CHAPTER 3

THE FUNCTIONS OF THE NERVOUS SYSTEM

THE CENTRAL NERVOUS SYSTEM

The nervous system is divided into two subunits

The central nervous system (CNS) includes the brain and the spinal cord.

The second part is the peripheral nervous system.

A neuron is a single neural cell.

A nerve is a bundle of axons running together like a multi-wire cable.

However, the term nerve is only used in the peripheral nervous system.

Inside the CNS, bundles of axons are called tracts.

A group of cell bodies is called a nucleus in the CNS and a ganglion in the peripheral nervous system.

The major structures of the forebrain are the two cerebral hemispheres, the thalamus, and the hypothalamus.

The large, wrinkled cerebral hemispheres dominate the brain’s appearance.

The longitudinal fissure that runs the length of the brain separates the two cerebral hemispheres, which are mirror images of each other in appearance.

The brain’s surface has many ridges and grooves that give it a very wrinkled appearance.

Each ridge is called a gyrus.

The groove or space between two gyri is called a sulcus or, if it is large, a fissure.

The outer surface is the cortex, which is made up mostly of the cell bodies of neurons.

Because cell bodies are not myelinated, the cortex looks grayish in color, which is why it is referred to as gray matter.

The cortex is only 1.5 – 4 mm thick, but the convolutions increase the amount of cortex by tripling the surface area.

The central nervous system is arranged in a hierarchy.

As you ascend from the spinal cord through the hindbrain and midbrain to the forebrain, the neural structures become more complex and so do the behaviors they control.

The hemispheres are divided into four lobes – frontal, parietal, occipital, and temporal – each named after the bone of the skull above it.

The frontal lobe is the area anterior to (in front of) the central sulcus and superior to (above) the lateral fissure.

The precentral gyrus, which extends the length of the central sulcus, is the location of the primary motor cortex, which controls voluntary (nonreflexive) movement.

The parts of the body are “mapped onto” the motor area of each hemisphere in the form of a homunculus, which means “little man.”

The secondary motor areas are located just anterior to the primary area.

THE CENTRAL NERVOUS SYSTEM

Broca’s area is located anterior to the motor area and along the lateral fissure.

Broca’s area controls speech production, contributing the movements involved in speech and grammatical structure.

The most anterior part of the frontal lobes – the prefrontal cortex – is the largest region in the human brain, twice as large as in chimpanzees, and it accounts for 29% of the total cortex.

The prefrontal cortex is involved in planning and organization, impulse control, adjusting behavior in response to rewards and punishments, and some forms of decision making.

During the 1940s and 1950s surgeons performed tens of thousands of lobotomies, a surgical procedure that disconnected the prefrontal area from the rest of the brain.

Initially the surgeries were performed on very disordered schizophrenics, but many overly enthusiastic doctors lobotomized patients with much milder problems.

The surgery calmed agitated patients, but the benefits came at a high price; the patients often became emotionally blunted, distractible, and childlike in behavior.

Now psychosurgery, the use of surgical intervention to treat cognitive and emotional disorders, is generally held in disfavor, unlike brain surgery to treat problems such as tumors.

The parietal lobes are located superior to the lateral fissure and between the central sulcus and the occipital lobe.

The primary somatosensory cortex, located on the postcentral gyrus, processes the skin senses (touch, warmth, cold, and pain), and the senses that inform us about body position and movement.

The somatosensory cortex also is organized as a homunculus, but in this case the size of each area depends on the sensitivity in that part of the body.

Each of the lobes contains association areas, which carry out further processing beyond what the primary area does, often combining information from other senses.

Parietal lobe association areas receive input from the body senses and from vision.

They help the person identify objects by touch, determine the location of the limbs, and locate objects in space.

Damage to the posterior parietal cortex may produce neglect, a disorder in which the person ignores objects, people, and activity on the side opposite the damage.

The lateral fissure separates the temporal lobe from the frontal and parietal lobes.

The temporal lobes contain the auditory projection area, visual and auditory association areas, and an additional language area.

The auditory cortex, which receives sound information form the ears, lies on the superior (uppermost) gyrus of the temporal lobe.

Just posterior to the auditory cortex is Wernicke’s area, which interprets language input arriving from the nearby auditory and visual areas.

