Download D.U.C. Assist. Lec. Faculty of Dentistry General Physiology Ihsan

D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
Physiology of Nervous system
Sensory (ascending) & Motor (descending) Pathways
Before discussing the ascending and descending pathways, we need to
give an orientation to the various areas of the cortex. (Figure 1) is a map
of the human cerebral cortex, showing that it is divided into about 50
distinct areas called Brodmann’s areas based on histological structural
differences.
Note in the (figure 1) the large central fissure (also called central sulcus)
that extends horizontally across the brain. In general, sensory signals
from all sensation system terminate in the cerebral cortex immediately
posterior to the central fissure in area called post central gyrus
(somatosensory area).somatosensory area divided in to two regions :
 primary somatosensory area (or somatosensory area I)
 somatosensory area association (or somatosensory area II)
Conversely, the portion of the cerebral cortex anterior to the central
fissure and constituting the posterior half of the frontal lobe is called pre
central gyrus (the motor cortex) and is devoted almost entirely to
control of muscle contractions and body movements. A major share of
this motor control is in response to somatosensory signals received from
the sensory portions of the cortex, which keep the motor cortex informed
at each instant about the positions and motions of the different body parts.
Visual signals terminate in the occipital lobe, and auditory signals in the
temporal lobe.
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
Sensory (ascending) pathways
The sensory signals are carried through one of these sensory pathways:
(1) The dorsal column–medial lemniscal system: fine touch , vibration
and proprioceptor to cerebral cortex.
(2) The anterolateral spinothalamic system:
 anterior spinothalamic pathway :crude touch and pressure to
cerebral cortex.
 lateral spinotthalamic pathway : pain and temperature to cerebral
cortex.
(3) The Spinocerebellar pathway: proprioceptor to cerebellum.
 anterior spinocerebellar pathway
 posterior spinocerebellar pathway
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
The Dorsal Column–Medial Lemniscal System (DCML)
Note in Figure bellow that axon of sensory neuron entering the dorsal
ganglion root pass uninterrupted up to the dorsal column which consists
of two fasciculus (gracilis and cuneatus) , where they synapse in the
dorsal column . From there, second-order neurons decussate immediately
to the opposite side of the brain stem and continue upward through the
medial lemniscus to the thalamus.
In the thalamus, the medial lemniscal fibers terminate in the thalamic
sensory relay area, called the ventrobasal nuclei complex. From the
ventrobasal complex, third-order nerve fibers project, as shown in the
Figure mainly to the postcentral gyrus of the cerebral cortex (primary
somatosensory area), which is also called somatic sensory area I
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
The Anterolateral spinothalamic Pathway
The anterolateral system (ALS) transmits nociceptive, thermal, crude
touch information to higher brain centers. generally by a sequence of
three neurons , The neuron sequence consists of: A first order neuron
whose cell body is located in a dorsal root ganglion. It transmits sensory
information from peripheral structures to the dorsal posterior horn of the
spinal cord.A second order neuron whose cell body is located within the
dorsal horn of the spinal cord, and whose axon usually decussates and
ascends:
(1) Most pain signals and thermal signals terminate in the reticular
nuclei of the brain stem and from there are relayed to the
intralaminar nuclei complex of the thalamus where the pain
signals are further processed.(lateral spinpthalamic).
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
(2) The tactile signals (crude touch and pressure) are transmitted
mainly into the ventrobasal complex, terminating in some of the
same thalamic nuclei where the dorsal column tactile signals
terminate. From here, the signals are transmitted to the
somatosensory cortex along with the signals from the dorsal
columns. (anterior spinothalamic)
The spinocerebellar pathways: proprioceptor to cerebellum .
•anterior spinocerebellar pathway
•posterior spinocerebellar pathway
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
Motor Pathways
CNS transmits motor commands in response to sensory information
Motor commands are delivered by the:
Somatic nervous system (SNS): directs contraction of skeletal muscles
Autonomic nervous system (ANS): directs the activity of glands, smooth
muscles, and cardiac muscle.
There are two major descending tracts
 Corticospinal tract: Conscious control of skeletal muscles
 Subconscious tract: Subconscious regulation of balance, muscle
tone, eye, hand, and upper limb position
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
Corticospinal (Pyramidal) Tract
The corticospinal tract mainly originates from the primary motor cortex
After leaving the cortex, axons of this tract enter the posterior limb of the
internal capsule and pass caudally through the brain stem to the ventral
surface of the medulla, where they are decussate in the medullary
pyramids. At the junction of the medulla and spinal cord, most of the
fibers cross the midline to enter the lateral funiculus of the spinal cord
and form the lateral corticospinal tract, which extends throughout the
length of the cord.
The fibers that do not cross continue as far as the thoracic spinal cord in
the ventral corticospinal tract.
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
The Subconscious Motor Tracts (extra pyramidal)
Consists of four tracts involved in monitoring the subconscious motor
control:
 Vestibulospinal tracts: Send information to the inner ear to
monitor position of the head. Vestibular nuclei respond by altering
muscle tone, neck muscle contraction, and limbs for posture and
balance.
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D.U.C.
Faculty of Dentistry
Second grade
General Physiology
Assist. Lec.
Ihsan Dhari
Lec.7
 Tectospinal tracts: Send information to the head, neck, and upper
limbs in response to bright and sudden movements and loud noises
The tectum area consists of :
•Superior colliculi: receives visual information
•Inferior colliculi: receives auditory information
 Reticulospinal tracts: Send information to cause eye movements
and activate respiratory muscles.
 Rubrospinal tracts: Send information to the flexor and extensor
muscles.
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