The Human Central Nervous System

Index to this page
The central nervous system is made up of the

The spinal cord

The brain

White Matter vs. Gray Matter

Both the spinal cord and the brain consist of

In the spinal cord, the white matter is at the surface, the gray matter inside.

In the brain of mammals, this pattern is reversed. However, the brains of "lower" vertebrates like fishes and amphibians have their white matter on the outside of their brain as well as their spinal cord.

The Meninges

Both the spinal cord and brain are covered in three continuous sheets of connective tissue, the meninges. From outside in, these are the The region between the arachnoid and pia mater is filled with cerebrospinal fluid (CSF).

The Interstitial Fluid of the Central Nervous System

The cells of the central nervous system are bathed in a fluid, called cerebrospinal fluid (CSF), that differs from that serving as the interstitial fluid (ISF) of the cells in the rest of the body.

CSF returns to the blood through lymphatic vessels draining the brain.

In mice, the flow of CSF increases by 60% when they are asleep. Perhaps one function of sleep is to provide the brain a way of removing potentially toxic metabolites accumulated during waking hours.

The Spinal Cord


31 pairs of spinal nerves arise along the spinal cord. These are "mixed" nerves because each contain both sensory and motor axons. However, within the spinal column, The spinal cord carries out two main functions: The interneurons carrying impulses to and from specific receptors and effectors are grouped together in spinal tracts.

Crossing Over of the Spinal Tracts

Impulses reaching the spinal cord from the left side of the body eventually pass over to tracts running up to the right side of the brain and vice versa. In some cases this crossing over occurs as soon as the impulses enter the cord. In other cases, it does not take place until the tracts enter the brain itself.

The Brain

The brain of all vertebrates develops from three swellings at the anterior end of the neural tube of the embryo. From front to back these develop into the The human brain receives nerve impulses from
Link to table listing the cranial nerves

The Hindbrain

The main structures of the hindbrain (rhombencephalon) are the

Medulla oblongata

The medulla looks like a swollen tip to the spinal cord. Nerve impulses arising here The neurons controlling breathing have mu (µ) receptors, the receptors to which opiates, like heroin, bind. This accounts for the suppressive effect of opiates on breathing. [Discussion] Destruction of the medulla causes instant death.

Pons

The pons seems to serve as a relay station carrying signals from various parts of the cerebral cortex to the cerebellum. Nerve impulses coming from the eyes, ears, and touch receptors are sent on the cerebellum. The pons also participates in the reflexes that regulate breathing.

The reticular formation is a region running through the middle of the hindbrain (and on into the midbrain). It receives sensory input (e.g., sound) from higher in the brain and passes these back up to the thalamus. The reticular formation is involved in sleep, arousal (and vomiting).

Cerebellum

The cerebellum consists of two deeply-convoluted hemispheres. Although it represents only 10% of the weight of the brain, it contains as many neurons as all the rest of the brain combined.

Its most clearly-understood function is to coordinate body movements. People with damage to their cerebellum are able to perceive the world as before and to contract their muscles, but their motions are jerky and uncoordinated.

So the cerebellum appears to be a center for learning motor skills (implicit memory). Laboratory studies have demonstrated both long-term potentiation (LTP) and long-term depression (LTD) in the cerebellum.

The Midbrain

The midbrain (mesencephalon) occupies only a small region in humans (it is relatively much larger in "lower" vertebrates). We shall look at only three features:
Link to discussion of how various psychoactive chemicals act on synapses within the central nervous system.

The midbrain along with the medulla and pons are often referred to as the "brainstem".

The Forebrain

The human forebrain (prosencephalon) is made up of

Diencephalon

We shall consider four of its structures: the

The Cerebral Hemispheres

Each hemisphere of the cerebrum is subdivided into four lobes visible from the outside: Hidden beneath these regions of each cerebral cortex is

Mapping the Functions of the Brain

It is estimated that the human brain contains some 86 billion (8.6 x 1010) neurons averaging 10,000 synapses on each; that is, almost 1015 connections. How to unravel the workings of such a complex system?

Several methods have been useful.

Histology

Microscopic examination with the aid of selective stains has revealed many of the physical connections created by axons in the brain.

The Electroencephalograph (EEG)

This device measures electrical activity (brain "waves") that can be detected at the surface of the scalp. It can distinguish between, for example, sleep and excitement. It is also useful in diagnosing brain disorders such as a tendency to epileptic seizures.

