When a skeletal muscle sarcomere shortens during contraction, which of the following keeps the same length?

The distance between successive Z lines

A drug blocks Ca²⁺ release through the ryanodine receptors (RyR1) of skeletal-muscle sarcoplasmic reticulum. A nerve–muscle preparation is treated with it and the motor nerve is stimulated. What is observed?

Normal end-plate and muscle action potentials are recorded, but the fibre develops little or no tension

No end-plate potential is recorded, because acetylcholine release from the motor nerve terminal is blocked

Normal end-plate and muscle action potentials are recorded, but the fibre develops little or no tension

Contraction is normal, because Ca²⁺ entering through the T-tubule voltage sensors alone activates troponin C

The fibre goes into a sustained rigor contracture, because its cross-bridges can no longer detach from actin

Nematodes move by whip-like dorsoventral bending and cannot crawl peristaltically as earthworms do. What is the main reason?

Their body wall has longitudinal muscles only, with no circular layer to antagonise them

They have no fluid-filled body cavity, so their muscles have nothing incompressible to act against

Their bodies are divided by septa into rigid segments that cannot change shape independently

They are enclosed in a rigid chitinous exoskeleton that prevents any change in body diameter

Their body wall has longitudinal muscles only, with no circular layer to antagonise them

Which bones of the human skeleton form mainly by intramembranous ossification, without a cartilage model?

The flat bones of the skull vault, such as the parietals

The femur and the other long bones of the upper and lower limbs

The flat bones of the skull vault, such as the parietals

The vertebrae of the lumbar region

The phalanges of the fingers and toes

Tetrodotoxin is applied to a squid giant axon. Which result is expected?

Action potentials are abolished, while the resting potential changes little

The resting potential collapses to 0 mV, because the K⁺ leak channels are blocked

Action potentials are greatly prolonged, because repolarisation by K⁺ channels fails

Action potentials continue normally, but transmitter release at the synapse is prevented

Which of the following statements about nervous systems are correct?

Annelids and arthropods have a ventral, solid nerve cord bearing segmental ganglia

Sympathetic preganglionic neurons leave the spinal cord in its thoracic and lumbar segments

Annelids and arthropods have a ventral, solid nerve cord bearing segmental ganglia

Postganglionic parasympathetic fibres release noradrenaline at most of their targets

Sympathetic preganglionic neurons leave the spinal cord in its thoracic and lumbar segments

The nerve net of Hydra is organised around a dorsal brain that coordinates the tentacles

In a neuron, E_{K} = −90 mV and E_{Na} = +60 mV, yet the resting membrane potential is −70 mV. Which explanation fits these values?

The resting membrane is far more permeable to K⁺ than to Na⁺, and a small Na⁺ leak pulls it positive of E_{K}

The Na⁺/K⁺-ATPase directly generates the whole −70 mV by moving three Na⁺ out for every two K⁺ in

The resting membrane is equally permeable to Na⁺ and K⁺, so the potential settles between the two values

The resting membrane is far more permeable to K⁺ than to Na⁺, and a small Na⁺ leak pulls it positive of E_{K}

Cl⁻ is pumped into the cell, which holds the inside more negative than E_{K} would allow on its own

Which hormone, although made from the amino acid tyrosine, acts mainly through a nuclear receptor that changes gene transcription?

Antidiuretic hormone (vasopressin)

Which of the following statements about the vertebrate endocrine system are correct?

ADH and oxytocin are made by hypothalamic neurons and released from the posterior pituitary

Prolactin secretion is held down mainly by dopamine from the hypothalamus

In iodine deficiency the plasma TSH concentration falls

ADH and oxytocin are made by hypothalamic neurons and released from the posterior pituitary

Aldosterone is secreted by the zona fasciculata of the adrenal cortex

Prolactin secretion is held down mainly by dopamine from the hypothalamus

The corpora allata are surgically removed from an early-instar silkworm larva, leaving its brain and prothoracic glands intact. What happens at the following moults?

The larva moults precociously into a miniature pupa and then a miniature adult

The larva stops moulting altogether, because without the corpora allata no ecdysone is produced

The larva passes through extra larval moults and grows into a giant larva

The larva moults precociously into a miniature pupa and then a miniature adult

The larva dies at the next moult, because PTTH can no longer be released from the brain

Which of the following statements about the human male reproductive system are correct?

