Structure of neurons, brain, spinal cord, reflex actions, major sense organs

1. Introduction to the Nervous System

1. The nervous system is responsible for coordination, communication, and response to stimuli.
2. It is divided into:
   • Central Nervous System (CNS): Includes the brain and spinal cord.
   • Peripheral Nervous System (PNS): Includes nerves outside the CNS.
   • Autonomic Nervous System (ANS): Controls involuntary actions (sympathetic & parasympathetic systems).

2. Structure of a Neuron

1. The neuron is the structural and functional unit of the nervous system.
2. A neuron consists of:
   • Cell body (Soma): Contains the nucleus and cytoplasm.
   • Dendrites: Short extensions that receive signals.
   • Axon: A long fiber that transmits impulses away from the cell body.
   • Myelin Sheath: Fatty covering that insulates the axon.
   • Synapse: The junction between two neurons for signal transmission.

3. The Human Brain

1. The brain is the control center of the body.
2. It is protected by the skull and surrounded by cerebrospinal fluid.
3. The brain is divided into three major parts:
   • Cerebrum: Controls voluntary actions, intelligence, memory, and emotions.
   • Cerebellum: Maintains balance, posture, and coordination.
   • Medulla Oblongata: Controls involuntary functions like breathing and heartbeat.

4. The Spinal Cord

1. The spinal cord is a long, cylindrical structure running through the vertebral column.
2. It acts as a pathway for nerve impulses between the brain and body.
3. It controls reflex actions and involuntary movements.

5. Reflex Actions

1. A reflex action is an automatic and rapid response to a stimulus.
2. The pathway followed by a reflex is called the reflex arc.
3. Example: Pulling back the hand when touching something hot.

6. Major Sense Organs

1. There are five major sense organs: Eyes, Ears, Nose, Tongue, and Skin.
2. They help in perceiving different stimuli from the environment.

6.1 Eyes (Organ of Vision)

1. The eyes detect light and help in vision.
2. Main parts of the eye:
   • Cornea: Transparent layer that focuses light.
   • Iris: Controls the size of the pupil.
   • Pupil: Regulates light entry.
   • Retina: Contains photoreceptors (rods for dim light, cones for color vision).
   • Optic Nerve: Transmits visual signals to the brain.

6.2 Ears (Organ of Hearing and Balance)

1. The ears help in hearing and maintaining balance.
2. Main parts of the ear:
   • Outer Ear: Includes pinna and auditory canal.
   • Middle Ear: Contains ossicles (malleus, incus, stapes) that amplify sound.
   • Inner Ear: Contains the cochlea (for hearing) and semicircular canals (for balance).

6.3 Nose (Organ of Smell)

1. The nose detects smell and aids in respiration.
2. Olfactory receptors in the nasal cavity sense different odors.

6.4 Tongue (Organ of Taste)

1. The tongue detects taste and aids in speech.
2. Different taste buds sense:
   • Sweet – Tip of the tongue.
   • Sour – Sides of the tongue.
   • Salty – Front and sides of the tongue.
   • Bitter – Back of the tongue.
   • Umami – Meaty or savory taste.

6.5 Skin (Organ of Touch)

1. The skin is the largest sense organ.
2. It detects pressure, temperature, and pain.
3. Different receptors include:
   • Meissner’s Corpuscles: Detect light touch.
   • Pacinian Corpuscles: Detect deep pressure.
   • Thermoreceptors: Detect temperature changes.
   • Nociceptors: Detect pain.

7. Nervous System Disorders

1. Alzheimer’s Disease: Progressive memory loss due to neuron degeneration.
2. Parkinson’s Disease: Movement disorder caused by dopamine deficiency.
3. Epilepsy: Seizures due to abnormal brain activity.
4. Multiple Sclerosis (MS): Autoimmune disorder affecting nerve function.

8. Summary of the Nervous System and Sense Organs

9. Conclusion

1. The nervous system is essential for body coordination and communication.
2. Sense organs help in perceiving external stimuli.
3. Proper brain function is crucial for survival and response to the environment.