The Nervous System

The nervous system is the master controller and communication network of the body. It is a complex, highly organized system that coordinates our actions, reflexes, and sensations, allowing us to interact with and respond to the world around us.

Structure of a Neuron

The fundamental unit of the nervous system is the neuron, a specialized cell designed to transmit information.

Cell Body (Soma or Cyton): The main part of the neuron, containing the nucleus and other organelles. It is the metabolic center of the neuron.
Dendrites: These are short, branched extensions of the cell body that receive electrochemical signals (neurotransmitters) from other neurons and transmit them towards the cell body.
Axon: A long, slender projection that carries nerve impulses away from the cell body to other neurons, muscles, or glands.
Myelin Sheath: Many axons are covered by a fatty, insulating layer called the myelin sheath, which is produced by Schwann cells. This sheath is not continuous and has gaps called Nodes of Ranvier. The myelin sheath allows for saltatory conduction, where the nerve impulse "jumps" from one node to the next, significantly increasing the speed of transmission.
Axon Terminals (Terminal Buttons): These are the branched endings of an axon. They contain synaptic vesicles filled with neurotransmitters, which are released into the synapse to transmit the signal to the next neuron.

Divisions of the Nervous System

The nervous system is broadly divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

The CNS is the body's command center, consisting of the brain and the spinal cord.

The Brain

The brain is the primary control center of the nervous system, responsible for higher-order functions like thought, memory, and emotion.

Cerebrum: The largest part of the brain, divided into two cerebral hemispheres (left and right). It is the center of intelligence, memory, consciousness, and voluntary actions.
Cerebellum: Located at the back of the brain, the cerebellum coordinates voluntary muscle movements, posture, and balance.
Medulla Oblongata: Part of the brainstem, it controls vital involuntary functions like heartbeat, breathing, and blood pressure.
Thalamus: Acts as a relay station for sensory information, directing it to the appropriate areas of the cerebrum for processing.
Hypothalamus: Located below the thalamus, it regulates body temperature, hunger, thirst, and the sleep-wake cycle. It also controls the pituitary gland, linking the nervous system to the endocrine system.
Pons: Part of the brainstem, it relays signals between the cerebrum and the cerebellum and plays a role in sleep and arousal.

Gray and White Matter: The CNS is composed of two types of tissue:

Gray Matter: Consists mainly of neuron cell bodies, dendrites, and unmyelinated axons. In the brain, it forms the outer layer (cerebral cortex), while in the spinal cord, it forms the central H-shaped core.
White Matter: Composed primarily of myelinated axons. In the brain, it is found beneath the cortex, while in the spinal cord, it surrounds the gray matter.

The Spinal Cord

The spinal cord is a long, thin, tubular bundle of nervous tissue that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It serves as a pathway for nerve impulses between the brain and the rest of the body and is the center for coordinating many reflexes.

Peripheral Nervous System (PNS)

The PNS consists of all the nerves that branch out from the brain and spinal cord, connecting the CNS to the rest of the body. It is further divided into the somatic and autonomic nervous systems.

Somatic Nervous System: Responsible for carrying sensory and motor information to and from the CNS. It is associated with voluntary control of body movements via skeletal muscles.
Autonomic Nervous System (ANS): Regulates involuntary bodily functions such as heart rate, digestion, respiratory rate, and pupillary response. It is further divided into:
- Sympathetic Nervous System: Prepares the body for "fight or flight" responses during times of stress.
- Parasympathetic Nervous System: Controls "rest and digest" functions, conserving energy.

Actions: Voluntary and Involuntary

Voluntary Actions: These are actions that are under the conscious control of the individual, such as walking, talking, and writing. They are initiated by the cerebrum.
Involuntary Actions: These are actions that are not under conscious control, such as the beating of the heart and the digestion of food. They are controlled by the autonomic nervous system.

Reflex Action

A reflex action is a rapid, involuntary, and stereotyped response to a stimulus. It is a protective mechanism that allows the body to react quickly to potentially harmful situations.