Sertoli cells are the main target of FSH and secrete inhibin

Meiosis of the male germ cells is completed in the epididymis

The scrotum keeps the testes slightly warmer than the core of the body

Sertoli cells are the main target of FSH and secrete inhibin

In early human pregnancy the corpus luteum does not regress and menstruation does not occur. What keeps the corpus luteum secreting progesterone?

hCG from the trophoblast of the embryo, which acts on the corpus luteum like LH

A second LH surge released by the pituitary in the middle of the luteal phase

A rise in FSH that converts the corpus luteum back into a growing follicle

Oxytocin from the posterior pituitary, which stops the endometrium from shedding

hCG from the trophoblast of the embryo, which acts on the corpus luteum like LH

How are male honeybees (drones) produced?

By parthenogenesis from unfertilised eggs, so that drones are haploid

From fertilised eggs whose larvae are fed only on royal jelly by nurse workers

By sequential hermaphroditism, in which older workers change sex

From fertilised eggs that lose one X chromosome during cleavage

Comparative Anatomy and Physiology II: Skeleton, Muscle, Nerves, Hormones and Reproduction

This chapter completes Section XL4.7 of XL4 Zoology, “Animal Anatomy and Physiology”, whose heading also names the muscular, nervous, endocrine, reproductive and skeletal systems. It compares hydrostatic, exo- and endoskeletons and teaches bone and joints; the sarcomere and excitation–contraction coupling; nervous-system plans, membrane and action potentials and synapses; hormone classes and the hypothalamic–pituitary axes, with insect moulting as the comparative case; and reproductive strategies with the hormonal control of the human testis and menstrual cycle. The numericals are sarcomere band widths, Nernst potentials, conduction velocity from latencies, hormone half-life and the timing of ovulation.

1. Skeletal systems: hydrostatic, exo- and endoskeletons

A hydrostatic skeleton is fluid in a closed compartment pressed on by muscles. In an earthworm, circular and longitudinal muscles act as antagonists on the coelomic fluid of each septum-bounded segment, giving peristaltic crawling; a nematode has longitudinal muscles only, so it can merely thrash in dorsoventral waves. An exoskeleton, the arthropod cuticle of chitin and protein (stiffened in crustaceans with calcium carbonate), protects and gives muscles internal levers, but must be shed (ecdysis) for growth and grows disproportionately heavy as size rises. An endoskeleton grows with the animal: calcareous ossicles of mesodermal origin in echinoderms, cartilage and bone in vertebrates, cartilage alone in sharks and rays.

Bone is type I collagen (tensile strength) mineralised with hydroxyapatite, Ca10(PO4)6(OH)2 (compressive strength). Compact bone is built of osteons (Haversian systems): a central canal carrying vessels, concentric lamellae, and osteocytes in lacunae linked by canaliculi. Osteoblasts lay down matrix and become osteocytes; osteoclasts, multinucleate cells of the monocyte–macrophage lineage, resorb it, so bone is remodelled throughout life. Cartilage is avascular and heals poorly: hyaline (articular surfaces, fetal skeleton), elastic (ear pinna, epiglottis) and fibrocartilage (intervertebral discs, pubic symphysis). Flat bones of the skull vault form by intramembranous ossification directly in mesenchyme; long bones and vertebrae by endochondral ossification, replacing a cartilage model and lengthening at the epiphyseal plates until these fuse after puberty.

The adult human skeleton
Part	Bones	Make-up
Axial skeleton	80	Skull 22, ear ossicles 6, hyoid 1, vertebral column 26, sternum 1, ribs 24 (7 true, 3 false, 2 floating pairs)
Appendicular skeleton	126	Pectoral girdles 4, upper limbs 60, pelvic girdles 2, lower limbs 60
Vertebral column	33 in the child, 26 in the adult	7 cervical, 12 thoracic, 5 lumbar, 5 sacral fused into the sacrum, usually 4 coccygeal fused into the coccyx

Joints are fibrous (skull sutures, immovable), cartilaginous (intervertebral discs, pubic symphysis, slightly movable) or synovial (freely movable, with a fluid-filled capsule): ball-and-socket at shoulder and hip, hinge at the elbow, pivot between atlas and axis, saddle at the base of the thumb. Ligaments join bone to bone, tendons muscle to bone. Almost all mammals, the giraffe included, have seven cervical vertebrae; sloths and manatees are the classic exceptions.

