Untitled Quiz

• Study notes on organization of the nervous system, brain, and spinal cord, including their subdivisions and relationships.

Central Nervous System (CNS)

• Consists of the Brain and Spinal Cord

Peripheral Nervous System (PNS)

• Consists of Cranial Nerves and Spinal Nerves

Neuron Structure

• Cell Body: Contains the nucleus, cytoplasm, and is the metabolic center
• Axon: Conducts nerve impulses away from the cell body
• Dendrite: Receives signals from other neurons
• Synapse: Junction between neurons where communication occurs
• Myelin: Fatty substance insulating axons, speeding up signal transmission
• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal conduction

Neuron Types

• Multipolar: One axon, multiple dendrites; primarily motor neurons
• Bipolar: One axon, one dendrite; sensory neurons in the retina
• Unipolar: One process that bifurcates to act as both an axon and a dendrite

Terminology

• Nucleus: Cluster of neurons in the CNS
• Ganglion: Cluster of neurons in the PNS
• Tract: Bundle of axons in the CNS
• Nerve: Bundle of axons in the PNS
• Afferent: Signals toward the CNS
• Efferent: Signals away from the CNS

CNS Cells

• Astrocytes: Support neurons and maintain the blood-brain barrier
• Oligodendrocytes: Produce myelin in the CNS
• Microglia: Act as immune cells and remove debris
• Ependymal cells: Line ventricles and produce cerebrospinal fluid (CSF)

PNS Cells

• Schwann cells produce myelin in the PNS
• Satellite cells support neurons in ganglia

Brain Organization - Forebrain (Prosencephalon)

• Telencephalon (cerebrum):
• Two cerebral hemispheres connected by the corpus callosum;
• Other pathways comprise mostly superficial gray matter (neurons and glia)
• Deep white matter contains axons
• Consists of 5 lobes: frontal, parietal, temporal, occipital, and insular
• Basal ganglia are interconnected deep structures (nuclei) involved in motor planning and programming

Brain Organization - Diencephalon

• Thalamus: Major relay center for sensory function, reciprocally connected with the cortex
• Hypothalamus: Maintains homeostasis/equilibrium, motivated behaviors, and functions with endocrine and autonomic systems

Brain Organization - Midbrain (Mesencephalon)

• Major structures include cerebral peduncles, substantia nigra, red nucleus, oculomotor and trochlear nuclei, and superior and inferior colliculi (III-IV)

Brain Organization - Hindbrain (Rhombencephalon)

• Metencephalon:
• Pons: Contains ascending (medial lemniscus) and descending (pyramidal fibers) fiber tracts; also contains trigeminal, abducens, and facial motor nuclei (V-VII)
• Cerebellum: Processes proprioceptive information from muscles, tendons, and joints, and coordinates motor activity; it contains three pairs of peduncles (stalks)

Brain Organization - Hindbrain (Rhombencephalon)

• Mylencephalon:
• Medulla oblongata: Contains numerous ascending and descending fiber tracts and sensory relay nuclei (nucleus gracilis and cuneatus). Contains motor nuclei for cranial nerves IX, X, and XII
• The term "brainstem" refers to the midbrain, pons, and medulla (excluding the cerebellum)

Cerebral Cortex

• Precentral gyrus
• Central sulcus
• Postcentral gyrus

Spinal Cord

• Lateral fissure

Spinal Cord Organization

• 31 segments and pairs of spinal nerves
• 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segment

Spinal Segment Composition

• White matter: Composed of nerve cell processes, white due to myelin
• Gray matter: Composed of nerve cell bodies, appear gray due to lack of myelin
• Dorsal horn: Sensory nerve processes bringing information into the spinal cord
• Dorsal root ganglion: Nerve cell bodies of unipolar, sensory neurons
• Ventral root: Nerve cell processes of efferent fibers on their way to muscles; motor neurons located here
• Spinal nerve (mixed): Mixture of incoming sensory and outgoing motor processes, 31 pairs
• Dorsal ramus: Mixture of sensory and motor fibers that branch posteriorly
• Ventral ramus: Mixture of sensory and motor fibers that branch anteriorly; involved in nerve plexuses

Meninges

• Dura mater: Superficial layer, thick and tough with a double layer in the skull and single layer around the spinal cord
• Arachnoid: Middle layer, spider web-like and delicate
• Pia mater: Deepest layer

