Coordination, Response & Homeostasis

Mammalian Nervous System

  • CNS: brain + spinal cord; PNS: rest of nerves. Functions: detect, transmit, coordinate/regulate. Impulses: electrical signals on neurones.

Types of Neurones

  • Sensory: receptors → CNS (cell body off middle).

  • Relay: entirely in CNS (short, many dendrites).

  • Motor: CNS → effectors (large cell body at one end).

  • All have myelin sheath (for saltatory conduction) and dendrites (for connections).

Reflex Arc

  • Sequence: stimulus → receptor → sensory → relay (spinal cord) → motor → effector → response. Automatic, fast, protective; brain gets copy.

Synapse

  • Gap where impulses cross chemically. Neurotransmitters released from vesicles, diffuse across cleft, bind receptors → new impulse. Ensures one-way transmission.

Sense Organs & Receptors

  • Skin: pressure, temp, pain. Tongue: taste. Nose: smell. Ear: sound & balance. Eye: light. Pathway: receptor → sensory → CNS → (relay) → motor → effector.

The Eye

  • Cornea (refracts), iris (pupil control), lens (focus), retina (rods & cones), optic nerve. Pupil reflex: adjusts for light intensity. Accommodation: adjusts lens for near/distant vision. Rods: low light; Cones: colour, fovea.

Hormones & Endocrine System

  • Hormones: blood-borne chemicals from glands to target cells. Examples:

    • Adrenaline: 'fight or flight', increases heart/breathing/glucose.

    • Insulin: lowers blood glucose (glucose → glycogen).

    • Glucagon: raises blood glucose (glycogen → glucose).

    • Thyroxine: metabolic rate.

    • Testosterone/Oestrogen: secondary sexual traits.

  • Nervous vs Hormonal: electrical/fast/short vs chemical/slower/long.

Blood Glucose Homeostasis (Negative Feedback)

  • High glucose: pancreas releases insulin → cells take up glucose, liver stores as glycogen → normal.

  • Low glucose: pancreas releases glucagon → liver converts glycogen to glucose → normal.

  • Type 1 diabetes: no insulin production; managed with insulin injections, diet, exercise.

Temperature Homeostasis

  • Set point ≈ 37∘C37^{\circ}C by hypothalamus/skin.

  • Hot: vasodilation, sweating, hairs flat → heat loss.

  • Cold: vasoconstriction, shivering, hairs erect, less sweating → conserve/generate heat.

Tropisms in Plants

  • Positive: towards stimulus; Negative: away.

  • Shoots: +phototropism, −gravitropism.

  • Roots: −phototropism, +gravitropism.

  • Auxin controls growth: in light, moves to shaded side → faster elongation → shoot bends to light. In gravity, accumulates lower side; promotes in shoots, inhibits in roots.