Nervous coordination

Unit 6 Nervous coordination

Key unit competence: Examine the structure of neurons and explain the mechanisms of impulse transmission.

Key ideas

• Coordination links receptors and effectors so that the organism responds as a whole. The nervous system gives fast, short-lived, precise responses using electrical impulses. Divisions: central nervous system (CNS) — brain and spinal cord; peripheral nervous system (PNS) — cranial and spinal nerves, divided into the somatic (voluntary, skeletal muscles) and autonomic (involuntary: sympathetic — "fight or flight", noradrenaline; parasympathetic — "rest and digest", acetylcholine) systems.
• Brain: cerebrum (two hemispheres; thinking, memory, voluntary movement, sensory areas, speech), cerebellum (balance, posture, coordination of movement), medulla oblongata (breathing, heart rate, blood pressure, reflexes like swallowing and coughing), hypothalamus (temperature, osmoregulation, links to the pituitary), thalamus (relays sensory information). The spinal cord has central grey matter (cell bodies, butterfly-shaped) and outer white matter (myelinated axons); dorsal roots carry sensory fibres and ventral roots motor fibres.
• Neurones: sensory (receptor to CNS; cell body in the dorsal root ganglion), relay/intermediate (within CNS), motor (CNS to effector; cell body in CNS, long axon). Structural types: unipolar, bipolar, multipolar. Parts: cell body (nucleus, Nissl granules — rough ER for neurotransmitter synthesis), dendrites (receive impulses), axon (carries impulses away), myelin sheath made by Schwann cells (insulates), nodes of Ranvier (gaps), synaptic knobs.
• Resting potential (about −70 mV, inside negative): the sodium–potassium pump actively moves 3 Na⁺ out for 2 K⁺ in using ATP; the membrane is more permeable to K⁺ (many open K⁺ channels), so K⁺ diffuses out; large negative organic ions remain inside. The membrane is polarised.
• Action potential: a stimulus opens some voltage-gated Na⁺ channels; if threshold (about −55 mV) is reached, many more open and Na⁺ floods in: depolarisation to about +40 mV. Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ diffuses out: repolarisation. Too many K⁺ leave: hyperpolarisation (about −80 mV). The pump restores the resting potential. During the refractory period (absolute, then relative) a new action potential cannot (or can only with a strong stimulus) be produced, so impulses travel in one direction and are separate.
• All-or-nothing law: if threshold is reached, the action potential is always the same size; stimulus strength is coded by frequency of impulses and the number of neurones stimulated. Propagation: local currents depolarise the next region of the membrane. In myelinated axons, depolarisation occurs only at the nodes of Ranvier, so the impulse "jumps" (saltatory conduction), which is much faster (up to 120 m s⁻¹) than in unmyelinated axons (about 1 m s⁻¹). Larger axon diameter and higher temperature also increase speed.
• Cholinergic synapse: an action potential arrives at the presynaptic knob, opening voltage-gated Ca²⁺ channels; Ca²⁺ enters, causing vesicles of acetylcholine (ACh) to fuse with the membrane (exocytosis); ACh diffuses across the synaptic cleft (about 20 nm) and binds to receptors on the postsynaptic membrane, opening Na⁺ channels; this gives an excitatory postsynaptic potential (EPSP); if threshold is reached, an action potential starts. Acetylcholinesterase breaks ACh into choline and ethanoic acid, which are reabsorbed and recycled. Mitochondria supply ATP.
• Functions of synapses: ensure one-way transmission (transmitter only in presynaptic knob, receptors only on postsynaptic membrane); summation — spatial (several presynaptic neurones) and temporal (repeated impulses from one neurone); inhibition (inhibitory synapses cause hyperpolarisation, e.g. GABA); filter out low-level stimuli; allow convergence and divergence; memory and learning; fatigue protects effectors from overstimulation. The neuromuscular junction is a synapse between a motor neurone and a muscle fibre (motor end plate) using ACh.
• Drugs and synapses: nicotine mimics ACh (stimulates receptors); curare and some snake venoms block ACh receptors (paralysis); organophosphate insecticides and nerve gases inhibit acetylcholinesterase (continuous stimulation, spasms); cocaine and amphetamines block reuptake of dopamine/noradrenaline; morphine/heroin bind endorphin receptors; alcohol and barbiturates enhance inhibitory GABA; caffeine is a stimulant.
• Reflex arc: stimulus → receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector → response (e.g. withdrawal from a hot object, knee jerk, which is monosynaptic with no relay neurone). Reflexes are rapid, automatic and protective. Spinal reflexes are coordinated by the spinal cord; cranial reflexes (blinking, pupil reflex) by the brain. Conditioned reflexes are learned. Voluntary responses are conscious, controlled by the cerebrum, slower and variable; involuntary responses are automatic. Forms of response: muscle contraction, gland secretion, change in behaviour.

