Structure of the Nervous System

Structure of the Nervous System

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Basic Features of the Nervous System

Neuraxis

  • Neuraxis: An imaginary line drawn through the center of the central nervous system, extending from the bottom of the spinal cord to the front of the forebrain.

Orientation Terms

  1. Anterior (Rostral): In reference to the central nervous system, this term indicates a location near or towards the head.

  2. Posterior (Caudal): In relation to the central nervous system, it indicates a position near or towards the tail.

  3. Rostral: Means "toward the beak", indicating a direction along the neuraxis that moves towards the front of the face.

  4. Caudal: Means "toward the tail", indicating a direction along the neuraxis that moves away from the front of the face.

  5. Dorsal: Meaning "toward the back"; denotes a direction perpendicular to the neuraxis towards the top of the head or back.

  6. Ventral: Meaning "toward the belly"; pertains to a direction perpendicular to the neuraxis toward the bottom of the skull or the front surface of the body.

  7. Lateral: Indicates a position toward the side of the body, away from the middle.

  8. Medial: Indicates a position toward the middle of the body, away from the side.

  9. Ipsilateral: Refers to being located on the same side of the body.

  10. Contralateral: Refers to being located on opposite sides of the body.

Sectional Terms

  • Cross Section or Frontal Section: A transverse cut through the brain made at right angles to the neuraxis, resembling a slice taken parallel to the forehead.

  • Horizontal Section: A slice through the brain made parallel to the ground.

  • Sagittal Section: A slice through the brain made parallel to the neuraxis and perpendicular to the ground.

  • Midsagittal: The plane through the neuraxis perpendicular to the ground, which divides the brain into two symmetrical halves.

Meninges

  • Meninges: The three layers of tissue encasing the central nervous system:

    • Dura Mater: The outermost layer of the meninges, characterized by its toughness and flexibility.

    • Arachnoid Membrane: The middle layer of the meninges, located between the dura mater and the pia mater, named from the Greek word for "spider".

    • Pia Mater: The innermost layer of the meninges, adjacent to the surface of the brain, noted for being thin and delicate.

Subarachnoid Space

  • The Subarachnoid Space is a fluid-filled area that cushions the brain, located between the arachnoid membrane and the pia mater.

Cerebrospinal Fluid (CSF)

  • Cerebrospinal Fluid: A clear fluid similar to blood plasma that fills both the ventricular system of the brain and the subarachnoid space surrounding the brain and spinal cord. It supports the weight of the brain and reduces shock caused by sudden head movements. The brain contains approximately 125 ml of CSF, with a half-life of about three hours.

Choroid Plexus

  • The Choroid Plexus is a highly vascular tissue protruding into the ventricles, responsible for the production of cerebrospinal fluid.

Ventricles

  • Ventricles: Four hollow spaces (termed "little bellies") located within the brain. They consist of:

    • Lateral Ventricles: The two ventricles situated in the center of the telencephalon.

    • Third Ventricle: Located in the center of the diencephalon.

    • Fourth Ventricle: Positioned between the cerebellum and the dorsal pons, is centrally located in the metencephalon.

  • The Cerebral Aqueduct: Connects the third and fourth ventricles, situated in the center of the mesencephalon.

Development of the Central Nervous System

  • Development begins 18 days post-conception, where the Neural Tube forms by the twenty-first day, giving rise to the brain and spinal cord. The neural tube closes by day 28, forming the forebrain, midbrain, and hindbrain.

Neural Tube

  • The Neural Tube is a hollow tube closed at the rostral end that forms from ectodermal tissue during early embryonic development; it serves as the origin for the central nervous system.

Ventricular Zone

  • The Ventricular Zone is a cell layer lining the inside of the neural tube, which contains founder cells that divide to produce the central nervous system.

Cerebral Cortex

  • The Cerebral Cortex is the outermost layer of gray matter in the cerebral hemispheres, approximately 3 mm thick, often referred to as "bark".

Radial Glia

  • Radial Glia are specialized glial cells with fibers extending radially outward from the ventricular zone to the cortical surface, guiding neurons during brain development.

Founder Cells and Division Types

  1. Symmetrical Division: A division of a founder cell producing two identical founder cells, increasing the ventricular zone size and subsequently the brain development.

  2. Asymmetrical Division: Where one founder cell generates another founder cell and a neuron, which migrates away towards its final location in the brain.

Apoptosis

  • Apoptosis: Literally means "falling away"; refers to cell death induced by an internal chemical signal within the cell.

Cortical Structures

  • Fissure: A significant groove in the brain's surface, larger than a sulcus.

  • Sulcus: A smaller groove in the cerebral hemisphere's surface, distinct from fissures.

  • Gyrus: A convolution of the cortex of the cerebral hemispheres that is separated by sulci or fissures.

The Forebrain-Telencephalon

Major Structures

  • Forebrain: The most rostral part of the brain, encompassing both the telencephalon and diencephalon.

  • Cerebral Hemisphere: One of the two larger portions of the forebrain, covered by the cerebral cortex, while the subcortical region contains the limbic system and basal ganglia.

Lobes of the Cerebral Cortex

  1. Frontal Lobe: Anterior portion of the cerebral cortex, rostral to the parietal lobe and dorsal to the temporal lobe.

  2. Parietal Lobe: Located caudal to the frontal lobe and dorsal to the temporal lobe; housing the primary somatosensory cortex.

