Lecture 4: GROSS ANATOMICAL FEATURES OF THE BRAINSTEM AND FOREBRAIN Flashcards
The Long Axis of the CNS Bends at the _____
The Long Axis of the CNS Bends at the Cephalic Flexure
The human central nervous system (CNS) is composed of the ___ and ____ .
The human central nervous system (CNS) is composed of the brain and spinal cord
sagittal plane
divides the brain into 2 symmetrical halves
Parasagittal planes are those parallel to the sagittal plane.
Coronal planes
also called frontal planes
are parallel to the long axis of the body and perpendicular to the sagittal plane (e.g., a vertical plane passing through both ears).
Axial planes
also called transverse or horizontal planes
are perpendicular to the long axis of the body.
directional terms such as anterior, dorsal, and rostral change their meanings relative to one another in different parts of the nervous system.
The reason for this is that walking upright necessitates a bend of about 80 degrees in going from the long axis of the spinal cord and brainstem to the long axis of the cerebrum.
This bend is a consequence of the cephalic flexure, which appears early in the embryological development of the nervous system and persists in the mature brain.
Dorsal-ventral terminology ignores this bend, as though we had a linear CNS and walked around on all fours.
The terms anterior and superior, in contrast, retain a constant meaning relative to the normal upright orientation of the body as a whole. This means, for example, that the ventral surface of the spinal cord is also its anterior surface, but the ventral surface of the diencephalon is its inferior surface.
Rostral-caudal terminology. Anatomically, rostral means “toward the nose.” However, it also has a functional connotation (implying “toward the telencephalon”), so that the posterior end of the cerebral hemispheres could be considered rostral to all parts of the diencephalon.
Use of anterior-posterior and superior-inferior (or dorsal-ventral) terminology in reference to the cerebrum avoids any ambiguity.
cephalic flexure
The bend of about 80 degrees between the long axis of the brainstem and spinal cord and the anterior-posterior axis of the cerebrum.
The cephalic flexure is visible at the junction between the brainstem and the diencephalon.
brainstem
the midbrain, pons, and medulla.
midbrain
The most rostral of the 3 subdivisions of the brainstem.
The midbrain is tubular in plan and features a great variety of structures: the superior and inferior colliculi in its roof (tectum), aqueduct and periaqueductal gray, oculomotor and trochlear nuclei and pretectal area, upper part of the reticular formation, red nuclei, substantia nigra, and cerebral peduncles.
A small region of enormous importance, which is continuous with the diencephalon.
pons
The second of the 3 parts of the brainstem, continuous rostrally with the midbrain and caudally with the medulla. The pons is overlain by the cerebellum and includes an enlarged basal region.
medulla
medulla oblongata
The most caudal of the 3 subdivisions of the brainstem, continuous rostrally with the pons and caudally with the spinal cord.
This small structure is important out of proportion to its size: it is crucial to vital functions (respiratory, cardiovascular, visceral activity) and other integrative activities; most sensory and motor tracts of the CNS run rostrally and caudally through it.
corpus callosum
Latin for “hard body”
a massive curvilinear bridge of commissural fibers, shaped in sagittal sections like an overturned canoe.
The corpus callosum interconnects most cortical areas of the 2 cerebral hemispheres and joins them functionally, providing the substrate for a unitary consciousness.
has an enlarged and rounded posterior splenium, a body, and an anterior, curved genu that tapers gently into a ventrally directed rostrum, which merges into the lamina terminalis (where development of the corpus callosum started).
The nervous system develops embryologically from a ____ ___; the cavity of the tube persists in adults as a system of ____.
The nervous system develops embryologically from a neuroectodermal tube; the cavity of the tube persists in adults as a system of ventricles, part of which is apparent in the sagittal plane.
third ventricle
The single, median, vertically oriented cavity of the diencephalon, separating the thalamus and hypothalamus of the 2 hemispheres.
