Download important ascending tracts

IMPORTANT ASCENDING TRACTS
POSTERIOR COLUMN
- PROPRIOCEPTION - is the sense of the relative position of neighbouring
parts of the body and strength of effort being employed in movement. it is
provided by proprioceptors in skeletal striated muscles (muscle spindles)
and tendons (Golgi tendon organ) and the fibrous capsules in joints.
SPINOTHALAMIC TRACT
-is a sensory pathway from exteroceptors of the skin to the thalamus. The
spinothalamic tract consists of two adjacent pathways: anterior and lateral. The
anterior spinothalamic tract carries information about crude touch. The lateral
spinothalamic tract conveys pain and temperature.
PROPRIOCEPTION
Joint capsules, tactile and pressure receptors send a signal through
the posterior root ganglia up through the gracile fasciculus for
lower body sensory impulses and the cuneate fasciculus for upper
body impulses. Once the gracile fasciculus reaches the gracile
nucleus, and the cuneate fasciculus reaches the cuneate nucleus in
the lower medulla oblongata, they begin to cross over as the
internal arcuate fibers. Upon reaching the opposite side, they
become the medial lemniscus, which is the second part of
the posterior column–medial lemniscus pathway. Medial
lemniscus terminates in ventral posterolateral nucleus in thalamus
which projects to the parietal lobe.
PAIN AND TEMPERATURE
The spinothalamic tract, like the dorsal column-medial lemniscus tract, uses
three neurons to convey sensory information from the periphery to conscious
level at the cerebral cortex.
Pseudounipolar neurons in the dorsal root ganglion have axons that lead from
the skin into the dorsal spinal cord where they ascend or descend one or two
vertebral levels via Lissauer's tract and then synapse with secondary neurons in
either the substantia gelatinosa of Rolando or the nucleus proprius (dorsal
horn). These secondary neurons are called tract cells.
The axons of the tract cells cross over (decussate) to the other side of the spinal
cord via the anterior white commissure, and to the anterolateral corner of the
spinal cord . The axons travel up the length of the spinal cord into
the brainstem, joining the medial lemniscus. Then the way to the cortex remains
the same like in proprioceptive tract.
OPTIC TRACT
The optic tract is a part of the visual system in the brain. It is a
continuation of the optic nerve that relays information from
the optic chiasm to the ipsilateral lateral geniculate nucleus
(LGN), pretectal nuclei, and superior colliculus.
The lateral geniculate nucleus is a relay center in the thalamus for
the visual pathway. It receives a major sensory input from
the retina.
Optic nerve – optic chiasm – optic tract – lateral geniculate
nucleus – optic radiation – area striata which surrounds calcarine
fissure (Brodman 17) (medial part of occipital lobe).
DESCENDING TRACTS
The descending tracts originate from different
cortical areas and from brain stem nuclei. The
descending pathway carry information associated
with maintenance of motor activities such as
posture, balance, muscle tone, and visceral and
somatic reflex activity.
CORTICOSPINAL TRACT
The pyramidal tracts include both the corticospinal and corticobulbar
tracts. These are aggregations of upper motor neuron nerve fibres that
travel from the cerebral cortex and terminate either in
the brainstem (corticobulbar) or spinal cord (corticospinal) and are
involved in control of motor functions of the body.
The corticobulbar tract conducts impulses from the brain to the cranial
nerves. These nerves control the muscles of the face and neck and are
involved in facial expression, mastication, swallowing, and other
functions.
The corticospinal tract is involved in voluntary movement. The majority
of fibres of the corticospinal tract cross over in the medulla, resulting in
muscles being controlled by the opposite side of the brain.
The pyramidal tracts are named because they pass through
the pyramids of the medulla.
CORTICOSPINAL TRACT
Precentral gyrus (Brodman 4) – corona radiata –
anterior 2/3 the posterior part of internal capsule
– cerebral peduncles in midbrain – pyramids on
the border between the pons and medulla –
decussation on the border between the medulla
oblongata and medulla spinalis – motor cells in
anterior horn of the spinal cord.
In brainstem - different pathway for corticobulbar
tract.
