Central Nervous System: Spinal Cord and Brain Lecture Flashcards

Overview of the Central Nervous System

  • Primary Components: The study focuses on the central nervous system (CNS), specifically the nerves and the brain (covered in Chapters 12-13).

  • Functional Relationship:

    • Brain: Contains the highest reflex centers and sensory receptors. It receives sensory input over cranial nerves and provides motor output over cranial nerves.

    • Spinal Cord: Functioning as an interface between the brain and effectors. It receives sensory input over spinal nerves and produces motor output over spinal nerves via reflex centers in the spinal cord.

  • Effectors: The terminal targets for motor commands, including:

    • Muscles.

    • Glands.

    • Adipose tissue.

Spinal Cord Anatomy and Gross Structures

  • Sections and Regional Organization:

    • Cervical Spinal Nerves: C1 through C8.

    • Thoracic Spinal Nerves: T1 through T12.

    • Lumbar Spinal Nerves: L1 through L5.

    • Sacral Spinal Nerves: S1 through S5.

    • Coccygeal Nerve: (Co1Co1).

  • Key Longitudinal Features:

    • Cervical Enlargement: Innervates the shoulder and upper limbs.

    • Lumbar Enlargement: Innervates the pelvis and lower limbs.

    • Conus Medullaris: The tapered, inferior tip of the spinal cord (located near L1-L2 in adults).

    • Cauda Equina: A bundle of spinal nerve roots extending below the conus medullaris, resembling a horse\'s tail.

    • Filum Terminale: A slender strand of fibrous tissue extending from the conus medullaris to the coccyx.

  • Surface Anatomy:

    • Posterior Median Sulcus: A shallow longitudinal groove on the posterior surface.

    • Anterior Median Fissure: A deep groove along the anterior surface.

Spinal Meninges and Protective Structures

  • Meningeal Layers (Superficial to Deep):

    1. Dura Mater: The tough, fibrous outermost layer. In the spinal cord, it is separated from the vertebrae by the epidural space, which contains adipose tissue.

    2. Arachnoid Mater: The middle layer, featuring the subarachnoid space filled with cerebrospinal fluid (CSF).

    3. Pia Mater: The delicate innermost layer, firmly attached to the surface of the spinal cord. It includes blood vessels and denticulate ligaments that stabilize the cord.

  • Subspaces and Structures:

    • Subarachnoid Space: Contains CSF and blood vessels. It is the target for clinical procedures such as a lumbar puncture.

    • Epidural Space: Space between the dura mater and the vertebral canal walls; contains fat and connective tissue.

    • Denticulate Ligaments: Extensions of the pia mater that anchor the spinal cord to the dura mater.

  • Lumbar Puncture (Spinal Tap):

    • The needle enters the subarachnoid space, typically passing through the supraspinous ligament and ligamentum flavum.

    • Performed lower in the vertebral column (e.g., L3-L4 or L4-L5) to avoid the spinal cord and sample CSF from the cauda equina region.

Microscopic Anatomy: Gray and White Matter

  • Definitions:

    • Nerve: A bundle of axons located in the Peripheral Nervous System (PNS).

    • Tracts: A bundle of axons located in the Central Nervous System (CNS).

    • Nuclei: A collection of cell bodies in the CNS.

    • Ganglion: A collection of cell bodies in the PNS.

  • Gray Matter (Cell bodies, neuroglia, and unmyelinated axons):

    • Organization: Arranged into "horns."

      • Posterior Gray Horn: Contains somatic and visceral sensory nuclei.

      • Lateral Gray Horn: Contains visceral motor nuclei (present in thoracic and lumbar segments).

      • Anterior Gray Horn: Contains somatic motor nuclei.

    • Commissures: The gray commissures (anterior and posterior) surround the central canal and allow for cross-communication.

  • White Matter (Myelinated and unmyelinated axons):

    • Organization: Arranged into columns or funiculi (Posterior, Lateral, and Anterior).

    • Tract Characteristics: Axons in a tract share uniform diameter and myelination, relaying the same type of info in the same direction.

    • Ascending Tracts: Relay sensory information toward the brain (e.g., Fasciculus gracilis, Fasciculus cuneatus, Spinothalamic tracts, Spinocerebellar tracts).