The third ventricle is confluent anteriorly with both lateral ventricles through the interventricular foramina and posteriorly with the fourth ventricle through the aqueduct. It has 4 small outpocketings.
lateral ventricle
The large central cavity of each cerebral hemisphere, following a C-shaped course throughout the hemisphere and derived from the lumen of the embryonic telencephalic vesicle.
interventricular foramen
AKA, the foramen of Monro
The narrow orifice between each lateral ventricle and the third ventricle.
cerebral aqueduct
AKA, the aqueduct of Sylvius
narrow channel through the midbrain that connects the third ventricle to the fourth ventricle. It is filled with CSF.
fourth ventricle
The aqueduct is continuous with the fourth ventricle of the pons and medulla, and the fourth ventricle is continuous with the microscopically tiny central canal of the caudal medulla and the spinal cord.
The most caudal of the brain ventricles, shaped like a tent with a peaked roof protruding into the overlying cerebellum and a diamond-shaped floor formed by the upper surface of the pons and rostral medulla.
Confluent with the third ventricle via the cerebral aqueduct and open to subarachnoid space through 3 foramina: one median aperture (of Magendie) and two lateral apertures (of Luschka).
central canal
The narrow, functionless remnant of the lumen of the spinal part of the embryonic neural tube, lined by ependyma and usually obstructed by epithelial debris. It runs the length of the spinal cord, contains traces of cerebrospinal fluid, and opens into the fourth ventricle at the obex of the medulla.
Humans, Relative to Other Animals, Have Large Brains
One impressive feature of the human brain is its size, and our distinctively human mental capacities are commonly attributed to this. The human brain weighs about 400g at birth, and this weight triples during the first 3 years of life (resulting from the addition of myelin and the growth of neuronal processes rather than the addition of neurons). The rate of growth then slows, and the maximum brain weight of around 1400g is reached at about age 11. This weight holds steady until about age 50, when a slow decline sets in. The weight of 1400g is only an average; brain weights for normal individuals range from 1100g (or less) to around 1700g. This large range is surprising, and its significance is not well understood; there is only a modest correlation between brain size and mental ability.
Part of the reason for the large human brain is simply a reflection of body size: big animals tend to have big brains. Elephants, for example, have 5000-g brains. Similarly, the size difference between the bodies of human males and females explains, at least in part, the fact that male brains are slightly larger than female brains. However, this is not the whole story; many animals that are larger than humans nevertheless have smaller brains. Overall, then, relative to body size, humans have larger brains than most other animals. It is tempting to attribute our mental abilities to our relatively large brains, but this is an oversimplification. Relative to body size, dolphins, some small primates and rodents, and even some fish have larger brains than we do. The key differences between human brains and those of other animals appear to be more complex neuronal interconnections and a selective increase in the size of certain areas of the cerebral cortex thought to be involved in higher functions.
Characteristic Gross Anatomical Features of The Medulla
The Medulla Includes Pyramids, Olives, and Part of the Fourth Ventricle
Characteristic Gross Anatomical Features of The Pons
The Pons Includes the Basal Pons, Middle Cerebellar Peduncles, and Part of the Fourth Ventricle
Characteristic Gross Anatomical Features of The Midbrain
The Midbrain Includes the Superior and Inferior Colliculi, the Cerebral Peduncles, and the Cerebral Aqueduct
The Brainstem Intro
The spinal cord continues rostrally into the brainstem, which performs spinal cord–like functions for the head. The brainstem contains the lower motor neurons for the muscles of the head and does the initial processing of general afferent information concerning the head.
It also does much more, reflecting the additional functions of the cranial nerves attached to it, as well as some distinctive built-in brainstem functions.
The Brainstem Has Conduit, Cranial Nerve, and Integrative Functions
Brainstem activities may be divided (roughly) into 3 general types: conduit functions, cranial nerve functions, and integrative functions.
The Brainstem’s Conduit Functions
The only way for ascending tracts to reach the thalamus or cerebellum, or for descending tracts to reach the spinal cord, is by passing through the brainstem. Many of these tracts, however, are not straight-through affairs, and identifiable relay nuclei in the brainstem are frequently involved.
