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 regulates every thought, action, and emotion through electrical impulses. 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. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves. Connects the CNS to limbs and organs.

Functional Classification:

• Sensory (Afferent) Division: Carries impulses from sensory receptors to the CNS. Keeps the CNS informed of events 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 stress-related activities ("fight or flight").

    • Parasympathetic Division: Promotes rest and digestion ("rest and digest").

Key Terms: integration (processing and interpreting sensory input), response (activation of effectors).

Part II: Nervous Tissue

Neuroglia and Neurons

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

• Neurons: Specialized cells for transmitting 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: Region where the axon arises from the cell body.

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

Structural Classification of Neurons:

• Multipolar: Many dendrites, one axon (most common in CNS).

• Bipolar: One dendrite, one axon (found in special senses).

• Unipolar: Single process that splits into two branches (sensory neurons in PNS).

Functional Classification of Neurons:

• Sensory (Afferent) Neurons: Carry impulses toward the CNS.

• Motor (Efferent) Neurons: Carry impulses away 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 of neurons to respond to stimuli and convert them into nerve impulses.

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

• Action Potential: The electrical event that propagates along the neuron’s membrane.

Steps in Generating a Nerve Impulse:

1. Stimulus causes depolarization of the neuron membrane.

2. If threshold is reached, an action potential is generated.

3. Impulse travels down the axon to axon terminals.

4. Neurotransmitters are released at the synapse to stimulate the next cell.

Reflexes: Rapid, automatic responses to stimuli. Types include:

• Somatic Reflexes: Involve skeletal muscles.

• Autonomic Reflexes: Involve smooth muscle, cardiac muscle, and glands.

Part IV: Anatomy of the Brain and Spinal Cord

Brain Regions and Functions

• Cerebrum: Largest part; responsible for higher functions such as thought, memory, and voluntary movement. Divided into lobes: frontal, parietal, temporal, occipital.

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

• Brain Stem: Includes midbrain (visual/auditory reflexes), pons (breathing), and medulla oblongata (vital centers for heart rate and blood pressure).

• Cerebellum: Coordinates movement and balance.

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

Part V: Peripheral Nervous System

Nerve Structure and Classification

• Nerves: Bundles of axons in the PNS. Classified as:

  • Sensory (Afferent) Nerves: Carry impulses toward the CNS.

  • Motor (Efferent) Nerves: Carry impulses away from the CNS.

  • Mixed Nerves: Contain both sensory and motor fibers.

• Spinal nerves divide into dorsal and ventral rami after formation.

Part VI: Autonomic Nervous System

Sympathetic and Parasympathetic Divisions

• Autonomic Nervous System (ANS): Regulates involuntary activities of smooth muscle, cardiac muscle, and glands.

• Sympathetic Division: Mobilizes body systems during activity ("fight or flight").

• Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest ("rest and digest").

Part VII: Homeostatic Imbalances of the Nervous System

Common Disorders

• Multiple Sclerosis: Autoimmune disease causing demyelination of CNS neurons.

• Hydrocephalus: Accumulation of cerebrospinal fluid (CSF) in the brain.

• Concussion: Temporary brain dysfunction due to trauma.

• Cerebrovascular Accidents (Stroke): Disruption of blood supply to the brain.

• Paralysis: Loss of muscle function.

• Cerebral Palsy: Motor impairment due to brain damage.

• Spina Bifida: Incomplete formation of vertebrae and spinal cord.

Part VIII: Special Senses

Eye and Vision

• Eye Anatomy: Includes structures such as the cornea, lens, retina, and optic nerve.

• Physiology of Vision: Light is refracted by the cornea and lens to focus on the retina, where photoreceptors (rods and cones) convert light into nerve impulses.

Hearing, Balance, Smell, and Taste

• Ear Anatomy: Divided into external, middle, and inner ear. Contains structures for hearing (cochlea) and balance (vestibule, semicircular canals).

• Physiology of Hearing: Sound waves cause vibrations that are transmitted through the ossicles to the cochlea, where they are converted to nerve impulses.

• Smell and Taste: Chemoreceptors in the nasal cavity and taste buds respond to chemical stimuli.

Part IX: Homeostatic Imbalances of the Special Senses

Common Disorders

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

• Night Blindness: Often due to vitamin A deficiency.

• Color Blindness: Genetic inability to distinguish certain colors.

• Cataracts: Clouding of the lens.

• 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 ("cross-eyed").

• Presbyopia: Age-related loss of near vision.

Summary Table: Divisions of the Nervous System

Division	Main Structures	Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, processing, and control
Peripheral Nervous System (PNS)	Cranial and Spinal Nerves	Communication between CNS and body
Somatic Nervous System	Skeletal Muscles	Voluntary movement
Autonomic Nervous System (ANS)	Cardiac and Smooth Muscles, Glands	Involuntary control
Sympathetic Division	Various organs	"Fight or flight" response
Parasympathetic Division	Various organs	"Rest and digest" response

Key Equations

• Nerve Impulse (Action Potential):

$V = IR$

Where V is voltage, I is current, and R is resistance. This equation describes the relationship between electrical potential and current in nerve fibers.

• Refraction of Light (Vision):

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

Where n is the refractive index and θ is the angle of incidence/refraction. This equation explains how light bends as it passes through different media in the eye.

Study Tips

• Keep track of vocabulary words and their definitions.

• Take good notes and prepare for quizzes and exams by reviewing key concepts and structures.

• Use flashcards for memorizing cranial nerves, brain regions, and special senses.

Additional info: This guide is based on a syllabus and lesson objectives for a college-level Anatomy & Physiology course, focusing on the nervous system and special senses. It is suitable for exam preparation and covers all major topics outlined in the provided materials.