CORTICOSPINAL TRACT
The nerve axons traveling down the tract are referred to as upper
motor neurons. These axons travel down the tracts in the white
matter of the spinal cord until they reach the vertebral level of the
muscle that they will innervate. At this point, the
axons synapse with lower motor neurons. The majority of axons
do not directly synapse with lower motor neurons, but instead
synapse with an interneuron that then synapses with a lower motor
neuron. This generally occurs in the anterior horn of the spinal
cord.
DORSAL ROOT
Information from the skin, skeletal muscle and joints is relayed to
the spinal cord by sensory cells located in the dorsal root ganglia.
The dorsal root fibers are the axons originated from the primary
sensory dorsal root ganglion cells. Each ascending dorsal root
axon, before reaching the spinal cord, bifurcates into ascending
and descending branches entering several segments below and
above their own segment.
VENTRAL ROOT
Ventral root fibers are the axons of motor and visceral efferent
fibers and emerge from poorly defined ventral lateral sulcus as
ventral rootlets. The ventral rootlets from discrete spinal cord
section unite and form the ventral root, which contain motor nerve
axons from motor and visceral motor neurons.
The α motor nerve axons innervate the extrafusal muscle fibers
while the small motor neuron axons innervate the intrafusal
muscle fibers located within the muscle spindles. The visceral
neurons send preganglionic fibers to innervate the visceral organs.
All these fibers join the dorsal root fibers distal to the dorsal root
ganglion to form the spinal nerve.
SPINAL NERVE ROOTS
The spinal nerve roots are formed by the union of dorsal and
ventral roots within the intervertebral foramen, resulting in a
mixed nerve joined together and forming the spinal nerve. Spinal
nerve rami include the dorsal primary nerves (ramus), which
innervates the skin and muscles of the back, and the ventral
primary nerves (ramus), which innervates the ventral lateral
muscles and skin of the trunk, extremities and visceral organs. The
ventral and dorsal roots also provide the anchorage and fixation of
the spinal cord to the vertebral cauda.
BLOOD SUPPLY OF THE SPINAL CORD
The arterial blood supply to the spinal cord in the upper cervical
regions is derived from two branches of the vertebral arteries, the
anterior spinal artery and the posterior spinal arteries.
At the level of medulla, the paired anterior spinal arteries join to
form a single artery that lies in the anterior median fissure of the
spinal cord.
The posterior spinal arteries are paired and form an anastomotic
chain over the posterior aspect of the spinal cord. A plexus of
small arteries, the arterial vasocorona, on the surface of the cord
constitutes an anastomotic connection between the anterior and
posterior spinal arteries.
THIS ARRANGEMENT PROVIDES UNINTERRUPTED
BLOOD SUPPLIES ALONG THE ENTIRE LENGTH OF
THE SPINAL CORD.
At spinal cord regions below upper cervical levels, the anterior and
posterior spinal arteries narrow and form an anastomotic network
with radicular arteries. The radicular arteries are branches of the
cervical, trunk, intercostal & iliac arteries. The radicular arteries
supply most of the lower levels of the spinal cord. There are
approximately 6 to 8 pairs of radicular arteries supplying the
anterior and posterior spinal cord.
CEREBELLOPONTINE ANGLE
The cerebellopontine angle is a potential space in the posterior
cranial fossa. Its boundaries are as follows:
- Anteriorly: Posterior fossa of the temporal bone
- Posteriorly: Anterior surface of the cerebellum
- Medially: Inferior olive
- Superiorly: Inferior border of the pons and cerebellar peduncle
- Inferiorly: The cerebellar tonsil
The trigeminal nerve is visible superior to the cerebellopontine
angle, whereas the IXth, Xth, and XIth nerves course inferiorly.
Other important structures within the cerebellopontine angle
include the anterior inferior cerebellar artery (AICA), flocculus,
and lateral aperture of the fourth ventricle (foramen of Luschka).
The labyrinthine artery is usually a branch of the AICA and
supplies the cochlea and labyrinth.
CEREBELUM, CIRCLE OF
WILLIS AND DURAL BRAIN
SINUSES
LOCATION
The term cerebellum is from latin meaning the
little brain. It is a part of the hindbrain situated in
the posterior cranial fossa.
It is also present behind the pons and medulla
oblongata, seperated from two structures by the
cavity of fourth ventricle.
It is covered by tentorium cerebelli and is
connected to brain stem by three cerebellar
peduncles.
Consists of two laterally, large hemispheres which are
united by midline VERMIS.