    • Descending Tracts: Relay motor commands down the spinal cord (e.g., Corticospinal tracts, Reticulospinal tracts, Vestibulospinal tract, Rubrospinal tract, Tectospinal tract).

Spinal Nerves, Connective Tissue, and Distribution

  • Connective Tissue Layers (Ordered outer to inner):

    1. Epineurium: Dense network of collagen fibers surrounding the entire nerve.

    2. Perineurium: Divides the nerve into bundles of axons called fascicles.

    3. Endoneurium: Delicate connective tissue surrounding individual axons (even Schwann cells).

  • Nerve Roots and Rami:

    • Roots: Proximal branches connecting the nerve to the cord.

      • Dorsal Root: Contains axons of sensory neurons; features the Dorsal Root Ganglion (cell bodies of sensory neurons).

      • Ventral Root: Contains axons of motor neurons.

    • Rami: Distal branches once the nerve exits the intervertebral foramen.

      • Dorsal Ramus: Innervates the skin and muscles of the back.

      • Vral Ramus: Innervates the ventrolateral body surface, body wall, and limbs.

      • Communicating Rami (Rami Communicantes): White and gray rami that connect to the sympathetic ganglion (part of the Autonomic Nervous System).

  • Sympathetic Trunk: A chain of ganglia that allows for rapid activation of multiple structures during a "fight-or-flight" response.

  • Dermatomes: Specific regions of the skin monitored by a single pair of spinal nerves. These are clinically significant for diagnosing nerve damage or infections like Shingles.

Sensory and Motor Information Pathways

  • General Rule: Dorsal = Sensory = Arriving (In); Ventral = Motor = Exiting (Out).

  • Sensory Pathway (Afferent):

    1. Receptors: Categorized as Interoceptors (internal organs), Proprioceptors (position/muscles), and Exteroceptors (external environment).

    2. Signal Route: Receptor —> Spinal Nerve —→ Dorsal Ramus (back) or Ventral Ramus (limbs/body wall)—→Dorsal Root Ganglion —→ Dorsal Root ——> Posterior Gray Horn.

    3. Visceral Route: Via sympathetic nerve —→ Rami communicantes ——> Dorsal root.

  • Motor Pathway (Efferent):

    1. Somatic Command: Anterior gray horn —→ Ventral root —→ Spinal nerve —→ Dorsal or Ventral ramus —→ Skeletal Muscle.

    2. Visceral Command: Lateral gray horn —→ Ventral root —→ White ramus communicans —→ Sympathetic ganglion —→ Target (organs) or Gray ramus communicans —→ Spinal nerve —→ Body wall effectors.

  • Decussation: The crossing of nerve fibers from one side of the body/brain to the other. Evolutionarily, the spinal cord is noted to have turned 180180 degrees during invertebrate-to-vertebrate transition.

Development of the Brain and Nervous System

  • Neural Tube Formation:

    • Originates from surface ectoderm forming the neural plate.

    • Plate invaginates into a neural groove with neural folds.

    • Folds fuse to form the neural tube (CNS).

    • Neural crest cells migrate to form the PNS.

  • Primary Brain Vesicles (4 weeks):

    1. Prosencephalon (Forebrain).

    2. Mesencephalon (Midbrain).

    3. Rhombencephalon (Hindbrain).

  • Secondary Brain Vesicles (5 weeks) and Adult Structures:

    • Telencephalon: Becomes Cerebrum (cerebral hemispheres) and lateral ventricles.

    • Diencephalon: Becomes Thalamus, Hypothalamus, Epithalamus, and third ventricle.

    • Mesencephalon: Becomes the Brain stem (midbrain) and cerebral aqueduct.

    • Metencephalon: Becomes the Pons and Cerebellum; associated with the fourth ventricle.

    • Myelencephalon: Becomes the Medulla oblongata; associated with the fourth ventricle.

Neuronal Circuits and Processing

  • Neuron Statistics:

    • Sensory Neurons: 10 million

    • Motor Neurons: half a million

    • Interneurons: 20 billion (organized into neuronal pools).

  • Circuit Types:

    • Divergence: One neuron stimulates many. Used for broad distribution (e.g., sensory info entering CNS).

    • Parallel Processing: Several neurons process the same info simultaneously for different responses.

    • Serial Processing: Info relayed stepwise from one neuron or pool to the next.

