Biology VII: Animal Form and Function II (Defence, Coordination, Senses, and Homeostasis)
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The Immune System
- Innate immunity (non-specific): skin and mucous membranes, acid in the stomach, lysozyme, phagocytes (neutrophils, macrophages), inflammation, fever, interferons, complement.
- Adaptive immunity (specific): lymphocytes. B cells produce antibodies (humoral response; antibody structure with variable and constant regions). T cells include helper T cells, cytotoxic T cells, and regulatory T cells (cell-mediated response). Antigen-presenting cells activate T cells. Memory cells give a faster, stronger secondary response.
- Immunity types: natural active (infection), artificial active (vaccination), natural passive (maternal antibodies, breast milk), and artificial passive (antiserum). Vaccines include live attenuated, inactivated, subunit, toxoid, and mRNA types. Herd immunity protects those who cannot be vaccinated. Sri Lanka has a national immunization programme.
- Disorders: allergy (hypersensitivity), autoimmune diseases (type 1 diabetes, rheumatoid arthritis), immunodeficiency (HIV infects helper T cells and causes AIDS), transplant rejection.
- Blood transfusion and organ transplantation involve tissue matching.
The Nervous System
- Neuron structure: cell body, dendrites, axon, myelin sheath, nodes of Ranvier, synaptic terminals. Types: sensory, motor, and interneurons.
- Resting potential: about −70 mV, maintained by the Na⁺/K⁺ pump and K⁺ leak channels.
- Action potential (Hodgkin & Huxley, 1952): a stimulus above threshold opens voltage-gated Na⁺ channels (depolarization), then K⁺ channels open (repolarization), with a brief refractory period. It is all-or-nothing. Saltatory conduction in myelinated axons is faster.
- Synapse: an action potential causes Ca²⁺ entry and release of neurotransmitters (for example acetylcholine) that bind receptors on the next cell. Summation and inhibition integrate signals.
- Organization: the central nervous system (brain and spinal cord) and the peripheral nervous system; the autonomic system has sympathetic (fight or flight) and parasympathetic (rest and digest) divisions. Reflex arc: receptor, sensory neuron, relay neuron, motor neuron, effector.
- Brain regions: cerebrum (higher functions; lobes), cerebellum (balance, coordination), medulla oblongata (breathing, heart rate), hypothalamus (homeostasis, hormones), thalamus, and the limbic system.
- Disorders: stroke, Parkinson's disease, Alzheimer's disease, epilepsy, multiple sclerosis, depression (a medical condition that is treatable).
Sense Organs
- Eye: cornea, aqueous humour, iris, lens, vitreous humour, retina (rods for dim light and cones for colour), fovea, optic nerve, blind spot. Accommodation: the ciliary muscles change the shape of the lens. Defects: myopia, hypermetropia, astigmatism, cataract.
- Ear: outer, middle (ossicles), and inner ear (cochlea for hearing; semicircular canals and vestibule for balance).
- Other senses: taste, smell, touch (skin receptors), temperature, pain. The skin has protective, sensory, and thermoregulatory roles.
The Endocrine System
| Gland | Main hormones and effects |
|---|---|
| Hypothalamus and pituitary | Releasing hormones; growth hormone; TSH; ACTH; FSH and LH; prolactin; ADH and oxytocin |
| Thyroid | Thyroxine (metabolic rate); calcitonin |
| Parathyroid | Parathyroid hormone (raises blood calcium) |
| Adrenal | Adrenaline (acute stress); cortisol (long-term stress); aldosterone |
| Pancreas (islets) | Insulin (lowers blood glucose); glucagon (raises it) |
| Gonads | Testosterone; oestrogen; progesterone |
Hormones act through negative feedback (most) or positive feedback (for example oxytocin in childbirth). Disorders: diabetes mellitus (type 1: insulin deficiency from autoimmune destruction of beta cells; type 2: insulin resistance, strongly linked with lifestyle and a major health issue in Sri Lanka), hyperthyroidism, hypothyroidism (goitre), Cushing's and Addison's diseases.
Homeostasis
Homeostasis maintains a stable internal environment through receptors, a control centre, and effectors.
- Thermoregulation: the hypothalamus detects changes; heat loss through sweating, vasodilation; heat gain by shivering, vasoconstriction.
- Blood glucose control by insulin and glucagon.
- Osmoregulation by ADH and the kidney; pH control by buffers, lungs, and kidneys.
Common Mistakes
- Confusing depolarization and repolarization.
- Stating that hormones travel through ducts (endocrine glands are ductless).
- Mixing up the effects of insulin and glucagon.
CHAPTER 10