1.0 The Structural Unit: The Neuron
The nervous system is composed of specialized cells called Neurons. These are unique because they have lost the ability to divide (centrioles are absent) but have gained the incredible ability to transmit electrical signals.
Anatomy of a Nerve Cell
- Cyton (Cell Body): Contains a well-defined nucleus and granular cytoplasm. It features Nissl's Granules, which are associated with protein synthesis.
- Dendrites: Branch-like extensions that receive chemical signals and convert them into electrical impulses, carrying them towards the cyton.
- Axon: The longest extension that carries impulses away from the cyton. It is surrounded by a fatty Myelin Sheath.
- Nodes of Ranvier: Constrictions in the myelin sheath that allow the impulse to "jump" (Saltatory conduction), greatly increasing speed.
- Axon Terminals: Bulb-like structures at the end of the axon that release neurotransmitters into the synapse.
2.0 Types of Neurons & Nerves
In the ICSE syllabus, you must distinguish between the functional types of neurons and the structural types of nerves.
Functional Classification of Neurons
Structure of a Nerve
A nerve is like an underground cable consisting of several bundles (fascicles) of nerve fibers (axons) held together by connective tissue.
- Sensory Nerve: Contains only sensory fibers (e.g., Optic Nerve).
- Motor Nerve: Contains only motor fibers (e.g., Nerve to eye muscles).
- Mixed Nerve: Contains both sensory and motor fibers (e.g., Spinal Nerves).
The Synapse & Neurotransmitters
A Synapse is the functional junction between two neurons. Since there is no physical contact, the electrical impulse is converted into a chemical signal using Acetlycholine or Dopamine. This ensures that the impulse travels in only one direction.
Stimulus is an agent or a sudden change in the environment that causes a reaction in the organism. The reaction of the organism to a stimulus is called the Response.
Nerve impulses are not just "electricity." They are electro-chemical waves. The speed of a nerve impulse in a myelinated fiber can reach up to 120 meters per second!
3.0 The Central Nervous System: The Brain
The brain is lodged in the bony Cranium (skull). It is further protected by three membranous layers called Meninges and a cushioning fluid.
Protective Barriers
- Meninges: Consists of the Dura mater (outer), Arachnoid (middle), and Pia mater (inner).
- Cerebrospinal Fluid (CSF): Found between the meninges and in brain ventricles; it protects against shocks and provides nutrition.
Major Parts of the Brain
The brain is divided into the Forebrain, Midbrain, and Hindbrain. For your exams, focus heavily on the functions of the Cerebrum, Cerebellum, and Medulla Oblongata.
Gray Matter vs. White Matter (In Brain)
In the brain, the Gray Matter (containing cell bodies) is on the Outer side (Cortex), while the White Matter (containing axons) is on the Inner side. This is the opposite of the spinal cord arrangement!
Why is the cerebrum highly folded into Gyri and Sulci?
Ans: To increase the surface area to accommodate more neurons, thereby increasing intelligence/processing power.
If the Medulla Oblongata is damaged, it results in instant death because it controls the vital autonomous systems of the heart and lungs.
4.0 The Spinal Cord
Extending from the Medulla Oblongata, the spinal cord is protected within the Vertebral Column. Like the brain, it is covered by meninges and contains CSF.
Internal Structure (Cross Section)
Unlike the brain, the spinal cord has Gray Matter on the INSIDE (butterfly-shaped) and White Matter on the OUTSIDE.
- Central Canal: A small cavity in the center containing CSF.
- Dorsal Root: Carries sensory fibers into the cord.
- Ventral Root: Carries motor fibers out of the cord.
5.0 Reflex Action & Reflex Arc
A Reflex Action is an immediate, involuntary response to a stimulus without the intervention of the will (conscious mind).
The Reflex Arc
This is the shortest pathway taken by an impulse to travel from the receptor to the effector. In ICSE, the sequence is vital:
Natural vs. Conditioned Reflexes
Ivan Pavlov's Experiment: He trained a dog to associate a bell (stimulus) with food. Eventually, the dog salivated just by hearing the bell. This is the classic example of a Conditioned Reflex.
Your brain "finds out" about a reflex action (like touching a hot plate) after the spinal cord has already moved your hand. This delay is why you feel the pain a split second after you've pulled away!
6.0 The Peripheral Nervous System (PNS)
The PNS consists of nerves that emerge from the brain and the spinal cord. In ICSE, you must remember the exact count of these nerve pairs.
7.0 The Autonomic Nervous System (ANS)
The ANS controls involuntary activities of internal organs (visceral organs). It is divided into two antagonistic (opposing) systems that work like a "gas pedal" and a "brake."
Sympathetic System
The "Fight or Flight" system. It prepares the body for emergencies.
- Dilates pupils
- Increases heartbeat
- Inhibits digestion
Parasympathetic System
The "Rest and Digest" system. It re-establishes normalcy after an emergency.
- Constricts pupils
- Decreases heartbeat
- Stimulates digestion
Anatomy of a Spinal Nerve
Every spinal nerve is a Mixed Nerve. It attaches to the spinal cord via two roots:
1. Dorsal Root: Contains a ganglion and only sensory fibers.
2. Ventral Root: No ganglion; contains only motor fibers.
In "Name the Following," if asked for the nerve that carries impulses to the brain from the eye, it is the Optic Nerve. From the ear, it is the Auditory Nerve. Both are Cranial Nerves.
The Vagus nerve is the longest cranial nerve. It travels all the way from the brainstem to the abdomen, controlling the heart, lungs, and digestive tract!