The Nervous System: Structure, Function, and Special Senses

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. Key structures:

  • Cell Body: Contains the nucleus and organelles.

  • Dendrites: Receive incoming signals and carry them toward the cell body.

  • Axon: Conducts impulses away from the cell body. Each neuron has one axon, which may branch into axon terminals.

  • Myelin Sheath: Insulating layer around axons, produced by Schwann cells (PNS) or oligodendrocytes (CNS), increasing impulse speed.

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

Nerve Impulse Generation and Conduction

• 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 axon. Generated when a stimulus causes depolarization of the neuron's membrane.

• Synapse: Junction between two neurons or a neuron and an effector cell, where neurotransmitters facilitate impulse transmission.

Reflexes: Rapid, predictable, involuntary responses to stimuli. Types include:

• Somatic Reflexes: Activate skeletal muscles.

• Autonomic Reflexes: Regulate smooth muscle, heart, and 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 functions such as reasoning, memory, and voluntary movement.

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

• Brain Stem: Composed of midbrain, pons, and medulla oblongata. Controls vital functions (breathing, heart rate).

• 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 (CSF).

Part V: Peripheral Nervous System

Nerves and Their Classification

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

• Spinal Nerves: 31 pairs, connect the spinal cord to the body.

• Mixed Nerves: Contain both sensory and motor fibers.

• Sensory Nerves: Carry impulses toward the CNS.

• Motor Nerves: Carry impulses away from the CNS.

Part VI: Autonomic Nervous System

Sympathetic and Parasympathetic Divisions

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

• Parasympathetic Division: Conserves energy ("rest and digest"). Decreases heart rate, stimulates digestion, promotes relaxation.

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 CSF 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 posture, 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

Structure and Function of the Eye

• Layers of the Eye: Sclera (outer), choroid (middle), retina (inner).

• Cornea: Transparent, allows light entry.

• Lens: Focuses light on the retina.

• Retina: Contains photoreceptors (rods for dim light, cones for color vision).

• Optic Nerve: Transmits visual information to the brain.

Vision Physiology: Light is refracted by the cornea and lens to focus images on the retina. The retina converts light into electrical signals sent to the brain.

Part IX: Hearing, Balance, Smell, and Taste

Ear Anatomy and Physiology

• 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: Cochlea (hearing), vestibule and semicircular canals (balance).

Special Senses: Smell (olfaction) and taste (gustation) rely on chemoreceptors that respond to chemical stimuli.

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.

• Cataracts: Clouding of the lens, leading to blurred vision.

• Glaucoma: Increased intraocular pressure, damaging 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 ("crossed eyes").

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

Summary Table: Divisions of the Nervous System

Division	Main Components	Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, command center
Peripheral Nervous System (PNS)	Cranial and Spinal Nerves	Communication lines between CNS and body
Somatic Nervous System	Somatic motor (voluntary)	Conducts impulses to skeletal muscles
Autonomic Nervous System (ANS)	Visceral motor (involuntary)	Regulates smooth muscle, cardiac muscle, glands
Sympathetic Division	Part of ANS	Mobilizes body systems during activity
Parasympathetic Division	Part of ANS	Conserves energy, promotes housekeeping functions

Key Equations and Concepts

• Resting Membrane Potential: $V_m = -70\ \text{mV}$ (typical for neurons)

• Ohm's Law (for nerve conduction): $V = IR$

• Action Potential Threshold: $\text{If}\ V_m \geq V_{\text{threshold}},\ \text{action potential is generated}$

Study Tips

• Keep track of new vocabulary and their definitions.

• Take organized notes and make flashcards for key concepts.

• Use self-assessment quizzes to test your understanding.