The Brainstem’s Cranial Nerve Functions
The cranial nerves contain the head’s equivalent of spinal nerve fibers, and those involved in the special senses of olfaction, sight, hearing, equilibrium, and gustation, or taste.
The olfactory and optic nerves project directly to the telencephalon and diencephalon, respectively, but the others project to or emerge from the brainstem. Thus a wide assortment of sensory and motor nuclei related to cranial nerve function can be found at various brainstem levels.
The ___ and ___ cranial nerves project directly to the telencephalon and diencephalon, respectively, but the others project to or emerge from the _____.
The olfactory and optic cranial nerves project directly to the telencephalon and diencephalon, respectively, but the others project to or emerge from the brainstem.
Cranial Nerve I
Sensory Function only.
Smell.
Cranial Nerve II
Sensory Function only.
Sight.
Cranial Nerve III
oculomotor nerve
Motor Function only.
Eye movements, pupil and lens function.
Cranial Nerve IV
Motor Function only.
Eye movements
Cranial Nerve V
trigeminal nerve
Sensory Function:
Facial sensation.
Motor Function:
Chewing.
enters the brainstem at a midpontine level
Cranial Nerve VI
abducens nerve
Motor Function only.
Eye movements.
Cranial Nerve VII
facial nerve
Sensory Function:
Taste.
Motor Function:
Facial expression.
consists of 2 parts: a larger and more medial motor root and a smaller sensory root (sometimes referred to as the intermediate nerve).
Cranial Nerve VIII
vestibulocochlear nerve
Sensory Function only.
Hearing, equilibrium.
Located slightly lateral to the facial nerve.
Has 2 parts: a vestibular division and a more lateral cochlear division.
Cranial Nerve IX
glossopharyngeal nerve
Sensory Function:
Taste.
Motor Function:
Swallowing.
Cranial Nerve X
vagus nerve
Sensory Function:
Thoracic & abdominal viscera.
Motor Function:
Speech, swallowing;
thoracic and abdominal viscera.
Cranial Nerve XI
Motor Function only.
Head and shoulder movements
Cranial Nerve XII
hypoglossal nerve
Motor Function only.
Tongue movements
The rootlets of the hypoglossal nerve (XII) emerge from the anterolateral sulcus, mainly in the rostral medulla. In the rostral medulla, the column dorsal to the hypoglossal rootlets is enlarged to form an oval swelling called the olive.
reticular formation
The central region of the brainstem, occupying most of the tegmentum of the midbrain, pons, and medulla with a complex netlike fabric of nerve cell bodies and interwoven processes.
Its myriad multimodal afferents, profusely collateralizing efferents running upward and downward to every level of the CNS, and involvement in virtually every activity from visceral functions to consciousness make it a core integrating structure of the brain.
medulla anatomy
The medulla is vaguely scoop shaped. The “handle” corresponds to the caudal or closed portion, containing a central canal continuous with that of the spinal cord. The open portion of the scoop corresponds to the rostral or open medulla, in which the central canal expands into the fourth ventricle. The apex of the V-shaped caudal fourth ventricle, where it narrows into the central canal, is called the obex.
pyramidal decussation
The site, located at the spinomedullary junction, at which most fibers in each pyramid cross the midline to form the contralateral lateral corticospinal tract.
anterior median fissure
??
The anterior median fissure is briefly interrupted by the pyramidal decussation at the junction between spinal cord and brainstem, but then it continues rostrally to the edge of the pons, separating the two pyramids.
olive
Protuberance on the lateral aspect of the medulla, just dorsolateral to the pyramid, caused by the underlying inferior olivary nucleus.
The rootlets of the glossopharyngeal (IX) and vagus (X) nerves emerge from a shallow lateral groove dorsal to the olive.
spinal tract of the trigeminal nerve
the spinal tract of the trigeminal nerve is the head’s equivalent of Lissauer’s tract.
cuneate tubercle
A swelling on the dorsolateral aspect of the mid-medulla overlying the cuneate nucleus, which mediates that part of the posterior column-medial lemniscus pathway carrying tactile and proprioceptive information from the arm and upper body.
gracile tubercle
A swelling, just caudal to the obex, located dorsomedially on the medulla overlying the nucleus gracilis, which mediates that part of the posterior column–medial lemniscus pathway carrying tactile and proprioceptive information from the leg and lower body.
vestibular area
Four elaborately subdivided secondary sensory nuclei of the vestibular division of the eighth cranial nerve in the floor of the fourth ventricle.
