Overview
This lecture covers how organisms detect and respond to changes, focusing on the nervous and endocrine systems, reflexes, sense organs, homeostasis, and plant tropic responses.
Mammalian Nervous System
- The nervous system has two parts: Central Nervous System (CNS—brain and spinal cord) and Peripheral Nervous System (PNS—nerves and neurones).
- Electrical impulses transmit via neurones, coordinating body functions.
Types of Neurones
- Sensory neurones carry impulses from sense organs to CNS.
- Relay (interneurone) connects sensory and motor neurones within the CNS.
- Motor neurones carry impulses from CNS to effectors (muscles/glands).
Reflex Actions & Arc
- Reflex action is a fast, automatic response to a stimulus via a reflex arc.
- The simple reflex arc pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
- Synapses (gaps between neurones) transmit impulses via neurotransmitters and ensure one-way transmission.
Sense Organs & Eye Structure
- Sense organs are groups of receptor cells responding to stimuli (light, sound, touch, temperature, chemicals).
- Main eye parts: cornea (refracts light), iris (controls light entry), lens (focuses), retina (contains rods/cones), optic nerve, and pupil.
Eye Responses: Pupil Reflex & Accommodation
- Pupil reflex adjusts pupil size for light intensity: radial muscles contract in low light, circular muscles contract in high light.
- Accommodation changes lens shape for focusing: ciliary muscles and suspensory ligaments adjust for near/distant objects.
Rods, Cones & Fovea
- Rods detect low-light, black & white; cones detect color in bright light.
- Fovea is where cones are concentrated for detailed vision.
Hormones & Endocrine System
- Hormones are chemicals produced by glands and transported in blood to target organs.
- Key glands: adrenal (adrenaline), pancreas (insulin/glucagon), testes (testosterone), ovaries (oestrogen).
- Adrenaline prepares the body for action (increases heart rate, blood glucose, pupil size).
Homeostasis & Negative Feedback
- Homeostasis keeps internal conditions constant (e.g., temperature, glucose).
- Negative feedback: deviations from a set point trigger responses to restore balance.
Blood Glucose Regulation
- Pancreas releases insulin (lowers glucose) or glucagon (raises glucose) to control blood sugar.
- Diabetes results when this control fails, leading to hyper/hypoglycaemia.
Temperature Regulation
- Body maintains 37°C for optimal enzyme function; too high denatures enzymes.
- Temperature control involves skin, brain, and changes to blood flow (vasoconstriction when cold, vasodilation when hot).
Tropic Responses in Plants
- Auxin is a plant hormone controlling growth responses (tropisms).
- Gravitropism: roots grow towards gravity; phototropism: shoots grow towards light.
- Auxin distribution causes differential cell elongation for bending toward/away from stimuli.
Key Terms & Definitions
- Neurone — nerve cell transmitting electrical impulses.
- Reflex Arc — pathway for automatic responses involving sensory, relay, and motor neurones.
- Synapse — gap between neurones where neurotransmitters pass impulses.
- Homeostasis — maintenance of stable internal environment.
- Negative Feedback — process returning conditions to set point when deviated.
- Auxin — plant hormone controlling growth via tropisms.
- Vasoconstriction — narrowing blood vessels to reduce heat loss.
- Vasodilation — widening blood vessels to increase heat loss.
Action Items / Next Steps
- Review diagrams of neurones, reflex arc, eye structure, and skin layers.
- Practice past exam questions, especially on reflexes and homeostasis.
- Read textbook sections on coordination, endocrine system, and plant responses.