Download lateral horns of gray matter

Chapter 13: Central
Nervous System
COVERINGS OF THE BRAIN
AND SPINAL CORD
 Two


protective coverings (Figure 13-2)
Outer covering is bone; cranial bones encase
the brain and vertebrae encase the spinal
cord (Figure 13-1)
Inner covering is the meninges; the meninges
of the cord continue inside the spinal cavity
beyond the end of the spinal cord
COVERINGS OF THE BRAIN
AND SPINAL CORD: MENINGES

Meninges have three membranous layers
(Figure 13-3)

Dura mater: strong, white, fibrous tissue; outer layer
of meninges and inner periosteum of the cranial
bones; has three important extensions
• Falx cerebri
 Projects downward into the longitudinal fissure between
the two cerebral hemispheres
 Dural sinuses: function as veins, collecting blood from
brain tissues for return to the heart
 Superior sagittal sinus—one of several dural sinuses
• Falx cerebelli: separates the two hemispheres of the
cerebellum
• Tentorium cerebelli: separates the cerebellum from the
cerebrum
COVERINGS OF THE BRAIN AND
SPINAL CORD: MENINGES (cont.)

Meninges have three membranous layers (cont.)
 Arachnoid mater: delicate, cobweblike layer between
the dura mater and pia mater
 Pia mater: innermost, transparent layer; adheres to
the outer surface of the brain and spinal cord;
contains blood vessels; beyond the spinal cord, forms
a slender filament called filum terminale
 Several spaces exist between and around the
meninges
• Epidural space: located between the dura mater and inside the bony
covering of the spinal cord; contains a supporting cushion of fat and
other connective tissues (virtually absent around brain because
dura is continuous with periosteum of bone)
• Subdural space: located between the dura mater and arachnoid
mater; contains lubricating serous fluid
• Subarachnoid space: located between the arachnoid and pia mater;
contains a significant amount of cerebrospinal fluid (CSF)

CEREBROSPINAL FLUID
Functions



Provides a supportive, protective cushion
Reservoir of circulating fluid monitored by the brain
to detect changes in the internal environment
Fluid spaces


CSF: found within the subarachnoid space around
the brain and spinal cord and within the cavities and
canals of the brain and spinal cord
Ventricles: four fluid-filled spaces within the brain
(Figure 13-4)
• First and second ventricles (lateral): one located in each
hemisphere of the cerebrum
• Third ventricle: thin, vertical pocket of fluid below and
medial to the lateral ventricles
• Fourth ventricle: tiny, diamond-shaped space where the
cerebellum attaches to the back of the brainstem
CEREBROSPINAL FLUID (cont.)

Formation and circulation of CSF (Figure 13-5)

Occurs by separation of fluid from blood in the
choroid plexuses
• Fluid from the lateral ventricles seeps through the
interventricular foramen (of Monro) into the third ventricle
• From the third ventricle fluid goes through the cerebral
aqueduct (of Sylvius) into the fourth ventricle
• From the fourth ventricle fluid goes to two different areas
 Some fluid flows directly into the central canal of the
spinal cord
 Some fluid leaves the fourth ventricle through openings
in its roof and goes into the cisterna magna, a space that
is continuous with the subarachnoid space
• Fluid circulates in the subarachnoid space and then is
absorbed into venous blood through the arachnoid villi
SPINAL CORD

Structure of the spinal cord (Figure 13-6)




Lies within the spinal cavity and extends from the
foramen magnum to the lower border of the first
lumbar vertebra
Oval-shaped cylinder that tapers slightly from above
downward
Two bulges, one in the cervical region and one in the
lumbar region
Anterior median fissure and posterior median sulcus
are two deep grooves; anterior fissure is deeper and
wider
SPINAL CORD (cont.)

Nerve roots
• Fibers of dorsal nerve root


Carry sensory information into the spinal canal
Dorsal root ganglion: cell bodies of unipolar, sensory
neurons make up a small region of gray matter in the
dorsal nerve root
• Fibers of ventral nerve root


Carry motor information out of the spinal cord
Cell bodies of multipolar motor neurons are in the gray
matter of the spinal cord
SPINAL CORD (cont.)



