The Human Nervous System
Control and coordination in humans are primarily achieved through the nervous system, which responds to stimuli in the environment. The fundamental unit of the nervous system is the Neuron (nerve cell), which transmits information in the form of electrical impulses. A neuron consists of a cell body, dendrites (which receive signals), and an axon (which conducts signals away). The microscopic gap between two neurons is called a synapse, where electrical signals are converted into chemical signals (neurotransmitters) to cross the gap.
Reflex Action
A reflex action is a sudden, involuntary, and rapid response to a stimulus (e.g., pulling your hand away from a hot object). The pathway taken by nerve impulses in a reflex action is called a Reflex Arc. The signal travels from the receptor to the spinal cord via a sensory neuron, and an immediate response is sent back via a motor neuron to the muscle (effector), bypassing the brain's conscious thought processes to save time and prevent injury.
The Human Brain
The brain is the main coordinating center of the body. It is divided into three major parts:
- Forebrain: The largest and main thinking part of the brain. It includes the cerebrum, which controls voluntary actions, memory, sensory processing, and intellect.
- Midbrain: Connects the forebrain and hindbrain and controls involuntary actions like the change in pupil size.
- Hindbrain: Comprises the pons, medulla (controls involuntary actions like blood pressure, salivation, and vomiting), and the cerebellum (responsible for precision of voluntary actions and maintaining posture and balance).
Coordination in Plants
Plants lack a nervous system, but they respond to environmental stimuli through chemical coordination (plant hormones or phytohormones) and movement.
- Tropic Movements: Directional growth movements in response to stimuli, such as Phototropism (response to light), Geotropism (response to gravity), and Hydrotropism (response to water).
- Plant Hormones: Auxins (promote cell elongation), Gibberellins (help in growth of the stem), Cytokinins (promote cell division), and Abscisic Acid (inhibits growth, causes wilting of leaves).
Hormones in Animals
Chemical coordination in animals is regulated by the Endocrine System, consisting of ductless glands that secrete chemical messengers called hormones directly into the blood. Important glands include:
- Thyroid Gland: Secretes Thyroxin, regulating carbohydrate, protein, and fat metabolism. Requires iodine.
- Pancreas: Secretes Insulin, which regulates blood sugar levels. Deficiency causes diabetes.
- Adrenal Gland: Secretes Adrenaline, the "fight or flight" hormone that prepares the body for emergency situations.
- Pituitary Gland: The "master gland" that secretes growth hormone and regulates other endocrine glands.