Next Lesson - Cells and Components of the Nervous System
Core
Neuroanatomy is complex, and it is closely linked to the anatomy of the head and neck. Articles on head and neck anatomy can be found in our Head and Neck unit here.
Basic Components of the Nervous System
The largest division in the nervous system is between the central and peripheral nervous systems.
The central nervous system (CNS) is made up of the brain and the spinal cord. The brain itself is the cerebral hemispheres, the brainstem and the cerebellum together.
The peripheral nervous system (PNS) is the nervous tissue that lies outside the brain and spinal cord. It includes the dorsal and ventral roots of the spinal nerves, the spinal and cranial nerves themselves, and ganglia (clusters of nerve cell bodies outside the CNS). The optic nerve is an exception: it develops as an outgrowth of the brain, so it is counted as part of the CNS.
Grey matter and white matter are terms used to describe different portions of the central nervous system depending on which cell types are located within that portion. For more information on the different cell types of the nervous system, please see here.
Grey matter contains neuronal cell bodies, dendrites, axons and synapses, alongside glial cells (the supporting cells of the nervous system) and blood vessels. It is found in the cerebral cortex and in deep nuclei (clusters of nerve cell bodies deep within the brain).
White matter is rich in axons, many of them myelinated, and also contains glial cells and blood vessels. Myelin gives white matter its pale appearance.
Peripheral Nervous System ‘Matter’
The terms grey matter and white matter are specific to the central nervous system, so cannot be used when referring to the peripheral nervous system.
The peripheral nervous system equivalent to grey matter is a ‘ganglion’, because this is a collection of cell bodies in the peripheral nervous system.
The equivalent term to white matter in the peripheral nervous system is a peripheral nerve, because a peripheral nerve is a bundle of axons held together by layers of connective tissue.
The spinal cord is divided into 31 segments. Each segment supplies a specific region of skin with sensory innervation (nerve supply), called a dermatome, and a specific group of muscles with motor innervation, called a myotome, on each side of the body. Each segment connects with a spinal nerve through two nerve roots, the short bundles of axons that join the spinal nerve to the cord. The dorsal (sensory) root carries sensory axons into the spinal cord, and the ventral (motor) root carries motor axons out of it.
The spinal cord has a central core of grey matter with an outer shell of white matter.
There are a number of divisions of white matter in the spinal cord based on where the axons in the white matter are travelling to or from.
A tract is an anatomically and functionally defined portion of white matter that connects two distinct regions of grey matter. Impulses travel along the axons of a tract in one direction only. A tract is usually either sensory (ascending, carrying signals up to the brain) or motor (descending, carrying commands down from the brain).
A fasciculus (plural fasciculi) is a subdivision of a tract that serves one distinct part of the body. For example, the fasciculus gracilis carries sensation from the lower half of the body and the fasciculus cuneatus carries sensation from the upper half.
A funiculus (plural funiculi) is a segment of white matter that contains multiple distinct tracts. Because there are multiple tracts present, impulses can travel in different directions.
Diagram - Cross Section of the Spinal Cord
SimpleMed original by Dr. Maddie Swannack
In the above diagram, the sensory tracts are shown on the left (in the blue) and the motor tracts are shown on the right (in the pink). It is important to remember that all tracts occur on both sides of the spinal cord, as marked with the outlines.
Like the white matter, the grey matter of the spinal cord is organised into regions. Rexed’s laminae are ten numbered layers (I to X) of spinal grey matter, defined by the appearance of their cells rather than by their position. Laminae I to VI make up the dorsal horn, and laminae VIII and IX make up the ventral horn.
The ventral horn contains the motor neurons that send signals out to skeletal muscle. It is easiest to imagine it as one continuous column that links the ventral horns of all spinal levels. Different levels of the ventral horn ‘column’ supply different specific muscles. For example, the upper lumbar levels (mainly L1 to L3) supply the iliopsoas muscle, the main hip flexor.
A nucleus is a grey matter structure made up of a collection of functionally related cell bodies. This is a different meaning of nucleus from the nucleus inside a cell.
The cortex is a folded sheet of grey matter on the surface of the brain. It contains more than neuronal cell bodies: it also holds dendrites, synapses, glia and blood vessels.
