Chp 7 The Structure of the Nervous System

THE STRUCTURE OF THE NERVOUS SYSTEM

Chapter Overview

By the end of this lecture, students should be able to discuss various essential components and structures of the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The key points include:

  • Understanding the composition and significance of CNS and PNS.

  • Clarifying the role and organization of cerebrospinal fluid (CSF) and ventricles.

  • Defining the neuraxis and its directional terms.

  • Discussing the different approaches to slice a brain.

  • Explaining the meningeal layers and their protective function for the CNS.

  • Describing the developmental stages of the CNS from the embryonic neural tube through to its fully developed state.

  • Identifying and understanding the organization and functions of various CNS subdivisions.

Importance of Neuroanatomy

Understanding neuroanatomy is crucial for a comprehensive grasp of neuroscience. The structures described in this chapter will be integral throughout the course. It is encouraged to memorize the divisions of the brain, their structures, and functions early on to facilitate understanding later. Suggested study techniques include using flashcards, visual aids, drawing diagrams, partaking in study groups, and utilizing online video resources.

Gross Organization of the Mammalian Nervous System

The nervous system of mammals exhibits notable similarities to the human brain, notably in cortical development and neuronal complexity. Structures are mirrored across the left and right hemispheres, albeit with distinct functions attributed to each side. The major branches of the nervous system are:

  • Peripheral Nervous System (PNS)

    • Autonomic (involuntary): includes sympathetic and parasympathetic systems.

    • Somatic (voluntary)

  • Central Nervous System (CNS): comprising the spine and brain, further categorized into distinct hemispheres which reflect bilateral asymmetry in function.

Anatomical References
The Neuraxis

The neuraxis is defined as an imaginary line traversing the CNS, stretching from the bottom of the spinal cord to the anterior portion of the forebrain. Directional terms associated with the neuraxis include:

  • Dorsal: top

  • Ventral: bottom

  • Rostral: toward the nose

  • Caudal: toward the tail

  • Medial: towards the center

  • Lateral: outward to the sides

  • Ipsilateral: same side of the body

  • Contralateral: opposite side of the body.

The Human Neuraxis

The human neuraxis, while fundamentally similar, requires adjustments in terminology due to its orientation; it ascends along the spine as though in a crawling position, curving towards the eyes.

Methods of Brain Dissection

Different slicing techniques produce various views of the brain:

  • Coronal: slices down like cutting bread.

  • Horizontal: slices in line with the eyes, resembling a hamburger bun.

  • Sagittal: slices through the midline, akin to cutting a hot dog bun.

The Central Nervous System (CNS)

The CNS is comprised of:

Major Structures

  1. Cerebrum: The rostral-most and largest brain part, also referred to as the telencephalon, divided into two hemispheres.

  2. Cerebellum: Located posterior to the cerebrum; essential for movement control and coordination, also known as "little brain."

  3. Brain Stem: The stalk from which the cerebrum and cerebellum extend; crucial for relaying information and regulating vital functions such as breathing, consciousness, and body temperature. It is considered one of the most primitive yet vital components.

  4. Spinal Cord: Encased by the vertebral column; serves as the primary information conduit between the body and brain. It communicates through spinal nerves via two branches:

    • Dorsal Roots: bring sensory information into the spinal cord (afferent projections).

    • Ventral Roots: transmit motor information away from the spinal cord (efferent projections).

Peripheral Nervous System (PNS)

The PNS consists of all spinal nerves innervating skin, joints, and muscles under voluntary control (somatic PNS), alongside the autonomic (visceral PNS) which innervates internal organs, blood vessels, and glands. Sensory fibers deliver visceral function information to the CNS, while motor fibers manage muscle contraction and gland secretion.

Meninges

The meninges are protective membranes encompassing both the brain and spinal cord, comprising three layers:

  • Dura Mater: the tough outer layer, literally meaning "tough mother."