Interneurons are located in the spinal cord’s
gray matter core
Spinal nerve: a single mixed nerve on each
side of the spinal cord where the dorsal and
ventral nerve roots join together
Cauda equina: bundle of nerve roots
extending (along with the filum terminale)
from the conus medullaris (inferior end of
spinal cord) (Figure 13-7)
SPINAL CORD (cont.)

Gray matter
• Columns of gray matter extend the length of the
cord
• Consists predominantly of cell bodies of
interneurons and motor neurons
• In transverse section, looks like an H, with the
limbs called the anterior, posterior, and lateral
horns of gray matter; crossbar of H is the gray
commissure
SPINAL CORD (cont.)

White matter
• Surrounds the gray matter and is subdivided in
each half on the cord into three funiculi: anterior,
posterior, and lateral white columns
• Each funiculus consists of a large bundle of axons
divided into tracts
• Names of spinal tracts indicate the location of the
tract, the structure in which the axons originate,
and the structure in which they terminate
SPINAL CORD (cont.)
 Functions

of the spinal cord
Provides conduction routes to and from the
brain
• Ascending tracts conduct impulses up the cord to
the brain
• Descending tracts conduct impulses down the cord
from the brain
• Bundles of axons comprise all tracts
SPINAL CORD (cont.)

Provides conduction routes to and from the
brain (cont.)
• Tracts are both structural and functional
organizations of nerve fibers


Structural: all axons of any one tract originate in the
same structure and terminate in the same structure
Functional: all axons that comprise one tract serve one
general function
SPINAL CORD (cont.)

Provides conduction routes to and from the
brain (cont.)
• Important ascending (sensory) tracts (Figure 13-8)





Lateral spinothalamic tracts: crude touch, pain, and
temperature
Anterior spinothalamic tracts: crude touch, pressure
Fasciculi gracilis and cuneatus: discriminating touch and
conscious kinesthesia
Spinocerebellar tracts: subconscious kinesthesia
Spinotectal: touch
SPINAL CORD (cont.)
• Important descending (motor) tracts (Figure 13-8)






Lateral corticospinal tracts: voluntary movements on
opposite side of the body
Anterior corticospinal tracts: voluntary movements on
same side of body
Reticulospinal tracts: maintain posture during movement
Rubrospinal tracts: transmit impulses that coordinate
body movements and maintenance of posture
Tectospinal tracts: head and neck movements during
visual reflexes
Vestibulospinal tracts: coordination of posture and
balance
• Spinal cord: reflex center for all spinal reflexes,
which are located in the gray matter of the cord
BRAIN

Structures of the brainstem (Figures 13-9 and
13-10)

Medulla oblongata
• Lowest part of the brainstem
• Part of the brain that attaches to spinal cord; located just
above the foramen magnum
• A few centimeters in length and separated from the pons
above by a horizontal groove
• Composed of white matter and a network of gray and white
matter called the reticular formation network
• Pyramids: two bulges of white matter located on the ventral
side of the medulla; formed by fibers of the pyramidal tracts
• Olive: oval projection located lateral to the pyramids
• Nuclei: clusters of neuron cell bodies located in the reticular
formation
BRAIN (cont.)
 Structures

of the brainstem (cont.)
Pons
• Located above the medulla and below the
midbrain
• Composed of white matter and reticular formation

Midbrain
BRAIN (cont.)
• Located above the pons and below the
cerebrum; forms the midsection of the brain
• Composed of white tracts and reticular
formation
• Extending divergently through the midbrain
are cerebral peduncles, which conduct
impulses between the midbrain and
cerebrum
• Corpora quadrigemina: landmark in
midbrain





Composed of two inferior colliculi and two
superior colliculi
Forms the posterior, upper part of the midbrain
that lies just above the cerebellum
Inferior colliculus contains auditory centers
Superior colliculus contains visual centers
Red nucleus and substantia nigra: clusters of cell
bodies of neurons involved in muscular control
BRAIN (cont.)

Functions of the brainstem



Performs sensory, motor, and reflex functions
Spinothalamic tracts, fasciculi cuneatus and
gracilis, spinoreticular tracts, corticospinal and
reticulospinal tracts pass through brainstem
Nuclei in medulla contain reflex centers
• Of primary importance: cardiac, vasomotor, and
respiratory centers
• Nonvital reflexes: vomiting, coughing, sneezing, etc.