2. Muscle: the sarcomere and excitation–contraction coupling

Three kinds of vertebrate muscle
Feature	Skeletal	Cardiac	Smooth
Striation and cells	Striated; long multinucleate fibres	Striated; branched cells joined by intercalated discs with gap junctions	Unstriated; spindle-shaped, one nucleus
Control	Voluntary, by somatic motor neurons	Involuntary and myogenic; autonomic nerves alter the rate	Involuntary; autonomic nerves, hormones, stretch
Ca²⁺ acts on	Troponin C	Troponin C	Calmodulin, which activates myosin light-chain kinase
Tetanus	Possible	Impossible: the long refractory period outlasts the twitch	Slow, sustained tonic contraction

A sarcomere runs from one Z line to the next. Thick myosin filaments occupy the central A band; thin actin filaments, anchored at the Z lines, extend inwards. The I band holds thin filaments only, the H zone thick filaments only. In the sliding-filament model neither filament shortens: overlap increases, so the I band and H zone narrow while the A band stays constant. With thick filaments 1.6 µm and thin filaments 1.0 µm long, a 2.4 µm sarcomere has I = 2.4 − 1.6 = 0.8 µm and H = 2.4 − 2 × 1.0 = 0.4 µm; at 2.0 µm the H zone vanishes. Tension is greatest at intermediate lengths, where the most myosin heads can reach actin (the length–tension relation).

In excitation–contraction coupling acetylcholine at the neuromuscular junction opens nicotinic receptors, and the end-plate potential fires a muscle action potential that runs into the T tubules. There dihydropyridine receptors, acting as voltage sensors, mechanically open the ryanodine receptors (RyR1) of the sarcoplasmic reticulum. Ca²⁺ binds troponin C, tropomyosin moves off the myosin-binding sites on actin, and cross-bridges cycle: ATP binding detaches myosin, ATP hydrolysis re-cocks the head, and release of Pi drives the power stroke. Relaxation needs ATP too, for the SERCA pump that returns Ca²⁺ to the reticulum; without ATP myosin stays bound (rigor mortis). In cardiac muscle, Ca²⁺ entering through L-type channels triggers release from the reticulum (calcium-induced calcium release).

Force is graded by recruiting motor units (a motor neuron and all the fibres it supplies; small in eye muscles, large in postural muscles), smallest first, and by firing faster until twitches fuse into tetanus. Slow oxidative (type I) fibres are red, rich in myoglobin and mitochondria, and resist fatigue; fast glycolytic fibres are pale and tire quickly. In flies, bees and beetles the asynchronous flight muscle, activated by stretch, contracts many times per nerve impulse, allowing hundreds of wingbeats per second; dragonflies and locusts have synchronous flight muscle, one contraction per impulse.

3. Nervous systems, membrane potentials and synapses

Cnidarians such as Hydra have a diffuse nerve net without a brain; flatworms have cerebral ganglia and a ladder of longitudinal cords; annelids and arthropods a ventral, solid, double nerve cord with segmental ganglia; chordates a dorsal, hollow, single nerve cord. Bilateral symmetry and forward movement brought cephalisation, the gathering of neurons and sense organs at the head. The vertebrate autonomic system has a sympathetic division (thoracolumbar outflow, short preganglionic fibres, noradrenaline at most targets; the adrenal medulla is a modified ganglion releasing adrenaline) and a parasympathetic one (craniosacral outflow, long preganglionic fibres, acetylcholine on muscarinic receptors at the targets).

The resting potential, about −70 mV in a mammalian neuron, arises mainly because the resting membrane is far more permeable to K⁺ than to Na⁺, with the gradients kept up by the Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in per ATP). Each ion's equilibrium potential follows the Nernst equation, E = (RT/zF) ln([ion]out/[ion]in), which at 37 °C becomes E = (61.5/z) log10([out]/[in]) mV. For K⁺ at 5 mM outside and 140 mM inside, E_K = 61.5 × log10(5/140) ≈ −89 mV; for Na⁺ at 145 and 12 mM, E_Na ≈ +67 mV; for Ca²⁺, z = 2 halves the factor. The membrane potential lies between the equilibrium potentials, weighted by permeability (the Goldman–Hodgkin–Katz equation): near E_K at rest, towards E_Na at the peak of a spike.