It also generates spoken language through Broca’s area and written language by the way of the motor cortex.

The inferior temporal cortex, in the lower part of the lobe as the name implies, plays a major role in the visual identification of objects.

The occipital lobes are the location of the visual cortex, which is where the visual information is processed.

The visual cortex contains a map of visual space because adjacent receptors in the back of the eye send neurons to adjacent cells in the visual cortex.

Deep within the brain the thalamus lies just below the lateral ventricles, where it receives information from all of the sensory systems except olfaction (smell) and relays it to the respective cortical projection areas.

The hypothalamus, a smaller structure just inferior to the thalamus, plays a major role in controlling emotion and motivated behaviors like eating, drinking and sexual activity.

The hypothalamus exerts this influence largely through its control of the autonomic nervous system.

The hypothalamus also influences the body’s hormonal environment through its control over the pituitary gland.

The pituitary is known as the master gland because it controls other glands in the body.

Just posterior to the thalamus is the pineal gland.

The pineal secretes melatonin, a hormone that induces sleep.

It controls seasonal cycles in nonhuman animals and participates with other structures in controlling daily rhythms in humans.

A couple of inches below the brain’s surface the longitudinal fissure ends in the corpus callosum, a dense band of fibers that carry information between hemispheres.

During development the hollow interior of the nervous system develops into cavities called ventricles in the brain and central canal in the spinal cord.

The ventricles are filled with cerebrospinal fluid, which carries material from the blood vessels to the central nervous system, and transports waste materials in the other direction.

The midbrain contains structures that have secondary roles in vision, audition and movement.

The superior colliculi help guide eye movements and fixation of gaze.

The inferior colliculi help locate the direction of sounds.

One of the structures involved in movement is the substantia nigra, which projects to the basal ganglia to integrate movements.

Another is the ventral tegmental area, which plays a role in the rewarding effects of food, sex, drugs and so on.

The hindbrain is composed of the medulla, the pons, and the cerebellum.

The medulla forms the lower part of the hindbrain.

Its nuclei are involved with control of essential life processes such as cardiovascular activity and respiration (breathing).

The pons contains centers related to sleep and arousal, which are part of the reticular formation.

The word pons means “bridge” in Latin, which reflects the fact that sensory neurons pass through on their way to the thalamus, and motor neurons pass through between the cortex and the cerebellum.

The reticular formation is a collection of many nuclei running through the middle of the hindbrain and the midbrain.

Besides its role in sleep and arousal, it contributes to attention and to aspects of motor activity, including reflexes and muscle tone.

The cerebellum is the second most distinctive appearing brain structure.

Perched on the back of the brain stem, it is wrinkled and divided down the middle like the cerebral hemispheres – thus its name, which means “little brain.”

The most obvious function is in refining movements initiated by the motor cortex by controlling their speed, intensity and direction.

It also plays a role in motor learning, and research implicates it in other cognitive processes and in emotion.

The spinal cord is a finger-sized cable of neurons that carries commands from the brain to the muscles and organs, and sensory information into the brain.

Sensory neurons enter the spinal cord through the dorsal root of each spinal nerve.

The axons of the motor neurons pass out of the spinal cord through the ventral root.

In some cases sensory neurons from the dorsal side connect with motor neurons, either directly or through an interneuron.

This pathway produces a simple, automatic movement in response to a sensory stimulus, called a reflex.

Both the brain and spinal cord are enclosed in a protective three-layered membrane called the meninges.

The space between the meninges and the CNS is filled with cerebrospinal fluid, which cushions the neural tissue from the trauma of blows and sudden movement.

The blood-brain barrier, which limits the passage between the bloodstream and the brain, provides constant protection from toxic substances and from neurotransmitters circulating in the blood.

The peripheral nervous system (PNS) is made up of the cranial nerves that enter and leave the underside of the brain, and the spinal nerves that connect to the sides of the spinal cord at each vertebra.

The PNS can be divided into the somatic nervous system and the autonomic nervous system (ANS).

The somatic nervous system includes the motor neurons that operate the skeletal muscles – that is, the ones that move the body – and the sensory neurons that bring information into the central nervous system from the body and the outside world.

The autonomic nervous system regulates general activity levels in the body, and controls smooth muscle (stomach, blood vessels, etc.), the glands, and the heart and other organs.

