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SUMMARY PART 1: Fundamentals of human neuropsychology
- Instelling
- Universiteit Leiden (UL)
Kolb & Whishaw (2015). Fundamentals of human Neuropsychology (7th edition). PART 1: chapters 3, 4, 5, 6, 7, 8, 9, 10, 13, 14.
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Kolb & Whishaw (2015). Fundamentals of human Neuropsychology (seventh edition).PART 1: Chapters 3, 4, 5, 6, 7, 8, 9, 10, 13, 14
Chapter 3: Nervous system organization
The human brain is composed of more than 100 billion neurons that engage in information processing.
Each neuron receives as many as 15,000 connections from other cells. The neurons in the brain are
organized in layers as well as in groups called nuclei (from the Latin nux, meaning “nut”), groups of cells
forming clusters that can be visualized with special stains to identify a functional grouping. Some brain
nuclei are folded, and others have distinctive shapes and colors. Within nuclei, cells that are close
together make most of their connections with one another.
3.1 Neuroanatomy: Finding Your Way Around the Brain
Describing Locations in the Brain
The locations of the layers, nuclei, and pathways of the brain can be described by their placement with
respect to other body parts of the animal, with respect to their relative locations, and with respect to a
viewer’s perspective.
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,The nervous system, like the body, is symmetrical, with a left side and a right side.
Ipsilateral: structures that lie on the same side.
Contralateral: if they lie on opposite sides.
Bilateral: if one of them lies in each hemisphere.
Proximal: structures that are close to one another.
Distal: those far from one another.
Afferent: any movement toward a brain structure.
Efferent. any movement away from a brain structure.
3.2 Overview of Nervous System Structure and Function
From an anatomical viewpoint, the central
nervous system (CNS) consists of the brain and
the spinal cord. The peripheral nervous system
(PNS) encompasses everything else. The PNS
has two divisions:
1. The somatic nervous system (SNS)
consists of all the spinal and cranial nerves
to and from the sensory organs and the
muscles, joints, and skin. The SNS
produces movement and transmits
incoming sensory information to the CNS,
including vision, hearing, pain, temperature,
touch, and the position and movement of
body parts.
2. The autonomic nervous system (ANS)
balances the body’s internal organs to “rest
and digest” through the parasympathetic
(calming) nerves or to “fight and flee” or
engage in vigorous activity through the
sympathetic (arousing) nerves.
Support and protection
The brain and spinal cord are supported and
protected from injury and infec- tion in four ways:
1. The brain is enclosed in a thick bone, the skull, and the spinal cord is encased in a series of
interlocking bony vertebrae. Thus, the CNS lies within bony encasements, whereas the PNS,
although connected to the CNS, lies outside them. The PNS, although more vulnerable to injury
because it lacks bony protection, can renew itself after injury by growing new axons and dendrites,
whereas self-repair is much more limited within the CNS.
2. Within the bony case enclosing the CNS is a triple-layered set of membranes, the meninges. The
outer dura mater (from the Latin, meaning “hard
mother”) is a tough double layer of tissue
enclosing the brain in a kind of loose sack. The
middle arachnoid membrane (from the Greek,
meaning “resembling a spider’s web”) is a very
thin sheet of delicate tissue that follows the
contours of the brain. The inner pia mater (from
the Latin, meaning “soft mother”) is a moderately
tough tissue that clings to the surface of the brain.
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, 3. The brain and spinal cord are cushioned from shock and sudden changes of pressure by the
cerebrospinal fluid that circulates in the four ventricles inside the brain, in the spinal column, and
within the subarachnoid space in the brain’s enclosing membranes. Cerebral spinal fluid is
continually being made and drained off into the circulatory system. If the outflow is blocked, as
occurs in a congenital condition called hydrocephalus (literally, water brain), severe mental
retardation and even death can result.
4. The brain and spinal cord are protected from many chemical substances circulating in the rest of
the body by the blood–brain barrier. To form this barrier, the cells of the capillaries—the very
small blood vessels—form tight junctions with one another, thus preventing many blood-borne
substances from crossing from the capillaries into the CNS tissues.
Blood supply
The brain receives its blood
supply from two internal carotid
arteries and two vertebral
arteries that course up each
side of the neck. The four
arteries connect at the base of
the brain, where they enter the
skull. From there, the cerebral
arteries branch off into several
smaller arteries that irrigate the
brainstem and cerebellum and
give rise to three arteries that
irrigate the forebrain.
The anterior cerebral artery
(ACA) irrigates the medial and
dorsal part of the cortex, the
middle cerebral artery (MCA) irrigates the lateral surface of the cortex, and the posterior cerebral
artery (PCA) irrigates the ventral and posterior surfaces of the cortex.
