The Nervous System

Introduction

The nervous system is the master control and communication system of the body. It is responsible for every thought, action, and emotion, using electrical impulses to communicate rapidly and efficiently. This study guide covers the organization, structure, and function of the nervous system, including its special senses and common homeostatic imbalances.

Part I: Organization of the Nervous System

Structural and Functional Classification

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and command center of the body.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS, such as cranial and spinal nerves. It connects the CNS to limbs and organs.

• Sensory (Afferent) Division: Carries impulses from sensory receptors to the CNS. Keeps the CNS informed of changes inside and outside the body.

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs (muscles and glands). Subdivided into:

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Further divided into:

    • Sympathetic Division: Prepares the body for 'fight or flight' responses.

    • Parasympathetic Division: Promotes 'rest and digest' activities.

Key Terms: afferent (toward CNS), efferent (away from CNS), integration (processing and interpreting sensory input).

Part II: Nervous Tissue

Neuroglia and Neurons

• Neuroglia (Glial Cells): Support, insulate, and protect neurons. Types include astrocytes, microglia, ependymal cells, oligodendrocytes (CNS), Schwann cells, and satellite cells (PNS).

• Neurons: Specialized cells that transmit nerve impulses. Each neuron has a cell body (with nucleus), dendrites (receive signals), and a single axon (sends signals away from the cell body).

• Axon Hillock: The region where the axon originates from the cell body.

• Axon Terminals: Branches at the end of the axon that form synapses with other cells.

Functional Classification of Neurons:

• Sensory (Afferent) Neurons: Carry impulses from receptors to the CNS.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors.

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Part III: Nerve Impulse and Reflex Physiology

Generation and Conduction of Nerve Impulses

• Irritability: Ability to respond to a stimulus and convert it into a nerve impulse.

• Conductivity: Ability to transmit the impulse to other neurons, muscles, or glands.

• Action Potential: The electrical event that propagates along the membrane of a neuron. Initiated when a threshold stimulus depolarizes the membrane.

• Synapse: The junction between two neurons or a neuron and an effector cell, where neurotransmitters facilitate signal transmission.

Reflexes: Rapid, predictable, involuntary responses to stimuli. Types include somatic (involving skeletal muscles) and autonomic (involving smooth/cardiac muscle or glands).

Part IV: Anatomy of the Brain and Spinal Cord

Brain Structure and Function

• Cerebrum: Largest part, divided into hemispheres and lobes (frontal, parietal, temporal, occipital). Responsible for higher brain functions such as thought, memory, and voluntary movement.

• Diencephalon: Includes thalamus (sensory relay), hypothalamus (homeostasis), and epithalamus.

• Brain Stem: Includes midbrain, pons, and medulla oblongata. Controls vital functions such as heart rate, breathing, and blood pressure.

• Cerebellum: Coordinates movement and balance.

Spinal Cord: Approximately 17 inches long, provides a two-way conduction pathway to and from the brain. Protected by vertebrae, meninges, and cerebrospinal fluid. Contains 31 pairs of spinal nerves.

Part V: Peripheral Nervous System

Nerves and Their Classification

• Cranial Nerves: 12 pairs, primarily serve the head and neck.

• Spinal Nerves: 31 pairs, serve the rest of the body.

• Mixed Nerves: Contain both sensory and motor fibers.

• Sensory Nerves: Carry impulses toward the CNS only.

• Motor Nerves: Carry impulses away from the CNS only.

Part VI: Autonomic Nervous System

Sympathetic and Parasympathetic Divisions

• Sympathetic Division: Mobilizes the body during activity ("fight or flight"). Increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Conserves energy and promotes housekeeping functions ("rest and digest"). Slows heart rate, stimulates digestion.

Part VII: Homeostatic Imbalances of the Nervous System

Common Disorders

• Multiple Sclerosis: Autoimmune destruction of myelin in the CNS, leading to impaired nerve transmission.

• Hydrocephalus: Accumulation of cerebrospinal fluid in the brain, increasing intracranial pressure.

• Concussion: Temporary brain dysfunction due to trauma.

• Cerebrovascular Accidents (Stroke): Disruption of blood supply to the brain, causing tissue death.

• Paralysis: Loss of muscle function due to nervous system injury.

• Cerebral Palsy: Group of disorders affecting movement and muscle tone, often due to brain damage before or at birth.

• Spina Bifida: Incomplete formation of the vertebral arches, potentially exposing the spinal cord.

Part VIII: Eye and Vision

Anatomy and Physiology of the Eye

• Special Senses: Include vision, hearing, balance, smell, and taste.

• Eye Structure: Composed of three layers (fibrous, vascular, and sensory) and filled with fluids (humors) to maintain shape.

• Light Refraction: Light is bent as it passes through the cornea and lens to focus on the retina.

• Common Disorders: Night blindness (vitamin A deficiency), color blindness (cone cell defect), cataracts (clouding of lens), glaucoma (increased intraocular pressure).

Part IX: Hearing, Balance, Smell, and Taste

Ear Anatomy and Function

• External Ear: Auricle and external auditory canal direct sound waves to the tympanic membrane (eardrum).

• Middle Ear: Contains ossicles (malleus, incus, stapes) that transmit vibrations to the inner ear.

• Inner Ear: Bony labyrinth with cochlea (hearing), vestibule, and semicircular canals (balance).

• Chemoreceptors: Detect chemicals for smell (olfactory receptors) and taste (taste buds).

Part X: Homeostatic Imbalances of the Special Senses

Common Disorders

• Conjunctivitis: Inflammation of the conjunctiva ("pink eye").

• Night Blindness: Poor vision in low light, often due to vitamin A deficiency.

• Color Blindness: Inability to distinguish certain colors, usually genetic.

• Glaucoma: Increased pressure in the eye, can damage the optic nerve.

• Otitis Media: Middle ear infection.

• Deafness: Can be conductive (mechanical) or sensorineural (nerve damage).

• Meniere's Syndrome: Inner ear disorder causing vertigo and hearing loss.

• Strabismus: Misalignment of the eyes ("cross-eyed").

• Presbyopia: Age-related loss of lens elasticity, affecting near vision.

Summary Table: Divisions of the Nervous System

Key Equations

• Nernst Equation (for membrane potential):

• Ohm's Law (for current flow):

Study Tips

• Keep track of vocabulary words and their definitions.

• Take good notes and use self-assessment quizzes to prepare for exams.

• Use diagrams and tables to visualize structures and functions.