Nervous System and Brain Anatomy Flashcards
General Functions and Structural Organization of the Nervous System
General Functions of the Nervous System:
Collects Information: Receptors detect stimuli (internal or external) and send sensory signals toward the spinal cord and brain.
Processes and Evaluates Information: The Central Nervous System (CNS) analyzes incoming sensory input to determine the appropriate response.
Initiates Response to Information: Motor output is directed from the CNS to effectors (muscles or glands) to bring about a response.
Functional Subdivisions of the Nervous System:
Sensory (Afferent) Nervous System: Receives sensory information from receptors and transmits it to the CNS.
Somatic Sensory: Detects stimuli from external environments (e.g., skin, skeletal muscle, joints, special senses).
Visceral Sensory: Detects stimuli from internal organs (e.g., heart, stomach, viscera).
Motor (Efferent) Nervous System: Transmits motor commands from the CNS to effector organs.
Somatic Motor: Initiates voluntary motor output to control skeletal muscles.
Autonomic Motor (Visceral Motor): Involuntarily controls cardiac muscle, smooth muscle, and glands. Subdivided into:
Sympathetic Nervous System
Parasympathetic Nervous System
Neuronal Structural and Functional Classifications
Structural Classifications of Neurons:
Unipolar Neurons: Possess a single short process extending from the cell body that bifurcates into a T-shape.
Peripheral Process: Extends from dendrites to the cell body.
Central Process: Extends from the cell body into the CNS.
Cell Body: Positioned off to the side of the main axon pathway.
Location: Most sensory neurons in the peripheral nervous system are unipolar.
Bipolar Neurons: Possess two processes extending directly from the cell body: one main dendrite and one axon (e.g., special senses like retina, olfactory epithelium).
Multipolar Neurons: Possess multiple dendrites and a single axon extending from the cell body. This is the most common structural type, including all motor neurons and interneurons.
Functional Classifications of Neurons:
Sensory (Afferent) Neurons: Conduct sensory input from receptors toward the CNS.
Motor (Efferent) Neurons: Conduct motor output away from the CNS to effector organs.
Interneurons (Association Neurons): Reside entirely within the CNS. They receive, process, and store information, establishing connections between sensory and motor pathways. All interneurons are structurally multipolar.
Organization and Structure of Nerves and Connective Tissue Coverings
Anatomy of a Myelinated Nerve Fiber:
Axon: Individual process carrying electrical impulses.
Myelin Sheath: Insulation wrapping around the axon formed by Schwann cells in the PNS (or Oligodendrocytes in the CNS).
Node of Ranvier (Neurofibril Node): Unmyelinated gaps along the axon situated between adjacent Schwann cells.
Connective Tissue Coverings of Nerves:
Endoneurium: Delicate layer of Areolar Connective Tissue wrapping around each individual myelinated axon; assists in electrical insulation.
Fascicles: Groups or bundles of myelinated axons grouped together.
Perineurium: Layer of Dense Irregular Connective Tissue wrapping around each individual fascicle; contains blood vessels supplying the nerve fibers.
Epineurium: Outer tough layer of Dense Irregular Connective Tissue wrapping around the outer perimeter of several fascicles to enclose the entire nerve.
Classifications of Nerves:
Structural Classification:
Cranial Nerves: Extend directly from the brain.
Spinal Nerves: Extend directly from the spinal cord.
Functional Classification:
Sensory Nerves: Contain exclusively sensory neurons transmitting signals to the CNS.
Motor Nerves: Contain exclusively motor neurons transmitting signals from the CNS.
Mixed Nerves: Contain both sensory and motor neurons. Most named nerves in the human body fall into this category. Individual axons within mixed nerves carry only one direction of information.
Functional Segments and Characteristics of Neurons
General Characteristics of Neurons:
Excitability: High responsiveness to stimuli; a stimulus induces rapid changes in local membrane potential.
