A&P Nervous System Study Guide: Your Ultimate Resource

1. Understanding the Nervous System: An Overview

The nervous system is the body’s command center, responsible for coordinating actions and transmitting signals between different parts of the body. This intricate network manages everything from breathing and blinking to complex thoughts and emotions. A solid understanding of its components and functions is vital for students and professionals in healthcare.

1.1. The Three Primary Functions of the Nervous System

The nervous system performs three crucial overlapping functions:

Sensory Input: Monitoring internal and external changes through millions of sensory receptors. These changes, known as stimuli, are gathered as sensory input.
Integration: Processing and interpreting sensory input to decide on the appropriate response. This complex analysis ensures coordinated bodily functions.
Motor Output: Effecting a response by activating muscles or glands (effectors) via motor output, translating decisions into actions.
Mental Activity: The brain controls all mental activity, including awareness, reasoning, and long-term memory.
Homeostasis: It depends on the nervous system’s capacity to identify, interpret, and respond to internal and external conditions. It can help stimulate or inhibit the activities of other systems to help maintain a constant internal environment.

1.2. Divisions of the Nervous System: Central and Peripheral

The nervous system is broadly divided into two main divisions:

Central Nervous System (CNS): Comprising the brain and spinal cord, the CNS serves as the integration and command center of the nervous system. It interprets sensory information and issues instructions based on past experiences and current conditions.
Peripheral Nervous System (PNS): The PNS includes all neural structures outside the CNS, mainly nerves extending from the brain and spinal cord. It provides pathways for communication between the CNS and the rest of the body.

1.3. Functional Classification: Sensory and Motor Divisions

The peripheral nervous system is further classified based on function:

Sensory (Afferent) Division: Conveys impulses from sensory receptors to the CNS.
- Somatic Sensory Fibers: Deliver impulses from the skin, skeletal muscles, and joints.
- Visceral Sensory Fibers: Transmit impulses from the visceral organs.
Motor (Efferent) Division: Carries impulses from the CNS to effector organs (muscles and glands).
- Somatic Nervous System: Allows voluntary control of skeletal muscles.
- Autonomic Nervous System (ANS): Regulates involuntary events such as heart rate, digestion, and gland secretion.
  - Sympathetic Division: Mobilizes the body during extreme situations (fight or flight).
  - Parasympathetic Division: Allows us to unwind and conserve energy (rest and digest).

2. Nervous Tissue: The Building Blocks

Nervous tissue, although complex, consists of two primary types of cells: supporting cells (neuroglia) and neurons (nerve cells).

2.1. Neuroglia: The Supporting Cast

Neuroglia, or glial cells, support, insulate, and protect neurons. Different types of neuroglia have specialized functions:

Astrocytes: Abundant, star-shaped cells that form a living barrier between capillaries and neurons, protecting neurons from harmful substances.
Microglia: Spiderlike phagocytes that dispose of debris, including dead brain cells and bacteria.
Ependymal Cells: Line the central cavities of the brain and spinal cord, circulating cerebrospinal fluid with their cilia.
Oligodendrocytes: Wrap their extensions tightly around nerve fibers, producing fatty insulating coverings called myelin sheaths in the CNS.
Schwann Cells: Form myelin sheaths around nerve fibers in the PNS.
Satellite Cells: Act as protective, cushioning cells in the PNS.

2.2. Neurons: The Message Transmitters

Neurons, or nerve cells, are highly specialized to transmit messages (nerve impulses) from one part of the body to another. Their structure supports this function:

Cell Body (Soma): The metabolic center of the neuron, containing a transparent nucleus.
Processes: Armlike fibers that vary in length:
- Dendrites: Convey incoming messages toward the cell body.
- Axon: Generates nerve impulses and conducts them away from the cell body.
Axon Hillock: The conelike region of the cell body where the axon arises.
Axon Terminals: Contain hundreds of tiny vesicles filled with neurotransmitters.
Synaptic Cleft: The tiny gap separating each axon terminal from the next neuron.
Myelin Sheaths: Whitish, fatty material covering most long nerve fibers, protecting and insulating the fibers and increasing the transmission rate of nerve impulses.
Nodes of Ranvier: Gaps or indentations in the myelin sheath formed by individual Schwann cells.

