Nervous System Organization, Structure, and Function: Study Guide for Anatomy & Physiology

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Nervous Tissue and Peripheral Nerve Organization

Overview of Nervous System Organization

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has distinct structures and functions essential for communication, integration, and response to stimuli.

CNS: Composed of the brain and spinal cord; responsible for processing and integrating information.
PNS: Consists of nerves and ganglia outside the CNS; transmits sensory and motor signals between the CNS and the rest of the body.
Sensory (Afferent) Division: Carries information toward the CNS.
Motor (Efferent) Division: Carries information away from the CNS.
Somatic Motor: Controls voluntary movements by targeting skeletal muscle.
Autonomic Motor: Regulates involuntary functions by targeting smooth muscle, cardiac muscle, and glands.

Neuron Structure and Function

Dendrite: Receives input from other neurons or sensory receptors.
Soma (Cell Body): Contains the nucleus and organelles; responsible for cell maintenance.
Nucleus & Nucleolus: Site of genetic material and ribosome production.
Nissl Bodies: Rough endoplasmic reticulum involved in protein synthesis.
Axon Hillock: Initiates action potentials.
Axon: Conducts electrical impulses away from the soma.
Myelin Sheath: Insulating layer that increases conduction speed.
Node of Ranvier: Gaps in myelin that facilitate rapid signal transmission (saltatory conduction).
Axon Terminal: Releases neurotransmitters to communicate with target cells.
Myoneural/Neuromuscular Junction: Site where a motor neuron communicates with a skeletal muscle fiber.

Example: Damage to the axon hillock prevents action potential initiation, disrupting communication with target tissues.

Supporting Cells (Neuroglia)

Cell Type	Location	Main Function
Astrocyte	CNS	Supports neural environment; maintains blood–brain barrier
Microglial cell	CNS	Immune surveillance; debris removal
Ependymal cell	CNS	Lines ventricles and central canal; produces cerebrospinal fluid (CSF)
Oligodendrocyte	CNS	Forms myelin on multiple axons
Satellite cell	PNS	Supports neuron cell bodies in ganglia
Schwann cell	PNS	Forms myelin on a single axon segment

Additional info: Oligodendrocytes can myelinate several axons, while Schwann cells myelinate only one segment of one axon.

Peripheral Nerve Coverings

Covering	What it Surrounds
Epineurium	Whole nerve
Perineurium	Fascicle (bundle of nerve fibers)
Endoneurium	Individual nerve fiber

Spinal Cord, Meninges, and Root Connections

Spinal Cord Structure and Function

Dura mater → Arachnoid mater → Pia mater: Three protective meninges from outermost to innermost; pia closely follows CNS surface.
Subarachnoid space: Located between arachnoid and pia; contains cerebrospinal fluid (CSF).
Anterior median fissure: Deep groove on the anterior surface for orientation.
Gray matter: Central, butterfly-shaped region; contains neuron cell bodies.
White matter: Surrounds gray matter; contains ascending (sensory) and descending (motor) pathways.
Dorsal horn: Receives and processes sensory input.
Ventral horn: Contains somatic motor neuron cell bodies.
Lateral horn: Contains autonomic motor neuron cell bodies (at selected levels).
Dorsal root: Carries sensory input toward the spinal cord.
Dorsal root ganglion: Contains sensory neuron cell bodies.
Ventral root: Carries motor output away from the spinal cord.
Mixed spinal nerve: Contains both sensory and motor axons after roots join.

Example: Damage to the dorsal root impairs sensory input but leaves motor output intact.

Brain Regions, Landmarks, and Communication Routes

Major Brain Regions and Their Functions

Region	Main Contribution
Frontal lobe	Planning, decision-making, voluntary motor control
Parietal lobe	Somatic sensory and spatial processing
Temporal lobe	Hearing, memory, language comprehension
Occipital lobe	Visual processing
Insula	Taste and visceral processing
Thalamus	Relays and processes sensory information to cortex
Hypothalamus	Homeostasis, autonomic regulation, endocrine control
Pineal gland	Melatonin production, circadian timing
Midbrain	Sensory–motor processing, eye movements, reflexes
Pons	Communication with cerebellum, breathing regulation
Medulla oblongata	Cardiovascular/respiratory regulation, protective reflexes
Cerebellum	Movement coordination, accuracy, timing, posture, balance

Spinothalamic pathway: Carries pain and temperature information toward the brain.
Corticospinal pathway: Carries voluntary motor commands from cortex to spinal motor circuits.
Cerebellar circuits: Adjust movement using sensory feedback.

