Learning Objectives
- Understand the nervous system and its functioning
- Learn about reflex actions and reflex arcs
- Understand the structure and function of the human brain
- Study the endocrine system and hormones
- Understand coordination in plants (tropic and nastic movements)
Key Concepts
Nervous System
The nervous system uses electrical impulses for fast, precise communication. The basic unit is the neuron (nerve cell).
Parts of a neuron: Dendrite (receives stimulus) → Cell body (contains nucleus) → Axon (carries impulse away) → Nerve endings/Synaptic terminals.
Synapse: The gap between two neurons. Impulse crosses as a chemical signal (neurotransmitter) at the synapse.
Reflex Action and Reflex Arc
A reflex action is an involuntary, rapid, automatic response to a stimulus. Examples: pulling hand from a hot object, knee-jerk reflex.
Reflex arc: Receptor → Sensory (afferent) neuron → Spinal cord (relay neuron) → Motor (efferent) neuron → Effector (muscle/gland).
Human Brain
Protected by the skull (cranium) and cerebrospinal fluid. Three main parts:
- Forebrain (Cerebrum): Largest part; centre of intelligence, memory, thinking, sensation, and voluntary actions. Has sensory and motor areas.
- Midbrain: Controls involuntary reflex actions related to sight and hearing.
- Hindbrain: Cerebellum (balance, posture, coordination); Medulla oblongata (involuntary actions like breathing, heartbeat, blood pressure); Pons (connects different parts of the brain).
Coordination in Plants
Plants show two types of movement:
Tropic movements (growth-dependent, directional):
- Phototropism: response to light (shoots grow toward light)
- Geotropism: response to gravity (roots grow downward)
- Hydrotropism: response to water (roots grow toward moisture)
- Chemotropism: response to chemicals (pollen tube toward ovule)
- Thigmotropism: response to touch (tendrils coiling around support)
Nastic movements (non-directional): Example: touch-me-not (Mimosa) leaves folding on touch.
Plant hormones: Auxin (growth, phototropism), Gibberellin (stem elongation), Cytokinin (cell division), Abscisic acid (inhibits growth, wilting).
Endocrine System (Hormonal Control)
Endocrine glands secrete hormones directly into the blood. Hormones are chemical messengers that act on target organs.
- Pituitary gland: Master gland; controls other glands; secretes growth hormone
- Thyroid gland: Thyroxine (regulates metabolism; needs iodine; deficiency causes goitre)
- Pancreas: Insulin (lowers blood sugar) and glucagon (raises blood sugar). Deficiency of insulin causes diabetes.
- Adrenal gland: Adrenaline (fight-or-flight response)
- Testes: Testosterone (male secondary sexual characters)
- Ovaries: Oestrogen (female secondary sexual characters)
Summary
Control and coordination in animals occur through the nervous system (fast, electrical) and endocrine system (slow, chemical). The brain is the main coordinating centre. Reflex arcs provide rapid automatic responses. Plants coordinate through hormones and respond to stimuli through tropic and nastic movements.
Important Terms
- Neuron
- The structural and functional unit of the nervous system
- Synapse
- The junction between two neurons where impulse is transmitted chemically
- Reflex Arc
- The pathway of nerve impulses in a reflex action
- Hormone
- A chemical substance secreted by endocrine glands that regulates body functions
- Tropism
- Directional growth movement of a plant part in response to an external stimulus
Quick Revision
- Neuron: Dendrite → Cell body → Axon → Synaptic terminals
- Reflex arc: Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector
- Cerebrum: thinking; Cerebellum: balance; Medulla: involuntary functions
- Plant hormones: Auxin, Gibberellin, Cytokinin, Abscisic acid
- Insulin controls blood sugar; its deficiency causes diabetes