Neurones and the reflex arc

• Sensory neurone — carries impulses from a receptor to the CNS. Cell body in the middle, on a side branch.
• Relay (intermediate) neurone — within the CNS; connects sensory to motor. Short, unmyelinated.
• Motor neurone — carries impulses from the CNS to an effector (muscle or gland). Large cell body in the CNS with many dendrites; a long axon.

Schwann cells wrap repeatedly around the axon, forming the myelin sheath. Myelin is a lipid, so it is an electrical insulator: the membrane can only depolarise at the gaps, the nodes of Ranvier. The impulse therefore jumps from node to node — saltatory conduction — which is far faster than the continuous conduction of an unmyelinated axon. Conduction is also faster in a wider axon and at a higher temperature.

Reflex arc: receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector. It is fast (few synapses) and involuntary — the brain is informed afterwards, which is why you have already pulled your hand back before you feel the pain.

The pupil reflex (8.2): in bright light the circular muscles of the iris contract and the radial muscles relax, constricting the pupil (an antagonistic pair, controlled by the autonomic nervous system). In dim light the radial muscles contract and the pupil dilates.

Your turn — conduction velocity

Resting potential and action potential

Resting potential (about −70 mV): the sodium–potassium pump actively transports 3 Na⁺ out for every 2 K⁺ in, using ATP. The membrane is far more permeable to K⁺ than to Na⁺ (potassium leak channels are open), so K⁺ diffuses back out. The result is a net excess of positive charge outside — the inside is negative. The axon is polarised.

Action potential:

1. A stimulus opens a few Na⁺ channels; if the threshold (about −55 mV) is reached, voltage-gated Na⁺ channels open.
2. Depolarisation: Na⁺ floods in down its electrochemical gradient. This opens still more Na⁺ channels — positive feedback — and the potential rises to about +40 mV.
3. Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ leaves, restoring the negative interior.
4. Hyperpolarisation: the K⁺ channels are slow to close, so the potential briefly overshoots below −70 mV. The Na⁺/K⁺ pump then restores the resting potential.

All-or-nothing: either the threshold is reached and a full-sized action potential fires, or nothing happens. A stronger stimulus does not make a bigger impulse — it makes them more frequent (and recruits more neurones). During the refractory period the Na⁺ channels cannot reopen, which ensures the impulse travels in one direction only and sets the maximum firing frequency.

Your turn — the size of the change

Your turn — maximum firing frequency

Resting or action potential?

Tap an event, then tap where it belongs.

The synapse — and what drugs do to it

Transmission across a cholinergic synapse:

1. The action potential arrives and depolarises the pre-synaptic membrane, opening voltage-gated Ca²⁺ channels.
2. Ca²⁺ enters, causing vesicles of acetylcholine to move to and fuse with the membrane: exocytosis.
3. The neurotransmitter diffuses across the synaptic cleft and binds receptors on the post-synaptic membrane.
4. Na⁺ channels open; if enough neurotransmitter binds, the threshold is reached and a new action potential fires.
5. Acetylcholinesterase hydrolyses the acetylcholine, so the post-synaptic membrane repolarises and the synapse is not permanently switched on. The products are reabsorbed and recycled using ATP.

Synapses ensure unidirectional transmission (only the pre-synaptic neurone has vesicles), and they allow summation (many weak stimuli combining), divergence and convergence.

Drugs (8.15): an agonist mimics the neurotransmitter (nicotine at acetylcholine receptors); an antagonist blocks the receptor; some drugs inhibit reuptake (cocaine blocks the reuptake of dopamine; SSRIs block the reuptake of serotonin, so it stays in the cleft for longer); others inhibit the breakdown enzyme (organophosphates inhibit acetylcholinesterase).

Your turn — total reflex time

Why is transmission one-way?

Detecting a stimulus — rods and cones

A receptor is a transducer: it converts one form of energy (here, light) into a nerve impulse. Light bleaches the pigment in a photoreceptor, and the resulting change in membrane permeability generates a generator potential. If it reaches the threshold, the bipolar neurone fires.

• Rods — contain rhodopsin, which is broken down by even low light intensity, so rods work in dim light. Many rods synapse onto one bipolar neurone (retinal convergence), so their generator potentials summate and reach the threshold in dim light: high sensitivity, but low acuity (the brain cannot tell which rod fired), and only monochrome vision.
• Cones — contain iodopsin, which needs a high light intensity to be broken down. Each cone typically has its own bipolar neurone, so two nearby cones send separate impulses: high visual acuity. Three types (red, green, blue) give colour vision. Concentrated at the fovea.

