Topic 6 - co-ordination

Stimuli and receptors

• Stimulus: detectable change in the environment.

• Receptors: cells that detect specific stimuli and generate a generator potential.

• Organism survival increases when appropriate responses occur.

Plant tropisms

• Tropism: plant growth response to a stimulus via growth.

  • Positive tropism: growth toward stimulus.

  • Negative tropism: growth away from stimulus.

• Main stimulus types: light (phototropism) and gravity (gravitropism).

• Growth factor: indoleacetic acid (IAA, an auxin).

  • Produced in tips of shoots and roots; diffuses to other cells.

  • In shoots: IAA increases cell wall plasticity→ results in cell elongation → shaded side elongates → shoot bends toward light (positive phototropism).

  • In roots: high IAA inhibits elongation → lower side inhibited → root bends downward (positive gravitropism), roots show negative phototropism.

• Gravitropism:

  • In shoots IAA causes cells on lower side to elongate → shoot grows upward (negative gravitropism relative to gravity when necessary).

  • In roots IAA inhibits lower-side elongation → root bends down (positive gravitropism).

Simple animal response

• Taxis: directional whole-body movement toward/away from a stimulus.

  • Positive taxis: toward stimulus.

  • Negative taxis: away from stimulus.

• Kinesis: non-directional change in activity (speed/turning rate) to stay in favorable conditions.

  • Increase turning in poor conditions to find better area; if surrounded by poor conditions, decrease turning to move straight.

Receptors and nervous system

• Reflex: rapid, automatic, involuntary response protecting tissues.

• Nervous system:

  • Central Nervous System (CNS): brain and spinal cord (coordination center).

  • Peripheral Nervous System (PNS): sensory and motor neurons (receptors and effectors).

• Reflex arc: receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector.

  • Typically three neurons, two synapses, fast response bypassing conscious brain.

Key human receptors

• Pacinian (Pacinian) corpuscle: pressure receptor in skin; sensory neuron wrapped in lamellae.

  • Stretch-mediated Na+ channels open when deformed → Na+ influx → generator potential.

• Rod cells (retina): detect low light, black-and-white, high sensitivity, retinal convergence (many rods → one bipolar cell) → low visual acuity.

• Cone cells (retina): three types with iodopsin (red, green, blue), require high light intensity for color vision, one cone → one bipolar cell → high visual acuity.

• Distribution: fovea dense with cones (high acuity); rods concentrated away from fovea.

Control of the heart

• Cardiac muscle is myogenic (contracts spontaneously); rate controlled by electrical conduction and nervous input.

• Conduction system:

  • Sinoatrial node (SAN): natural pacemaker in right atrium; emits depolarization wave.

  • Atrioventricular node (AVN): at atrial-ventricular border, delays signal.

  • Bundle of His → Purkinje fibers: conduct to ventricles; ventricles contract from apex upwards.

• Medulla controls heart rate via autonomic nervous system:

  • Sympathetic → increases SAN firing → increases heart rate.

  • Parasympathetic → decreases SAN firing → decreases heart rate.

• Receptors affecting heart rate:

  • Chemoreceptors detect blood pH (CO2/H+); low pH → sympathetic stimulation → increase heart rate.

  • Baroreceptors (pressure receptors) detect blood pressure; high pressure → parasympathetic stimulation → decrease heart rate; low pressure → sympathetic stimulation → increase heart rate.

Neuron structure and action potential

• Motor neuron structure:

  • Cell body: organelles, neurotransmitter synthesis.

  • Dendrites: receive inputs.

  • Axon: long conductive fiber; Schwann cells form myelin sheath; gaps = nodes of Ranvier.

• Resting potential: around −70 mV (inside negative). Maintained by Na+/K+ pump (3 Na+ out, 2 K+ in) and membrane permeability (more K+ channels open).

• Action potential:

  • Stimulus opens voltage-gated Na+ channels → depolarization.

  • Threshold at ~−55 mV → rapid Na+ influx → peak ~+40 mV.

  • Na+ channels close; K+ channels open → repolarization and hyperpolarization (≈−80 mV).

  • Restored by Na+/K+ pump to −70 mV.

• All-or-nothing principle: stimulus must reach threshold to trigger full action potential; larger stimulus increases frequency, not amplitude.

• Refractory period: Na+ channels recover; ensures discrete impulses, forward propagation, and limits impulse frequency.

Factors affecting conduction speed

• Myelination and saltatory conduction: action potentials jump node-to-node → faster conduction.

• Axon diameter: larger diameter → less internal resistance → faster conduction.

• Temperature: higher temperature → faster ion diffusion and enzyme activity → increased speed (within physiological limits).

Synapses and neuromuscular junctions

• Synapse process:

  • Action potential arrives at presynaptic knob → Ca2+ channels open → Ca2+ influx.

  • Vesicles fuse with membrane → neurotransmitter released into synaptic cleft.

  • Neurotransmitter binds postsynaptic receptors → opens ion channels (e.g., Na+) → postsynaptic potential.

  • Neurotransmitter is broken down by enzymes and recycled.

• Cholinergic synapse: uses acetylcholine (ACh); ACh broken down by acetylcholinesterase into acetate and choline.

• Summation:

  • Spatial: many presynaptic neurons converge on one postsynaptic neuron.

  • Temporal: one presynaptic neuron fires repeatedly over short time.

• Inhibitory synapses: open Cl− channels (or cause hyperpolarization) making AP less likely.

• Neuromuscular junction: motor neuron to muscle fiber; ACh always the transmitter; triggers muscle action potential → T-tubules → Ca2+ release from sarcoplasmic reticulum → contraction.

Skeletal muscle structure and sliding filament theory

• Key structures:

  • Myofibrils: cylindrical structures in muscle fibers composed of repeating sarcomeres.