Pons contains reflexes mediated by fifth, sixth,
seventh, and eighth cranial nerves and
pneumotaxic centers that help regulate respiration
Midbrain contains centers for certain cranial nerve
reflexes (visual and auditory)
BRAIN (cont.)

Structure of the cerebellum (Figure 13-11)






Second largest part of the brain; contains more
neurons than the rest of the nervous system
Located just below the posterior portion of the
cerebrum; transverse fissure separates these two
parts of the brain
Gray matter makes up the cortex and white matter
predominates in the interior
Arbor vitae: internal white matter of the cerebellum;
distinctive pattern similar to the veins of a leaf
Cerebellum has numerous sulci and delicate, parallel
gyri (folia)
Consists of the cerebellar hemispheres and the
vermis
BRAIN (cont.)

Structure of the cerebellum (cont.)

Internal white matter: composed of short and long
tracts
• Shorter tracts conduct impulses within the cerebellum
• Longer tracts conduct impulses to and from the cerebellum;
fibers enter or leave by way of three pairs of peduncles
 Inferior cerebellar peduncles: composed chiefly of tracts
into the cerebellum from the medulla and cord
 Middle cerebellar peduncles: composed almost entirely
of tracts into the cerebellum from the pons
 Superior cerebellar peduncles: composed principally of
tracts from dentate nuclei in the cerebellum through the
red nucleus of the midbrain to the thalamus
BRAIN (cont.)
 Structure

of the cerebellum (cont.)
Dentate nuclei
• Important pair of cerebellar nuclei, one of which is
located in each hemisphere
• Nuclei connected with thalamus and motor areas
of the cerebral cortex by tracts
• Through the tracts, cerebellar impulses influence
the motor cortex and the motor cortex influences
the cerebellum
Functions of the Cerebellum
 Provides
involuntary coordination of body
movements
 Helps control posture
 Controls balance and allows body
movements to be smooth and coordinated
BRAIN: DIENCEPHALON

Diencephalon (Figure 13-13)



Located between the cerebrum and the midbrain
Consists of several structures located around the
third ventricle: thalamus, hypothalamus, optic
chiasma, pineal gland, and several others
Thalamus
• Dumbbell-shaped mass of gray matter composed of many
nuclei
• Each lateral mass forms one lateral wall of the third
ventricle
• Intermediate mass extends through the third ventricle and
joins the two lateral masses
• Geniculate bodies: two of the most important groups of
nuclei comprising the thalamus; play role in processing
auditory and visual input
• Serves as a major relay station for sensory impulses on
their way to the cerebral cortex
BRAIN: DIENCEPHALON (cont.)

Thalamus (cont.)
• Plays two parts in mechanism responsible for
sensations


Impulses produce conscious recognition of the crude,
less-critical sensations of pain, temperature, and touch
Neurons relay all kinds of sensory impulses, except
possibly olfactory, to the cerebrum
• Plays part in the mechanism responsible for
emotions by associating sensory impulses with
feeling of pleasantness and unpleasantness
• Plays part in arousal mechanism
• Plays part in mechanisms that produce complex
reflex movements
BRAIN: DIENCEPHALON (cont.)

Hypothalamus
• Consists of several structures that lie beneath the
thalamus
• Forms floor of the third ventricle and lower part of lateral
walls
• Prominent structures found in the hypothalamus
 Supraoptic nuclei
 Paraventricular nuclei
 Mamillary bodies: posterior part of hypothalamus,
involved with olfactory sense
• Infundibulum: the stalk leading to the posterior lobe of
the pituitary gland
• Small but functionally important area of the brain;
performs many functions of greatest importance for
survival and enjoyment
• Links mind and body
• Links nervous system to endocrine system
BRAIN: DIENCEPHALON (cont.)

Hypothalamus (cont.)
• Summary of hypothalamic functions







Regulator and coordinator of autonomic activities
Major relay station between the cerebral cortex and lower
autonomic centers; crucial part of the route by which
emotions express themselves in changed bodily
functions
Synthesizes hormones secreted by posterior pituitary
and plays an essential role in maintaining water balance
Some neurons function as endocrine glands
Plays crucial role in arousal mechanism
Crucial part of mechanism regulating appetite
Crucial part of mechanism maintaining normal body
temperature
BRAIN: DIENCEPHALON (cont.)