At threshold, voltage-gated Na⁺ channels open in a positive-feedback burst and the membrane overshoots to about +30 mV; the Na⁺ channels then inactivate and delayed voltage-gated K⁺ channels repolarise and briefly hyperpolarise it. Inactivation causes the absolute refractory period, which keeps impulses one-way and caps their frequency; tetrodotoxin blocks the Na⁺ channels. The spike is all-or-none, so intensity is coded by frequency. Conduction velocity rises with axon diameter (the giant axons of squid and earthworms) and with myelination, which makes conduction saltatory between nodes of Ranvier (myelin from Schwann cells in peripheral nerves, oligodendrocytes in the CNS). It is measured by stimulating at two points: velocity = distance between them ÷ difference in latency, which cancels the fixed delays at the junction and in the muscle. Sites 250 mm apart with latencies of 7.5 and 3.5 ms give 250 ÷ 4.0 = 62.5 m s⁻¹.

At a chemical synapse the arriving spike opens voltage-gated Ca²⁺ channels, vesicles release transmitter, and receptors produce an excitatory (glutamate; acetylcholine at the neuromuscular junction) or inhibitory (GABA, glycine) postsynaptic potential that summates in space and time; the delay is about 0.5 ms and transmission is one-way. Acetylcholinesterase ends the acetylcholine signal (organophosphates inhibit it), curare blocks nicotinic receptors and botulinum toxin blocks release. Electrical synapses through gap junctions are almost instantaneous and can conduct both ways, which suits escape circuits such as the crayfish giant fibres. The knee jerk is a monosynaptic stretch reflex.

4. Endocrine systems: hormones, receptors and axes

Hormone classes
Class	Examples	Receptor and action
Peptide and protein	Insulin, glucagon, ADH, growth hormone	Cell surface (G-protein-coupled or tyrosine kinase); second messengers such as cAMP, IP3 and Ca²⁺; fast
Steroid	Cortisol, aldosterone, testosterone, oestradiol, ecdysone	Intracellular receptor that binds DNA and alters transcription; slower, longer-lasting
Catecholamine	Adrenaline, noradrenaline	Cell-surface adrenergic receptors, like peptides
Thyroid hormone	Thyroxine (T4), triiodothyronine (T3)	Nuclear receptor, like steroids, though made from tyrosine

The hypothalamus controls the anterior pituitary through releasing hormones carried in portal vessels: TRH → TSH → thyroid; CRH → ACTH → adrenal cortex; GnRH → FSH and LH → gonads; GHRH and somatostatin → growth hormone; prolactin is held down mainly by dopamine. The posterior pituitary releases ADH and oxytocin, which are made by neurons of the supraoptic and paraventricular nuclei and carried down their axons. End hormones feed back negatively: in iodine deficiency T4 falls, TSH rises and the thyroid enlarges into a goitre. The adrenal cortex makes aldosterone (zona glomerulosa), cortisol (fasciculata) and androgens (reticularis); islet β cells make insulin, α cells glucagon. PTH raises plasma Ca²⁺ (bone resorption, renal reabsorption, activation of vitamin D to calcitriol), and calcitonin from thyroid C cells lowers it. After secretion stops, a hormone's level falls as C = C0 × (1/2)^t/t½: peptides last minutes, protein-bound steroids and thyroid hormones longer (thyroxine about a week).

Insect moulting is the comparative case. Neurosecretory cells of the brain release PTTH, which drives the prothoracic glands to secrete ecdysone (converted to active 20-hydroxyecdysone); each ecdysone pulse triggers a moult. Juvenile hormone (JH) from the corpora allata decides what kind of moult: high JH gives a larval moult, low JH a pupal moult, no JH the adult. Removing the corpora allata from a young larva therefore gives a precocious miniature pupa and adult, and implanting active corpora allata into a last-instar larva gives an extra larval instar. In amphibians thyroid hormone drives metamorphosis, and tadpoles without a thyroid keep growing as tadpoles.

⚠️ Ecdysone triggers, JH decides

Ecdysone sets off every moult, larval, pupal or adult; juvenile hormone only chooses the outcome. A stem that removes the prothoracic glands stops moulting altogether; one that removes the corpora allata changes what the next moult produces.

5. Reproductive systems and their hormonal control

Asexual reproduction copies one parent: binary fission in protists, budding in Hydra, fragmentation in planarians, gemmules in sponges. Sexual species may have separate sexes or be hermaphrodite, simultaneously (earthworms, which still cross-fertilise) or sequentially (clownfish are male first, wrasses female first). Parthenogenesis produces young from unfertilised eggs: haploid honeybee drones, summer generations of aphids. Fertilisation is external in most bony fishes and frogs and internal in amniotes and insects. Embryos develop in laid eggs (oviparity: birds, monotremes), in eggs retained by the mother and fed by yolk (ovoviviparity: some sharks and snakes) or on maternal nourishment through a placenta (viviparity: eutherian mammals); marsupials are born after a short gestation and finish developing in the pouch.