    • Convergence: Several neurons converge on one postsynaptic neuron. Allows different sources to cause the same response or conscious/unconscious control (e.g., diaphragm control).

    • Reverberation: Axon collaterals loop back to stimulate presynaptic neurons. A positive feedback loop that continues until fatigue or inhibition occurs (e.g., memory formation in REM sleep).

Reflexes: Principles, Arcs, and Classifications

  • Reflex Arc Steps:

    1. Activation of a Receptor by stimulus.

    2. Activation of a Sensory Neuron.

    3. Information Processing in the CNS.

    4. Activation of a Motor Neuron.

    5. Response of a Peripheral Effector.

  • Classifications:

    • Development: Innate (genetically programmed, e.g., sucking) vs. Acquired (learned/conditioned, e.g., Pavlovian).

    • Nature of Response: Somatic (skeletal muscle) vs. Visceral/Autonomic (smooth/cardiac muscle, glands).

    • Complexity: Monosynaptic (one synapse, fastest) vs. Polysynaptic (multiple synapses/interneurons).

    • Processing Site: Spinal reflexes (spinal cord) vs. Cranial reflexes (brain).

Specific Spinal Reflexes and Clinical Testing

  • Stretch Reflex (Monosynaptic):

    • Example: Patellar reflex.

    • Involves Muscle Spindles (receptors). Intrafusal fibers are innervated by gamma motor neurons while extrafusal (skeletal) fibers are innervated by alpha motor neurons.

    • Maintain posture and muscle tone.

  • Withdrawal Reflex (Polysynaptic):

    • Example: Removing hand from heat.

    • Features Reciprocal Inhibition: When flexors are stimulated, extensors are inhibited.

  • Crossed Extensor Reflex:

    • A contralateral reflex where the stimulus occurs on one side and the response (e.g., straightening a leg for support) occurs on the opposite side.

  • Diagnostic Tests:

    • Babinski Sign: Plantar reflex in infants where toes fan out. In adults, the normal response is the Plantar Reflex (curling of toes). Presence of Babinski in adults indicates CNS damage.

    • Abdominal Reflex: Skin stroking results in navel movement toward the stimulus (T7−T12T7-T12).

    • Biceps Reflex: Tap to biceps tendon causes elbow flexion (C5,C6C5, C6).

    • Triceps Reflex: Tap to triceps tendon causes elbow extension (C6,C7C6, C7).

Ventricular System, Cerebrospinal Fluid (CSF), and Brain Meninges

  • Ventricular Chambers:

    • Lateral ventricles (one in each hemisphere) connected to the Third ventricle via the interventricular foramen.

    • Third ventricle (in diencephalon) connected to the Fourth ventricle via the cerebral aqueduct.

    • Fourth ventricle (brainstem) leads to the central canal of the spinal cord.

  • Cerebrospinal Fluid (CSF):

    • Production: Produced by the Choroid Plexus (specialized ependymal cells and capillaries).

    • Reabsorption: Reabsorbed into venous circulation through Arachnoid Granulations in the superior sagittal sinus.

    • Functions: Buoyancy, protection, and waste removal.

  • Brain Meninges:

    • Similar to spinal meninges but the Dura Mater has two layers (periosteal and meningeal).

    • Dural Sinuses: Large collecting veins between dural layers.

    • Dural Folds: Folds like the falx cerebri that stabilize the brain.

The Cerebrum: Structure and Functional Areas

  • Gross Anatomy:

    • Gyri: Folds/ridges.

    • Sulci: Shallow grooves.

    • Fissures: Deep grooves.

  • Lobe Specializations:

    • Frontal: Primary motor cortex (precentral gyrus), speech center (Broca\'s area), and prefrontal cortex.

    • Parietal: Primary sensory cortex (postcentral gyrus).

    • Temporal: Auditory and olfactory cortex.

    • Occipital: Visual cortex.

    • Insula: Deep lobe containing gustatory (taste), visceral sensory, and vestibular (balance) cortexes.

  • Homunculus: A "little man" mapping of the primary motor and sensory cortex where larger areas of the brain correspond to body parts with more motor units or sensory receptors (e.g., hands, lips).

Basal Nuclei and Hemispheric Lateralization

  • Basal Nuclei (aka Basal Ganglia):

    • Deep subcortical motor centers: Caudate nucleus, Putamen, and Globus pallidus (Lentiform nucleus).