Damage to the Brain

Many cases of brain damage from, for example, have provided important insights into the functions of various parts of the brain.

Example 1:

Battlefield injury to the left temporal lobe of the cerebrum interferes with speech.

Example 2: Phineas P. Gage

In 1848, an accidental explosion drove a metal bar completely through the frontal lobes of Phineas P. Gage. Not only did he survive the accident, he never even lost consciousness or any of the clearly-defined functions of the brain. However, over the ensuing years, he underwent a marked change in personality. Formerly described as a reasonable, sober, conscientious person, he became — in the words of those observing him — "thoughtless, irresponsible, fitful, obstinate, and profane". In short, his personality had changed, but his vision, hearing, other sensations, speech, and body coordination were unimpaired. (Similar personality changes have since been often observed in people with injuries to their prefrontal cortex.)

The photograph (courtesy of the Warren Anatomical Museum, Harvard University Medical School) shows Gage's skull where the bar entered (left) and exited (right) in the accident (which occurred 12 years before he died of natural causes in 1861).

Stimulating the exposed brain with electrodes

There are no pain receptors on the surface of the brain, and some humans undergoing brain surgery have volunteered to have their exposed brain stimulated with electrodes during surgery. When not under general anesthesia, they can even report their sensations to the experimenter.

Experiments of this sort have revealed a band of cortex running parallel to and just in front of the fissure of Rolando that controls the contraction of skeletal muscles. Stimulation of tiny spots within this motor area causes contraction of the muscles.

The area of motor cortex controlling a body part is not proportional to the size of that part but is proportional to the number of motor neurons running to it. The more motor neurons that activate a structure, the more precisely it can be controlled. Thus the areas of the motor cortex controlling the hands and lips are much larger than those controlling the muscles of the torso and legs.

A similar region is located in a parallel band of cortex just behind the fissure of Rolando. This region is concerned with sensation from the various parts of the body. When spots in this sensory area are stimulated, the patient reports sensations in a specific area of the body. A map can be made based on these reports.

When portions of the occipital lobe are stimulated electrically, the patient reports light. However, this region is also needed for associations to be made with what is seen. Damage to regions in the occipital lobe results in the person's being perfectly able to see objects but incapable of recognizing them.

The centers of hearing — and understanding what is heard — are located in the temporal lobes.

CT = X-ray Computed Tomography

This is an imaging technique that uses a series of X-ray exposures taken from different angles. Computer software can integrate these to produce a three-dimensional picture of the brain (or other body region). CT scanning is routinely used to quickly diagnose strokes.

PET = Positron-Emission Tomography

This imaging technique requires that the subject be injected with a radioisotope that emits positrons.

The images above (courtesy of Michael E. Phelps from Science 211:445, 1981) were produced in a PET scanner. The dark areas are regions of high metabolic activity. Note how the metabolism of the occipital lobes (arrows) increases when visual stimuli are received.

Similarly, sounds increase the rate of deoxyglucose uptake in the speech areas of the temporal lobe.

The image on the right (courtesy of Gary H. Duncan from Talbot, J. D., et. al., Science 251: 1355, 1991) shows activation of the cerebral cortex by a hot probe (which the subjects describe as painful) applied to the forearm (which forearm?).


Most cancers consume large amounts of glucose (cellular respiration is less efficient than in normal cells so they must rely more on the inefficient process of glycolysis). Therefore PET scanning with 18F-fluorodeoxyglucose is commonly used to monitor both the primary tumor and any metastases.

MRI = Magnetic Resonance Imaging

This imaging technique uses powerful magnets to detect magnetic molecules within the body. These can be endogenous molecules or magnetic substances injected into a vein.

fMRI = Functional Magnetic Resonance Imaging

fMRI exploits the changes in the magnetic properties of hemoglobin as it carries oxygen. Activation of a part of the brain increases oxygen levels there increasing the ratio of oxyhemoglobin to deoxyhemoglobin.

The probable mechanism:

Magnetoencephalography (MEG)

MEG detects the tiny magnetic fields created as individual neurons "fire" within the brain. It can pinpoint the active region with a millimeter, and can follow the movement of brain activity as it travels from region to region within the brain.

MEG is noninvasive requiring only that the subject's head lie within a helmet containing the magnetic sensors.

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1 November 2013