A fibre is a term used to describe an axon and its supporting cells. In the CNS, these supporting cells are oligodendrocytes, which form the myelin around axons.
An association fibre connects cortical regions within the same hemisphere.
A commissural fibre connects the left and right hemispheres of the brain, or the left and right halves of the cord.
A projection fibre connects the cerebral hemispheres with the cord or brainstem, and vice versa.
Diagram - The Three Main Types of Fibre
SimpleMed original by Dr. Maddie Swannack
The brainstem is split into three portions:
The midbrain is the most superior portion of the brainstem. It is responsible for eye movements and reflex responses to sound and vision.
The pons is the central portion of the brainstem. It relays signals from the cerebrum to the cerebellum, and it contains the nuclei of several cranial nerves, such as the trigeminal and facial nerves. It is also involved in breathing and in the regulation of sleep and arousal (being awake and alert).
The medulla is the most inferior portion of the brainstem. It contains the medullary pyramids, which carry the corticospinal tract (the motor pathway that runs from the cortex to the spinal cord). It also contains the respiratory and cardiovascular centres.
Diagram - The Midbrain Structures
Source illustration: OpenStax College, source, CC BY 4.0.
The gyri are the raised areas of the cerebral cortex, with the sulci between them. Together, they make up the bumpy surface of the brain that is familiar.
There are a number of important landmarks:
Central Sulcus - starts near the top of the hemisphere and runs downwards and forwards, roughly in the coronal plane. It separates the frontal and parietal lobes.
Precentral Gyrus - contains the primary motor cortex.
Postcentral Gyrus - contains the primary somatosensory cortex, the area that receives sensation from the body, such as touch.
Lateral or Sylvian Fissure - a deep sulcus (fissure) that separates the temporal lobe from the parietal and frontal lobes above it.
Calcarine Sulcus - a groove on the inner (medial) surface of the occipital lobe, at the back of the brain. The primary visual cortex, which receives visual information from the eyes, surrounds it.
Parahippocampal Gyrus - visible on the inferior aspect of the brain, the parahippocampal gyrus is an area of cortex next to the hippocampus (a structure in the temporal lobe that is important for memory). It plays an important role in memory encoding and retrieval.
Cingulate Gyrus - visible on the midline, the cingulate gyrus is part of the limbic system, a collection of structures involved in emotion, learning and memory.
Diagram - The Cortices of the Brain
Source illustration: BruceBlaus / Blausen.com staff (2014), source, CC BY 3.0.
The optic chiasm is where the two optic nerves meet. Fibres from the nasal (inner) half of each retina (the light-sensing layer at the back of the eye) cross to the opposite optic tract, while fibres from the temporal (outer) half stay on the same side. Compression or damage here causes characteristic loss of parts of the visual field.
The uncus is a region of the cerebral cortex of the temporal lobe. When the pressure inside the skull (intracranial pressure) is raised, the uncus can herniate (be pushed out of its normal place) through the tentorium cerebelli, the membrane that separates the cerebellum from the cerebrum above it, and compress the midbrain structures.
The medullary pyramids are paired structures within the medulla (also called the medulla oblongata), visible on the inferior aspect of the brain. They contain the descending corticospinal motor tracts.
Diagram - The Inferior Surface of the Brain
Base illustration: Henry Vandyke Carter, Gray’s Anatomy, source, public domain; annotated adaptation.
The corpus callosum is made up of commissural fibres that connect the two hemispheres of the brain.
The thalamus is a relay station. It passes sensory signals (from every sense except smell) and motor signals on to the cerebral cortex, and it helps to regulate consciousness and alertness.
The hypothalamus is essential for homeostatic control in the body.
The fornix is a C-shaped bundle of nerve fibres that carries signals out of the hippocampus, a structure in the temporal lobe that is important for memory.
The cerebellar tonsils are a region of the cerebellum. Like the uncus, they can herniate (be pushed out of their normal place) through the foramen magnum, the large opening at the base of the skull where the spinal cord begins, and compress the medulla.
Diagram - The Midline Structures of the Brain
Base illustration: Henry Vandyke Carter, Gray’s Anatomy, source, public domain.
Edited by: Dr. Marcus Judge
Reviewed by: Adrian Judge
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