Cerebellar surface is divided by numerous curve transverse
fissures giving it a laminated appearance.
One fissure named “horizontal fissure” extends around
dorsolateral border of each hemisphere from middle
cerebellar peduncle to vallecula, seperating superior and
inferior surface.
EXTERNAL SURFACE OF CEREBELLUM
The deepest fissure in the vermis is primary
fissure, which curves ventrolaterally in the
superior surface of the cerebellum to meet
horizontal fissure.
Primary fissure divides the cerebellum into
anterior and posterior lobe.
ARBOR VITAE
In latin “ tree of life” it is the white matter
of the cerebellum. It is so called because of the
tree like appearance.
It brings sensory and motor sensation to and from
cerebellum.
CEREBELLAR PEDUNCLES
The cerebellum is connected to the brain stem by
three peduncles
CEREBELLAR PEDUNCLE
Cerebellar peduncle is the part that connects cerebellum to
the brain stem. There are 6 cerebellar peduncles in total, 3
on the left and 3 on the right. It may refer to:
Superior cerebellar peduncle - primary output of the
cerebellum with mostly fibers carrying information to the
midbrain.
Middle cerebellar peduncle - carry input fibers from the
contralateral cerebral cortex.
Inferior cerebellar peduncle - receives ipsilateral
proprioceptive information from the spinal cord.
LOBES OF CEREBELLUM Divisions of lobes
Anatomical
• Flocculonodular lobe
• Anterior lobe
• Posterior lobe
Functional(Evolutionary)
•
Paleocerebellum
•
Neocerebellum
•
Archicerebellum
FUNCTIONS
Coordination of movement - the cerebellum
controls the timing and pattern of muscle
activation during movement.
Maintenance of equilibrium (in conjunction with
the vestibular system)
Regulation of muscle tone - modulates spinal cord
and brain stem mechanisms involved in postural
control.
DYSFUNCTION:
Cerebellar Ataxia - a disturbance that alters the direction
and extent of voluntary movements; abnormal gait and
uncoordinated movements.
Dysmetria - altered range of motion (misjudge distance).
Intention Tremor - oscillating motion, especially of head,
during movement.
Vestibular signs – nystagmus.
GROSS ANATOMICAL ORGANIZATION
1. Internal Organization:
A.Cerebellar Cortex - surface gray matter, sulci and
folia
B. White Matter - internal
C.Cerebellar Nuclei - three pairs located in white
matter:
- Fastigial
- Interpositus (Globose and Emboliform)
- Dentate
2. Cerebellar Lobes:
Anterior Lobe = Spinocerebellum (paleocerebellum)related to spinal cord, postural tone. Damage results in
forelimb hyperextension and hindlimb hip flexion
Posterior Lobe = Cerebrocerebellum (neocerebellum)damage results in hypotonia, hypermetria & intention
tremor
Flocculonodular Lobe = Vestibulocerebellum associated
with the vestibular system (eye movement, etc.); damage
results in dysequilibrium, wide based gait and nystagmus
3. Longitudinal Zones
VERMIS - most medali portion of cerebellum; associated with the
fastigial nucleus, concerned with regulation of muscle tone for posture
and locomotion.
PARAVERMIS - intermediate part of the cerebellum, associated with
the interpositus nucleus; participates in the control of an evolving
movement by utilizing proprioceptive sensory information generated by
the movement itself to correct errors in the movement.
HEMISPHERE - the largest and most lateral part of the cerebellum;
associated with the dentate nucleus; influences the output to the motor
cortex and permits fine delicate adjustments in muscle tone-> skilled
movement
CEREBELLAR CORTEX
Molecular Layer - most superficial, consisting of
axons of granule cells (parallel fibers) and dendrites
of PCs.
Purkinje Cell Layer - middle layer consisting of a
single layer of large neuronal cell bodies (Purkinje
cells).
Granule Cell Layer - deepest layer (next to white
matter) consisting of small neurons called granule
cells.
CELL TYPES
Cell Types and Afferent Fibers of the Cerebellar Cortex
Purkinje Cells - the only output neuron from the cortex utilizes GABA to inhibit neurons
in deep cerebellar nuclei.
Granule Cells - intrinsic cells of cerebellar cortex; use glutamate as an excitatory
transmitter; excites Purkinje cells via axonal branches called “parallel fibers”.
