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