Ventricular System

• Lateral ventricles (2): cavity of the telencephalon, extends to all lobes
• Interventricular foramen (of Monro)
• 3rd ventricle: Cavity of the diencephalon
• Cerebral aqueduct: Cavity of the mesencephalon
• 4th ventricle: Cavity of the metencephalon and myelencephalon
• Foramen of Magendie (1; posterior), foramen of Luschka (2; lateral): Continuous with subarachnoid space, exit site for CSF
• Central canal: Cavity of spinal cord
• Choroid plexus: Produces 60% of CSF in the lateral, third, and fourth ventricles

Direction of CSF Flow

• Lateral ventricle to interventricular foramina to third ventricle to cerebral aqueduct to fourth ventricle to foramen of Magendie or foramina of Luschka to subarachnoid space (SAS) to arachnoid granulations to superior sagittal sinus
• Lateral ventricles are connected to the third ventricle via interventricular foramina
• The third ventricle is connected to the fourth ventricle via the cerebral aqueduct
• The fourth ventricle releases CSF into the subarachnoid space via median and lateral apertures
• CSF flows through ventricles into the subarachnoid space and drains into the superior sagittal sinus.

Arachnoid Specializations

• Arachnoid granulations: Extensions of arachnoid through the dura into the venous sinus (superior sagittal sinus), allowing CSF to enter venous circulation

Dura Specializations

• Falx cerebri: Separates cerebral hemispheres in the midline
• Falx cerebelli: Small dural sheath at posterior cerebellum
• Tentorium cerebelli: Dural sheath separating occipital lobe from cerebellum

Additional Dura Structures

• Diaphragma sellae: Anchors pituitary into sella turcica
• Dural Venous sinuses: Superior, inferior, straight, transverse, and sigmoid sinuses, draining into the internal jugular vein

CSF

• Production: 60% from choroid plexus, 40% from ependymal cells lining the ventricles
• Function: Chemical buffer, physical buffer, buoyancy
• Rate of formation: 500 cc/day
• Total volume: 150 cc
• Constituents: Water, electrolytes, glucose, proteins (few), lymphocytes (few)

Cerebral Cortex Organization

• 6-layered structure with input vs output layers, relations to association, commissural, and projection fibers
• Features: 2-4 mm thick, highly convoluted gray substance comprising half of brain volume

Cerebral Cortex Divisions

• Neocortex (isocortex): 6 layers (~95% of all cortex)
• Allocortex: 3-5 layers with variable lamination patterns, including paleocortex and archicortex

Cerebral Cortex Lamination Pattern

• Layer 1: Molecular level; superficial, rich in horizontally oriented dendrites and axons
• Layer 2: External granular layer; mostly smaller neurons and few pyramidal cells; inputs from Layer II neurons
• Layer 3: External pyramidal cell layer; medium-sized pyramidal neurons with association/commissural axons terminating in layer II
• Layer 4: Internal granular layer; stellate neurons and few pyramidal neurons that receive thalamic input
• Layer 5: Internal pyramidal layer; large pyramidal neurons, including giant cells (Betz); axons become projection fibers
• Layer 6: Multiform layer; multiple cell types, including pyramidal cells projecting to the thalamus

Principal Fiber Types

• Association fibers: Interconnect portions of cerebral cortex in the same hemisphere; axons of pyramidal cells in layer 3 that terminate in layer 2
• Commissural fibers: Interconnect hemispheres; mainly the corpus callosum with 200-300 million axons from pyramidal cells in layer 3 terminating in layer 2
• Projection fibers: Axons project to the thalamus, deep forebrain, brainstem, or spinal cord; from large pyramidal cells in layer 5 (except those projecting to the thalamus)

Simplified Principal Fiber Types

• Association fibers: Connect areas within the same hemisphere
• Commissural fibers: Connect corresponding areas of both hemispheres
• Projection fibers: Connect the cortex to lower brain regions and the spinal cord

Cortical Layers/Fiber Types

• III → II association fibers
• III → commissural fibers
• V → SubC projection fibers
• VI → thalamus corticothalamic fibers
• Thal → IV thalamocortical fibers

Six-Layered Neocortex

• Input layers: II (external granular) and IV (internal granular)
• Output layers: III (external pyramidal) and V (internal pyramidal)
• Layer I (molecular) consists mostly of fibers
• Layer VI sends projections to the thalamus

Types of Cortex

• Primary:
• Precentral gyrus (motor)
• Postcentral gyrus (somatosensory)
• Cuneus and lingual gyrus (vision)
• Superior temporal gyrus (auditory)
• Piriform cortex (olfaction)
• Association:
• Unimodal or hetero-modal; responsible for higher-order processing and integration

