Neurons

Neurons
The billions of highly specialized cells that compose the nervous system are called neurons. A neuron is functionally divided into four components: cell body, axon, presynaptic terminal, and dendrites.

cell body

The cell body is the center of the neuron,
containing its nucleus; the nucleus contains DNA material (genes, chromosomes) and proteins. The human brain uses an estimated 30,000–40,000 genes, more than any other organ of the body

The axon and the dendrites are extensions from the cell body, serving as vehicles for the cell body to receive and transmit information from other neurons, as shown in Figure 3.2. The information carried by neurons is in the form of electrochemical nerve impulses; these impulses transmit information to and away from the cell
body. Each neuron has a single efferent nerve extension, the axon, which carries nerve impulses away from the cell body. The axon extends from the cell body for a distance of 1 mm to 1 m, at which point it arborizes into a number of terminal branches

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Axon

Each neuron has a single efferent nerve extension, the axon, which carries nerve impulses away from the cell body. The axon extends from the cell body for a distance of 1 mm to 1 m, at which point it arborizes into a number of terminal branches

The distal end of each terminal branch is a presyn-
aptic terminal. These terminals are the sites at which the axonal connection of one neuron corresponds with the dendritic extension of another neuron.

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Dendrites are
the afferent extensions of a neuron, meaning they bring nerve impulses into the cell
body from the axonal projections of other neurons.

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A single cell body contains a
number of dendritic extensions; many dendrites are studded with small protuber-
ances (called spines), which increase the surface area of the afferent connections of
the neuron (Noback et al., 2005).

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Neurons communicate by means of electrochemical nerve impulses that travel along the dendrite of one neuron and into its cell body, then along the axon to the
dendrite of another neuron

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synapse

The synapse is the site where two neurons meet. For
the two neurons to communicate, the nerve impulse must cross the synapse

Neurotransmitters

For the two neurons to communicate, the nerve impulse must cross the synapse. Neurotransmitters are chemical agents that help transmit information across the synap-
tic cleft, which is the space between the axon of the transmitting neuron and the
dendrite of the receiving neuron.

synaptogenesis

When a synapse is created, that is, when one neu-
ron forges a connection with another neuron, this is referred to as synaptogenesis

nervous tissue

The tissue formed by the linkages of thousands of neurons is called nervous tissue

The two primary types of nervous tissue are gray matter and white matter. Gray matter consists of the cell bodies of neurons and the dendrites. White matter
is the tissue that carries information among gray matter, consisting primarily of axonal fibers that carry information among gray matter tissues. Thus, gray matter is where information is generated and processed, whereas white matter serves as an
information conduit.

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Gray matter

Gray matter consists of the cell bodies of neurons and the dendrites.

Thus, gray matter is where information is generated and processed,

White matter

White matter
is the tissue that carries information among gray matter, consisting primarily of axonal fibers that carry information among gray matter tissues.

whereas white matter serves as an
information conduit.

myelin.

Neurons are sheathed in a coating called myelin. The myelin sheath contributes
to the rapid relay of nerve impulses, particularly within white matter. This sheath
also helps protect the neuron

Myelinization

refers to the growth of the myelin
sheath, a slow process that is not complete until late childhood.

Nervous System Divisions

Nervous System Divisions
As we mentioned previously, the human body has two major nervous systems:
the CNS and the PNS. The CNS consists of the brain and the spinal cord. The PNS
comprises the nerves that emerge from the brain and the spinal cord to innervate
the rest of the body.

Innervate

Innervate is the term in neuroscience that means “to sup-
ply nerves” to a particular region or part of the body

cranial nerves

The 12 pairs of nerves that
emerge from the brain are the cranial nerves.

spinal nerves

The 31 pairs of nerves that emerge
from the spinal cord are called spinal nerves.

The cranial and spinal nerves carry
information back and forth among the brain, the spine, and the rest of the body. This information includes sensory information carried to the brain by afferent pathways and motor information carried away from the brain by efferent pathways.

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Central Nervous System

Central Nervous System. The CNS consists of the brain and the spinal cord

The brain is essentially the chief executive operator of the entire CNS: It initiates and regulates virtually all motor, sensory, and cognitive processes.

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The spinal cord acts
primarily as a conduit of information, carrying not only sensory information from the body to the brain through afferent pathways, but also motor commands from the brain to the rest of the body through efferent pathways

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First shield is bone

Given the importance of the CNS to many human functions, its design includes a number of protective shields. The first shield is bone. Both the brain and the spinal cord are protected by bone; the skull covers the brain, and the vertebral column
covers the spinal cord.

The second shield

The second shield is a series of layered membranes

meninges

These meninges, which
comprise three layers, completely encase the CNS. These are sometimes called the
meningeal envelope

pia mater

The inside layer of membrane, called the pia mater, tightly
wraps around the brain and spinal cord and carries the blood vessels that serve the brain. It is a thin, transparent shield that gives the brain its bright pink color.