In the human male the testes sit in the scrotum, 2 to 3 °C below core temperature, which spermatogenesis needs. Seminiferous tubules hold the germ cells and the Sertoli cells, the FSH target, which nourish the germ cells, form the blood–testis barrier and secrete inhibin; between the tubules Leydig cells respond to LH by secreting testosterone. Sperm mature and are stored in the epididymis, pass along the vas deferens and mix with secretions of the seminal vesicles (rich in fructose), prostate and bulbourethral glands. Testosterone feeds back on GnRH and LH, inhibin on FSH.

The human menstrual cycle (28-day example)
Phase	Ovary	Hormones	Uterus
Menstrual, days 1–5	A new set of follicles starts growing	Oestradiol and progesterone low	Functional endometrium shed
Follicular, days 6–13	One dominant Graafian follicle	FSH; rising oestradiol	Endometrium rebuilt (proliferative)
Ovulation, about day 14	Follicle ruptures, releasing a secondary oocyte	LH surge, set off by sustained high oestradiol (positive feedback)	Endometrium thick
Luteal, days 15–28	Corpus luteum	Progesterone and oestradiol high; LH and FSH suppressed	Secretory endometrium, ready for implantation

Without implantation the corpus luteum regresses, progesterone falls and the endometrium is shed. After implantation the trophoblast secretes hCG, which acts like LH to keep the corpus luteum secreting progesterone until the placenta takes over; hCG in urine is the basis of pregnancy tests. The luteal phase is nearly constant at about 14 days, so cycle length varies through the follicular phase. Most mammals have oestrous cycles, receptive only in heat and resorbing the endometrium, whereas humans and some other primates menstruate; rabbits and cats are induced ovulators, ovulating in response to mating.

Key takeaways

Hydrostatic skeletons need antagonistic muscles acting on enclosed fluid (circular plus longitudinal in earthworms, longitudinal only in nematodes); exoskeletons must be moulted; endoskeletons grow. Bone is collagen plus hydroxyapatite, remodelled by osteoblasts and osteoclasts; the adult human has 206 bones, 80 axial and 126 appendicular.
In contraction the A band is constant while the I band and H zone narrow; I = S − thick length and H = S − 2 × thin length. Ca²⁺ released through RyR1 binds troponin C; ATP is needed both to detach myosin and to pump Ca²⁺ back.
Nerve net, ladder, ventral double cord and dorsal hollow cord mark the main nervous plans. E = (61.5/z) log10(out/in) mV at 37 °C; the resting potential sits near E_K because K⁺ permeability dominates.
Conduction velocity rises with diameter and myelination and is measured as distance ÷ difference in latency between two stimulation sites. Chemical synapses are one-way with about 0.5 ms delay; electrical synapses are fast and can run both ways.
Peptides and catecholamines act on surface receptors through second messengers; steroids and thyroid hormones on intracellular receptors that change transcription. Hypothalamic releasing hormones drive the anterior pituitary; ADH and oxytocin are made in the hypothalamus. In insects ecdysone triggers each moult and juvenile hormone decides its kind.
Reproductive strategies range from fission and budding to hermaphroditism, parthenogenesis and placental viviparity. LH drives Leydig cells and FSH Sertoli cells; an oestradiol-triggered LH surge causes ovulation, the corpus luteum makes progesterone, and hCG rescues it in pregnancy. The luteal phase is about 14 days.