The veins of the brain, through which spent blood returns to the heart, are classified as external and
internal cerebral and cerebellar veins. The venous flow does not follow the course of the major arteries but
instead follows a pattern of its own.
Neurons and glia
The brain has its origin in a single
undifferentiated cell called a neural stem
cell (also called a germinal cell). A stem
cell has extensive capacity for self-
renewal. To initially form a brain, it divides
and produces two stem cells, both of
which can divide again. Thus, may play a
role in brain repair after injuries such as
stroke or other trauma.
In the developing embryo, stem cells give
rise to progenitor cells that migrate and
act as precursor cells, giving rise to
nondividing, primitive types of nervous
system cells called blasts. Some blasts
differentiate into neurons; others
differentiate into the glia. These two basic
brain-cell types—neurons and glia—take
many forms and make up the entire adult brain.
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,Neurons differ chiefly in overall size, in the length and branching of their axons, and in the complexity of
their dendritic processes:
The simplest sensory neuron, a bipolar neuron consists of a cell body with a dendrite on one side and
an axon on the other. Somatosensory neurons, which project from the body’s sensory receptors into the
spinal cord, are modified so that the dendrite and axon are connected, which speeds information
conduction because messages do not have to pass through the cell body.
Interneurons within the brain and spinal cord link up sensory- and motor neuron activity in the CNS.
There are many kinds of interneurons and all have many dendrites that branch extensively but, like all
neurons, a brain or spinal-cord inter- neuron has only one axon, although it can branch as well
Motor neurons located in the brainstem project to facial muscles, and motor neurons in the spinal cord
project to other muscles of the body. Together, motor neurons are called the final common path because
all behavior produced by the brain is produced through them.
The various types of glial cells have different functions as well. Some are described in Table 3.1.
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,Gray, White, and Reticular Matter
Gray matter acquires its characteristic gray-brown color from the capillary blood vessels and neuronal cell
bodies that predominate there.
White matter consists largely of axons that extend from these cell bodies to form connections with
neurons in other brain areas. These axons are covered with an insulating layer of glial cells that are
composed of the same fatty substance (lipid) that gives milk its white appearance. As a result, an area of
the nervous system rich in axons covered with glial cells looks white.
Reticular matter (from the Latin rete, meaning “net”) contains a mixture of cell bodies and axons from
which it acquires its mottled gray and white, or netlike, appearance. Thus, with respect to our analogy
equating brain regions with communities and roads, communities are gray, roads are white, and reticular
matter is suburbia.
Layers, Nuclei, Nerves, and Tracts
Large, well-defined groups of cell bodies in the CNS form either layers or nuclei (clusters). Within the
PNS, such as clusters are called ganglia. Tracts (fiber pathways) are large collections of axon projecting
toward or away from a nucleus or layer in the CNS.
Tracts carry information from one place to another within the CNS; for ex- ample, the corticospinal
(pyramidal) tract carries information from the cortex to the spinal cord. Fibers and fiber pathways that
enter and leave the CNS are called nerves, such as the auditory nerve or the vagus nerve; but, after they
have entered the central nervous system, they, too, are called tracts.
3.3 The Origin and Development of the Central Nervous System
The developing brain is less complex than the adult brain and provides a clearer picture of the vertebrate
brain’s basic three-part structure.
The three regions of the primitive, developing brain are recognizable in as a series of three enlargements
at the end of the embryonic spinal cord. The adult brain of a fish, amphibian, or reptile is roughly
equivalent to this three-part brain:
1. The prosencephalon (“front brain”) is responsible for olfaction,
2. The mesencephalon (“middle brain”) is the seat of vision and hearing,
3. The rhombencephalon (hindbrain) controls movement and balance. Here, the spinal cord is
considered part of the hindbrain.
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,In mammals the prosencephalon develops further to form the cerebral hemispheres (the cortex and
related structures):
The telencephalon (“endbrain”) are the cerebral hemispheres collectively called.
The diencephalon (“between brain”) are the remaining old parts of the prosencephalon, including
the thalamus.
The back part of the brain also develops further. It is subdivided into the metencephalon (“across
brain”), which includes the enlarged cerebellum, and the myelencephalon (“spinal brain”), the
lower region of the brainstem.
The human brain is a more complex mammalian brain, retaining most of the features of other mammalian
brains and possessing especially large cerebral hemi- spheres. As we describe the major structures of the
CNS in the sections that follow, we group them according to the three-part scheme of forebrain,
brainstem, and spinal cord. These three subdivisions reinforce the concept of levels of function, with
newer levels partly replicating the work of older ones.
