Anatomy of the Nervous System and Embryonic Development

Early Embryonic Development of the Nervous System

The development of the nervous system begins with the formation of the neural plate, also known as the neuroectoderm. This initial structure arises from the ectoderm, the outermost germ layer. As development progresses, the neural plate undergoes changes to form the neural fold and the neural groove. Through a process identified as convergence, these folds eventually meet and fuse to form the neural tube. During this process, a specific population of cells known as the neural crest is also formed, which eventually gives rise to various peripheral structures while the neural tube itself becomes the basis for the central nervous system.

Primary and Secondary Vesicle Stages of Development

The embryonic development of the brain is categorized into distinct stages based on the formation of vesicles. In a three-to-four-week embryo, there are three primary brain vesicles: the Prosencephalon (Forebrain), the Mesencephalon (Midbrain), and the Rhombencephalon (Hindbrain). By the five-week embryo stage, these primary vesicles differentiate into five secondary brain vesicles. The Prosencephalon divides into the Telencephalon and the Diencephalon. The Mesencephalon remains as the Mesencephalon. The Rhombencephalon divides into the Metencephalon and the Myelencephalon.

These vesicles eventually develop into mature adult structures. The Telencephalon becomes the Cerebrum. The Diencephalon develops into the Eye cup, Thalamus, Hypothalamus, and Epithalamus. The Mesencephalon develops into the Midbrain. The Metencephalon gives rise to the Pons and the Cerebellum. Finally, the Myelencephalon develops into the Medulla oblongata.

Neural Tube Defects and Spina Bifida

Neural tube defects occur if the neural tube fails to close properly during early development. A primary example is spina bifida, a condition that occurs within the first $4$ weeks of embryonic development. There are several forms of spina bifida, including Spina Bifida Occulta, Meningocele, and Myelomeningocele. These conditions represent various degrees of failure in the closure of the neural tube and the subsequent formation of the vertebral column and spinal cord.

Major Divisions of the Nervous System

The nervous system is structurally divided into two main components. The Central Nervous System (CNS) consists of the Brain and the Spinal cord. The Peripheral Nervous System (PNS) consists of the structures outside the CNS, specifically Nerves and Ganglia. Within the brain, there are four major regions: the Cerebrum, the Diencephalon, the Brain stem, and the Cerebellum. The brain stem itself is composed of the midbrain and the hindbrain components.

Anatomy and Function of the Cerebrum

The cerebrum is the largest part of the brain, accounting for most of the brain mass. It is responsible for many higher-level neurological functions, including memory, emotion, and consciousness. Structurally, the cerebrum is characterized by the cerebral cortex, which contains the prominent folds or "wrinkles" of the brain. The cerebrum is divided into left and right cerebral hemispheres by the longitudinal fissure. Communication between these two hemispheres is facilitated by the corpus callosum, which serves as the major pathway of communication.

The cerebrum is composed of the cerebral cortex and three sets of internal nuclei: the basal nuclei, the basal forebrain, and the limbic cortex. The surface of the cerebral cortex is defined by ridges called gyri (singular: gyrus) and grooves called sulci (singular: sulcus).

Lobes of the Cerebral Cortex

The cerebral cortex is organized into distinct lobes, each associated with specific functions:

• Frontal lobe: Responsible for voluntary motor functions, planned movements, speech, and short-term memory.
• Parietal lobe: Responsible for somatosensation, proprioception (the sense of self-movement and body position), and kinesthesia.
• Temporal lobe: Responsible for auditory sensation and memory.
• Occipital lobe: Responsible for primary visual perception.
• Insular lobe: Located deep within the lateral sulcus, it is involved in emotion, pain, and the sense of taste.

Brodmann’s Areas and Cytoarchitecture

In the early $1900’s$, a German neuroscientist named Korbinian Brodmann studied the cytoarchitecture of the cerebral cortex. Based on the microscopic organization of cells, he divided the cortex into $52$ different regions, which are now referred to as Brodmann’s Areas. These areas are used to map specific functions to precise anatomical locations on the cortex.

Subcortical Structures

Subcortical nuclei are found beneath the cerebral cortex and serve to augment cortical processes. These include:

• Nuclei of the basal forebrain: Involved in learning and memory.
• Hippocampus: Critical for long-term memory formation.
• Amygdala: Involved in memory and emotional behavior.
• Basal nuclei: Involved in cognitive processing and the planning of movements. The basal nuclei include the Caudate, Putamen, and Globus pallidus.

The Diencephalon

The diencephalon serves as the connection between the cerebrum and the rest of the nervous system, with the notable exception of olfaction (smell). It consists of several components:

• Thalamus: Relays sensory and motor signals to the cortex and regulates consciousness, sleep, and alertness.
• Hypothalamus: Regulates homeostasis, produces neurohormones, and is involved in memory and emotion.
• Epithalamus: Contains the pineal gland, which is responsible for the production of melatonin.
• Subthalamus: Contains the subthalamic nucleus, which is considered part of the basal nuclei.

The Brain Stem and Hindbrain

The brain stem is composed of the midbrain and parts of the hindbrain. The midbrain is involved in vision, hearing, motor control, sleep and wakefulness, alertness, and temperature regulation. The hindbrain includes the Pons and the Medulla oblongata. The Pons serves as the main connection with the cerebellum and conducts signals from the brain to the cerebellum and medulla. The Medulla oblongata is responsible for the regulation of the cardiovascular and respiratory systems, as well as sleep and wakefulness.

The Cerebellum

Often referred to as the ‐‐little brain,‐‐ the cerebellum accounts for approximately $10\%$ of the total brain mass. Its primary function is to compare information from the cerebrum with sensory feedback from the periphery via the spinal cord, ensuring coordinated movement and balance.

The Spinal Cord: Gray and White Matter

The spinal cord is divided into regions that correspond to the regions of the vertebral column. Anatomically, it separates into gray matter and white matter. The gray matter is shaped like a capital ‐‐H‐‐ or a butterfly and is divided into horns:

• Posterior Horn: Responsible for sensory processing.
• Anterior Horn: Sends motor signals to skeletal muscle.
• Lateral Horn: Serves as the central component of the sympathetic division of the Autonomic Nervous System (ANS).

The white matter of the spinal cord is organized into columns: Posterior, Anterior, and Lateral. Within these columns are tracts:

• Ascending tracts: These carry sensory information up to the brain. These tracts are found in the posterior, anterior, and lateral columns.
• Descending tracts: These carry motor commands from the brain down to the body. These tracts are found in the anterior and lateral columns, but notably, they are not found in the posterior column.