2.3. Neuron Classification: Functional and Structural

Neurons can be classified by function or structure:

Functional Classification:
- Sensory (Afferent) Neurons: Carry impulses from sensory receptors to the CNS.
- Motor (Efferent) Neurons: Carry impulses from the CNS to viscera, muscles, or glands.
- Interneurons (Association Neurons): Connect motor and sensory neurons in neural pathways.
Structural Classification:
- Multipolar Neurons: Have several processes; the most common type, including all motor and association neurons.
- Bipolar Neurons: Have two processes (an axon and a dendrite); rare in adults, found in some special sense organs.
- Unipolar Neurons: Have a single process emerging from the cell body that divides into proximal and distal processes.

3. The Central Nervous System: Brain and Spinal Cord

The CNS consists of the brain and spinal cord, the body’s control center.

3.1. The Brain: The Command Center

The brain, the largest and most complex mass of nervous tissue, is divided into four major regions:

Cerebral Hemispheres: The most superior part of the brain, responsible for higher cognitive functions.
- Gyri: Elevated ridges of tissue on the surface of the cerebral hemispheres.
- Sulci: Shallow grooves separating the gyri.
- Fissures: Deeper grooves that separate large regions of the brain, such as the longitudinal fissure separating the cerebral hemispheres.
- Lobes: Each hemisphere is divided into lobes named after the cranial bones that lie over them: frontal, parietal, temporal, and occipital.
- Cerebral Cortex: The superficial layer of gray matter responsible for speech, memory, logical and emotional response, consciousness, interpretation of sensation, and voluntary movement.
- Cerebral White Matter: Composed of fiber tracts carrying impulses to, from, and within the cortex.
- Basal Nuclei: Islands of gray matter that help regulate voluntary motor activities by modifying instructions sent to the skeletal muscles.
Diencephalon: Sits atop the brain stem and is enclosed by the cerebral hemispheres. Key structures include the thalamus, hypothalamus, and epithalamus.
Brain Stem: About the size of a thumb in diameter and approximately 3 inches long, connecting the brain to the spinal cord. Its structures are the midbrain, pons, and medulla oblongata.
- Midbrain: Extends from the mammillary bodies to the pons inferiorly.
- Pons: A rounded structure that protrudes just below the midbrain, primarily composed of fiber tracts.
- Medulla Oblongata: The most inferior part of the brain stem, containing nuclei that regulate vital visceral activities such as heart rate, blood pressure, and breathing.
- Reticular Formation: A diffuse mass of gray matter extending the entire length of the brain stem, involved in motor control of visceral organs and playing a role in consciousness and the sleep/wake cycle (reticular activating system, RAS).
Cerebellum: Projects dorsally from under the occipital lobe of the cerebrum, providing precise timing for skeletal muscle activity and controlling balance and equilibrium.

3.2. Protecting the CNS: Meninges, CSF, and Blood-Brain Barrier

The CNS is protected by several mechanisms:

Meninges: Three connective tissue membranes covering and protecting the CNS structures:
- Dura Mater: The outermost, leathery layer.
- Arachnoid Mater: The middle, weblike layer.
- Pia Mater: The innermost, delicate layer that clings tightly to the surface of the brain and spinal cord.
Cerebrospinal Fluid (CSF): A watery “broth” similar in composition to blood plasma, forming a protective cushion around the CNS.
Blood-Brain Barrier: Composed of the least permeable capillaries in the body, separating neurons from bloodborne substances and maintaining a stable chemical environment for the brain.

3.3. The Spinal Cord: The Information Highway

The spinal cord, a glistening white continuation of the brain stem, extends from the foramen magnum of the skull to the first or second lumbar vertebra.

Function: Provides a two-way conduction pathway to and from the brain and serves as a major reflex center.
Structure: Enclosed within the vertebral column, cushioned and protected by the meninges.
Spinal Nerves: Thirty-one pairs of spinal nerves arise from the cord and exit from the vertebral column.
Cauda Equina: The collection of spinal nerves at the inferior end of the vertebral canal.

3.4. Gray and White Matter of the Spinal Cord

The spinal cord consists of gray and white matter:

Gray Matter: Looks like a butterfly or a letter H in cross-section, with dorsal (posterior) and ventral (anterior) horns.
White Matter: Composed of myelinated fiber tracts, divided into dorsal, lateral, and ventral columns, each containing fiber tracts with the same destination and function.
- Sensory (Afferent) Tracts: Conduct sensory impulses to the brain.
- Motor (Efferent) Tracts: Carry impulses from the brain to skeletal muscles.

4. The Peripheral Nervous System: Nerves and Ganglia

The PNS consists of nerves and scattered groups of neuronal cell bodies (ganglia) found outside the CNS.