Reflex Arc: The neural pathway that mediates a reflex action. It typically includes:
1. Receptor: Detects the stimulus.
2. Sensory Neuron (Afferent Neuron): Transmits the nerve impulse from the receptor to the CNS.
3. Interneuron (Association Neuron): Located in the CNS, it processes the signal and transmits it to a motor neuron.
4. Motor Neuron (Efferent Neuron): Carries the nerve impulse from the CNS to the effector.
5. Effector: The muscle or gland that responds to the stimulus.
Natural vs. Acquired Reflexes:
- Natural (Innate) Reflexes: These are reflexes that are present from birth, such as sneezing, coughing, and blinking.
- Acquired (Conditioned) Reflexes: These are reflexes that are learned through experience, such as salivating at the sight of food.

Cornea: The transparent outer layer at the front of the eye that refracts light.
Iris: The colored part of the eye that controls the size of the pupil.
Pupil: The opening in the center of the iris that allows light to enter.
Lens: A transparent, biconvex structure that focuses light onto the retina.
Retina: The light-sensitive layer at the back of the eye containing photoreceptor cells (rods and cones).
Optic Nerve: Transmits visual information from the retina to the brain.

Vision

Accommodation: The process by which the eye changes the shape of its lens to focus on objects at different distances.
Adaptation: The ability of the eye to adjust to different levels of light.
Stereoscopic Vision: The ability to perceive depth and three-dimensional structure due to the slightly different images received by each eye.

Defects of the Eye and Corrective Measures

Myopia (Nearsightedness): The eye focuses images in front of the retina instead of on the retina. Corrected with a concave lens.
Hyperopia (Farsightedness): The eye focuses images behind the retina. Corrected with a convex lens.
Presbyopia: Age-related loss of accommodation. Corrected with bifocal or progressive lenses.
Astigmatism: The cornea is irregularly shaped, causing blurred vision. Corrected with a cylindrical lens.
Cataract: The lens becomes cloudy, obstructing vision. Corrected by surgically replacing the lens.

The Ear

The ear is the organ of hearing and balance.

Parts and Functions

Outer Ear:
- Pinna (Auricle): The visible part of the ear that collects sound waves.
- Auditory Canal: A tube that directs sound waves to the eardrum.
Middle Ear:
- Tympanic Membrane (Eardrum): Vibrates in response to sound waves.
- Ossicles (Malleus, Incus, Stapes): Three tiny bones that amplify and transmit vibrations from the eardrum to the inner ear.
Inner Ear:
- Cochlea: A spiral-shaped, fluid-filled tube containing hair cells that convert sound vibrations into nerve impulses.
- Vestibular System (Semicircular Canals, Utricle, Saccule): Responsible for maintaining balance and equilibrium.

Hearing and Balance

Perception of Sound: Sound waves cause the eardrum to vibrate. These vibrations are amplified by the ossicles and transmitted to the cochlea, where they are converted into nerve impulses that are sent to the brain via the auditory nerve.
Role in Balance: The vestibular system detects changes in head position and movement, sending signals to the brain to help maintain balance.

Nerve Impulse Transmission

The transmission of a nerve impulse is an electrochemical process.

Resting Potential: In a resting neuron, the inside of the membrane is negatively charged relative to the outside (approximately -70mV). This is maintained by the sodium-potassium pump, which actively transports 3 Na+ ions out of the cell for every 2 K+ ions it pumps in.
Action Potential (The "All-or-None" Concept): When a neuron is stimulated above a certain threshold (typically around -55mV), it fires an action potential. This is an "all-or-none" event, meaning that the strength of the action potential is always the same, regardless of the strength of the stimulus.
Depolarization: During an action potential, voltage-gated Na+ channels open, and Na+ ions rush into the cell, causing the inside of the membrane to become positively charged (depolarization).
Repolarization: After a brief period, the Na+ channels close, and voltage-gated K+ channels open. K+ ions rush out of the cell, restoring the negative charge inside the membrane (repolarization).
Hyperpolarization: The K+ channels are slow to close, so for a brief period, the membrane becomes even more negative than the resting potential (hyperpolarization).
Refractory Period: During this time, the neuron cannot fire another action potential. This ensures that the action potential travels in only one direction.
Synaptic Transmission: When the action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synapse. These chemical messengers diffuse across the synapse and bind to receptors on the next neuron, either exciting or inhibiting it.
Synaptic Delay: The brief delay in the transmission of a nerve impulse across a synapse.

Nervous System