Example: Lesion in the cerebellum leads to poor coordination and balance (ataxia).

Key Brain Landmarks

Gyrus/Sulcus: Raised fold/groove on the cerebral cortex.
Longitudinal fissure: Separates cerebral hemispheres.
Central sulcus: Separates frontal and parietal lobes.
Lateral sulcus: Borders temporal lobe; insula lies deep within.
Parieto-occipital sulcus: Medial boundary between parietal and occipital lobes.
Precentral gyrus: Primary motor cortex (anterior to central sulcus).
Postcentral gyrus: Primary somatosensory cortex (posterior to central sulcus).
Corpus callosum: Connects cerebral hemispheres.
Vermis/Arbor vitae: Connects cerebellar hemispheres/branching cerebellar white matter.

Sensory Receptors, Peripheral Nerves, and Reflexes

Classification of Sensory Receptors

Receptor Type	Information Detected
Mechanoreceptor	Touch, pressure, vibration, stretch
Thermoreceptor	Temperature
Photoreceptor	Light
Chemoreceptor	Chemicals
Nociceptor	Potentially damaging stimuli (pain)
Exteroceptor	External environment
Interoceptor	Internal conditions
Proprioceptor	Body position and movement

Note: A receptor can be classified by both the type of stimulus and its location.

Peripheral Nerve Organization and Body Maps

Term	Main Relationship
Dorsal/Ventral roots	Sensory input/Motor output
Dorsal/Ventral rami	Both mixed; supply back/limbs and anterolateral trunk
Plexus	Network of ventral rami regrouping fibers into peripheral nerves
Dermatome	Skin territory associated with a spinal sensory level
Myotome	Muscle group or movement associated with a spinal motor level
Peripheral nerve distribution	Structures supplied by a named nerve

Plexuses: Cervical (C1–C4): neck; Brachial (C5–T1): upper limb; Lumbar (L1–L4): anterior/medial lower limb; Sacral (L4–S4): posterior lower limb.

Major Limb Nerves and Landmarks

Nerve	Identification Relationship
Ulnar	Passes posterior to medial epicondyle
Median	Anterior forearm toward palm; passes through carpal tunnel
Radial	Posterior arm near humerus; branches to lateral/posterior forearm
Femoral	Anterior thigh, deep to inguinal ligament
Sciatic	Large posterior thigh nerve; divides into tibial and common fibular
Common fibular	Passes around fibular neck
Tibial	Posterior leg; passes behind medial ankle toward sole

Reflex Arcs

Withdrawal Reflex: Sequence: Receptor → sensory neuron → spinal interneurons → motor neuron → skeletal muscle.
Reflexes can occur before conscious perception due to direct spinal cord processing.

Example: Stepping on a sharp object triggers a withdrawal reflex before pain is consciously felt.

Cranial Nerves and Somatic Motor Pathways

Cranial Nerve Overview

Number & Name	Type	Main Functions
I Olfactory	Sensory	Smell
II Optic	Sensory	Vision
III Oculomotor	Motor	Most eye movements, eyelid elevation, pupil constriction
IV Trochlear	Motor	Superior oblique muscle (eye movement)
V Trigeminal	Mixed	Facial sensation, chewing
VI Abducens	Motor	Lateral rectus muscle (eye abduction)
VII Facial	Mixed	Facial expression, anterior tongue taste
VIII Vestibulocochlear	Sensory	Hearing, equilibrium
IX Glossopharyngeal	Mixed	Posterior tongue taste, swallowing
X Vagus	Mixed	Visceral regulation, voice, swallowing
XI Accessory	Motor	Sternocleidomastoid and trapezius muscles
XII Hypoglossal	Motor	Tongue movement

Example: Damage to the abducens nerve (VI) impairs lateral eye movement.

Summary Table: CNS vs. PNS Myelin

Feature	CNS	PNS
Myelinating Cell	Oligodendrocyte	Schwann cell
Number of Axons Myelinated	Multiple	One
Regeneration Capacity	Limited	Greater

Additional info: Myelin damage in the CNS (e.g., multiple sclerosis) often leads to more severe and less reversible deficits compared to the PNS.

Key Concepts and Applications

Tracing Communication: Be able to trace sensory and motor pathways, label neuron structures, and predict effects of lesions at different points.
Clinical Application: Use anatomical and functional clues to interpret clinical cases (e.g., loss of sensation, motor deficits, reflex changes).
Comparisons: Distinguish between roots, rami, plexuses, and body maps (dermatome, myotome).

Example: Interruption of the ventral root causes motor deficits without affecting sensation.