Coordination and Control

Two parallel systems

Animals coordinate function through a fast, point-to-point nervous system and a slower, broadcast endocrine system. Punjab Textbook Board Biology XII Chapter 16 and Campbell 12e Chapters 48–49 unify the two with a common logic: stimulus → receptor → integration → effector. The human nervous system has roughly 86 billion neurons (Herculano-Houzel, 2009), of which ≈16 billion are cortical. Glial cells outnumber neurons in white matter and provide myelination (oligodendrocytes in CNS, Schwann cells in PNS), buffering, and immune surveillance.

The neuron — resting and action potentials

A typical neuron rests at −70 mV, set by K⁺ leak channels and the Na⁺/K⁺-ATPase (3 Na⁺ out / 2 K⁺ in per ATP). When threshold (≈ −55 mV) is reached, voltage-gated Na⁺ channels open in milliseconds, depolarising the membrane to about +30 mV. Inactivation of Na⁺ channels and delayed opening of voltage-gated K⁺ channels repolarise it, with a brief afterhyperpolarisation. The action potential is all-or-none and propagates without decrement; in myelinated axons it jumps node-to-node (saltatory conduction), reaching velocities of 100 m/s in large mammalian motor fibres versus ≈1 m/s in unmyelinated C fibres. Hodgkin and Huxley's 1952 squid-axon work, recognised by the 1963 Nobel Prize, formalised the quantitative model.

Synaptic transmission

At a chemical synapse, action-potential arrival opens voltage-gated Ca²⁺ channels in the presynaptic terminal; Ca²⁺ influx triggers SNARE-mediated vesicle fusion and release of neurotransmitter into the 20–40 nm cleft. Acetylcholine, glutamate, GABA, glycine, dopamine, serotonin, and noradrenaline cover most MDCAT-relevant transmission. Acetylcholine at the neuromuscular junction binds nicotinic receptors (ligand-gated cation channels), while muscarinic receptors are G-protein-coupled. Excitatory postsynaptic potentials (EPSPs) summate spatially and temporally; inhibitory postsynaptic potentials (IPSPs) hyperpolarise. Acetylcholinesterase hydrolyses ACh in <1 ms; organophosphate insecticides irreversibly inhibit it, a mechanism shared with nerve agents.

Reflex arcs and CNS organisation

The simplest pathway is the monosynaptic stretch reflex (e.g., the patellar tendon reflex): muscle spindle Ia afferent → α-motor neuron in the spinal cord → quadriceps contraction. Polysynaptic reflexes such as the withdrawal reflex include interneurons that mediate reciprocal inhibition of antagonists. The CNS comprises brain and spinal cord; the peripheral nervous system splits into somatic (voluntary) and autonomic (involuntary) divisions. The autonomic system itself divides into sympathetic (thoracolumbar, noradrenergic at target, "fight or flight") and parasympathetic (craniosacral, cholinergic, "rest and digest"). Cardiac vagal tone, for example, normally keeps resting heart rate ~70 bpm — denervation raises intrinsic SA node rate to ~100 bpm.

Endocrine control and the hypothalamic–pituitary axis

The hypothalamus releases peptide releasing/inhibiting hormones into the hypophyseal portal system, controlling six anterior-pituitary hormones: GH, TSH, ACTH, FSH, LH, and prolactin. The posterior pituitary releases oxytocin and ADH, synthesised in hypothalamic neurons. The thyroid produces T3/T4 (regulating BMR; iodine-deficient regions including parts of northern Pakistan show endemic goitre rates documented by the Aga Khan University surveys). The adrenal cortex produces glucocorticoids (cortisol, peaks ~16 µg/dL at 8 AM), mineralocorticoids (aldosterone), and androgens; the medulla produces adrenaline and noradrenaline. Pancreatic islets release insulin (β-cells), glucagon (α-cells), and somatostatin (δ-cells); insulin lowers blood glucose by promoting GLUT4 translocation in muscle and adipose tissue. Type 1 diabetes is autoimmune β-cell destruction; Type 2 involves insulin resistance — both are leading MDCAT clinical scenarios.