The brain begins as a tube, and, even after it folds and matures, its interior remains “hollow.” The four
prominent pockets created by the folding of this hollow interior in the brain are called ventricles
(“bladders”) and are numbered 1 through 4. The “lateral ventricles” (first and second) form C-shaped
lakes underlying the cerebral cortex, whereas the third and fourth ventricles extend into the brainstem
and spinal cord. All are filled with cerebrospinal fluid, which is produced by ependymal glial cells located
adjacent to the ventricles (see Table 3.1). Cerebral spinal fluid flows from the lateral ventricles out through
the fourth ventricle and eventually drains into the circulatory system.
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,3.4 The Spinal Cord
We begin our description of neuroanatomy with the spinal cord. It is structurally the simplest part of the
CNS, and the basic plan of the spinal cord is also seen in the plan of the brainstem. Along with the spinal
cord, we also de- tail the functions of the somatic and the autonomic nervous systems.
Spinal-Cord Structure and the Spinal Nerve Anatomy
The spinal cord lies inside the bony spinal-
column vertebrae, which are categorized
into five regions from top to tail: cervical (C),
thoracic (T), lumbar (L), and sacral (S).
The segments, called dermatomes (“skin
cuts”), encircle the spinal column as a stack
of rings.
Each spinal segment is connected by SNS
spinal nerve fibers to the body dermatome
of the same number, including the organs
and musculature that lie within the
dermatome. In the main, the cervical
segments control the forelimbs, the thoracic
segments control the trunk, and the lumbar
segments control the hind limbs.
Afferent fibers entering the dorsal part of the
spinal cord (posterior in humans) bring
information from the sensory receptors of
the body. These spinal nerve fibers
converge as they enter the spinal cord,
forming a strand of fibers referred to as a
dorsal root. Efferent fibers leaving the
ventral (anterior in humans) part of the
spinal cord, carrying information from the
spinal cord to the muscles, form a similar
strand of spinal nerves known as a ventral
root.
The spinal cord itself consists of white
matter, or tracts, arranged so that, with a
few exceptions, the dorsally located tracts
are sensory and the ventrally located tracts
are motor. The spinal tracts carry
information to and from the brain. The inner
part of the cord consists of gray matter; that
is, it is composed largely of neural cell bodies, which, in this case, organize movements and give rise to
the ventral roots. In cross section, this gray-matter region has the shape of a butterfly
Spinal-Cord Function and the Spinal Nerves
Today, the principle that the dorsal part of the spinal cord is sensory and the ventral part is motor is called
the Bell–Magendie law. Because of the segmental structure of the spinal cord and the body, rather good
inferences can also be made about the location of spinal-cord dam- age or disease on the basis of
changes in sensation or movement in particular body parts.
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, Spinal-cord injury
Persons whose spinal cords are cut so that they no longer have control over their legs are paraplegic; if
the cut is higher on the cord, making them unable to use their arms either, they are quadriplegic.
A main effect of spinal-cord injury is to server connections between the cord and the brain. Research on
spinal-cord injury has three main objectives:
1. Following the injury, damage in the cord takes hours to days to develop. Arresting the
degenerative processes can make an important contribution to sparing function.
2. Inducing fibers to regrow across the damaged section of the spinal cord can restore information.
Approaches to establishing regrowth involve removing scar tissue, inducing fibers to regrow by
pharmacological treatments, and implanting glial cells in damaged regions to stimulate axon
regrowth.
3. Developing aids to movement such as brain-computer interfaces (BCI) and similar marriages of
directed neural activity and technology can bypass injury and aid in restoring movement.
Spinal reflexes and sensory integration
The SNS consists of the spinal and cranial nerves that produce movement and transmit incoming sensory
information to the CNS. Sensory information plays a central role in eliciting different kinds of movements
organized by the spinal cord.
Movements dependent only on spinal-cord function are referred to as reflexes, specific movements
elicited by specific forms of sensory stimulation. There are many kinds of sensory receptors in the body,
including receptors for pain, temperature, touch and pressure, and the sensations of muscle and joint
movement. The size of the spinal nerve fiber coming from each kind of receptor is distinctive; generally,
pain and temperature fibers are smaller, and those for touch and muscle sense are larger.
The stimulation of pain and temperature receptors in a limb usually produces flexion movements that
bring the limb inward, toward the body and away from injury. If the stimulus is mild, only the distal part of
the limb flexes in response to it but, with successively stronger stimuli, the size of the movement increases
until the whole limb is drawn back.
The stimulation of fine touch and muscle receptors in a limb usually produces extension movements,
which extend the limb outward, away from the body. Because each of the senses has its own receptors,
fibers, connections, and reflex movements, each sense can be thought of as an independent sensory
system.
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