Conductivity: Ability to propagate electrical signals along the plasma membrane via sequential opening of voltage-gated ion channels.
Secretion: Release of chemical neurotransmitters from synaptic vesicles into a synaptic cleft in response to conductive electrical activity.
Extreme Longevity: Neurons remain functional throughout an individual's entire lifetime.
Amitotic: Adult neurons lose mitotic activity and cannot divide (with rare exceptions).
Four Functional Segments of a Neuron:
Receptive Segment: Composed of dendrites and the cell body (soma). Neurotransmitter reception triggers postsynaptic potentials (Excitatory Postsynaptic Potentials [EPSPs] or Inhibitory Postsynaptic Potentials [IPSPs]).
Initial Segment: Composed of the Axon Hillock. Site of signal summation where postsynaptic potentials are added to determine if threshold potential is reached to fire an action potential.
Conductive Segment: Composed of the length of the Axon and its collateral branches. Responsible for propagating the action potential down the cell.
Transmissive Segment: Composed of Synaptic Knobs (axon terminals). Responsible for releasing neurotransmitters from synaptic vesicles in response to the arriving action potential.
Central Nervous System Organization: Gray and White Matter
Central Nervous System Components: Composed of the Brain and Spinal Cord.
White Matter:
Composed primarily of myelinated axons traveling up and down the spinal cord and throughout the brain.
Myelinated within the CNS specifically by Oligodendrocytes.
Form CNS Tracts: Axon pathways within the brain that perform functions similar to nerves, but lack connective tissue wrappings (endoneurium, perineurium, epineurium).
Examples of White Matter Regions:
Arbor Vitae: Tree-like white matter structure inside the cerebellum.
Corpus Callosum: Deep white matter tract connecting cerebral hemispheres.
Internal Capsule: Tracts passing between cerebral cortex and subcortical structures.
Funiculi: White matter columns in the spinal cord.
Gray Matter:
Composed of unmyelinated structures: neuron cell bodies, dendrites, and unmyelinated axons.
Examples of Gray Matter Regions:
Cerebral Cortex: Outer surface layer of the cerebrum.
Cerebral Nuclei: Deep clusters of gray matter within the brain.
Horns: Gray matter regions in the center of the spinal cord (anterior, posterior, lateral horns).
Peripheral Ganglia:
A ganglion is a collection of neuron cell bodies located outside the CNS within the PNS (e.g., Posterior Root Ganglion, containing cell bodies of sensory neurons surrounded by supportive satellite cells).
Ventricles and Cerebrospinal Fluid Flow
Ventricles of the Brain: Interconnected cavities located within the brain tissue that circulate Cerebrospinal Fluid (CSF).
Lateral Ventricles (Left and Right): Located within the cerebral hemispheres.
Interventricular Foramen: Channels connecting each lateral ventricle to the third ventricle.
Third Ventricle: Narrow midline cavity located within the diencephalon.
Cerebral Aqueduct: Canal running through the midbrain that connects the third ventricle to the fourth ventricle.
Fourth Ventricle: Located between the pons/medulla oblongata anteriorly and the cerebellum posteriorly.
Choroid Plexus: Specialized networks of blood capillaries and ependymal cells located within the ventricles that continuously produce CSF.
Cranial Meninges, Spaces, and Clinical Conditions
Functions of Cranial Meninges: Separate and support soft brain tissue, enclose and protect blood vessels supplying the brain, and help contain and circulate cerebrospinal fluid.
Three Connective Tissue Layers (Deep to Superficial):
Pia Mater: Delicate layer of Areolar Connective Tissue. Adheres tightly to every contour, gyrus, and sulcus of the brain surface.
Arachnoid Mater: Web-like layer composed of collagen and elastic fibers.
Membrane Layer: Deep layer of the arachnoid mater.
Trabeculae: Column-like extensions spanning across the subarachnoid space.
Subarachnoid Space: Space situated between the pia mater and arachnoid membrane filled with CSF and major blood vessels.