The second layer is the arachnoid mater, a delicate membrane separated from the pia mater by the subarachnoid space.

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dura mater

The third and outermost layer is the dura mater
(literally, “hard mother”). The dura mater consists of thick, fibrous tissue that com-
pletely encases the brain and the spinal cord.

cerebrospinal fluid (CSF).

The third shield is a layer of fluid called cerebrospinal fluid (CSF).

CSF circu-
lates between the two innermost layers of the meninges—the pia mater and the
arachnoid mater—within the subarachnoid space. CSF carries chemicals important
to metabolic processes, but it is also an important buffer against jolts to the CNS.

spinal tap

Perhaps you have heard of a procedure called a spinal tap (not the fictional rock band, which is perhaps a more amusing connotation). Also called a lumbar puncture, a spinal tap involves inserting a needle between two of the lower (lumbar)
vertebrae and extracting CSF from the subarachnoid space. It is a procedure often used to diagnose meningitis, which is an infection or inflammation of the meninges. [Meningitis is also very serious, so it is important to know the symptoms. Typically, these include headache, neck stiffness, high fever, and altered mental state
(Glimåker et al., 2015).]

Meningitis

It is a procedure often
used to diagnose meningitis, which is an infection or inflammation of the menin-
ges. [Meningitis is also very serious, so it is important to know the symptoms. Typ-
ically, these include headache, neck stiffness, high fever, and altered mental state
(Glimåker et al., 2015).]

Peripheral Nervous System

Peripheral Nervous System.

The PNS is the system of nerves connected to the
brainstem and the spinal cord. These nerves carry sensory information to the CNS
and motor commands away from the CNS, thus controlling nearly all voluntary and
involuntary activity of the human body.

The PNS consists of two sets of nerves: cranial nerves and spinal nerves. The 12 pairs of cranial nerves run between the brainstem and the facial and neck regions and are particularly important for speech, language, and hearing. The cranial nerves transmit information concerning four of the five senses (vision, hearing, smell, and
taste) to the brain. They also carry motor impulses from the brain to the face and neck muscles, including those activating the tongue and the jaw, both of which are involved with speech.

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cranial nerves

cranial nerves run between the brainstem and the facial and neck regions and are particularly important for speech, language, and hearing. The cranial nerves transmit information concerning four of the five senses (vision, hearing, smell, and taste) to the brain. They also carry motor impulses from the brain to the face and
neck muscles, including those activating the tongue and the jaw, both of which are involved with speech.

The seven cranial nerves most closely involved with speech
and language production are the following:
• Trigeminal (V): Facial sensation; jaw movements, including chewing
• Facial (VII): Taste sensation; facial movements, including smiling
• Acoustic (VIII): Hearing and balance
• Glossopharyngeal (IX): Tongue sensation; palatal and pharyngeal movement,
including gagging
• Vagus (X): Taste sensation; palatal, pharyngeal, and laryngeal movement, in-
cluding voicing
Accessory (XI): Palatal, pharyngeal, laryngeal, head, and shoulder movement
• Hypoglossal (XII): Tongue movement

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The 31 pairs of spinal nerves run between the spinal cord and all peripheral
areas of the human body, including the arms and the legs. These nerves mediate reflexes, sensory activity, and conscious (volitional) motor activity.

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contralateral.

An important fea-
ture of the CNS and PNS is that almost everything is organized to be contralateral.
This means the right side of the brain processes information from the left side of
the body, and vice versa. In the simplest terms, damage to the left side of the brain
will affect the functioning of the right side of the body

What are the Major Structures and
Functions of the Human Brain?

What are the Major Structures and
Functions of the Human Brain?
The brain is the commander in chief, or mediator, of the entire human body, and
it is viewed as the most complex and sophisticated organ of the human body.
The relatively small volume and murky gray appearance of the brain belie its
significance to the human species’ capacity for thought and language. Weighing
only about 2 lb (1,100–1,400 g) and comprising about 2% of the total weight of

the body (Jerison, 2012), the brain is extraordinarily important to the entire functioning of the human body and mind. In fact, the human brain—and its capacity for abstract thought and language—differentiates humans most significantly from other species

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The growth of the human brain in both size and weight is one of
the most important evolutionary changes in the anatomy of the human species. Proportionally, the relative size of the human brain and its sheer demand for energy (consuming one-fifth of the metabolic resources of the body) far exceed
those of any other mammal (Jerison, 2012).

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The most important evolutionary change in the human brain, accounting for these increases in weight and mass, is the enlargement of the outer layers of the brain

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These enlarged regions are called the neocortex, meaning “new cortex” (or, more literally, “new rind”), which has grown over the original human brain. The neocortex controls most of the functions that exemplify human thought and language, including speech, language, reasoning, planning, and problem solving. Recent research has exposed deficits in regions of the neocortex of children with
autism spectrum disorder, which may help us understand why these brain-based functions are so impaired in this population (Stoner et al., 2014)