4.1. Structure of a Nerve

A nerve is a bundle of neuron fibers found outside the CNS:

Endoneurium: A delicate connective tissue sheath surrounding each fiber.
Perineurium: A coarser connective tissue wrapping that binds groups of fibers into fascicles.
Epineurium: A tough fibrous sheath that binds all the fascicles together to form the cordlike nerve.
Mixed Nerves: Carry both sensory and motor fibers.
Sensory (Afferent) Nerves: Carry impulses toward the CNS.
Motor (Efferent) Nerves: Carry only motor fibers.

4.2. Cranial Nerves: Serving the Head and Neck

The twelve pairs of cranial nerves primarily serve the head and neck, emerging directly from the brain. They are named and numbered (I-XII) based on their location and function:

Olfactory (I): Carries impulses for the sense of smell.
Optic (II): Carries impulses for vision.
Oculomotor (III): Controls eye movement, eyelid movement, lens shape, and pupil size.
Trochlear (IV): Controls one external eye muscle (superior oblique).
Trigeminal (V): Conducts sensory impulses from the face and activates chewing muscles.
Abducens (VI): Controls the lateral rectus muscle, which rolls the eye laterally.
Facial (VII): Activates facial expression muscles and the lacrimal and salivary glands; carries sensory impulses from taste buds.
Vestibulocochlear (VIII): Transmits impulses for balance and hearing.
Glossopharyngeal (IX): Controls swallowing and saliva production; carries sensory impulses from taste buds and pressure receptors of the carotid artery.
Vagus (X): Carries sensory and motor impulses to the pharynx, larynx, and abdominal and thoracic viscera; promotes digestive activity and regulates heart activity.
Accessory (XI): Activates the sternocleidomastoid and trapezius muscles.
Hypoglossal (XII): Controls tongue movements and carries sensory impulses from the tongue.

4.3. Spinal Nerves and Nerve Plexuses

The thirty-one pairs of human spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord. Each spinal nerve divides into dorsal and ventral rami, containing both sensory and motor fibers.

Dorsal Rami: Serve the skin and muscles of the posterior body trunk.
Ventral Rami: Form the intercostal nerves (T1-T12) and the nerve plexuses:
- Cervical Plexus (C1-C5): Serves the diaphragm (phrenic nerve), skin, and muscles of the shoulder and neck.
- Brachial Plexus (C5-T1): Serves the muscles and skin of the shoulder, thorax, and upper limb (axillary, radial, median, musculocutaneous, and ulnar nerves).
- Lumbar Plexus (L1-L4): Serves the lower abdomen, anterior and medial thigh muscles, and the skin of the anteromedial leg and thigh (femoral and obturator nerves).
- Sacral Plexus (L4-S4): Serves the lower trunk and posterior surface of the thigh, leg, and foot (sciatic, common fibular, and tibial nerves; superior and inferior gluteal nerves).

4.4. The Autonomic Nervous System: Regulating Automatic Functions

The autonomic nervous system (ANS) is the motor subdivision of the PNS that controls body activities automatically, regulating cardiac muscle, smooth muscles, and glands. It consists of two divisions:

Parasympathetic Division: Allows us to “unwind” and conserve energy (rest and digest).
- Preganglionic neurons are located in brain nuclei of cranial nerves III, VII, IX, and X and in the S2-S4 levels of the spinal cord (craniosacral division).
Sympathetic Division: Mobilizes the body during extreme situations (fight or flight).
- Preganglionic neurons are in the gray matter of the spinal cord from T1 through L2 (thoracolumbar division).

5. Physiology of the Nervous System: How It Works

The physiology of the nervous system involves a complex journey of impulses.

5.1. Nerve Impulses: The Electrical Signals

Neurons have two major functional properties:

Irritability: The ability to respond to a stimulus and convert it into a nerve impulse.
Conductivity: The ability to transmit the impulse to other neurons, muscles, or glands.

Key concepts include:

Resting Neuron Membrane: The plasma membrane of a resting neuron is polarized, with fewer positive ions on the inner face than on the outer surface.
Action Potential Initiation and Generation: The permeability properties of the cell’s plasma membrane change briefly due to neurotransmitters.
Depolarization: The inward rush of sodium ions changes the polarity of the neuron’s membrane.
Graded Potential: The local inside is now more positive, and the outside is less positive.
Nerve Impulse (Action Potential): A long-distance signal propagated over the entire axon, or it doesn’t happen at all (all-or-none response).
Repolarization: The outflow of positive ions restores the electrical conditions at the membrane to the polarized state.
Saltatory Conduction: Nerve impulse jumps from node to node along myelinated fibers, increasing the transmission rate.