Arachnoid Villi (Granulations): Projections of the arachnoid layer punching through the meningeal layer of the dura mater into dural venous sinuses, enabling CSF reabsorption into the venous system.
Dura Mater: Tough, heavy outer layer composed of Dense Irregular Connective Tissue.
Periosteal Layer: Outer layer that fuses to the internal periosteum of skull bones.
Meningeal Layer: Inner layer extending inward toward the brain.
Cranial Dural Septa: Double layers of meningeal dura mater that extend as flat partitions into the cranial cavity (e.g., Falx Cerebri).
Dural Venous Sinuses: Large blood-filled venous spaces formed where the periosteal and meningeal layers of dura mater separate.
Subdural Space and Subdural Hematoma:
Normally, the dura mater sits directly against the arachnoid mater.
The boundary between them is a potential space called the Subdural Space.
Traumatic head injuries can cause venous bleeding into this potential space, forming an actual fluid-filled space known as a Subdural Hematoma.
Brain Anatomy: Cerebrum, Fissures, and Lobes
Major Brain Regions:
Cerebrum: Location of consciousness, complex thought, higher-order reasoning, memory, sensory perception, and voluntary skeletal muscle control.
Cerebellum: Coordinates and fine-tunes voluntary motor functions; stores muscle memory.
Surface Topography of the Cerebrum:
Gyri (singular: Gyrus): Raised tissue folds on the surface of the brain, maximizing functional cortical area within the skull.
Sulci (singular: Sulcus): Shallow depressions separating adjacent gyri.
Fissures: Deep grooves in brain tissue.
Longitudinal Fissure: Deep groove along the sagittal plane separating the left and right cerebral hemispheres.
Transverse Fissure: Deep groove separating the occipital lobes of the cerebrum from the cerebellum.
Five Cerebral Lobes:
Frontal Lobe: Contains the precentral gyrus. Controls voluntary motor functions, concentration, verbal communication, decision making, planning, and personality.
Parietal Lobe: Contains the postcentral gyrus. Responsible for processing and evaluating general sensory information.
Temporal Lobe: Responsible for processing auditory (hearing) and olfactory (smell) inputs.
Occipital Lobe: Responsible for processing incoming visual information.
Insula: Deep lobe (internal to lateral sulcus) responsible for memory and interpretation of taste (gustatory stimuli).
Cerebral Lateralization and Functional Cortical Areas
Corpus Callosum: Large horizontal white matter tract connecting left and right cerebral hemispheres to allow interhemispheric communication.
Cerebral Lateralization: Functional specialization of each cerebral hemisphere.
Left Hemisphere (Categorical Hemisphere):
Controls motor functions of the right side of the body / right hand.
Receives visual input from the right visual field.
Processes feeling shapes with the right hand.
Specialized for verbal memory, speech production (motor speech area / Broca's area), and superior language and mathematical comprehension (Wernicke area).
Right Hemisphere (Representational Hemisphere):
Controls motor functions of the left side of the body / left hand.
Receives visual input from the left visual field.
Processes feeling shapes with the left hand.
Specialized for memory for shapes (with limited language comprehension), musical ability, recognition of faces, and spatial relationships.
Primary Motor vs. Primary Somatosensory Cortices:
Precentral Gyrus (Frontal Lobe): Houses the Primary Motor Cortex (Somatic Motor Area). Controls voluntary skeletal muscle activities.
Postcentral Gyrus (Parietal Lobe): Houses the Primary Somatosensory Cortex. Receives somatic sensory stimuli from skin receptors and proprioceptors in joints/muscles.
Cortical Homunculi Topographic Maps (Medial to Lateral Spatial Layout):
Primary Motor Cortex Map: Toes, Foot, Leg, Knee, Hip, Trunk, Shoulder, Arm, Elbow, Wrist, Hand, Fingers, Thumb, Eye, Face, Lips, Jaw, Tongue, Pharynx.
