Neuron Structure and Nervous System Organization

Neuron Structure

• Dendrites are tapered extensions of the cell body that collect information from other neurons. They contain cytoskeleton, mitochondria, and other important organelles.
• Soma (cell body) is the main part of the neuron, with a nucleus, Golgi apparatus, Nissl substance, cytoskeleton, and mitochondria. It synthesizes macromolecules and integrates electrical signals.
• Axon is a single, cylindrical extension that can be many centimeters long. It conducts information to other neurons and may be myelinated or unmyelinated. It contains cytoskeleton, mitochondria, and transport vesicles.
• Axon terminals (synaptic endings) are vesicle-filled structures that transmit information to other neurons. They typically connect to dendrites or the soma of other neurons, but other configurations exist.

Nervous System Organization

• The nervous system is divided into the somatic nervous system and the visceral nervous system.
• Somatic nervous system controls voluntary movements and conscious sensations.
• Visceral nervous system regulates autonomic functions (e.g., heart rate, breathing, digestion).

Brain Structure

• The cerebral hemispheres are the largest part of the brain and are covered by the cerebral cortex, which consists of six layers of gray matter.
• Gray matter is composed of cell bodies, while white matter contains myelinated axons.
• The cerebral cortex is divided into four lobes: frontal, parietal, occipital, and temporal.
• Frontal lobe is responsible for planning, movement, and complex thought.
• Parietal lobe processes sensory information, including touch, temperature, and pain.
• Occipital lobe is responsible for vision.
• Temporal lobe processes auditory information and memory.
• Internal capsule is a V-shaped structure consisting of axons connecting the cerebral cortex to other brain regions.
• Corpus callosum is a thick band of white matter connecting the two hemispheres and enabling communication between them.

Ventricular System

• The ventricular system is a network of cavities within the brain filled with cerebrospinal fluid (CSF).
• Lateral ventricles are the largest cavities and are located within the cerebral hemispheres.
• Third ventricle is located in the diencephalon and connects to the lateral ventricles through the interventricular foramina (of Monro).
• Fourth ventricle is located below the cerebellum and connects to the third ventricle through the cerebral aqueduct (of Sylvius).
• Choroid plexus is a network of modified ependymal cells that produce CSF.

Meninges

• Meninges are three layers of connective tissue that surround the brain and spinal cord.
• Dura mater, the outermost layer, is tough and fibrous and has two layers: periosteal layer and meningeal layer.
• Arachnoid mater, the middle layer, has a web-like structure and is separated from the dura mater by the subdural space.
• Pia mater, the innermost layer, is a delicate membrane that closely adheres to the brain and spinal cord.
• The subarachnoid space is located between the arachnoid mater and the pia mater and contains CSF.

Cerebral Vasculature

• Blood supply to the brain comes from the internal carotid arteries and vertebral arteries.

• Internal carotid arteries provide blood to the anterior circulation and branch into the anterior cerebral arteries (ACAs) and middle cerebral arteries (MCAs).

• Anterior communicating artery connects the two ACAs.

• Vertebral arteries provide blood to the posterior circulation.

• Posterior communicating arteries connect the internal carotid arteries to the posterior cerebral arteries (PCAs), which are branches of the vertebral arteries.

• The circle of Willis is a ring-shaped network of arteries that supplies blood to the brain and provides an alternative route for blood flow if one artery is blocked.### Arterial Supply to the Brain

• The posterior cerebral cortex receives blood supply from the vertebrobasilar system, which starts with the vertebral arteries branching off from the subclavian arteries.

• The vertebral arteries ascend through the foramen transversarium of the cervical vertebrae before entering the foramen magnum and joining to form the basilar artery.

• The basilar artery runs along the ventral brainstem and gives rise to the posterior cerebral artery (PCA) at the midbrain level.

• The PCA perfuses the inferior and medial temporal and occipital lobes and gives rise to the posterior communicating artery, which connects it to the internal carotid artery.

Venous Drainage of the Brain

• Superficial and deep veins of the cerebral hemispheres drain into the dural venous sinuses before reaching the internal jugular vein.
• The superior sagittal sinus runs along the superior edge of the falx cerebri, continuing posteriorly to drain into the transverse sinuses bilaterally.
• Each transverse sinus becomes the sigmoid sinus, which exits the jugular foramen to become the internal jugular vein.
• The inferior sagittal sinus, located along the inferior margin of the falx cerebri, joins the great vein of Galen to form the straight sinus.
• The confluence of the sinuses, where the straight sinus, superior sagittal sinus, and occipital sinus meet, drains into the transverse sinuses.
• The cavernous sinus, located on either side of the hypophysial fossa, receives drainage from other sinuses and the ophthalmic veins.
• The cavernous sinus drains via the superior petrosal sinus to the transverse sinus and the inferior petrosal sinuses to the internal jugular vein.

Superficial Veins

• Three main superficial veins drain the cerebral cortex:
• Superficial middle cerebral vein: runs parallel to the lateral fissure and drains the temporal lobe into the cavernous sinus.
• Superior Anastomotic Vein (of Trolard): travels perpendicularly to the superficial middle cerebral vein across the parietal lobe and drains into the superior sagittal sinus.
• Inferior Anastomotic Vein (of Labbé): travels perpendicularly to the superficial middle cerebral vein and drains the temporal lobe into the transverse sinus.

Deep Veins

• Deep veins drain into the great cerebral vein (of Galen) before reaching dural venous sinuses.
• The anterior cerebral vein and deep middle cerebral vein travel adjacent to the ACA and MCA.
• These veins join to form the basal vein (of Rosenthal), which encircles the lateral aspect of the midbrain.
• The internal cerebral veins, formed at the interventricular foramen by the septal and thalamostriate veins, join the basal veins to form the great cerebral vein (of Galen).

Thalamus

• The thalamus is a large, egg-shaped mass of gray matter derived from the diencephalon.
• It primarily serves as a relay center for pathways projecting to the cerebral cortex and also acts as a gatekeeper, regulating information transfer.
• Sensory, motor, limbic, and modulatory signals synapse within thalamic nuclei.
• The thalamus is adjacent to the interventricular foramen, hypothalamus, posterior limb of the internal capsule, body of the lateral ventricle, and the midbrain.
• The two thalamic masses are connected by the interthalamic adhesion.

Thalamic Nuclei

• Thalamic nuclei are classified into four groups based on their location relative to the internal medullary lamina: anterior, medial, lateral, and intralaminar.
• Thalamic nuclei are also divided into three functional classes: relay, intralaminar, and reticular.
• Relay nuclei, which have reciprocal excitatory connections with the cortex, comprise most of the thalamus.
• Relay nuclei are further classified as specific or nonspecific based on their projections to specific areas of the primary sensory and motor cortex or more diffuse cortical projections.
• The specific relay nuclei, primarily located in the lateral thalamus, are involved in sensory modalities (except olfaction) before reaching the primary cortical targets.
• Intralaminar nuclei are associated with arousal and attention.
• The reticular nucleus, located at the periphery of the thalamus, acts as an inhibitory gatekeeper.
• The thalamus receives vascular supply from penetrating branches of the ACA, anterior choroidal artery, lenticulostriate arteries of the MCA, and thalamoperforator arteries of the PCAs.

This quiz covers the foundational aspects of neuron structure, including dendrites, soma, axons, and axon terminals. It also explores the division of the nervous system into somatic and visceral components. Test your understanding of how neurons function and their role in the nervous system.