Functions

• Primary motor cortex controls voluntary movements
• Primary sensory cortex processes sensory input
• Homunculus represents the somatotopic organization of body parts

Major Motor System

• Maps the motor system from the precentral gyrus through the brainstem to spinal cord white matter.
• Corticospinal tract (pyramidal tract)
• Originates in precentral gyrus
• Descends through internal capsule, cerebral peduncles, medulla pyramids, where most fibers decussate at the pyramidal decussation
• Controls limb movements
• Rubrospinal tract
• Originates in the red nucleus of the midbrain
• Facilitates flexor movements in the upper limbs
• Vestibulospinal and reticulospinal tracts
• Involved in posture and balance

Blood Supply

• Circle of Willis and its major branches/vascular territories: anterior, middle, and posterior cerebral arteries.
• Internal Carotid Artery (2)
• Anterior cerebral artery (2)
• Anterior communicating artery (1)
• Middle cerebral artery (2)
• Lenticulostriate arteries (many small)
• Ophthalmic artery
• Anterior choroidal artery
• Basilar Artery (1)
• Posterior cerebral artery (2)
• Posterior communicating artery (2)
• Superior cerebellar artery (2)
• Pontine arteries (4 pairs)
• Anterior Inferior Cerebellar Artery (AICA) (2)
• Vertebral Artery (2)
  • Posterior inferior cerebellar artery (PICA) (2)
  • Anterior spinal artery (1)
• Posterior Spinal Arteries (2)

Major Arteries

• Anterior cerebral artery (ACA): supplies medial, frontal, and parietal lobes
• Middle Cerebral artery (MCA): supplies lateral frontal, parietal, and temporal lobes
• Posterior Cerebral artery (PCA): supplies occipital libe

Sensory Systems

• Details regarding major ascending sensory systems and crossover points, and consequences of lesions
• Dorsal column-medial lemniscus pathway
• Carries fine tough, vibration, proprioceptions
• Crosses at the medulla
• Spinothalamic tract
• Carries pain and temperature
• Crosses at the spinal cord level
• Dorsal spinocerebellar tract
• Carries unconscious proprioception
• Does not cross

Spinal Cord Organization

• Organization of dorsal root entry zone (DREZ) in terms of fiber diameter and conduction velocity.
• DREZ is organized by fiber diameter and conduction velocity
• Spinal cord gray matter layer (rexed laminae)
• Lamina II (substantia gelatinosa): pain modulation
• Lamina IV: sensory relay
• Lamina VI: proprioceptive input

Spinal Reflexes

• Spinal reflexes and associated classes of sensory fiber type (I, II, III, IV)
• Reflex types
• Stretch reflex: mediated by la fibers
• Flexor withdrawl reflex: mediated by II and III fibers
• Sensory fiber types
• Ia & II: muscle spindles (proprioception)
• III & IV: free nerve endings (pain, temperature)

Sensory Receptor Types

• Sensory receptor types (Free Nerve Ending, Merkel, Meissner, Ruffini, Pacinian, Muscle spindle, GTO) and associated ascending sensory system
• Free nerve endings: pain and temperature (spinothalamic tract)
• Merkel discs: light tough and pressure (dorsal column- medial lemniscus pathway)
• Meissner corpuscles: fine touch, low-frequency vibration (dorsal column-medial leminsicus pathway
• Ruffini endings: skin stretch, deep pressure (dorsal column-medial lemniscus pathway)
• Pacinan corpuscles: deep pressure, high frequence vibration (dorsal column-medial lemniscus pathway)
• Muscle spindles: proprioception, muscle stretch (dorsal column-medial lemniscus pathway, spinocerebellar tract)
• Golgi tendon organs (GTOs) : proprioception, muscle tension (spinocerebellar tract)

Trigeminal System

• Trigeminal system for sensation : trigeminal nerve, spinal trigeminal nucleus, tigeminothalamic tract, VPM
• Contains
• First-order neurons in trigeminal ganglion
• Second-order neurons in spinal trigeminal nucleus
• Third-order neurons in VPM of thalamus, projecting to postcentral gyrus

Autonomic Nervous System

• Organization of autonomic nervous system: origins (thoracolumbar vs craniosacral), ganglia, targets, sympathetic vs parasympathetic responses
• Sympathetic (Thracolumbar)
• Short preganglionic, long postganglionic fibers
• Neurotransmitters: ACh (preganglionic), NE (postganglionic)
• Parasympathetic (craniosacral)
• Long preganglionic, short postganglionic fibers
• Neurotransmitter: ACh at both synapses