5.2. The Nerve Impulse Pathway: Communication Between Neurons

The events occurring at the synapse include:

Arrival: The action potential arrives at the axon terminal.
Fusion: The vesicle fuses with the plasma membrane.
Release: Neurotransmitter is released into the synaptic cleft.
Binding: The neurotransmitter binds to a receptor on the receiving neuron’s end.
Opening: The ion channel opens.
Closing: Once the neurotransmitter is broken down and released, the ion channel closes.

5.3. Autonomic Functioning: Balancing Act

Body organs served by the autonomic nervous system receive fibers from both divisions, resulting in antagonistic effects.

Cholinergic Fibers: Parasympathetic fibers release acetylcholine.
Adrenergic Fibers: Sympathetic postganglionic fibers release norepinephrine.

5.4. Sympathetic and Parasympathetic Divisions: Opposing Actions

Sympathetic Division: Often referred to as the “fight-or-flight” system, preparing the body for action during stressful or emergency situations.
Parasympathetic Division: Most active when the body is at rest, promoting normal digestion, elimination, and conserving body energy (the “resting-and-digesting” system).

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7. Common Neurological Disorders: A Clinical Perspective

Understanding the anatomy and physiology of the nervous system provides a foundation for comprehending neurological disorders. Here’s a brief overview of some common conditions:

7.1. Alzheimer’s Disease

A progressive neurodegenerative disorder characterized by memory loss, cognitive decline, and behavioral changes. It is associated with the accumulation of amyloid plaques and neurofibrillary tangles in the brain.

7.2. Parkinson’s Disease

A chronic, progressive movement disorder caused by the loss of dopamine-producing neurons in the substantia nigra. Symptoms include tremors, rigidity, bradykinesia (slow movement), and postural instability.

7.3. Multiple Sclerosis (MS)

An autoimmune disease that affects the brain and spinal cord, causing demyelination of nerve fibers. Symptoms vary widely but can include fatigue, vision problems, muscle weakness, and balance issues.

7.4. Stroke

Occurs when blood supply to the brain is interrupted, leading to brain cell damage. Types of stroke include ischemic (caused by a blood clot) and hemorrhagic (caused by bleeding in the brain).

7.5. Epilepsy

A neurological disorder characterized by recurrent seizures, which are caused by abnormal electrical activity in the brain.

7.6. Neuropathy

Damage to peripheral nerves, which can result from various causes, including diabetes, infections, and trauma. Symptoms include pain, numbness, tingling, and muscle weakness.

Understanding these disorders requires a solid grasp of normal nervous system function. CONDUCT.EDU.VN offers resources to help you understand the pathophysiology, symptoms, and management of these and other neurological conditions.

8. FAQs: A&P Nervous System Study Guide

Here are some frequently asked questions related to studying the anatomy and physiology of the nervous system:

What are the main divisions of the nervous system?
The main divisions are the central nervous system (CNS) and the peripheral nervous system (PNS).
What is the role of neuroglia in the nervous system?
Neuroglia support, insulate, and protect neurons.
What are the different types of neurons, and how do they function?
Sensory neurons carry impulses to the CNS, motor neurons carry impulses from the CNS, and interneurons connect motor and sensory neurons.
What is the structure and function of the brain stem?
The brain stem includes the midbrain, pons, and medulla oblongata, and it regulates vital visceral activities such as heart rate and breathing.
How is the central nervous system protected?
By the meninges (dura mater, arachnoid mater, pia mater), cerebrospinal fluid, and the blood-brain barrier.
What is the function of the spinal cord?
The spinal cord provides a two-way conduction pathway to and from the brain and serves as a major reflex center.
What are cranial nerves, and what do they do?
Cranial nerves emerge from the brain and primarily serve the head and neck, controlling various sensory and motor functions.
What is the autonomic nervous system (ANS), and what are its divisions?
The ANS controls body activities automatically and includes the sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.
How do nerve impulses work?
Nerve impulses involve changes in the electrical charge across the neuron’s membrane, including depolarization and repolarization.
What is the role of neurotransmitters in synaptic transmission?
Neurotransmitters are released into the synaptic cleft, bind to receptors on the receiving neuron, and transmit the nerve impulse.

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