Primary Somatosensory Cortex Map: Genitals, Toes, Foot, Leg, Hip, Trunk, Neck, Head, Shoulder, Elbow, Forearm, Wrist, Hand, Fingers, Thumb, Eye, Nose, Face, Upper lip, Teeth/Gums/Jaw, Lower lip, Tongue, Pharynx, Intra-abdominal.
Relative Cortical Representation Rank: Trunk, Hand, Foot.
Diencephalon Structures and Functions
Overview: Acts as a central structural and functional relay center for sensory and motor pathways; regulates visceral activities.
Major Components:
Thalamus: Principal relay center for all incoming sensory information (except olfaction) bound for the cerebral cortex.
Hypothalamus: Master control center for body homeostasis. Regulates the Autonomic Nervous System (ANS), Endocrine system, body temperature, food intake, water intake, emotional behavior, and circadian rhythms. Contains the Infundibulum (stalk attaching to the Pituitary Gland) and Mammillary Bodies.
Epithalamus: Connects diencephalon to the limbic system. Contains the Habenular Nucleus and the Pineal Gland (secretes the hormone melatonin to regulate circadian rhythms).
The Limbic System, Reticular Activating System, and Memory Formations
The Limbic System ("Emotional Brain"):
Ring of structures encircle the diencephalon; involved in emotional processing, motivation, and memory formation. Interacts with the frontal lobe for logical thought.
Cingulate Gyrus: Receives inputs from all other regions of the limbic system.
Hippocampus: Key structure involved in long-term memory formation and consolidation.
Parahippocampal Gyrus: Cortical tissue associated directly with hippocampal functions.
Amygdaloid Body (Amygdala): Primary center for emotional memory, specifically processing fear, anxiety, and aggression.
Fornix: White matter tract (myelinated axons) connecting the hippocampus to other limbic structures.
Anterior Thalamic Nucleus & Mammillary Body: Regions involved in memory recall mechanisms.
Olfactory Bulb and Olfactory Tract: Process smell information directly into the limbic system.
Reticular Activating System (RAS):
Functions as the body's "Alarm Clock".
Processes visual, auditory, and touch/pain/temperature stimuli to activate the cerebral cortex and maintain consciousness.
Sends motor output down motor tracts to the spinal cord.
Memory Processing:
Short-Term Memory (STM): Characterized by limited duration (seconds to hours); requires a functional Hippocampus.
Long-Term Memory (LTM): Requires both the Amygdaloid Body and Hippocampus for encoding; permanently stored in sensory association areas of the cerebral cortex.
Encoding: The process of consolidating short-term memory traces into long-term memory storage.
Brainstem Anatomy and Vital Reflex Centers
Overview: Connects the cerebrum, diencephalon, and cerebellum to the spinal cord. Houses vital autonomic centers for basic life functions.
Midbrain:
Contains white matter tracts called Cerebral Peduncles.
Tectal Plate: Contains paired colliculi:
Superior Colliculi: Visual reflex centers (e.g., tracking moving objects visually).
Inferior Colliculi: Auditory reflex centers (e.g., turning head toward unexpected loud sounds).
Pons:
Contains white matter peduncles and neural tracts.
Houses the Pontine Respiratory Center: Works with medulla oblongata centers to control breathing rate.
Medulla Oblongata:
Anterior structures include Pyramids (site of decussation of pyramids, where motor tracts cross to opposite sides) and lateral Olives.
Contains Nucleus Gracilis and Nucleus Cuneatus.
Houses vital autonomic regulatory centers:
Medullary Respiratory Center: Controls basic breathing rhythm.
Cardiac Center: Regulates heart rate and force of contraction.
Vasomotor Center: Controls blood vessel smooth muscle diameter to regulate blood pressure via baroreceptor reflexes.
Detailed Breakdown of Cranial Nerves (CN I - XII)
CN I: Olfactory Nerve
Sensory Function: Olfaction (smell).
Somatic Motor Function: None.
