1.1 Human Nervous System
The human nervous system is an important control and coordination system in the body. It detects stimuli, sends information in the form of nerve impulses, processes data, and coordinates appropriate responses.
Structure of the Human Nervous System
The human nervous system is divided into two main parts:
- Central Nervous System (CNS): Consists of the brain and spinal cord. It acts as the integration and command center.
- Peripheral Nervous System (PNS): Consists of 12 pairs of cranial nerves (connecting the brain to sensory and motor organs in the head) and 31 pairs of spinal nerves (connecting the spinal cord to the rest of the body).
Voluntary and Involuntary Actions
- Voluntary Actions: Conscious actions controlled by the cerebrum in the brain.
Pathway: Stimulus → Receptor → Nerve impulse → Brain (Cerebrum) → Motor nerve → Effector (Muscle/Gland) → Response.
Examples: Reading, writing, speaking, dancing, walking.
- Involuntary Actions: Automatic actions that occur without conscious thought, controlled by the medulla oblongata or spinal cord.
- Controlled by Medulla Oblongata: Heartbeat, peristalsis, breathing, vasodilation.
- Reflex Actions (controlled by Spinal Cord): Rapid, automatic responses to dangerous stimuli to protect the body from injury (e.g., pulling hand away from a hot object, knee-jerk reflex).
1.2 Stimuli and Responses in Humans
Human Sense Organs and Sensory Receptors
Humans have five main sensory organs equipped with specific sensory receptors sensitive to distinct stimuli:
- Eye (Sight): Light receptors (Photoreceptors).
- Rods: Sensitive to light intensity (black and white vision in dim light).
- Cones: Sensitive to color vision (Red, Green, Blue) in bright light.
- Ear (Hearing): Auditory receptors sensitive to sound vibrations.
- Nose (Smell): Olfactory cells sensitive to airborne chemical substances dissolved in mucus.
- Tongue (Taste): Taste buds containing gustatory receptors sensitive to dissolved chemical substances.
5 Basic Tastes: Sweet, Sour, Salty, Bitter, and Umami (savory taste from glutamate).
- Skin (Touch): Cutaneous receptors in the dermis and epidermis:
- Pain receptor: Near skin surface (epidermis), detects tissue damage.
- Cold receptor: Detects cold temperatures.
- Heat receptor: Located deeper in dermis, detects heat.
- Touch receptor: Near skin surface, detects light touches.
- Pressure receptor: Deepest layer of dermis, detects heavy pressure.
Limitations of Senses and Vision Defects
- Optical Illusions: Brain misinterprets visual cues due to surrounding patterns or colors.
- Blind Spot: The point on the retina where the optic nerve exits; lacks photoreceptors.
- Short-sightedness (Myopia): Unable to focus on distant objects because the eyeball is too long or the lens is too thick. Images form in front of the retina. Corrected by concave lenses.
- Long-sightedness (Hyperopia): Unable to focus on near objects because the eyeball is too short or the lens is too thin. Images form behind the retina. Corrected by convex lenses.
- Astigmatism: Irregular curvature of the cornea or lens causing blurred vision at all distances. Corrected by cylindrical lenses.
1.3 Stimuli and Responses in Plants
Plants respond to stimuli through directional growth movements called tropisms and non-directional movements called nastic movements.
Tropisms
Tropisms are slow, directional responses dependent on the direction of the stimulus. Positive tropism is growth towards the stimulus; negative tropism is growth away from the stimulus.
- Phototropism: Response to light. Shoots are positively phototropic (grow towards light); roots are negatively phototropic.
- Geotropism / Gravitropism: Response to gravity. Roots are positively geotropic (grow downwards); shoots are negatively geotropic.
- Hydrotropism: Response to water. Roots are positively hydrotropic (grow towards water sources).
- Thigmotropism: Response to touch. Tendrils and climbing stems wrap around supports (e.g., cucumber, pea plant).
Nastic Movements
Rapid responses to external stimuli that do not depend on the direction of the stimulus. Example: Seismonastic movement in Mimosa pudica (touch-sensitive plant) closing its leaflets rapidly when touched as a defense mechanism.
1.4 Importance of Responses to Stimuli in Other Animals
Stereoscopic and Monocular Vision
- Stereoscopic (Binocular) Vision: Both eyes located at the front of the head. Overlapping fields of vision create a 3D image with accurate depth perception. Common in predators (e.g., eagles, tigers, humans).
- Monocular Vision: Eyes located on opposite sides of the head. Wide field of vision with minimal overlap; useful for early detection of predators. Common in prey animals (e.g., rabbits, deer, pigeons).
Stereophonic Hearing
Hearing using both ears, allowing animals to determine the precise direction and location of a sound source by analyzing time and loudness differences between both ears.
Specialized Sensory Organs in Animals
- Electric Fields: Sharks and electric eels detect weak electrical signals produced by prey.
- Pheromones: Insects (e.g., ants, moths) release chemical signals for communication and mating.
- Echolocation: Bats and dolphins emit high-frequency sound waves (ultrasound) to navigate and hunt in pitch darkness.
- Lateral Line System: Fish detect water vibrations and pressure changes.