IAA and phytochrome

Phototropism: IAA (indoleacetic acid, an auxin) is made at the shoot tip and transported down. It is redistributed away from the light to the shaded side, where it causes cell elongation (by loosening the cell wall). The shaded side grows faster, so the shoot bends towards the light — a positive phototropic response. In the root, the same auxin inhibits elongation, so roots show positive gravitropism.

Phytochrome: a light-sensitive pigment that exists in two interconvertible forms.

P_fr is the active form. In daylight, Pfr accumulates. It promotes germination of light-sensitive seeds and controls flowering: in long-day plants Pfr promotes flowering; in short-day plants Pfr inhibits it — which is why what actually matters is the length of the uninterrupted dark period, during which Pfr slowly reverts to Pr.

Brain regions and brain imaging

• Cerebral hemispheres — the largest region; folded to increase surface area. Conscious thought, memory, language, personality, voluntary movement and the processing of sensory information.
• Hypothalamus — the homeostatic control centre: temperature, water balance, hunger, sleep; it also controls the pituitary gland, linking the nervous and endocrine systems.
• Cerebellum — coordination of movement, balance and posture; fine motor control.
• Medulla oblongata — the involuntary essentials: heart rate, ventilation rate, blood pressure, swallowing.

Nervous vs hormonal coordination (8.7): nervous — electrical impulses along neurones, very fast, very localised (a specific effector), short-lived effect. Hormonal — chemical messengers in the blood, slower, widespread (any cell with the right receptor), longer-lasting.

Match the imaging technique

Tap what it shows on the left, then the technique.

Critical periods, animal models and habituation

The critical period (8.10): a window early in development during which the nervous system must receive the right stimulation to develop normally. Hubel and Wiesel showed that if one eye of a kitten is deprived of light during this period, the neural connections serving that eye are lost and it remains functionally blind — even after the eye is reopened. In humans, an untreated squint or cataract in infancy can cause permanent amblyopia. The visual cortex shows plasticity: the connections that are used are strengthened, and those that are not are pruned.

Animal models (8.11–8.12): they made the discovery possible, but they raise real ethical problems. Arguments for: without them we would not understand visual development or have effective treatments; the animals’ welfare is regulated by law; the potential benefit is enormous. Arguments against: the animals cannot consent and suffer real harm; results in one species may not transfer to humans; and alternatives (cell culture, computer modelling, human imaging) increasingly exist.

Habituation (8.13): the simplest form of learning — a decreasing response to a repeated, harmless stimulus. Repeated stimulation means less Ca²⁺ enters the pre-synaptic neurone, so less neurotransmitter is released, the post-synaptic threshold is not reached, and the response fades. It is adaptive: it stops an animal wasting energy responding to a stimulus that carries no threat.

When brain chemicals go wrong

Parkinson’s disease — the dopamine-secreting neurones of the substantia nigra die, so dopamine levels fall. Because dopamine is needed for the control of movement, the result is tremor, muscle rigidity and slow movement (bradykinesia). Treatments raise dopamine activity: L-dopa (a precursor that, unlike dopamine, can cross the blood–brain barrier and is then converted to dopamine), dopamine agonists, and MAO-B inhibitors which slow the breakdown of dopamine.

Depression is associated with low levels of serotonin. SSRIs (selective serotonin reuptake inhibitors) block the reuptake transporter, so serotonin stays in the synaptic cleft for longer and is more likely to bind post-synaptic receptors.

Note the careful language: associated with. A low serotonin level and depression correlate — establishing which causes which is much harder, and it is exactly the kind of point SNAB expects you to make.

From the genome to behaviour

Genome sequencing (8.16): the outcomes are used to identify genes associated with disease, to develop new drugs, and to move towards personalised medicine — matching a drug to a patient’s genotype. Ethical questions: who owns the data, could insurers or employers use it, and how do you counsel someone about a risk rather than a certainty?

Drugs from GM organisms (8.17–8.18): a human gene (e.g. for insulin or a clotting factor) is cut out with a restriction enzyme, joined into a plasmid vector using DNA ligase, and taken up by a bacterium, which then makes the human protein. GM plants and animals (“pharming”) can make more complex proteins in milk or leaves. Benefits: pure, unlimited, no risk of disease transmission from donated human tissue, and no ethical objection from those who reject animal-derived insulin. Risks and objections: escape of the transgene into wild populations, unknown long-term effects, the welfare of GM animals, and the concentration of patents in a few companies.

Nature and nurture (8.19): the classic methods are twin studies — comparing concordance in monozygotic (genetically identical) and dizygotic twins, and especially MZ twins reared apart — and adoption studies, comparing a child with their biological and adoptive parents. A higher MZ than DZ concordance implies a genetic contribution; that concordance is never 100 % implies an environmental contribution. Almost every trait is both.

Interpreting a twin study

The big ideas to know

You have covered the whole of SNAB Topic 8. Press Finish to see your score.