  • Arachnoid: a softer middle layer resembling a spiderweb.

  • Pia Mater: a thin layer that closely adheres to the surface of the brain and spinal cord, closely following its contours (sulci and gyri).
    There exists a subarachnoid space between the arachnoid and pia mater, filled with cerebrospinal fluid (CSF).

Ventricular System

The ventricular system consists of hollow interconnected chambers within the brain:

  • Ventricles: spaces through which cerebrospinal fluid flows; includes lateral ventricles (largest), third ventricle, cerebral aqueduct, and fourth ventricle. The fluid is produced in the choroid plexus and exits the ventricular system into the subarachnoid space via kecil openings near the cerebellum. CSF is absorbed by blood vessels at arachnoid villi, and improper flow can result in conditions such as hydrocephalus.

Developmental Anatomy

The Central Nervous System beginnings trace back to early embryonic development as…

Development Stages

  1. Zygote: The fertilization product.

  2. Blastocyst: A hollow sphere of pluripotential cells.

  3. Embryo: A three-layered organism with developing features.

  4. Neurulation: Begins around the 17th day post-conception, with the thickening of the ectoderm to form the neural plate and subsequent folding to create the neural tube — this structure will become the CNS.

Brain Formation

By approximately day 28, three primary brain regions form around interconnected chambers, specifically:

  • Forebrain (prosencephalon)

  • Midbrain (mesencephalon)

  • Hindbrain (rhombencephalon)

Differentiation Process

  1. Forebrain: Develops into optic vesicles (retina), telencephalon (cerebral cortex, basal ganglia, and limbic system), and diencephalon (thalamus and hypothalamus).

  2. Midbrain: Differentiates into the tectum (superior and inferior colliculi) and tegmentum (substantia nigra, which plays a role in movement).

  3. Hindbrain: Comprises three structures:

    • Cerebellum: Control of movement.

    • Pons: Relays descending axons from the cortex to the cerebellum.

    • Medulla Oblongata: Site for vital life function regulation and sensory/motor signal relay.

CNS Organization Post-Development

  • Major divisions include:

    • Forebrain: Comprised of telencephalon (including the cerebral cortex, basal ganglia, limbic system) and diencephalon (thalamus, hypothalamus).

    • Midbrain (mesencephalon) with its distinguished tectum and tegmentum.

    • Hindbrain with cerebellum, pons, and medulla oblongata.

Telencephalon Structures

The telencephalon comprises the cerebral cortex (with sulci, gyri, and fissures) and subcortical regions. Specific lobes functionally include:

  • Frontal Lobe: Houses the primary motor cortex and forms plans and strategies through the prefrontal cortex.

  • Parietal Lobe: Contains the primary somatosensory cortex and processes bodily sensations.

  • Temporal Lobe: Comprises primary auditory cortex and association cortices for auditory and visual processing.

  • Occipital Lobe: Contains the primary visual cortex and visual association areas.

Hemispheric Lateralization

The left and right hemispheres perform complementary, yet distinct functions. The left hemisphere is often associated with analytical tasks, language processing, and detail orientation while the right hemisphere specializes in synthesis, spatial navigation, and social cues. The corpus callosum connects the two hemispheres, facilitating inter-hemispheric communication.

Limbic System and Basal Ganglia

The limbic system, including the hippocampus (for memory) and amygdala (for emotional responses), is crucial for emotional regulation and memory formation. Basal ganglia are also vital for movement control and reward-based learning.

Thalamus and Hypothalamus Functions

The thalamus serves primarily as a sensory relay, interfacing between sensory organs and appropriate cognition centers. The hypothalamus regulates homeostasis and the autonomic nervous system while managing endocrine functions related to growth and stress responses.

Key Takeaways

Understanding the complex structure and functions of the nervous system is foundational for further studies in neuroscience. The interplay of brain structures, their development stages, and functional divisions highlights the intricate organization essential for human behavior, cognition, and emotional regulation.