BRAIN
Meninges
- Dura mater (outer layer)
 Toughest layer composed of predominantly collagen fibers, blood vessels, and sensory nerves
o Innervated primarily by the Trigeminal n. (CN V)
o Only the meninges has sensory innervation (the brain does not perceive pain)
 Only in the cranium, 3 folds of the dura mater enclose 3 venous sinuses
o Venosus sinuses receive veins draning the brain and skull bones, convey blood to the
maxillary, internal jugular, and vertebral veins, and are not accompanied by arteries
o Falx cerebri
 Located between the two cerebral hemispheres
 Contains the sagittal venous sinus
o Tentorium cerebelli membranaceum
 Found between the cerebrum and cerebellum
 Contains the transverse venous sinus
o Diaphragm sellae
 Separated the pituitary gland from the brain
 Contains the cavernous venous sinus
 Dura mater is separated from the bony vertebral canal by adipose tissue, creating the epidural space
that is not present within the cranium (because dura mater is firmly attached to the internal surface)
 Sub-dural space is filled with a thin fluid-like film
o Space can become enlarged due to leaking of fluid (i.e. from sub-dural hemorrhage)
- Arachnoid mater
 Thin membrane closely associated with dura mater due to the centrifugal pressure exerted by CSF
 Sub-arachnoid space contains CSF and blood vessels that supply or drain the brain
o Cisterna cerebello-medullaris
 Enlarged region of subarachnoid space located b/w cerebellum and medulla oblongata
 Primary site for CSF sampling
Clinical Notes: Epidural anesthetics injected into the epidural space are lipid-soluble and tend to stay localized within the few spinal
segments, blocking the conduction of impulses in spinal nerves at the injection site.
Epidural hemorrhage is usually trauma-induced, resulting from the breakage of epidural blood vessels (i.e. middle meningeal a.). Subdural hemorrhage is usually due to the breakage of veins within any of the dural venous sinuses. Subarachnoid space bleeding is due
to the breakage of any vessels within this space (i.e. cerebral, cerebellar, and spinal blood vessels).
The cisterna cerebello-medullaris is the primary site for CSF sampling. The needle is placed at the center of the imaginary triangle
drawn by connecting the lines between the wings of the atlas and the external occipital protuberance. Prominent nuchal tubercles may
interfere with needle placement.
Lumbar CSF sampling is more likely to provide helpful information on spinal cord diseases, though is usually not attempted in small
animals. The needle is placed in the L4-L5 or L5-L6 space at 45° angle.
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Pia mater
 Thin membrane whose surface blends with the CNS
 In the brain, forms the choroid plexus (no choroid plexus in the spinal cord)
 Denticulate ligaments
o Formed by fused strands of arachnoid and pia mater that extend to the dura mater
o Help suspend the spinal cord within the dura mater
Forebrain
Cerebrum (telencephalon)
- Consists of two cerebral hemispheres divided by the dorsal longitudinal fissure (contains the falx cerebri)
- Separated from the cerebellum by the transverse fissure (contains the tentorium cerebelli)
- Each hemisphere has gyri (outward folds) and sulci (inward folds)
- Each hemisphere can be divided into lobes named for their functionality
 Frontal lobe (part of the motor cortex)
 Parietal lobe
o Part of the motor and sensory cerebral cortex
 Occipital lobe (visual cortex)
 Temporal lobe (auditory cortex)
 Olfactory lobe
o Olfactory bulb
o Olfactory peduncle projection pathway that
joins bulb to cerebral hemisphere
o Lateral olfactory tract
 Olfaction input to piriform lobe
o Medial olfactory tract
 Piriform lobe (“paleopallium”)
o Limbic system integration
o Receives input from the lateral olfactory tract
o Rhinal sulcus
 Separates this phylogenetically older cerebrum from new cerbrum (“neopallium”)
- Composed of both gray and white matter
 Cerebral cortex is the outer gray matter that consists of unmyelinated cell bodies
 Inner white matter layer that consists of 3 types of myelinated nerve fibers
o Association fibers
 Course from one gyrus to another within the same hemisphere
o Projection fibers
 Course b/w the cerebrum and the diencephalon
 Descend from the telencephalon to the brain stem and spinal cord
 Deep collection of these fibers forms an internal capsule that courses superficially to
become the corona radiata
o Commissural fibers
 Connect the two hemispheres
 These fibers collectively constitute the corpus callosum, which is divided into 3 parts
o Genu (rostral bend)
o Body (middle trunk)
o Splenium (caudal bend)
- Septum pellucidum (thin, vertical sheet of tissue ventral to the corpus callosum)
- Cingulate gyrus
 Part of limbic system
 Gyrus immediately dorsal to the corpus callosum
- Fornix
