about by chemical changes) traveling along a nerve fibre.
• Potential Difference: The electrical force/voltage measured in volts ($V$) or millivolts ($mV$). $1000\,mV = 1\,V$.
• Distribution of Ions:
  • Extracellular Fluid: High concentration of sodium ($Na^+$) and chloride ($Cl^-$) ions.
  • Intracellular Fluid: Low concentration of $Na^+$; high concentration of potassium ($K^+$) and large negative organic ions.
• Resting Membrane Potential: The potential of unstimulated nerve cells is approximately $-70\,mV$, meaning the inside is more negative than the outside. The membrane is considered polarised.
• Maintenance of Resting Potential:
  • Permeability: The membrane is $10 \times$ more permeable to $K^+$ than $Na^+$ due to leakage channels.
  • Sodium-Potassium Pump: Uses active transport (ATP) to move $3\,Na^+$ out of the cell for every $2\,K^+$ moved in.
  • Organic Ions: Large negative ions inside the cell cannot diffuse out.

Action Potentials and Transmission

• Action Potential Definition: Rapid depolarisation and repolarisation of the membrane lasting approximately $1\,ms$.
• Steps of an Action Potential:
  1. Depolarisation: Stimulation causes some $Na^+$ channels to open. If the potential increases by $15\,mV$ to the threshold of $-55\,mV$, voltage-gated $Na^+$ channels open. $Na^+$ floods in, making the inside positive ($+40\,mV$).
  2. Repolarisation: $Na^+$ channels close; voltage-gated $K^+$ channels open. $K^+$ flows out, making the inside negative again.
  3. Hyperpolarisation: $K^+$ channels stay open longer than needed, dropping potential below $-70\,mV$.
• Refractory Period: A brief interval from the threshold $-55\,mV$ until returning to $-70\,mV$ where the membrane cannot be stimulated again.
• Conduction Types:
  • Unmyelinated Fibres: Depolarisation in one area triggers the next, traveling away from the stimulus point at a maximum speed of $2\,m/s$.
  • Myelinated Fibres (Saltatory Conduction): The impulse jumps from node to node because myelin acts as an insulator. Speed can reach up to $140\,m/s$.
• All-or-None Response: A stimulus either reaches the threshold to trigger an impulse of constant magnitude, or it does not. The impulse does not weaken with distance.
• Determining Stimulus Strength: The CNS distinguishes intensity via:
  1. The number of nerve fibres depolarised.
  2. The frequency of impulses (impulses per unit of time).

Synaptic Transmission

• Synapse: The small gap between the axon terminal of one neuron and the dendrite/cell body of another.
• Transmission Process:
  1. Nerve impulse reaches the axon terminal and opens voltage-gated calcium ($Ca^{2+}$) channels.
  2. $Ca^{2+}$ enters the pre-synaptic terminal from the extracellular fluid.
  3. Synaptic vesicles fuse with the membrane, releasing neurotransmitters via exocytosis.
  4. Neuromitters diffuse across the synaptic cleft and bind to receptors on the post-synaptic membrane.
  5. Ligand-gated protein channels open, sodium ions enter, and a new action potential is initiated.
• Removal: Neurotransmitters are removed via reabsorption, enzymatic degradation, or diffusion.
• Examples: Acetylcholine, adrenaline, dopamine, histamine.
• Neuromuscular Junction: A synapse between an axon and a skeletal muscle cell.

Sensory Receptors

• Thermoreceptors: Respond to heat and cold. Peripheral thermoreceptors are in the skin; central thermoreceptors are in the hypothalamus (monitoring core blood temperature).
• Osmoreceptors: Located in the hypothalamus; sensitive to osmotic pressure and blood plasma concentration.
• Chemoreceptors: Sensitive to chemicals (odours, tastes) and internal levels of the pH, oxygen ($O_2$), and carbon dioxide ($CO_2$) in blood vessels.
• Touch Receptors (Mechanoreceptors): Located in skin. Some are close to the surface for light touch (lips, fingertips); others are deep for pressure and vibration.
• Pain Receptors (Nociceptors): Stimulated by tissue damage, poor blood flow, or excessive heat/chemicals. They adapt very little or not at all to ensure awareness of damage.

Reflexes and the Reflex Arc

• Reflex Properties:
  1. Requires a stimulus.
  2. Involuntary.
  3. Rapid.
  4. Stereotyped (same response every time).
• Spinal Reflex: Coordinated by the spinal cord without brain involvement at the moment of response.
• Reflex Arc Components:
  1. Receptor: Reacts to stimulus and initiates impulse.
  2. Sensory Neuron: Carries impulse to the CNS.
  3. Interneurons/Synapse: Processes and directs impulse (may involve one or more synapses).
  4. Motor Neuron: Carries impulse to the effector.
  5. Effector: Muscle or gland that carries out the response.
• Innate vs. Acquired: Innate reflexes (blinking, suckling) are genetic; acquired reflexes (riding a bike, braking a car) are learned through repetition.

Comparison of Nervous and Endocrine Systems

Questions & Discussion

• Otto Loewi's Experiment (1921): Loewi proved chemical transmission by stimulating the vagus nerve of a frog's heart (Heart A), causing it to slow down. He transferred the surrounding salt solution to another heart (Heart B), which then also slowed down, proving a chemical (originally called "vagusstoff," now known as acetylcholine) was released.
• Chemical Effects on Transmission:
  • Stimulants: Caffeine and benzedrine stimulate transmission at the synapse.
  • Depressants: Anaesthetics and hypnotics depress transmission.
  • Nerve Agents: Organophosphates (found in nerve gases and insecticides) cause acetylcholine buildup at neuromuscular junctions, leading to continuous muscle contraction and respiratory failure.
• Daphnia Heart Rate Study: Using the translucent "water flea" Daphnia, researchers can observe how chemicals like ethanol (depressant), caffeine, and nicotine (stimulants) affect metabolic rates and heartbeats in real-time.
• Clinical Significance: Doctors test reflexes like the patellar (knee jerk) or Achilles (ankle jerk) to identify nervous system impairment or damage to the spinal cord.