Nervous system, reflex arcs, endocrine system, and plant movements
Living organisms need to respond to changes in their environment (stimuli) and coordinate all body functions. This is achieved through two systems: the Nervous System (quick responses) and the Endocrine System (hormones — slower, longer-lasting responses).
A neuron (nerve cell) is the structural and functional unit of the nervous system.
Parts: Cell body (contains nucleus) → Dendrites (receive signals, branch-like) → Axon (transmits signals outward, long fibre) → Synaptic terminal (connects to next neuron)
Signal travels as: Dendrites → Cell body → Axon → Synaptic terminal → next neuron
Sensory neurons: Carry signals from sense organs (eyes, skin) to brain/spinal cord
Motor neurons: Carry signals from brain/spinal cord to muscles/glands
Interneurons: Connect sensory and motor neurons (in brain and spinal cord)
A reflex action is a rapid, automatic, involuntary response to a stimulus — controlled by spinal cord without brain involvement. This saves time in emergencies!
Reflex arc path:
Stimulus → Receptor → Sensory neuron → Spinal cord (interneuron) → Motor neuron → Effector (muscle/gland) → Response
You touch a hot object → heat receptors in skin detect it → sensory neurons send signal → reaches spinal cord → spinal cord sends signal back via motor neuron → muscles contract → hand pulled away!
The brain is informed after the withdrawal — you feel pain a moment later. This is why the withdrawal is so fast!
Forebrain (Cerebrum):
• Largest part; controls conscious activities
• Thinking, reasoning, memory, voluntary movements, interpretation of senses
Midbrain: Connects forebrain to hindbrain; controls eye movements and vision/hearing relay
Hindbrain:
• Cerebellum: Balance and coordination of movements (fine motor control)
• Medulla oblongata: Controls involuntary actions — heartbeat, breathing, blood pressure, vomiting, salivation
• Pons: Relay station between cerebellum and rest of brain; helps in breathing regulation
Plants respond to stimuli by growing towards or away from the stimulus.
Phototropism: Response to light
• Positive (shoot grows toward light) → auxin redistributes to shaded side → shaded side grows more → bends toward light
• Negative (root grows away from light)
Geotropism: Response to gravity
• Root: positive geotropism (grows downward)
• Shoot: negative geotropism (grows upward)
Hydrotropism: Root grows toward water (positive)
Thigmotropism: Response to touch (tendrils wrap around support)
Chemotropism: Response to chemicals (pollen tube grows toward ovule)
Auxin (plant hormone) is produced at shoot tip. When light hits one side, auxin migrates to the shaded side. Cells on shaded side elongate more. This makes the shoot bend toward the light.
Think: Auxin = plant's "growth serum" that makes cells elongate.
Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream. They are produced in small quantities but have large effects. They are non-cellular (no physical connection needed).
| Gland | Hormone(s) | Function |
|---|---|---|
| Pituitary (master gland) | Growth hormone, TSH, FSH, LH, ADH | Controls other endocrine glands; growth; water balance |
| Thyroid | Thyroxine | Controls metabolic rate; requires iodine; deficiency = goitre |
| Pancreas | Insulin, Glucagon | Regulates blood sugar; deficiency of insulin = diabetes mellitus |
| Adrenal | Adrenaline | "Fight or Flight" hormone; increases heart rate, BP, breathing in emergency |
| Testes (male) | Testosterone | Male secondary sexual characters (puberty) |
| Ovaries (female) | Oestrogen, Progesterone | Female secondary characters; menstrual cycle; pregnancy |