Topic 8 - Coordination, Response and Gene Technology

Schwann cells wrap repeatedly around peripheral axons to form a lipid-rich myelin sheath. Gaps between cells are nodes of Ranvier, where ion channels allow action potentials to be regenerated. Myelin insulates internodes and enables faster saltatory conduction.

The basic pathway is stimulus → receptor → sensory neurone → CNS → motor neurone → effector. The CNS integrates information and the effector produces a response through muscle contraction or gland secretion.

Bright light activates retinal receptors; impulses reach the CNS and motor neurones stimulate circular iris muscles so the pupil constricts. In low light, radial muscles contract and the pupil dilates.

Antagonistic effectors allow opposite outcomes, while a rapid neural pathway limits exposure to harmful conditions.

The spinal cord has central butterfly-shaped grey matter containing neurone cell bodies, dendrites and many synapses. Surrounding white matter contains bundles of myelinated axons running up and down the cord. Sensory axons enter through a dorsal route and motor axons leave through a ventral route.

In a withdrawal reflex, a pain receptor activates a sensory neurone, a relay neurone excites the flexor motor neurone and an inhibitory pathway relaxes the antagonist extensor. Information also ascends through white matter so pain can be perceived after the response begins.

When membrane depolarisation reaches threshold, voltage-gated sodium channels open and sodium ions enter. Potassium ions then leave to repolarise the membrane; the signal propagates as neighbouring sections reach threshold.

The all-or-nothing action potential is regenerated along the axon, so its amplitude does not fade with distance. Refractory periods help maintain one-way transmission.

A stronger stimulus is represented by more frequent action potentials, not a larger action potential, once threshold has been crossed.

In a myelinated axon, the sheath insulates the membrane between nodes of Ranvier. Depolarisation is regenerated at the nodes, so the impulse appears to jump along the axon by saltatory conduction.

The insulation reduces ion leakage and increases conduction speed compared with a non-myelinated axon, where the membrane is activated continuously.

Damage to myelin can slow or disrupt signalling even when the axon and its cell body remain present, because the normal node-to-node pattern is lost.

An arriving action potential opens presynaptic voltage-gated Ca$^{2+}$ channels. Calcium entry triggers acetylcholine-containing vesicles to fuse with the membrane. Acetylcholine diffuses across the cleft, binds complementary postsynaptic receptors and opens ion channels; sufficient depolarisation triggers a new action potential. Acetylcholinesterase hydrolyses acetylcholine so stimulation is brief.

Vesicles and release machinery are presynaptic, while receptors are postsynaptic, making chemical transmission one-way. Synaptic delay, summation and inhibition allow integration rather than simple electrical continuity.

In darkness, rhodopsin is intact and cGMP-gated cation channels remain open. Sodium and other cations enter the rod outer segment, keeping the cell relatively depolarised and causing continuous neurotransmitter release onto bipolar cells.

A photon absorbed by rhodopsin changes retinal and separates/activates the opsin component. The signalling cascade lowers cGMP, so cation channels close. Sodium entry falls while potassium loss continues, hyperpolarising the rod and reducing neurotransmitter release.

The change in transmitter alters bipolar-cell activity and then ganglion-cell firing. Ganglion-cell axons form the optic nerve, where information is carried as action potentials; the rod itself signals mainly by graded membrane-potential change.

Many rods converge onto shared pathways, giving high sensitivity in dim light but lower spatial resolution and no colour discrimination compared with cones.

Habituation occurs when repeated harmless stimulation produces a smaller response. The animal saves energy because fewer calcium ions enter the presynaptic neurone, less neurotransmitter is released and the postsynaptic cell is less likely to reach threshold.

A snail may take less time to re-emerge after repeated gentle touches. If the sound or touch becomes stronger, the response can return because the stimulus is no longer identical.

The change is in synaptic transmission, not a loss of all sensory ability. The animal can still respond when the stimulus becomes relevant.

The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system (PNS) carries information between the CNS and receptors or effectors. Sensory and motor pathways connect the two.

This division separates integration from transmission. A receptor detects a stimulus, the CNS coordinates a response, and motor neurones carry the output to muscles or glands.

A skin receptor sends an impulse through a sensory neurone in the PNS to the spinal cord; a motor pathway then activates an effector.

