# **Main Components of the Nervous System** The human nervous system is divided into two main parts: the ***Central Nervous System (CNS)*** and the ***Peripheral Nervous System (PNS)***. | System | Components | Main Function | | :--- | :--- | :--- | | **Central Nervous System (CNS)** | Brain and Spinal Cord | Acts as the body's coordinating center for processing information. | | **Peripheral Nervous System (PNS)**| Nerves branching from CNS (Cranial & Spinal Nerves) | Transmits information from sensory receptors to the CNS and carries commands from the CNS to muscles and glands (effectors). | ## **Central Nervous System (CNS)** The CNS consists of the brain and spinal cord, both of which are hollow and protected by bone and membranes. - **Protection:** - *Bone:* The brain is enclosed by the **cranium**, and the spinal cord is enclosed by the **vertebral column**. - *Meninges:* Three protective membranes cover the CNS: 1. **Dura mater:** The tough outer layer, next to the bone. 2. **Arachnoid mater:** The middle, web-like membrane. 3. **Pia mater:** The delicate inner layer, adhering to the surface of the brain and spinal cord. - *Cerebrospinal Fluid (CSF):* Found between the arachnoid and pia mater, this fluid cushions the CNS from physical shock. ### **The Brain** The brain is the primary control center of the nervous system and is divided into three main parts. **1. Forebrain** The largest and most complex part of the brain, consisting of the cerebrum, thalamus, and limbic system. - ***Cerebrum:*** - *Structure:* The largest part of the brain, divided into two **cerebral hemispheres** connected by the **corpus callosum**. The surface, or **cerebral cortex**, is highly folded into ridges (*gyri*) and grooves (*sulci*). - *Lobes:* Each hemisphere has four lobes: **frontal, parietal, temporal, and occipital**. - *Functional Areas:* - **Sensory Area:** Receives impulses from the body. - **Association Area:** Interprets and analyzes information. - **Motor Area:** Controls voluntary responses. - *Higher Functions:* Responsible for memory, reasoning, judgment, thoughts, and dreams. - ***Thalamus:*** - *Function:* Acts as a relay station for all sensory impulses (except smell), directing them to the appropriate regions of the cerebrum. - ***Limbic System:*** - A set of structures involved in emotion, memory, and homeostasis. - **Hypothalamus:** Regulates homeostasis (hunger, thirst, body temperature, blood pressure) and controls the pituitary gland, linking the nervous and endocrine systems. - **Amygdala:** Controls emotions such as love, hate, anger, fear, and sexual arousal. - **Hippocampus:** Crucial for converting short-term memory into long-term memory. **2. Midbrain** - *Function:* Acts as a relay station for nerve tracts between the cerebrum, spinal cord, and cerebellum. It contains the *reticular formation*, which connects the hindbrain with the forebrain. **3. Hindbrain** Located at the base of the brain, it controls vital bodily functions. - ***Cerebellum:*** Controls equilibrium (balance), body position, and coordinates complex muscle actions like walking and running. It is also involved in learning and memory for behaviors. - ***Pons:*** Acts as a bridge connecting the cerebellum, medulla, and cerebrum. It helps control the rate and pattern of heartbeat and breathing. - ***Medulla Oblongata:*** Controls automatic functions such as heartbeat, blood pressure, respiration, and swallowing. ### **The Spinal Cord** A vital link that extends from the medulla oblongata down the vertebral column. - *Function:* Transmits nerve impulses between the brain and the rest of the body. - *Structure:* - A cross-section reveals an outer region of **white matter** (myelinated nerve fibers) and a central, H-shaped region of **grey matter** (neuron cell bodies and unmyelinated fibers). - The arrangement of white and grey matter is opposite to that in the brain. - A tiny **central canal** filled with CSF runs through the center. - **Spinal nerves** emerge from the dorsal (back) and ventral (front) roots of the grey matter. ## **Peripheral Nervous System (PNS)** The PNS consists of all the nerves that branch out from the CNS, connecting it to the rest of the body. It includes cranial and spinal nerves. ### **Cranial Nerves** - There are **12 pairs** of cranial nerves that arise directly from the brain. - They transmit signals between the brain and the head, neck, and torso, controlling functions like smell, sight, taste, hearing, and facial movement. | # | Name | Type | Function | |:--|:---|:---|:---| | I | Olfactory nerve | Sensory | Sense of smell | | II | Optic nerve | Sensory | Sense of sight | | III | Oculomotor nerve | Motor | Movement of eyes | | IV | Trochlear nerve | Motor | Rotation of the eye ball | | V | Trigeminal nerve | Mixed | Functioning of the facial parts | | VI | Abducens nerve | Mixed | Rotation of the eye ball | | VII | Facial nerve | Mixed | Facial expression and sense of taste | | VIII | Auditory nerve | Sensory | Sense of hearing and balance | | IX | Glossopharyngeal nerve | Mixed | Ability to taste and swallow | | X | Vagus nerve | Mixed | Digestion and heart rate | | XI | Accessory nerve | Motor | Shoulder and neck movement | | XII | Hypoglossal nerve | Motor | Ability to move tongue | ### **Spinal Nerves** - There are **31 pairs** of spinal nerves that emerge from the spinal cord. - Each spinal nerve is a *mixed nerve*, containing both sensory and motor fibers. - They are categorized by the region of the vertebral column they emerge from: - **8 Cervical** pairs - **12 Thoracic** pairs - **5 Lumbar** pairs - **5 Sacral** pairs - **1 Coccygeal** pair ## **Subdivisions of the PNS** The PNS is further divided based on function. 1. **Somatic Nervous System:** - Controls *conscious* and *voluntary* activities. - Connects the CNS to the skin and skeletal muscles. 2. **Autonomic Nervous System (ANS):** - Controls *unconscious* and *involuntary* activities. - Connects the CNS to visceral organs (heart, stomach, glands). - It has two opposing divisions: | Division | Function | State | Key Actions | | :--- | :--- | :--- | :--- | | **Sympathetic** | Prepares the body for emergency situations | "Fight or Flight" | Increases heart rate, blood pressure; diverts blood to muscles. | | **Parasympathetic** | Returns the body to a normal, resting state | "Rest and Digest" | Slows heart rate; stimulates digestion; conserves energy. | ## **Sensory Receptors and their Working** Specialized cells that detect stimuli from the internal and external environment. - **Olfactory Receptors (Smell):** *Chemoreceptors* in the upper nasal cavity that are stimulated by dissolved chemicals (gases). - **Taste Receptors (Gustation):** *Taste buds* on the tongue's papillae contain taste cells that detect four primary sensations: sweet, sour, salty, and bitter. - **Sensory Receptors in Skin:** - ***Mechanoreceptors (Touch/Pressure):*** - *Meissner's corpuscles & Merkel disks:* Detect light touch. - *Pacinian corpuscles:* Detect deep pressure. - ***Thermoreceptors:*** Detect changes in temperature (cold and heat). - ***Nociceptors (Pain):*** Free nerve endings that respond to tissue damage or extreme stimuli. ## **Application: Nerve Gas and Acetylcholinesterase** - Nerve gases like *sarin* are potent inhibitors of the enzyme **acetylcholinesterase**. - Normally, this enzyme breaks down the neurotransmitter *acetylcholine* in muscle synapses, allowing muscles to relax after contraction. - By blocking the enzyme, nerve gas causes acetylcholine to build up, leading to continuous muscle contraction. - If this affects the diaphragm, breathing becomes impossible, leading to death.