 Part of the limbic system
 Collection of fibers that connect the hippocampus to the mammillary bodies and cerebrum
- Hippocampus (“archepallium”)
 Part of the limbic system and a specialized region of the cerebral cortex
- Caudate nucleus
 Englargement on the floor of the lateral ventricle
 Part of the basal nuclei
 Monitors motor coordination
Diencephalon
- Thalamus
 Major processing and routing area for all sensory information to the brain (except the sense of smell)
 Interthalamic adhesion (b/w the left and right sides of the thalamus)
 Lateral geniculate nucleus
o Receives fibers of the optic tract
o Performs visual information processing and projection to the visual cortex
o Connected to the rostral colliculus of the midbrain via the brachium of the rostral colliculus
 Medial geniculate nucleus
o Functions as a part of the auditory system
o Connected to the caudal colliculus of the midbrain via the brachium of the caudal colliculus
- Hypothalamus
 Mammillary bodies are caudal to the tuber cinereum and demarcate the most caudal extent of the
hypothalamus on the ventral surface of the diencephalon
- Epithalamus
 Contains the pineal gland
o Small gland located on the roof of the III ventricle
o Produces melatonin
 Contains the habenular nucleus
- Optic chiasma
 Formed by decussation of CN II (optic nn.)
 Optic tracts course laterally and dorsocaudally from the chiasm to enter the lateral geniculate nucleus
- Hypophysis (pituitary gland)
 Located caudal to the optic chiam
 Attached to the tuber cinereum of the hypothalamus by the infundibulum
 The lumen of the infundibulum communicated with the third ventricle of the diencephalon
Mesencephalon (midbrain)
- Tectum
 Portion of the midbrain dorsal to the cerebral aquaduct
 Composed of four groups of neuronal cell bodies (colliculi)
o Rostral colliculus (paired)
 Functions with the visual system and processes the pupillary light reflex
 Brachium of the rostral colliculus connects midbrain to lateral geniculate nucleus
o Caudal colliculus (paired) – functio in the auditorys system
 Brachium of the caudal colliculus connects midbrain to medial geniculate nucleus
- Cerebral peduncle
 Portion of the mesencephalon ventral to the cerebral aqueduct
- Provides attachment for CN III (oculomotor n.)
- CN IV (trochlear n.) exits the rostral medullary velum and courses on the lateral surface of the midbrain
Metencephalon
Cerebellum
- Transverse cerebral fissure separates the cerebellum from the cerebrum
- Controls balance and coordinates postural and locomotor activities
 Unlike the cerebrum, it does not initiate voluntary movement
 Lesions in the cerebellum result in loss of balance, staggering or high-stepping (hypermetria) gait
- Exhibits convolutions, or folia, that are equivalent to the gyri of the cerebrum
- Consists of three major lobes
 Lateral cerebellar hemispheres (two lateral, larger lobes)
 Vermis (median, narrow lobe)
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Composed of both gray and white matter
 White matter is arranged in a highly-branched pattern called the arbor vitae cerebelli
o Arbor vitae contains important nuclei
 Gray matter surrounds the arbor vitae
Connected to the brain stem by three cerebellar peduncles on each side
 Rostral cerebellar peduncle (medial)
 Middle cerebellar peduncle (lateral)
 Caudal cerebellar peduncle (middle)
Pons
- Provides communication between the cerebrum and cerebellum
- Composed of transverse fibers that course into the middle cerebellar peduncles
- Provides attachment for CN V (trigeminal n.) along the caudolateral aspect
Myelencephalon
Medulla oblongata
- Dorsal surface of the medulla (caudal to IV ventricle)
 Dorsal median sulcus (central groove)
 Fasciculus gracilis
o Longitudinal bulge lateral to the dorsal median sulcus
o Involved in pelvic limb proprioception
 Dorsal intermediate sulcus
o Groove lateral to the fasciculus gracilis
 Fasciculus cuneatus
o Longitudinal bulde lateral to the dorsal intermediate sulcus
o Involved in thoracic limb proprioception
- Trapezoid body
 Transverse band that separates the pons and the medulla oblongata
 Involved in auditory function
o Contains decussating (cross-over) fibers from cochlear nuclei
- Pyramids are a pair of longitudinal fibers that course either side of the ventral median fissue
 Decussation of pyramid fibers caudally results in complete obliteration of the fissure
 The pyramids serve as the descending corticospinal tract system
- Sits atop the rhomboid fossa (floor of the IV ventricle)
 Median sulcus (central groove of the fossa)
 Sulcus limitans (paired lateral grooves of the fossa)
o Sensory regions are dorsal to this sulcus
o Motor regions are ventral to this sulcus
 Rostral medullary velum
o Thin membrane forming the rostral roof of the IV ventricle
o Contains decussating fibers of CN IV (trochlear n.)