Practice questions (18)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

When a skeletal muscle sarcomere shortens during contraction, which of the following keeps the same length?
1. The I band
2. The H zone
3. The A band
4. The distance between successive Z lines
Show answer
Answer: C — The A band
The A band is the length of the thick filaments, which do not shorten, so it stays constant. The I band and H zone are the regions where thin and thick filaments do not overlap; as the filaments slide, overlap grows and both narrow, and the Z lines move closer. The I band is the tempting choice because it contains thin filaments, which also keep their length, but the band itself is defined by the non-overlapping part, which shrinks.
In a skeletal muscle sarcomere the thick filaments are 1.6 µm long and each thin filament extends 1.0 µm from its Z line. Ignore the bare central zone of the thick filament. When the sarcomere is 2.4 µm long, what percentage of the thick-filament length lies alongside thin filaments?
Numerical answer — type the value.
Show answer
Answer: 75
The H zone, where thick filaments have no thin filaments beside them, is the sarcomere length minus the two thin filaments: 2.4 − 2 × 1.0 = 0.4 µm. The overlapped part of the thick filament is 1.6 − 0.4 = 1.2 µm, which is 1.2 ÷ 1.6 = 75% of its length. At 2.0 µm the H zone would vanish and overlap would be 100%.
A drug blocks Ca²⁺ release through the ryanodine receptors (RyR1) of skeletal-muscle sarcoplasmic reticulum. A nerve–muscle preparation is treated with it and the motor nerve is stimulated. What is observed?
1. No end-plate potential is recorded, because acetylcholine release from the motor nerve terminal is blocked
2. Normal end-plate and muscle action potentials are recorded, but the fibre develops little or no tension
3. Contraction is normal, because Ca²⁺ entering through the T-tubule voltage sensors alone activates troponin C
4. The fibre goes into a sustained rigor contracture, because its cross-bridges can no longer detach from actin
Show answer
Answer: B — Normal end-plate and muscle action potentials are recorded, but the fibre develops little or no tension
Transmission at the junction and the muscle action potential do not depend on the reticulum, so both are normal; what fails is the step that links them to contraction, Ca²⁺ release through RyR1, so tension is lost. In skeletal muscle the dihydropyridine receptors act mainly as voltage sensors coupled mechanically to RyR1, and the Ca²⁺ they admit is far too little to activate contraction, which rules out the voltage-sensor option. Rigor requires ATP depletion, not a block of Ca²⁺ release.
Nematodes move by whip-like dorsoventral bending and cannot crawl peristaltically as earthworms do. What is the main reason?
1. They have no fluid-filled body cavity, so their muscles have nothing incompressible to act against
2. Their bodies are divided by septa into rigid segments that cannot change shape independently
3. They are enclosed in a rigid chitinous exoskeleton that prevents any change in body diameter
4. Their body wall has longitudinal muscles only, with no circular layer to antagonise them
Show answer
Answer: D — Their body wall has longitudinal muscles only, with no circular layer to antagonise them
Peristalsis needs circular and longitudinal muscles acting alternately on the fluid of separate compartments, as in the septate earthworm. Nematodes have only longitudinal muscles working against a pressurised pseudocoelom and an elastic cuticle, so contraction on one side simply bends the body. They do have a fluid-filled body cavity (the pseudocoelom), they are not segmented, and their cuticle is collagenous, not chitinous.
Which bones of the human skeleton form mainly by intramembranous ossification, without a cartilage model?
1. The femur and the other long bones of the upper and lower limbs
2. The flat bones of the skull vault, such as the parietals
3. The vertebrae of the lumbar region
4. The phalanges of the fingers and toes
Show answer
Answer: B — The flat bones of the skull vault, such as the parietals
Intramembranous ossification turns mesenchyme directly into bone and builds the flat bones of the skull vault. Long bones, vertebrae and phalanges form by endochondral ossification, replacing a hyaline cartilage model; the femur is the tempting choice only because it is the textbook example of the other process.
At 37 °C a neuron's intracellular K⁺ concentration is 140 mM. Extracellular K⁺ rises from 4 mM to 8 mM. By how many millivolts does the K⁺ equilibrium potential become less negative? Use E = 61.5 log10([K⁺]out/[K⁺]in) mV and give the answer to one decimal place.
Numerical answer — type the value.
Show answer
Answer: 18.5
E before = 61.5 log10(4/140) and E after = 61.5 log10(8/140). The change is 61.5 × [log10(8/140) − log10(4/140)] = 61.5 × log10 2 = 61.5 × 0.301 = 18.5 mV, independent of the internal concentration. E_K moves from about −95 to −76 mV, which is why a high plasma K⁺ depolarises excitable cells.