Pineal gland
• Located just above the corpora quadrigemina of
the midbrain
• Involved in regulating the body’s biological clock
(Figure 13-14)
• Produces melatonin as a “timekeeping” hormone



Melatonin is made from the neurotransmitter serotonin
Levels increase when sunlight is absent and decrease
when sunlight is present, thus regulating the circadian
(daily) biologic clock (Figure 13-15)
Melatonin is the “sleep hormone”
BRAIN: CEREBRUM
 Structure

of the cerebrum
Cerebral cortex
• Largest and uppermost division of the brain;
consists of right and left cerebral hemispheres,
each divided into five lobes (Figure 13-16)





Frontal lobe
Parietal lobe
Temporal lobe
Occipital lobe
Insula (island of Reil)
• Cerebral cortex: outer surface composed of six
layers of gray matter
• Gyri: convolutions; some are named: precentral
gyrus, postcentral gyrus, cingulate gyrus, and
hippocampal gyrus
• Sulci: shallow grooves
BRAIN: CEREBRUM (cont.)

Cerebral cortex (cont.)
• Fissures: deeper grooves that divide each cerebral
hemisphere into lobes; four are prominent




Longitudinal fissure: deepest fissure; divides cerebrum
into two hemispheres
Central sulcus (fissure of Rolando): groove between
frontal and parietal lobes
Lateral fissure (fissure of Sylvius): groove between
temporal lobe below and parietal lobes above; island of
Reil lies deep in lateral fissure
Parietooccipital fissure: groove that separates occipital
lobe from parietal lobes
BRAIN: CEREBRUM (cont.)

Cerebral tracts and basal nuclei
• Basal nuclei

Structure: islands of gray matter located deep inside the
white matter of each hemisphere (Figure 13-18); include
the following:
• Caudate nucleus
• Lentiform nucleus: consists of putamen and
pallidum
• Amygdaloid nucleus

Function: regulation of voluntary (conscious) motor
control related to posture, walking, and other repetitive
movements; possible roles in thinking and learning
BRAIN: CEREBRUM (cont.)

Cerebral tracts and basal nuclei (cont.)
• Three types of cerebral tracts make up cerebrum’s
white matter (Figure 13-17)



Projection tracts: extensions of the sensory
spinothalamic tracts and motor corticospinal tracts
Association tracts: most numerous cerebral tracts;
extend from one convolution to another in the same
hemisphere
Commissural tracts: extend from one convolution to a
corresponding convolution in the other hemisphere;
comprise the corpus callosum and anterior and posterior
commissure
BRAIN: CEREBRAL CORTEX
 Functions

of the cerebral cortex
Certain areas of the cerebral cortex engage in
predominantly one particular function (Figures
13-19 and 13-20)
• Postcentral gyrus: mainly general somatic sensory
area; receives impulses from receptors activated
by heat, cold, and touch stimuli
• Precentral gyrus: chiefly somatic motor area;
impulses from neurons in this area descend over
motor tracts and stimulate skeletal muscles
• Transverse gyrus: primary auditory area
• Occipital lobe: primary visual area
BRAIN: CEREBRAL CORTEX
(cont.)

Sensory functions of the cortex
• Somatic senses: sensations of touch, pressure,
temperature, proprioception, and similar
perceptions that require complex sensory organs
• Cortex contains a somatic sensory map of the
body
• Information sent to primary sensory areas is
relayed to sensory association areas and other
parts of the brain
• The sensory information is compared and
evaluated, and the cortex integrates separate bits
of information into whole perceptions
BRAIN: CEREBRAL CORTEX
(cont.)