Parasympathetic Motor Function: None.
CN II: Optic Nerve
Sensory Function: Vision.
Somatic Motor Function: None.
Parasympathetic Motor Function: None.
CN III: Oculomotor Nerve
Sensory Function: None.
Somatic Motor Function: Innervates four extrinsic eye muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) and the levator palpebrae superioris muscle (elevates eyelid).
Parasympathetic Motor Function: Innervates sphincter pupillae muscle in eye to cause pupil constriction; contracts ciliary muscles to make lens of eye more rounded for near vision.
CN IV: Trochlear Nerve
Sensory Function: None.
Somatic Motor Function: Innervates superior oblique eye muscle.
Parasympathetic Motor Function: None.
CN V: Trigeminal Nerve
Sensory Function: General sensory from anterior scalp, nasal cavity, nasopharynx, entire face, most of oral cavity, teeth, anterior two-thirds of tongue, part of auricle of ear, and cranial meninges.
Somatic Motor Function: Muscles of mastication, mylohyoid, digastric (anterior belly), tensor tympani, tensor veli palatini.
Parasympathetic Motor Function: None.
CN VI: Abducens Nerve
Sensory Function: None.
Somatic Motor Function: Innervates lateral rectus eye muscle.
Parasympathetic Motor Function: None.
CN VII: Facial Nerve
Sensory Function: Taste from anterior two-thirds of tongue.
Somatic Motor Function: Muscles of facial expression, digastric (posterior belly), stylohyoid, stapedius.
Parasympathetic Motor Function: Increases secretion from lacrimal gland of eye, submandibular salivary glands, and sublingual salivary glands.
CN VIII: Vestibulocochlear Nerve
Sensory Function: Hearing (cochlear branch); equilibrium/balance (vestibular branch).
Somatic Motor Function: None.
Parasympathetic Motor Function: None.
CN IX: Glossopharyngeal Nerve
Sensory Function: General sensory and taste from posterior one-third of tongue, general sensory from part of pharynx, visceral sensory from carotid bodies.
Somatic Motor Function: One pharyngeal muscle (stylopharyngeus).
Parasympathetic Motor Function: Increases secretion from parotid salivary gland.
CN X: Vagus Nerve
Sensory Function: Visceral sensory information from heart, lungs, and most abdominal organs; general sensory information from external acoustic meatus, tympanic membrane, part of pharynx, laryngopharynx, and larynx.
Somatic Motor Function: Most pharyngeal muscles; all laryngeal muscles.
Parasympathetic Motor Function: Innervates smooth muscle and glands of heart, lungs, larynx, trachea, and most abdominal organs.
Visceral Parasympathetic Physiology:
Slows heart rate, constricts lung bronchioles, increases digestive secretions, and stores carbohydrates ("Rest & Digest").
Intrinsic firing rate of SA node without autonomic input is . Parasympathetic stimulation via muscarinic receptors induces Inhibitory Postsynaptic Potentials (IPSPs), slowing resting heart rate to .
Impairment of vagus nerve functions is a known cause of gastroparesis.
CN XI: Accessory Nerve
Sensory Function: None.
Somatic Motor Function: Trapezius muscle, sternocleidomastoid muscle.
Parasympathetic Motor Function: None.
CN XII: Hypoglossal Nerve
Sensory Function: None.
Somatic Motor Function: Intrinsic and extrinsic tongue muscles.
Parasympathetic Motor Function: None.
Summary of Cranial Nerves with Autonomic (Parasympathetic) Function:
Oculomotor (CN III): Controls ciliary muscles (lens shape) and iris sphincter muscle (pupil size).
Facial Nerve (CN VII): Controls lacrimal glands (tears), facial mucosal glands, submandibular salivary glands, and sublingual salivary glands.
Glossopharyngeal (CN IX): Controls parotid salivary gland.
Vagus Nerve (CN X): Controls thoracic and abdominal visceral organs.