  3. Temporal Lobe: Found rostral to the occipital lobe and below the parietal and frontal lobes.

  4. Occipital Lobe: Positioned caudal to the parietal and temporal lobes, located posteriorly from the central sulcus.

Functions of the Frontal Lobe

  • Primary Motor Cortex: Involved in the control of voluntary muscle movements.

  • Prefrontal Cortex: Underpins decision-making, impulse control, planning, and organizing.

Functions of the Parietal Lobe

  • Primary Somatosensory Cortex: Receives sensory input from the skin and muscles, involved in perceptual processing and consciousness via attention mechanisms.

  • Unilateral Neglect: A condition resulting from damage to the right parietal lobe, leading to a lack of awareness or attention to a part of space.

Right Parietal Lobe Damage: The Rubber Hand Illusion

  • If a hidden hand and a rubber hand are stroked simultaneously, the person may come to perceive the rubber hand as their own. If the strokes are asynchronous, the illusion fails.

Functions of the Occipital Lobe

  • Primary Visual Cortex: An area primarily responsible for processing visual information.

  • Calcarine Fissure: A significant landmark located in the occipital lobe, hosting the major visual areas of the cortex.

Functions of the Temporal Lobe

  • Primary Auditory Cortex: The region in the superior temporal lobe receiving and processing auditory information.

  • Inferior Temporal Cortex: A secondary area involved in auditory processing.

The Neocortex and Limbic System

  • The Neocortex: The most recently evolved portion of the cortex encompassing primary sensory, motor, and association areas.

  • The Limbic Cortex: An older cortex part of the limbic system situated near the medial edge of the cerebral hemispheres, contributing mainly to emotion, memory, and behavior regulation.

    • Cingulate Gyrus: A strip of limbic cortex situated just above the corpus callosum.

Basal Ganglia

  • Part of the telencephalon, comprising the caudate nucleus, globus pallidus, and putamen; critically involved in motor control and coordination. Degeneration of these structures can lead to movement disorders such as Parkinson's disease, characterized by tremors and rigidity.

Diencephalon

  • Located between the telencephalon and mesencephalon, surrounding the third ventricle, containing major structures including the thalamus and hypothalamus.

Thalamus

  • The largest part of the diencephalon, containing nuclei that relay information to specific areas of the cerebral cortex, fundamental in sensory perception.

    • Lateral Geniculate Nucleus: Associated with the vision pathway.

    • Medial Geniculate Nucleus: Related to auditory processing.

    • Ventrolateral Nucleus: Facilitates the coordination of motor functions.

Hypothalamus

  • Controls vital autonomic functions, the pituitary gland, and behaviors associated with survival (feeding, fighting, fleeing, and reproduction).

Pituitary Gland

  • Known as the "master gland" of the endocrine system; the anterior portion is regulated by hypothalamic hormones, while the posterior releases hormones made by the hypothalamus.

The Midbrain - Mesencephalon

  • Encases the cerebral aqueduct and holds structures essential for vision and hearing, tegmentum, and pathways associated with the reticular formation which functions in arousal and attention.

Tectum & Tegmentum

  • The Tectum: Dorsal part of the midbrain housing the superior and inferior colliculi responsible for visual and auditory reflexes, respectively.

  • The Tegmentum: Includes the periaqueductal gray matter, reticular formation, red nucleus, and substantia nigra, all of which are significant in motor control and responses to stimuli.

The Hindbrain - Metencephalon

  • Comprises the caudal region of the brain, incorporating the metencephalon and myelencephalon.

  • Cerebellum: Known as the "little brain", positioned dorsal to the pons, it plays a crucial role in motor coordination and regulating movements.

    • It processes sensory inputs and adjusts motor outputs, with considerable damage leading to impaired motor function.

  • Pons: Sits rostrally to the medulla, helping regulate sleep and arousal.

  • Medulla Oblongata: The most caudal segment of the brain, crucial in regulating autonomic functions like heart rate, respiration, and reflexes.

Spinal Cord

  • Composed of nervous tissue, it extends down from the medulla. Responsible for transmitting signals between the brain and the rest of the body.

Spinal Roots

  • Dorsal Roots: Contain incoming sensory fibers; Ventral Roots: Contain outgoing motor fibers.

  • Cauda Equina: A group of spinal roots located caudally to the termination of the spinal cord.

Cranial Nerves

  • Twelve pairs of peripheral nerves that emerge directly from the brain, serving sensory and motor functions primarily in the head and neck region. The Vagus Nerve is the largest cranial nerve, contributing significantly to parasympathetic functions in the thoracic and abdominal cavities.

Cranial Nerve Mnemonics

  1. Olfactory: Smell

  2. Optic: Vision

  3. Oculomotor: Eye movements

Functions Summary

  • "Some Say Marry Money But My Brother Says Big Brains Matter More" - Narrative summarizing the sensory or motor functionality of cranial nerves.

Peripheral Nervous System

  • Somatic Nervous System: Controls voluntary movements and sensory information transmission to CNS.

  • Autonomic Nervous System: Governs involuntary bodily functions, subdivided into:

    • Sympathetic Division: Prepares the body for stress, engaging fight-or-flight responses.

    • Parasympathetic Division: Promotes rest, supports bodily functions during relaxed states, commonly referred to as the craniosacral system.