In a nerve conduction study a motor nerve is stimulated at the elbow and at the wrist, 252 mm apart, and the muscle response is recorded in the hand. The latency is 7.8 ms from the elbow and 3.6 ms from the wrist. What is the conduction velocity of the nerve segment in m/s?
Numerical answer — type the value.
Show answer
Answer: 60
Both latencies include the same junctional and muscle delays, so the difference, 7.8 − 3.6 = 4.2 ms, is the time to travel the 252 mm between the sites. Velocity = 252 mm ÷ 4.2 ms = 60 mm per ms = 60 m s⁻¹. Dividing the distance by either latency alone would underestimate the velocity.
Tetrodotoxin is applied to a squid giant axon. Which result is expected?
1. Action potentials are abolished, while the resting potential changes little
2. The resting potential collapses to 0 mV, because the K⁺ leak channels are blocked
3. Action potentials are greatly prolonged, because repolarisation by K⁺ channels fails
4. Action potentials continue normally, but transmitter release at the synapse is prevented
Show answer
Answer: A — Action potentials are abolished, while the resting potential changes little
Tetrodotoxin blocks voltage-gated Na⁺ channels, so the regenerative Na⁺ influx cannot occur and no spike fires; the resting potential, set mainly by K⁺ permeability, is little affected. Prolonged spikes from failed repolarisation are the effect of K⁺-channel blockers such as tetraethylammonium, the usual confusion. The toxin does not block K⁺ leak channels, and transmitter release fails only because no spike reaches the terminal.
Which of the following statements about nervous systems are correct?
1. Annelids and arthropods have a ventral, solid nerve cord bearing segmental ganglia
2. Postganglionic parasympathetic fibres release noradrenaline at most of their targets
3. Sympathetic preganglionic neurons leave the spinal cord in its thoracic and lumbar segments
4. The nerve net of Hydra is organised around a dorsal brain that coordinates the tentacles
Show answer
Answer: A — Annelids and arthropods have a ventral, solid nerve cord bearing segmental ganglia; C — Sympathetic preganglionic neurons leave the spinal cord in its thoracic and lumbar segments
The ventral solid cord with segmental ganglia is the annelid–arthropod plan, and the sympathetic outflow is thoracolumbar. Parasympathetic postganglionic fibres release acetylcholine onto muscarinic receptors; noradrenaline is the transmitter of most sympathetic postganglionic fibres. The cnidarian nerve net is diffuse and has no brain.
In a neuron, E_K = −90 mV and E_Na = +60 mV, yet the resting membrane potential is −70 mV. Which explanation fits these values?
1. The Na⁺/K⁺-ATPase directly generates the whole −70 mV by moving three Na⁺ out for every two K⁺ in
2. The resting membrane is equally permeable to Na⁺ and K⁺, so the potential settles between the two values
3. The resting membrane is far more permeable to K⁺ than to Na⁺, and a small Na⁺ leak pulls it positive of E_K
4. Cl⁻ is pumped into the cell, which holds the inside more negative than E_K would allow on its own
Show answer
Answer: C — The resting membrane is far more permeable to K⁺ than to Na⁺, and a small Na⁺ leak pulls it positive of E_{K}
By the Goldman–Hodgkin–Katz equation the membrane potential is a permeability-weighted compromise between equilibrium potentials. Lying 20 mV from E_K and 130 mV from E_Na, −70 mV shows K⁺ permeability dominating with a small Na⁺ leak. Equal permeabilities to Na⁺ and K⁺ would drag the potential far from E_K, to a value much less negative than −70 mV. The pump contributes only a few millivolts directly; its main role is to maintain the gradients. A potential more negative than E_K is the opposite of what is observed.
Which hormone, although made from the amino acid tyrosine, acts mainly through a nuclear receptor that changes gene transcription?
1. Adrenaline
2. Triiodothyronine (T3)
3. Glucagon
4. Antidiuretic hormone (vasopressin)
Show answer
Answer: B — Triiodothyronine (T3)
T3 is lipophilic, enters cells and binds thyroid-hormone receptors in the nucleus that act as transcription factors. Adrenaline is also made from tyrosine, which makes it the tempting answer, but it is water-soluble and acts on cell-surface adrenergic receptors through second messengers. Glucagon and ADH are peptides acting on surface receptors.
Which of the following statements about the vertebrate endocrine system are correct?
1. In iodine deficiency the plasma TSH concentration falls
2. ADH and oxytocin are made by hypothalamic neurons and released from the posterior pituitary
3. Aldosterone is secreted by the zona fasciculata of the adrenal cortex
4. Prolactin secretion is held down mainly by dopamine from the hypothalamus
Show answer
Answer: B — ADH and oxytocin are made by hypothalamic neurons and released from the posterior pituitary; D — Prolactin secretion is held down mainly by dopamine from the hypothalamus