In a shoot lit from one side, more auxin action on the shaded side promotes greater elongation, bending the shoot toward light. In a germinating cereal, embryo gibberellin reaches aleurone cells and induces transcription of amylase genes, mobilising endosperm starch.

1. Surface-sterilise cereal grains using the approved aseptic procedure and cut them to separate embryo-containing and endosperm portions.
2. Soak matched portions in the same gibberellin concentration; soak equivalent portions in sterile water as negative controls.
3. Place cut faces onto sterile starch-agar plates, with equal spacing and labelled positions.
4. Incubate sealed plates at a fixed safe temperature for 24–48 hours.
5. Add iodine according to the approved method and measure clear-zone diameter or area around each portion.
6. Repeat grains and compare means with spread.

Independent variables may be tissue type and gibberellin treatment. Control grain variety/age, portion size, solution volume/concentration, agar depth, incubation time and temperature. A known amylase positive control can confirm that the starch-iodine detection works.

Nervous control uses rapid electrical impulses and targeted neurotransmitter release; hormonal control uses chemicals carried in blood, usually with slower onset and longer-lasting, broader effects. Both use receptors, coordination centres and effectors.

A reflex can move a hand away quickly, while adrenaline or thyroxine changes metabolism across several tissues. The best system depends on speed, duration and distribution.

The systems can interact: neural signals can trigger hormone release, and hormones can alter the sensitivity or activity of nervous tissues.

Compare a patient with reference data or the same person across conditions. Increased fMRI or PET signal supports involvement or altered metabolism, not that one region alone causes the behaviour. Diagnosis combines imaging with clinical evidence.

Some disorders are associated with altered neurotransmitter levels or signalling. Parkinson’s involves reduced dopamine from lost neurones; antidepressant drugs can increase serotonin or noradrenaline signalling.

A drug may mimic a transmitter, supply a precursor, block breakdown or inhibit reuptake. These mechanisms change synaptic transmission, but symptoms usually involve networks and multiple contributing factors.

L-dopa is converted into dopamine in the brain, while an SSRI reduces serotonin reuptake so more remains in the synaptic cleft.

Recombinant bacteria can produce human insulin; a suitable promoter drives expression, fermenter conditions are controlled, and insulin is purified rather than administering the organism itself.

Recombinant DNA is formed when DNA from different sources is joined. A restriction enzyme cuts the desired gene and vector, complementary ends pair, and DNA ligase seals the sugar-phosphate backbone.

PCR can amplify the target before insertion, while a plasmid provides replication or expression signals in the host. The construct must be introduced into cells and selected or screened.

A gene coding for insulin can be joined to a bacterial plasmid; the plasmid is then transferred into bacteria where the gene may be expressed.

Use a selectable or screenable marker to identify candidate cells, then PCR or another DNA test to verify the construct. Measure RNA or protein to show expression, because vector entry and marker survival alone do not prove a functional therapeutic product.

A microarray uses labelled nucleic acids from samples to measure which genes are expressed relative to a reference. Complementary binding at spots produces a pattern that can be compared between tissues or conditions.

The result is a relative expression profile, not a direct measurement of every protein or a guaranteed cause. Controls, normalisation and probe specificity determine how confidently differences can be interpreted.

A gene spot with stronger signal in diseased tissue suggests higher transcript abundance, but follow-up experiments are needed to test whether the gene drives the phenotype.

Bioinformatics uses databases and computational tools to compare DNA, RNA or protein sequences, predict relationships and identify patterns. The output depends on the input sequence, reference database and comparison method.

Check alignment quality, sequence length, conserved regions and the scoring or statistical assumptions before interpreting a match.

A high sequence similarity can support common ancestry or a shared function, while a low-quality or partial sequence may produce a misleading result.

Genetic modification can add a useful characteristic, such as producing a medicine or resisting a pest. Its value depends on effectiveness, containment, ecological effects, ownership, welfare and who receives the benefit.

Benefits may include reliable therapeutic protein production or reduced losses; risks include unintended gene flow, resistance, ecological disruption and ethical objections to animal or food applications.

A GM microorganism making insulin is contained and purified, whereas a GM crop releasing a trait into wild relatives raises a different monitoring problem.