General Anatomy/Nervous System/Anatomical compartmentalization

Central nervous system (CNS)

The CNS is the first of two principal components of the Nervous System that can be divided by their anatomical compartmentalization. The CNS is enclosed by bony structures – the skull (Cranium) and the vertebral canal (Canalis vertebralis) – in order to provide maximal mechanical shelter. Furthermore, at the microscopical level all parts of the CNS are ensheated with a specific tissue, the meninges, that almost completely separate the CNS from the rest of the body. The meningeal envelope controls the import and export of nurishing substances, pivotal electrolytes, but also prevents entry of potentially harmful material like toxins, bacteria and viruses. Anatomically, the CNS consists of two major components, the spinal cord and the brain. In both parts two basic types of nervous tissue can be differentiated: gray and white matter.

Spinal cord (Medulla spinalis)

Gross anatomy:

The spinal cord is a tail-like structure that embedded in the vertebral canal of the spine. The adult spinal cord has a length of ca. 40 cm and weighs approximately 30 g. The spinal cord is caudally attached to the Medulla oblongata of the brain stem. It can be subdivided into five sections (cranio-caudal order): the cervical part (Pars cervicalis), thoracic part (Pars thoracica), lumbar part (Pars lumbalis), sacral part (Pars sacralis) and the coccygeal part (Pars coccygea). Most caudally the spinal cord constricts and ends in a conical structure, the Conus medullaris. In the course of the spinal cord, two swellings are apparent at the cervical and the lower lumbar level, the Intumescentia cervicalis and Intumescentia lumbosacralis. The swollen appearance is owed to the fact that these portions of the spinal cord provide extensive innervation of the upper and lower extremities (muscles, dermatomes, vasculature), which are supplied by these segments (C4-T1 and L1-L3).

Microscopic anatomy:

The spinal cord is composed of three distinct columns on each side: the ventral (anterior), lateral (or intermediate) and dorsal (posterior) column.

Internal organization & Function:

The spinal cord exhibits a archetypic organizational principle serving four distinct tasks:

(1) ascending tracts convey mainly sensory information to the brain.

(2) descending tracts carry centrally (in the brain) generated informations for controlling peripheral targets, e.g. skeletal muscles.

(3) local autonomic centres control visceral organs; their efferent commands are conveyed by the autonomic nervous system.

(4) local sensorymotoric centres are enabled to carry out basic programmes for controlling posture and fundamental movement routines.

Brain (Cerebrum)

The brain is the most complex organ of the body. Its complexity arises from hundreds of billions of nerve cells (neurons) that are clustered in different local compartments (Nuclei) and that interacts with each other via the process of neurotransmission. The brain can be further subdivided into three main sections that arise very early in embryonic development: the forebrain (Prosencephalon), midbrain (Mesencephalon) and hindbrain (Rhombencephalon).

Hindbrain (Rhombencephalon)

The hindbrain consists of three components.

Medulla oblongata (Myelencephalon)

Most caudally at the transition zone between brain and spinal cord, the Medulla oblongata is the first region that formally belongs to the brain. The internal structure and gross aspect of the Medulla oblongata nearly resembles the spinal cord's appearance, which led to the coining of an alternative name: Myelencephalon.

Pons

The Pons (bridge) bridges the Medulla oblongata with the midbrain region. Attached to the dorsal aspect of the Pons is the Cerebellum.

Cerebellum

The Cerebellum is a separate brain region only attached to the rest of the brain by three stalks (Pedunculi) on each side: the Pedunculus cerebellaris superior, Pedunculus cerebellaris medius and Pedunculus cerebellaris inferior. The Cerebellum exhibits a distinction of gray and white matter like the Prosencephalon. Two hemispheres are separated from each other by the interposed Vermis cerebelli. A different subdivision describes three cerebellar lobes, anterior (Lobus cerebellaris anterior), posterior (Lobus cerebellaris posterior)and the flocculonodular lobe (Lobus flocculonodularis). Additionally, the Cerebellum contains internal nuclei (Nuclei cerebelli) that are distinct from the cerebellar cortex: Ncl. fastigii (Ncl. medialis cerebelli), Ncl. dentatus (Ncl. lateralis cerebelli), Ncl. emboliformis (Ncl. interpositus anterior), and the Ncl. globosus (Ncl. interpositus posterior).

Terminological comment:

Pons and Cerebellum are occasionally referred to as the Metencephalon (not to be mistaken with Mesencephalon). They develop from the same embryological structure, which is still apparent in adulthood given their close topographical position and their interconnection via two of the three cerebellar peduncles.

The term "brain stem" is a useful description of the topographical and functional triad composed of Medulla oblongata, Pons and Metencephalon. These three components arise from developmentally distinct regions in the embryo but they represent an anatomical and functional continuum. The brain stem houses all cranial nerve nuclei.

Midbrain (Mesencepalon)

The midbrain is interposed between the hind- and the forebrain. It displays the same basic functional composition as found in the spinal cord and the hindbrain. Ventral areas are dedicated to motor function and accommodate tracts that convey motor information from the Prosencephalon. Dorsal regions of the midbrain are implicated in sensory information cicuits. The Mesencephalon can be further subdivided into three compartments, the ventral Basis or Pes mesencephali. The Tectum mesencephali represents the dorsal roof of the midbrain. Interposed between both is the Tegmentum mesencephali. In the Mesencephalon nuclei of some cranial nerves, whose ventrodorsal positions in the midbrain match the functional motor-sensory axis of the brain stem and the spinal column.

Forebrain (Prosencephalon)

The forebrain is the evolutionary youngest part of the CNS. It consists of two components.

Diencephalon

Thalamus

Hypothalamus

Telencephalon

Cortex

Basal ganglia

Peripheral nervous system (PNS)

PNS is an extent network of neurons that spreads the whole body and is out of bony structures – the skull (Cranium) and the vertebral canal (Canalis vertebralis). This network consists of Somatic & Autonomic ports based on functional compartmentalization. The Somatic part classifies in two groups.

Afferent division or sensory fibers

They are Axons of the primary afferent neurons that transmit the signals of stimuli in the form of action potential to the central portions of sensory system that lie in the central nerve system. Body of these neurons lie in the peripheral ganglions and nuclei in the CNS.

Efferent division or motor fibers

They are axons of efferent neurons that conduct motor signals from central nerve system to peripheral organs; muscles... and create motions in the target sites.