ADH and oxytocin are synthesised in the supraoptic and paraventricular nuclei and stored in the posterior pituitary, and prolactin is the one anterior pituitary hormone under mainly inhibitory (dopamine) control. In iodine deficiency T4 falls, negative feedback weakens and TSH rises, causing goitre. Aldosterone comes from the zona glomerulosa; the fasciculata makes cortisol.
The corpora allata are surgically removed from an early-instar silkworm larva, leaving its brain and prothoracic glands intact. What happens at the following moults?
1. The larva stops moulting altogether, because without the corpora allata no ecdysone is produced
2. The larva passes through extra larval moults and grows into a giant larva
3. The larva moults precociously into a miniature pupa and then a miniature adult
4. The larva dies at the next moult, because PTTH can no longer be released from the brain
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Answer: C — The larva moults precociously into a miniature pupa and then a miniature adult
Juvenile hormone from the corpora allata keeps moults larval. Without it, the ecdysone pulses from the intact prothoracic glands still trigger moults, but these become metamorphic, giving a small pupa and adult early. Extra larval instars are the result of the opposite experiment, implanting active corpora allata. Ecdysone comes from the prothoracic glands and PTTH from the brain, neither of which was removed.
Secretion of a peptide hormone stops abruptly when its plasma concentration is 160 pg/mL. Its plasma half-life is 8 minutes and its removal follows first-order kinetics. What is its plasma concentration, in pg/mL, 32 minutes later?
Numerical answer — type the value.
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Answer: 10
32 minutes is 32 ÷ 8 = 4 half-lives, so the concentration is 160 × (1/2)⁴ = 160 ÷ 16 = 10 pg/mL. Subtracting a fixed amount per half-life, a linear decline, is the usual error.
Which of the following statements about the human male reproductive system are correct?
1. Leydig cells respond to LH by secreting testosterone
2. Meiosis of the male germ cells is completed in the epididymis
3. The scrotum keeps the testes slightly warmer than the core of the body
4. Sertoli cells are the main target of FSH and secrete inhibin
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Answer: A — Leydig cells respond to LH by secreting testosterone; D — Sertoli cells are the main target of FSH and secrete inhibin
LH acts on Leydig cells to produce testosterone, and FSH acts on Sertoli cells, which secrete inhibin that feeds back on FSH. Meiosis is completed inside the seminiferous tubules; the epididymis is where sperm mature and are stored. The scrotum keeps the testes 2 to 3 °C cooler than the body core, not warmer.
A woman has regular 31-day menstrual cycles, with day 1 the first day of menstrual bleeding. Her luteal phase, counted from the day after ovulation to the last day of the cycle, lasts 14 days. On which day of the cycle does she ovulate?
Numerical answer — type the value.
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Answer: 17
The luteal phase occupies the last 14 days of the cycle, days 18 to 31, so ovulation falls on day 31 − 14 = 17. The same rule gives day 14 for a 28-day cycle; the extra three days of a longer cycle are added to the follicular phase, not the luteal one.
In early human pregnancy the corpus luteum does not regress and menstruation does not occur. What keeps the corpus luteum secreting progesterone?
1. A second LH surge released by the pituitary in the middle of the luteal phase
2. A rise in FSH that converts the corpus luteum back into a growing follicle
3. Oxytocin from the posterior pituitary, which stops the endometrium from shedding
4. hCG from the trophoblast of the embryo, which acts on the corpus luteum like LH
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Answer: D — hCG from the trophoblast of the embryo, which acts on the corpus luteum like LH
The trophoblast secretes hCG, which binds the LH receptor of the corpus luteum and sustains progesterone secretion until the placenta takes over. Pituitary LH is suppressed by luteal progesterone, so no second surge occurs; that is the tempting but wrong idea. FSH does not convert a corpus luteum into a follicle, and oxytocin acts on uterine contraction and milk ejection, not on maintaining the endometrium.
How are male honeybees (drones) produced?
1. By parthenogenesis from unfertilised eggs, so that drones are haploid
2. From fertilised eggs whose larvae are fed only on royal jelly by nurse workers
3. By sequential hermaphroditism, in which older workers change sex
4. From fertilised eggs that lose one X chromosome during cleavage
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Answer: A — By parthenogenesis from unfertilised eggs, so that drones are haploid
Honeybees are haplodiploid: unfertilised eggs develop parthenogenetically into haploid males, while fertilised eggs give diploid females. Feeding royal jelly to female larvae decides queen versus worker, not sex, which makes it the tempting distractor. Workers do not change sex, and honeybee sex is not determined by X chromosomes.