Collectively they project to the nuclei of extraocular muscles (mostly via the medial longitudinal fasciculus), the cerebellum, the reticular formation, the thalamus, and the spinal cord.
vagal trigones
A small elevation in the floor of the caudal fourth ventricle with boundaries forming a triangle just lateral to the hypoglossal trigone. Each vagal trigone is a fusiform swelling produced by the underlying dorsal motor nucleus of the vagus.
hypoglossal trigone
A triangular elevation in the floor of the caudal fourth ventricle formed by the underlying hypoglossal nucleus.
facial colliculus
A swelling in the floor of the fourth ventricle in the caudal pons, caused by the underlying internal genu of the facial nerve looping around the abducens nucleus.
The Pons Anatomy
The pons is dominated by the massive, transversely oriented structure on its ventral surface from which it derives its name. Pons is the Latin word for “bridge,” and this portion of it (the basal pons) looks like a bridge interconnecting the two cerebellar hemispheres. It is not, however, a direct interconnection. Rather, many of the fibers descending in each cerebral peduncle synapse in scattered nuclei of the ipsilateral half of the basal pons. These nuclei in turn project their fibers across the midline, after which they funnel into the middle cerebellar peduncle (brachium pontis) and finally enter the cerebellum.
middle cerebellar peduncle
The largest of the cerebellar peduncles, containing fibers from contralateral pontine nuclei that end as mossy fibers in almost all areas of cerebellar cortex.
Sometimes referred to as the brachium pontis (the “arm of the pons”).
basal pons
A mass of gray and white matter, oriented transversely and filled with transversely and longitudinally coursing fibers, on the anterior surface of the pons. The basal pons looks like a bridge (for which the pons was named) between the two cerebellar hemispheres, but in fact it is a key link between the cerebrum and cerebellum: corticopontine fibers end in its scattered pontine nuclei, which in turn project across the midline into the cerebellum via the middle cerebellar peduncle.
brachium conjunctivum
The superior cerebellar peduncle (brachium conjunctivum) forms much of the roof of the fourth ventricle in the pons.
It emerges from the cerebellum, moves toward the midline and the brainstem, and enters the brainstem near the junction between the pons and midbrain. At this same junction, the trochlear nerve (IV) emerges from the dorsal surface of the brainstem.
The superior cerebellar peduncle is covered in the rostral pons by a flattened band of fibers called the lateral lemniscus.
superior cerebellar peduncle (brachium conjunctivum)
The major efferent route from the cerebellum, containing projections from deep cerebellar nuclei on their way to the contralateral red nucleus and thalamus (VL/VA).
Also referred to as the brachium conjunctivum (a “joined-together arm,” named for its course through a decussation with its contralateral counterpart).
A descending limb leaves the superior cerebellar peduncle near its decussation and projects to the contralateral inferior olivary nucleus.
lateral lemniscus
A flattened ribbon of fibers on the lateral surface of the rostral pontine tegmentum, arising from the cochlear nuclei and superior olivary nuclei. The lateral lemniscus is part of the ascending auditory pathway, conveying information from both ears to the inferior colliculus.
The Midbrain Includes the Superior and Inferior Colliculi, the Cerebral Peduncles, and the Cerebral Aqueduct
The midbrain is characterized by four bumps (the paired superior and inferior colliculi) on its posterior surface and by the large cerebral peduncles on its anterior surface. The oculomotor nerve (III) emerges from the interpeduncular fossa between the peduncles.
brachium of the inferior colliculus
(usually shortened to inferior brachium)
a broad, low ridge extending rostrally from the inferior colliculus. This is a continuation of the ascending auditory pathway, projecting from the inferior colliculus to the thalamic relay nucleus for hearing (the medial geniculate nucleus).
