Download The reticular activating system (RAS)

The nervous system
Ahmed
L6
Dr.Abdul-Aziz
Regulation of Body Water
The hypothalamus regulates body water in two ways:
(1) by creating the sensation of thirst, an area called the thirst center is
located in the lateral hypothalamus. When the fluid electrolytes in either
this center or closely allied areas become too concentrated, the subject
develops an intense desire to drink water to return the electrolyte
concentration of the thirst center to normal.
(2) by controlling the excretion of water into the urine, When the body
fluids become too concentrated, the neurons of the supraoptic nuclei of
the hypothalamus secrete ADH .This hormone is transported to the
kidneys where it acts on the collecting ducts of the kidneys to cause
increased reabsorption of water.
Regulation of Uterine Contractility and of Milk Ejection from the
Breasts.
Stimulation of the paraventricular nuclei causes their neuronal cells to
secrete the hormone oxytocin, The main functions of oxytocin
1.it increase contractility of the myoepithelial cells surrounding the
alveoli of the breasts, which then causes the alveoli to empty their milk
through the nipples during baby suckling.
2. at time of labour, large quantities of oxytocin are secreted, and this
secretion helps to promote labor contractions that expel the baby.
Hypothalamic Control of Endocrine Hormone Secretion by the Anterior
Pituitary Gland.
The pituitary gland (hypophysis) can be viewed as a master gland
because it controls the secretion of several target endocrine glands. The
secretion of pituitary hormones is controlled by release factors from the
hypothalamus, giving rise to the concept of the hypothalamic-pituitary
axis. Negative feedback from the primary target gland modulates the
secretion of both pituitary and hypothalamic hormones. The pituitary
gland lies in a bony cavity, known as the sella turcica, located at the base
of the brain. It is connected to the median eminence of the
hypothalamus via the pituitary stalk. The pituitary gland has two major
lobes, the anterior pituitary and the posterior pituitary.
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The nervous system
Ahmed
L6
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Dr.Abdul-Aziz
The nervous system
Ahmed
L6
Dr.Abdul-Aziz
GENERATION OF CIRCADIAN RHYTHMS
Most of the body’s major controlled variables exhibit regular cyclical
variation throughout a 24-hour period. For example, body temperature
varies by about 1°C, and is lowest in the early morning and highest in the
evening. The plasma concentrations of several hormones also exhibit
striking circadian rhythms (e.g., cortisol levels peak about 8 AM, whereas
growth hormone levels are highest during sleep). Circadian rhythms are
generated by an endogenous body clock . The body clock is created by
neurons of the suprachiasmatic nucleus of the hypothalamus, the
activity of which oscillates spontaneously in a daily cycle. The
suprachiasmatic nucleus receives afferent fibers directly from the retina
via the retinohypothalamic tract, which provides information about
daylight. Air travel across time zones produces the unpleasant
sensations of “jet lag” (e.g., fatigue and being unable to eat and sleep
properly when in the new time zone). Jet lag is caused by the mismatch
between the endogenous circadian rhythms and the new time zone, and
several days are required to entrain the body clock to the new time.
THE RETICULAR FORMATION AND DIFFUSE
MODULATORY SYSTEMS
The Reticular Formation, is a complex network of nuclei (masses of gray
matter) and fibers that extend the length of the brain stem. The reticular
formation receives sensory signals, which it sends up to higher centers,
and motor signals, which it sends to the spinal cord. It can be roughly
categorized as three columns: median, medial, and lateral. An
important characteristic of neurons in the reticular formation is their
widespread connectivity throughout the CNS. The main function of
reticular formation
 Modulates sensation of pain.
 Modulates certain postural reflexes and muscle tone.
 Helps control breathing and heartbeat.
 Regulates level of brain arousal and consciousness.
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The nervous system
Ahmed
L6
Dr.Abdul-Aziz
The reticular activating system (RAS),
One portion of the reticular formation, called the reticular activating
system (RAS), it arouses the cerebrum via the thalamus and causes a
person to be alert. Without continuous transmission of nerve signals
from the lower brain into the cerebrum, the cerebrum becomes useless.
The reticular activating system of the brain stem activate the cerebral
part of the brain in two ways:
(I) by directly stimulating a background level of neuronal activity in wide
areas of the brain.
(II) by activating neurohormonal systems that release specific facilitory
or inhibitory hormone-like neurotransmitter substances into selected
areas of the brain(Neurohormonal Control of Brain Activity).
(I) Directly stimulating of background level of neuronal activity in
wide areas of the brain(Reticular Excitatory Area of the Brain Stem)
The central driving component of this system is an excitatory area
located in the reticular substance of the pons and mesencephalon.This
area transmits facilitory signals downward to the spinal cord to maintain
tone in the antigravity muscles and to control levels of activity of the
spinal cord reflexes. In addition to these downward signals, this area also
sends a profusion of signals in the upward direction. Most of these go
first to the thalamus, where they excite a different set of neurons that
transmit nerve signals to all regions of the cerebral cortex as well as to
multiple subcortical areas which inturn regulates level of brain arousal
and consciousness.
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The nervous system
Ahmed
L6
Dr.Abdul-Aziz
Figure: Excitatory-activating system of the brain.
The level of activity of the excitatory area in the brain stem, and
therefore the level of activity of the entire brain, is determined to a
great extent by the number and type of sensory signals that enter the
brain from the periphery. Moreover, additional feedback signals also
return from the cerebral cortex back to excitatory area in the brain stem.
Therefore, any time the cerebral cortex becomes activated by either
brain thought processes or motor processes, signals are sent from the
cortex to the brain stem excitatory area, which in turn sends still more
excitatory signals to the cortex. This helps to maintain the level of
excitation of the cerebral cortex or even to enhance it, thus leading to an
“awake” mind.
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