The Nervous System 🧠

The nervous system is a coordination network in animals that includes the brain, spinal cord, and nerves. It has two main divisions:

• Central Nervous System (CNS) 🧠: Comprising the brain and spinal cord.
• Peripheral Nervous System (PNS) 🌐: Consisting of nerves that connect the CNS to the rest of the body.

Nerves connected directly to the brain are called cranial nerves, while those joined to the spinal cord are known as spinal nerves. The nervous system transmits messages in the form of electrical impulses ⚡, also called nerve impulses.

Neurons are the functional units of the nervous system. They are classified into three main types:

• Sensory Neurons 👀 – Carry impulses from sense organs (receptors) to the CNS.
• Motor Neurons 💪 – Transmit impulses from the CNS to effectors (e.g., muscles and glands).
• Relay Neurons (Interneurons) 🔗 – Link sensory and motor neurons within the CNS.

Sensory Neurons 👀

Sensory neurons transmit impulses from receptors, which detect stimuli, to the CNS. Some key characteristics:

• Carry impulses from sense organs to the CNS.
• Have long dendrons and short axons.
• The cell bodies are not at the end but positioned along the neuron.

Motor Neurons 💪

Motor neurons send impulses from the CNS to effectors, which produce responses, such as glands secreting substances or muscles contracting. Key characteristics:

• Carry impulses from the CNS to effectors.
• Have long axons and short dendrites.
• Their cell bodies are located at the end of the neuron.

Relay Neurons (Interneurons) 🔗

Relay neurons, found in the CNS, connect sensory and motor neurons. They are multipolar, allowing multiple pathways for impulses.

A synapse is the junction between two neurons. Neurons are separated at the synapse by tiny gaps called synaptic gaps or synaptic clefts. Here's how an impulse crosses a synapse:

1. An impulse arrives at the synapse.
2. Vesicles containing neurotransmitters (chemical messengers) move to the end plates.
3. The vesicles release the neurotransmitter into the synaptic gap.
4. The neurotransmitter diffuses across the gap, restarting the impulse in the next neuron.

A reflex action is an automatic response to a stimulus that helps protect the body. Reflexes can be quick (e.g., withdrawing a hand from a hot surface 🔥) or slower (e.g., pupil constriction 👁️). Some examples of reflex actions and their survival values:

• Hand withdrawal from heat 🔥: Prevents burns or injuries.
• Knee-jerk reflex 🦵: Protects balance and stability.
• Blinking when an object nears the eye 👁️: Shields the eyes.
• Tear production when a foreign object enters the eye 😢: Cleanses and protects the eye.

Other reflexes include sneezing 🤧, salivation, peristalsis, vasoconstriction, and vasodilation. Reflex actions are classified as:

• Spinal Reflexes: Impulses pass through the spinal cord (e.g., knee-jerk).
• Cranial Reflexes: Impulses pass through the brain (e.g., pupil reflex).

The reflex arc is the pathway an impulse follows during a reflex action. The knee-jerk reflex arc illustrates this process.

A conditioned reflex is a learned response that occurs due to repeated exposure to a specific stimulus. Unlike unconditioned reflexes, a conditioned reflex relies on an association formed over time.

Pavlov’s Experiment 🐶 demonstrates this concept. Pavlov noted that dogs salivated when they saw food 🍖. After repeatedly ringing a bell 🔔 before feeding the dogs, they began salivating at the sound of the bell alone. The bell sound became a conditioned stimulus, and the salivation, a conditioned reflex.

Conditioning is used to train animals and humans for tasks. In humans, conditioned reflexes include activities like walking 🚶, responding to names, cycling 🚴, and driving 🚗.

The Human Brain and Its Functions 🧠

Brain Regions 🧠

• Forebrain: Contains the cerebrum, hypothalamus, pituitary gland, and olfactory lobe.
• Midbrain: Includes the optic lobe.
• Hindbrain: Consists of the medulla oblongata and cerebellum.

Functions of Brain Parts 🧠

• Medulla Oblongata: Controls automatic processes like heart rate ❤️, breathing 🌬️, and reflexes.
• Cerebellum: Coordinates movement and maintains posture.
• Hypothalamus: Regulates thirst 💧, hunger 🍔, and body temperature 🌡️, maintaining homeostasis.
• Pituitary Gland: Connects the nervous and endocrine systems, releasing hormones.
• Cerebrum: Responsible for intelligence 🧠, memory 🧠, language, and consciousness.

A drug is any substance that alters metabolic reactions. Drugs can be medicinal or harmful.

Drug Dependency and Abuse 🚫

• Dependency (Addiction): When the body cannot function normally without a drug.
• Tolerance: Needing higher doses for the same effect.
• Withdrawal Symptoms: Physical symptoms from stopping an addictive drug.

Classes of Abused Drugs 🚫

• Stimulants (e.g., cocaine, caffeine, nicotine): Increase alertness, heart rate ❤️, and energy levels.
• Depressants (e.g., alcohol, heroin): Reduce anxiety and pain, cause drowsiness 😴.
• Hallucinogens (e.g., marijuana, LSD): Distort perception and reality.

The Eye 👁️

The eye detects light, allowing for vision 👀.

Structure and Functions of Eye Parts

• Sclera: Protects the eye.
• Cornea: Refracts light to form an image on the retina.
• Choroid: Supplies nutrients to the retina and limits reflections.
• Retina: Contains rods and cones for light and color detection.
• Optic Nerve: Transmits visual information to the brain.

The Ear 👂

The ear detects sound and helps maintain balance ⚖️.

Structure and Functions of Ear Parts

• Outer Ear: Collects and directs sound to the ear canal.
• Middle Ear: Amplifies sounds 🔊.
• Inner Ear: Houses the cochlea (for hearing) and the vestibular apparatus (for balance).