 Caudal medullary velum
o Thin membrane over the obex that forms the caudal roof of the IV ventricle
 Obex
o Point that marks the apex of the IV ventricle
o Covered by the caudal medullary velum
o At this level, the IV ventricle is continuous with the central canal of spinal cord
- CN VI (abducent n.) attaches at the medial aspect of the trapezoid body
- CN VII (facial n.) attaches on the lateral surface of the trapezoid body
- CN VIII (vestibulocochlear n.) arises near CN VII on the lateral surface of the trapezoid body
- CN IX (glossopharyngeal n.) and X (vagus n.) attach on the lateral side of the medulla obongata
- CN XI (accessory n.) emerges from the lateral surface of the spinal cord
- Several rootlets of CN XII (hypoglossal n.) arise on the caudal medulla oblongata
CSF and Choroid Plexus
- Choroid plexus
 Compact mass of pia mater, blood vessels, and ependymal tissue
 Produces CSF
 Located within the telencephalon, diencephalon, myelencephalon
- CSF circulation
 Lateral ventricle
o Located with telencephalon (cerebrum)
o Curved cavity ventral to the corpus callosum
o Communicates with the III ventricle via the interventricular foramen
 III ventricle
o Located within the diencephalon
o Communicates with the IV ventricle via the mesencephalic (cerebral) aquaduct
 IV ventricle
o Located within the myelencephalon (medulla oblongata)
o Communicates with the central canal of the spinal cord via the obex
Arterial Supply
- Major arteries of the brain are not accompanied by veins
- Basilar a.
 Formed by terminal branches of the vertebral aa. (enter vertebral canal via transverse foramina of atlas)
 Courses along the midline of the ventral surface of the medulla and pons
 Divides into two branches that from the caudal portion of the arterial circle of the brain
- Ventral spinal a.
 Caudal continuation of the basilar a.
 Runs along the ventral aspect of the spinal cord
- Internal carotid a.
 Other main source of blood to the arterial circle of the brain
 Enters the middle cranial fossa under the rostral end of the petrous temporal bone
 Course rostrally through cavernous venous sinus, along the hypophysis and optic chiasm
 Emerge from the dura mater and divide into branches that anastomose to complete the arterial circle
o Rostral cerebral a.
 Terminal branch of the internal carotid a. at the rostral aspect of the circle
 Supplies cerebral gyri
 Branches supply nasal cavity and are sources for cilialry aa. that supply the eyeball
o Middle cerebral a.
 Courses dorsolaterally to supply lateral surface of cerebral hemisphere
o Caudal cerebral a.
 Courses caudodorsally over the thalamus and corpus collosum
 Supplies the medial surface of the cerebral hemisphere (caudal portion)
 Supplies the diencephalon and rostral mesencephalon
- Cerebral arterial circle
 Surrounds the pituitary gland, supplying it and the cerebrum
- Rostral cerebellar a.
- Caudal cerebellar a.
 Branch of the basilar a. near the middle medulla oblongata
 Courses dorsally to supply the caudal portion of the cerebellum