The autonomic nervous system is composed of two branches.

The sympathetic nervous system activates the body in ways that help it cope with demands, such as emotional stress and physical emergencies.

The parasympathetic nervous system slows the activity of most organs to conserve energy, but it also activates digestion to renew energy.

The nervous system begins as a hollow tube that later becomes the brain and spinal cord.

The nervous system begins development when the surface of the embryo forms a groove.

The edges of this groove curl upward until they meet, turning the groove into a tube.

Development of the nervous system then proceeds in four distinct stages: cell proliferation, migration, circuit formation, and circuit pruning.

During proliferation the cells that will become neurons divide and multiply at the rate of 250,000 new cells every minute.

Proliferation occurs in the ventricular zone, the area surrounding the hollow tube that will later become the ventricles and the central canal.

These newly formed neurons then migrate, moving from the ventricular zone outward to their final location.

They do so with the aid of specialized radial glial cells.

During circuit formation, the axons of developing neurons grow toward their target cells and form functional connections.

To find their way, axons form growth cones at their tip which sample the environment for directional cues.

Chemical and molecular signposts attract or repel the advancing axon, coaxing it along the way.

By pushing, pulling, and hemming neurons in from the sides, the chemical and molecular forces guide the neuron to intermediate stations and past inappropriate targets until they reach their final destinations.

The next stage of neural development, circuit pruning, involves the elimination of excess neurons and synapses.

Neurons that are unsuccessful in finding a place on a target cell, or that arrive late, die.

In a second step of circuit pruning, the nervous system refines its organization and continues to correct errors by eliminating large numbers of excessive synapses.

Synapses are strengthened or weakened depending on whether the presynaptic neuron and the postsynaptic neuron fire together.

It is thought that the postsynaptic neuron sends feedback to the presynaptic terminals in the form of neurotrophins, chemicals that enhance development and survival of neurons.

Later, the plasticity (ability to be modified) of these synapses decreases.

Fetal alcohol syndrome (FAS) which often produces mental retardation, is caused by the mother’s use of alcohol during a critical period of brain development.

FAS brains are often small and malformed, and neurons are dislocated.

During migration many cortical neurons fail to line up in columns as they normally would because the radial glial cells revert to their more typical glial form prematurely.

Other neurons continue migrating beyond the usual boundary of the cortex.

Stimulation continues to shape synaptic construction and reconstruction throughout the individual’s life.

Much of the change resulting from experience in the mature brain involves reorganization, a shift in connections that changes the function of an area of the brain.

IN THE NERVOUS SYSTEM

Johannes Müller’s doctrine of specific nerve energies states that each sensory projection area produces its own unique experiences regardless of the kind of stimulation it receives.

This is why you “see stars” when your rollerblades shoot out from under you and the back of your head (where the visual cortex is located) hits the pavement.

One reason neuroscientists are interested in the development of the nervous system is because they hope to find clues about how to repair the nervous system when it is damaged by injury, disease, or developmental error.

Regeneration, the growth of severed axons, occurs in amphibians and mammals’ peripheral nervous system. 

Myelin provides a guide tube for the sprouting end of a severed neuron to grow through, and the extending axon is guided to its destination much as it would be during development.

Another way the nervous system could repair itself is by neurogenesis, the birth of new neurons.

In adult mammalian brain produces new neurons, but so far as we know there is significant neurogenesis in only two areas.

One is the hippocampus, and the other is near the lateral ventricles, supplying the olfactory bulb.

The simplest neural recovery involves compensation as uninjured tissue takes over functions of lost neurons.

Presynaptic neurons sprout more terminals to form additional synapses with their targets and postsynaptic neurons add more receptors.

A more dramatic form of neural recovery involves reorganization of other brain areas.

In spite of Ramón y Cajal’s declaration that there is no regeneration in the central nervous system, workers a century later are pursuing several strategies for inducing self repair following damage.

Efforts include using neuron growth enhancers, counteracting forces that inhibit regrowth, and providing guide tubes or scaffolding for axons to follow.

The most exciting research uses stem cells to replace injured neurons.

Stem cells are undifferentiated cells that can develop into specialized cells such as neurons, muscle, or blood.

Placing embryonic stem cells into an adult nervous system encourages them to differentiate into neurons appropriate to that area.