  • Sarcomere: functional unit between Z-lines; contains actin (thin) and myosin (thick).

  • Sarcolemma: muscle cell membrane conducting action potentials.

  • Sarcoplasmic reticulum: stores/releases Ca2+ for contraction.

  • Sarcoplasm: cytoplasm with organelles, myoglobin, glycogen.

• Sarcomere bands:

  • Z-lines mark sarcomere ends.

  • A-band: length of myosin (does not change length during contraction).

  • I-band: region of actin not overlapping myosin (shortens during contraction).

  • H-zone: myosin-only region (decreases during contraction).

  • M-line: center of myosin.

• Sliding filament mechanics:

  • AP → Ca2+ release → tropomyosin shifts exposing myosin-binding sites on actin.

  • Myosin heads with ADP + Pi bind actin → crossbridge formation.

  • Power stroke: myosin pivots, pulling actin inward, ADP+Pi released.

  • ATP binds myosin head → detaches from actin.

  • ATP hydrolysis by myosin ATPase (activated by Ca2+) resets myosin head.

  • Cycle repeats while Ca2+ remains high.

  • Stop stimulation → Ca2+ pumped back into SR → tropomyosin blocks sites → muscle relaxes.

• ATP & phosphocreatine:

  • ATP hydrolysis powers detachment and Ca2+ transport.

  • Phosphocreatine regenerates ATP rapidly during short intense activity.

• Glycogen: stored glucose source for ATP production during respiration.

Muscle fibre types

• Slow-twitch (Type I):

  • High myoglobin, rich blood supply, many mitochondria.

  • Slow contraction, sustain aerobic respiration, endurance activities.

  • Lower glycogen stores, produce ATP slowly but more overall via aerobic pathways.

• Fast-twitch (Type II):

  • Thicker, more myosin filaments, large glycogen and phosphocreatine stores.

  • Fast, powerful contractions, suited to short bursts (sprinting, weightlifting).

  • Rely more on anaerobic respiration; rapid ATP production, smaller yield.

Homeostasis and negative feedback

• Homeostasis: physiological control systems maintain internal variables within narrow limits (temperature, pH, blood glucose, water potential).

• Negative feedback: mechanisms restore variables to set points; often separate mechanisms to raise or lower a level.

Blood glucose regulation

• Key organs and hormones:

  • Pancreatic islets (islets of Langerhans): beta cells release insulin; alpha cells release glucagon.

  • Insulin: released when blood glucose high; promotes glucose uptake and glycogen synthesis in liver and muscle; increases insertion of GLUT4 channels (facilitated diffusion).

  • Glucagon: released when blood glucose low; activates second messenger cascade in liver to promote glycogenolysis and gluconeogenesis.

  • Adrenaline: also stimulates glycogenolysis via same second messenger pathway.

• Important processes:

  • Glycogenesis: formation of glycogen from glucose.

  • Glycogenolysis: hydrolysis of glycogen to glucose.

  • Gluconeogenesis: synthesis of new glucose from non-carbohydrate sources (amino acids, glycerol).

• Second messenger model (glucagon/adrenaline):

  • Hormone binds receptor → activates adenylate cyclase → converts ATP to cyclic AMP (cAMP) → cAMP activates protein kinase → phosphorylates enzymes to hydrolyze glycogen.

Diabetes types and management

• Type 1: insufficient insulin production (often autoimmune destruction of beta cells); treated by insulin injections.

• Type 2: insulin resistance (target cells less responsive); linked to obesity and poor diet; managed by diet, exercise, and sometimes insulin.

Osmoregulation and the kidney

• Water potential regulation: keep blood water potential within safe limits to avoid cell swelling or shrinkage.

• Nephron functions:

  • Ultrafiltration (glomerulus → Bowman’s capsule): high hydrostatic pressure forces water and small solutes into capsule; large molecules (proteins, RBCs) remain in blood.

  • Proximal convoluted tubule (PCT): selective reabsorption of all glucose (via Na+-glucose co-transport), most water by osmosis; epithelial cells have microvilli and many mitochondria.

  • Loop of Henle: creates medullary Na+ gradient for water reabsorption.

    • Descending limb: permeable to water, impermeable to ions → water leaves by osmosis, filtrate concentrated.

    • Ascending limb: impermeable to water, actively transports Na+ out → medulla becomes hypertonic; lower limb thickness and presence/absence of aquaporins differ.

  • Distal convoluted tubule and collecting duct: fine-tune water reabsorption; collecting duct carries urine to ureter.

• Ultrafiltration filtration barriers:

  • Fenestrated capillary endothelium (blocks blood cells).

  • Basement membrane (protein-rich gel preventing large proteins).

  • Podocyte filtration slits (further restrict large molecules).

ADH and water balance mechanisms

• Osmoreceptors in hypothalamus detect blood water potential changes:

  • Low blood water potential → osmoreceptors shrink → hypothalamus stimulates increased ADH production.

  • High water potential → osmoreceptors swell → decreased ADH production.

• ADH pathway:

  • ADH produced in hypothalamus, stored/released from posterior pituitary into blood, travels to kidneys.

  • ADH binds receptors on distal convoluted tubule and collecting duct cells → activates intracellular enzyme cascade (phosphorylation) → vesicles fuse with membrane inserting aquaporins → increases membrane permeability to water.

  • Result: more water reabsorbed by osmosis → smaller volume, concentrated urine.

• Responses summary:

  • Excess water (high water potential): less ADH → collecting duct less permeable → large volumes dilute urine.

  • Water deficit (low water potential): more ADH → collecting duct more permeable → small volumes concentrated urine.

Key information

• Key resting potential value: −70 mV.

• Threshold potential for action potential: about −55 mV.

• Peak action potential: about +40 mV.