    • Functions: Provide subconscious control of skeletal muscle tone and coordinate learned movement rhythms. They do not initiate movement but modify it.

  • Hemispheric Lateralization:

    • Left Hemisphere: Dominant for speech, writing, language, mathematical calculation, and logical analysis.

    • Right Hemisphere: Dominant for spatial visualization, analysis by touch, and complex multi-sensory patterns.

The Diencephalon: Thalamus, Hypothalamus, and Limbic System

  • Thalamus:

    • The main relay center; all sensory info (except smell) passes through here before the cortex.

    • Involved in memory processing and relaying information between the cortex and cerebellum.

  • Hypothalamus:

    • The chief integration center for the Autonomic Nervous System (ANS).

    • Regulates: Body temperature, food intake, water balance (thirst), biological rhythms (circadian), and sleep-wake cycles.

    • Endocrine Function: Produces ADH and Oxytocin; controls the anterior pituitary gland.

  • Limbic System:

    • The functional group of nuclei/tracts along the border of the cerebrum and diencephalon.

    • Amygdala: Links emotions to memories and regulates "fight-or-flight" and heart rate.

    • Hippocampus: Critical for learning and the storage/retrieval of long-term memories. Affected by Alzheimer\'s and dementia.

Sleep, Brain Waves, and Homeostasis

  • Sleep Stages:

    • NREM Stage 1-2: Light sleep, metabolism slows.

    • NREM Stage 3-4: Deep sleep; neural activity slows to basal levels for restoration.

    • REM Stage 5: Rapid eye movement; brain waves mimic an alert state; time for analysis and eliminación of unneeded synaptic connections.

  • Electroencephalogram (EEG) Waves:

    • Alpha: Healthy, resting awake adults with eyes closed.

    • Beta: Concentrating or stressed adults.

    • Theta: Children or frustrated adults; transient in sleep.

    • Delta: Deep sleep in all ages; presence in awake adults indicates brain damage.

  • Narcolepsy: Caused by the immune destruction of hypothalamic nuclei that secrete Orexins (aka hypocretins), the "wake-up" chemicals.

Memory and Learning

  • Types of Memory:

    1. Declarative: Facts and data.

    2. Procedural: Skills and actions.

    3. Emotional.

  • Stages: Conversion from short-term to long-term memory involves factors like repetition, emotional state, and association with existing knowledge.

The Brainstem: Midbrain, Pons, and Medulla Oblongata

  • Midbrain (Mesencephalon):

    • Corpora Quadrigemina: Superior colliculi (visual reflexes) and Inferior colliculi (auditory reflexes).

    • Substantia Nigra: Produces dopamine to regulate basal nuclei. Degeneration causes Parkinson\'s Disease.

    • Red Nucleus: Subconscious control of upper limb position.

    • Reticular Activating System (RAS): Maintains consciousness.

  • Pons:

    • Relays information between the cerebrum and cerebellum.

    • Contains respiratory centers (Pneumotaxic and Apneustic) that modify breathing depth and rate.

  • Medulla Oblongata:

    • Contains vital visceral centers: Cardiovascular (heart rate) and Respiratory Rhythmicity (basic pace).

    • Decussation of Pyramids: The point where descending motor tracts cross over to the opposite side.

    • Features relay stations like the Olivary nucleus, Nucleus cuneatus, and Nucleus gracilis.

  • Reticular Formation: A functional system throughout the brainstem that filters repetitive stimuli and maintains cortical alertness.

The Cerebellum

  • Functions: Fine-tunes motor activities to ensure smooth, coordinated movement; maintains balance and posture.

  • Purkinje Cells: Large, highly branched neurons in the cerebellar cortex that receive massive amounts of input and project axons into the white matter.

  • Arbor Vitae: The tree-like branching pattern of cerebellar white matter.

  • Cerebellar Peduncles: Superior, Middle, and Inferior stalks that connect the cerebellum to the brainstem and relay info to/from the cortex.

White Matter Fibers and Communication

  • Association Fibers: Interconnect areas within the same hemisphere (e.g., Arcuate fibers, Longitudinal fasciculi).

  • Commissural Fibers: Connect the two hemispheres (e.g., Corpus Callosum, Anterior commissure).

  • Projection Fibers: Connect the cerebrum to lower brain centers or the spinal cord (e.g., Internal capsule).