Basket Cells - inhibitory interneuron; utilizes GABA to inhibit Purkinje cells.
Climbing Fibers - axons arising from the olivary nucleus; use glutamate and aspartate to
excite Purkinje cell and cerebellar nuclei neurons.
Mossy Fibers - all other axons that enter the cerebellum; excite granule cells (and
neurons in cerebellar nuclei).
MAJOR CEREBELLAR INPUTS (AXONS
ENTERING THE CEREBELLUM)
1.CLIMBING FIBER INPUTS = OLIVOCEREBELLAR FIBERS– arise exclusively
from the olivary nucleus of the caudal medulla; have a powerful excitatory effect on
Purkinje cells upon which they synapse.
2. MOSSY FIBER INPUTS:
A.Vestibulocerebellar fibers - arise mainly from the vestibular nerve and vestibular
nuclei; project to flocculonodular lobe and fastigial nucleus (coordinate head and eye
movement.
B.Spinocerebellar fibers- arise from spinal cord --> via spinocerebellar tracts-->go to
anterior lobe; make cerebellum aware of ongoing movements via proprioceptive input
from muscle spindles and joint receptors.
C.Cerebropontocerebellar fibers -arise from pyramidal cells in the cerebral cortex,
synapse on pontine nuclei which send their axons to the contralateral cerebellar cortex
via pontocerebellar fibers (form middle peduncle)- Alerts cerebellum regarding
anticipated movements.
MAJOR CEREBELLAR OUTPUTS (ARISE FROM
NEURONS IN DEEP CEREBELLAR NUCLEI):
1.Fastigial nucleus projections: (via inferior peduncle)-->
vestibular nuclei and reticular formation--> vestibulospinal and
reticulospinal tracts influence spinal motor neurons--> effect
extensor muscles related to maintaining posture and balance.
2.Interpositus nucleus projections: (via superior peduncle) -go
to red nucleus to influence rubrospinal tract - correct errors
related to the gross movements
3.Dentate nucleus projections: (via superior peduncle) ->
projects to thalamus to influence the output from the motor
cortex - makes delicate adjustments related to fine, skilled
movements
CIRCLE OF WILLIS
The circle of Willis (circulus arteriosus cerebri) is
an anastomotic system of arteries that sits at the
base of the brain. The “circle” was named after
Thomas Willis by his student Richard Lower.
Willis was the author of Cerebri Anatome, a book
that described and depicted this vascular ring.
Although such a vascular ring had been described
earlier, the name Willis has been eponymously
propagated.
The circle of Willis encircles the stalk of the
pituitary gland and provides important
communications between the blood supply of the
forebrain and hindbrain (ie, between the internal
carotid and vertebrobasilar systems). A complete
circle of Willis is present in most individuals,
although a well-developed communication
between each of its parts is identified in less than
half of the population.
The circle of Willis is formed when the internal carotid artery
(ICA) enters the cranial cavity bilaterally and divides into the
anterior cerebral artery (ACA) and middle cerebral artery (MCA).
The anterior cerebral arteries are then united by an anterior
communicating (ACOM) artery. These connections form the
anterior half (anterior circulation) of the circle of Willis.
Posteriorly, the basilar artery, formed by the left and right vertebral
arteries, branches into a left and right posterior cerebral artery
(PCA), forming the posterior circulation. The PCAs complete the
circle of Willis by joining the internal carotid system anteriorly via
the posterior communicating (PCOM) arteries
DURAL SINUSES
Dural venous sinuses are venous channels located intracranially between
the two layers of dura mater (endosteal layer and meningeal layer). They
can be conceptualised as trapped epidural veins. Unlike other veins in
the body they run alone, not parallel to arteries. Furthermore, they are
valveless, allowing for bidirectional blood flow in intracranial veins. It is
also important to note that the draining territories of intracranial veins
are different from those of major cerebral arteries.
Together the dural venous sinuses form the major drainage pathways
from the brain, predominantly to the internal jugular veins.
MAIN DURAL SINUSES
UNPAIRED
- superior sagittal sinus
- inferior sagittal sinus
- straight sinus
- occipital sinus
- intercavernous sinus
PAIRED
- transverse sinus
- sigmoid sinus
- inferior petrosal sinus
- superior petrosal sinus
- cavernous sinus
- sphenoparietal sinus
- basilar venous plexus