Motor functions of the cortex
• For normal movements to occur, many parts of the
nervous system must function
• Precentral gyrus: primary somatic motor area;
controls individual muscles
• Secondary motor area: in the gyrus immediately
anterior to the precentral gyrus; activates groups of
muscles simultaneously

Integrative functions of the cortex
CONSCIOUSNESS
 State
of awareness of one’s self, one’s
environment, and other human beings
(Figure 13-21)
 Depends on excitation of cortical
neurons by impulses conducted to them
by the reticular activating system
 Two current concepts about the reticular
activating system


Functions as arousal system for the
cerebral cortex
Functioning is crucial for maintaining
consciousness
LANGUAGE
 Ability
to speak and write words and
understand spoken and written words
 Speech centers: areas in the frontal,
parietal, and temporal lobes
 Left cerebral hemisphere contains speech
centers in approximately 90% of the
population; contained in either the right
hemisphere or both in the remaining 10%
 Aphasias: lesions in speech centers
EMOTIONS

Subjective experiencing and objective
expressing of emotions involve functioning of the
limbic system (Figure 13-22)
 Limbic system: also known as the emotional
brain


Most structures of limbic system lie on the medial
surface of the cerebrum (cingulate gyrus and
hippocampus)
Have primary connections with other parts of the
brain, such as the thalamus, fornix, septal nuclei,
amygdaloid nucleus, and hypothalamus
MEMORY
A
major mental activity
 Cortex is capable of storing and retrieving
both short- and long-term memory
 Temporal, parietal, and occipital lobes are
among the areas responsible for shortand long-term memory
 Structural changes in the neural pathways
of the cerebral cortex store long-term
memories
 Limbic system plays a key role in memory
SOMATIC SENSORY PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM

For the cerebral cortex to perform its
sensory functions, impulses must first be
conducted to the sensory areas by sensory
pathways (Figure 13-23)
 Three main pools of sensory neurons


Primary sensory neurons conduct impulses from
the periphery to the central nervous system
Secondary sensory neurons
• Conduct impulses from the cord or brainstem to the
thalamus
• Dendrites and cell bodies are located in the gray
matter of the cord and brainstem
• Axons ascend in tracts up the cord and through the
brainstem, terminating in the thalamus, where they
synapse with dendrites or cell bodies of tertiary
sensory neurons
SOMATIC SENSORY PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM
(cont.)
 Three
main pools of sensory neurons
(cont.)

Tertiary sensory neurons
• Conduct impulses from thalamus to the postcentral
gyrus of the parietal lobe
• Bundle of axons of tertiary sensory neurons form
the thalamocortical tracts
• Extend through the internal capsule to the cerebral
cortex
 Sensory
pathways to the cerebral cortex
are crossed
SOMATIC SENSORY PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM
(cont.)

Two sensory pathways conduct impulses that
produce sensations of touch and pressure

Medial lemniscal system
• Consists of tracts that make up the fasciculi cuneatus and
gracilis, and the medial lemniscus
• Axons of secondary sensory neurons make up medial
lemniscus
• Functions: transmit impulses that produce discriminating
touch and pressure sensations and kinesthesia

Spinothalamic pathway: functions are crude touch
and pressure sensation
SOMATIC SENSORY PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM
(cont.)

For the cerebral cortex to perform its motor
functions, impulses are conducted from its motor
areas to skeletal muscles by somatic motor
pathways
 Consist of motor neurons that conduct impulses
from the central nervous system to skeletal
muscles; some motor pathways are extremely
complex and others are very simple
SOMATIC MOTOR PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM
 Two
methods used to classify somatic
motor pathways: pyramidal and
extrapyramidal tracts (Figure 13-24)

Pyramidal tracts: also known as corticospinal
tracts
• Approximately three quarters of the fibers
decussate in the medulla and extend down the
cord in the crossed corticospinal tract located on
the opposite side of the spinal cord in the lateral
white column
• Approximately one quarter of the fibers do not
decussate but extend down the same side of the
spinal cord as the cerebral area from which they
came
SOMATIC MOTOR PATHWAYS IN
THE CENTRAL NERVOUS SYSTEM

Extrapyramidal tracts: much more complex
than pyramidal tracts
• Consist of all motor tracts from the brain to the
spinal cord anterior horn motor neurons except the
corticospinal tracts
• Within the brain, consist of numerous relays of
motor neurons between motor areas of the cortex,
basal nuclei, thalamus, cerebellum, and brainstem
• Within the spinal cord, some important tracts are
the reticulospinal tracts
• Conduction by extrapyramidal tracts plays a crucial
part in producing large, automatic movements
• Conduction by extrapyramidal tracts plays an
important part in emotional expressions
• Motor program: set of coordinated commands that
control the programmed motor activity mediated by
extrapyramidal pathways (Figure 13-25)