Common misconceptions

• "Nerve impulses are electric currents flowing along the axon like a wire." No: they are waves of depolarisation caused by ion movements across the membrane.
• "A stronger stimulus gives a bigger action potential." No: all-or-nothing; it gives a higher frequency.
• "Myelin speeds impulses by conducting electricity." No: myelin insulates; impulses jump between nodes of Ranvier.
• "Synapses slow the nervous system for no purpose." No: they give one-way flow, summation, filtering and integration.

Worked examples

Practice questions

1. Describe the structure of a motor neurone.

   Show answer

   Cell body in CNS with nucleus and Nissl granules; dendrites; long axon; myelin sheath of Schwann cells; nodes of Ranvier; motor end plates/synaptic knobs at effector.

   Common mistake: Placing the cell body in the middle of the axon.

2. Explain how the resting potential is maintained.

   Show answer

   Na⁺–K⁺ pump moves 3 Na⁺ out, 2 K⁺ in using ATP; membrane more permeable to K⁺, which diffuses out; negative ions stay inside; inside about −70 mV.

   Common mistake: Saying the membrane is permeable to Na⁺ at rest.

3. Describe the events of an action potential.

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   Stimulus reaches threshold; Na⁺ channels open; Na⁺ enters; depolarisation to +40 mV; Na⁺ channels close, K⁺ channels open; K⁺ leaves; repolarisation; hyperpolarisation; pump restores resting potential.

   Common mistake: Saying K⁺ enters during depolarisation.

4. Explain three factors affecting speed of conduction.

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   Myelination (saltatory conduction); axon diameter (larger diameter less resistance); temperature (faster ion diffusion up to a limit).

   Common mistake: Saying stimulus strength increases speed.

5. Describe transmission across a cholinergic synapse.

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   Action potential reaches knob; Ca²⁺ channels open, Ca²⁺ enters; vesicles fuse and release ACh; ACh diffuses across cleft; binds receptors; Na⁺ channels open; postsynaptic depolarisation; acetylcholinesterase breaks down ACh; products recycled.

   Common mistake: Leaving out calcium.

6. State five functions of synapses.

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   One-way transmission; summation; filter weak stimuli; inhibition; integration (convergence/divergence); learning and memory; prevent overstimulation (fatigue).

   Common mistake: Saying synapses speed up impulses.

7. Describe a reflex arc for withdrawing a hand from a hot object.

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   Thermoreceptors/pain receptors in skin; sensory neurone to spinal cord; relay neurone; motor neurone; biceps contracts; hand withdrawn; impulse also to brain for awareness.

   Common mistake: Saying the brain decides first.

8. Compare voluntary and involuntary responses.

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   Voluntary: conscious, cerebrum, learned, slower, variable, skeletal muscles. Involuntary: automatic, spinal cord or lower brain, fast, stereotyped, protective.

   Common mistake: Saying reflexes need conscious decision.

Matching and drag-and-drop

1. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. The myelin sheath is made by:	a. 3 Na⁺ out and 2 K⁺ in
   2. The resting potential of a neurone is about:	b. K⁺ ions leaving the axon
   3. Repolarisation is caused by:	c. Schwann cells
   4. The sodium–potassium pump moves:	d. −70 mV

   Show answer

   1 → c (Schwann cells) · 2 → d (−70 mV) · 3 → b (K⁺ ions leaving the axon) · 4 → a (3 Na⁺ out and 2 K⁺ in)

   • Wrapped around axon.
   • Inside negative.
   • K⁺ channels open.
   • Uses ATP.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   central nervous system  ·  0.02 s  ·  central system  ·  sympathetic system
   1. The "fight or flight" division of the autonomic system is the .
   2. The brain and spinal cord form the .
   3. An impulse travels 1.2 m at 60 m s⁻¹. The time taken is .
   Show answer

   1: sympathetic system · 2: central nervous system · 3: 0.02 s

   • Uses noradrenaline.
   • CNS.
   • 1.2 ÷ 60.

3. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. Grey matter of the spinal cord contains mainly:	a. Ca²⁺
   2. The enzyme that breaks down acetylcholine is:	b. cell bodies
   3. Entry of this ion triggers vesicle release at a synapse:	c. acetylcholinesterase
   4. Impulses jumping from node to node is:	d. saltatory conduction

   Show answer

   1 → b (cell bodies) · 2 → c (acetylcholinesterase) · 3 → a (Ca²⁺) · 4 → d (saltatory conduction)

   • Butterfly-shaped.
   • Into choline and ethanoic acid.
   • Exocytosis of ACh.
   • In myelinated axons.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   relay neurone  ·  mimics ACh at receptors  ·  motor neurone  ·  medulla oblongata
   1. Nicotine causes alertness because it .
   2. The knee-jerk reflex has no .
   3. Breathing and heart rate are controlled by the .
   Show answer

   1: mimics ACh at receptors · 2: relay neurone · 3: medulla oblongata

   • Stimulates postsynaptic neurones.
   • Monosynaptic.
   • Vital centres.

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