Lecture 4: GROSS ANATOMICAL FEATURES OF THE BRAINSTEM AND FOREBRAIN

Question 1

The Long Axis of the CNS Bends at the _____

Answer 1

The Long Axis of the CNS Bends at the Cephalic Flexure

Question 2

The human central nervous system (CNS) is composed of the ___ and ____ .

Answer 2

The human central nervous system (CNS) is composed of the brain and spinal cord

Question 3

sagittal plane

Answer 3

divides the brain into 2 symmetrical halves

Parasagittal planes are those parallel to the sagittal plane.

Question 4

Coronal planes

Answer 4

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).

Question 5

Axial planes

Answer 5

also called transverse or horizontal planes

are perpendicular to the long axis of the body.

Question 6

directional terms such as anterior, dorsal, and rostral change their meanings relative to one another in different parts of the nervous system.

Answer 6

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.

Question 7

cephalic flexure

Answer 7

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.

Question 8

brainstem

Answer 8

the midbrain, pons, and medulla.

Question 9

midbrain

Answer 9

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.

Question 10

pons

Answer 10

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.

Question 11

medulla

Answer 11

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.

Question 12

corpus callosum

Answer 12

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).

Question 13

The nervous system develops embryologically from a ____ ___; the cavity of the tube persists in adults as a system of ____.

Answer 13

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.

Question 14

third ventricle

Answer 14

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.

Question 15

lateral ventricle

Answer 15

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.

Question 16

interventricular foramen

Answer 16

AKA, the foramen of Monro

The narrow orifice between each lateral ventricle and the third ventricle.

Question 17

cerebral aqueduct

Answer 17

AKA, the aqueduct of Sylvius

narrow channel through the midbrain that connects the third ventricle to the fourth ventricle. It is filled with CSF.

Question 18

fourth ventricle

Answer 18

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).

Question 19

central canal

Answer 19

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.

Question 20

Humans, Relative to Other Animals, Have Large Brains

Answer 20

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.

Question 21

Characteristic Gross Anatomical Features of The Medulla

Answer 21

The Medulla Includes Pyramids, Olives, and Part of the Fourth Ventricle

Question 22

Characteristic Gross Anatomical Features of The Pons

Answer 22

The Pons Includes the Basal Pons, Middle Cerebellar Peduncles, and Part of the Fourth Ventricle

Question 23

Characteristic Gross Anatomical Features of The Midbrain

Answer 23

The Midbrain Includes the Superior and Inferior Colliculi, the Cerebral Peduncles, and the Cerebral Aqueduct

Question 24

The Brainstem Intro

Answer 24

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.

Question 25

The Brainstem Has Conduit, Cranial Nerve, and Integrative Functions

Answer 25

Brainstem activities may be divided (roughly) into 3 general types: conduit functions, cranial nerve functions, and integrative functions.

Question 26

The Brainstem’s Conduit Functions

Answer 26

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.

Question 27

The Brainstem’s Cranial Nerve Functions

Answer 27

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.

Question 28

The ___ and ___ cranial nerves project directly to the telencephalon and diencephalon, respectively, but the others project to or emerge from the _____.

Answer 28

The olfactory and optic cranial nerves project directly to the telencephalon and diencephalon, respectively, but the others project to or emerge from the brainstem.

Question 29

Cranial Nerve I

Answer 29

Sensory Function only.

Smell.

Question 30

Cranial Nerve II

Answer 30

Sensory Function only.

Sight.

Question 31

Cranial Nerve III

Answer 31

oculomotor nerve

Motor Function only.
Eye movements, pupil and lens function.

Question 32

Cranial Nerve IV

Answer 32

Motor Function only.

Eye movements

Question 33

Cranial Nerve V

Answer 33

trigeminal nerve

Sensory Function:
Facial sensation.

Motor Function:
Chewing.

enters the brainstem at a midpontine level

Question 34

Cranial Nerve VI

Answer 34

abducens nerve

Motor Function only.
Eye movements.

Question 35

Cranial Nerve VII

Answer 35

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).

Question 36

Cranial Nerve VIII

Answer 36

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.

Question 37

Cranial Nerve IX

Answer 37

glossopharyngeal nerve

Sensory Function:
Taste.

Motor Function:
Swallowing.

Question 38

Cranial Nerve X

Answer 38

vagus nerve

Sensory Function:
Thoracic & abdominal viscera.

Motor Function:
Speech, swallowing;
thoracic and abdominal viscera.

Question 39

Cranial Nerve XI

Answer 39

Motor Function only.

Head and shoulder movements

Question 40

Cranial Nerve XII

Answer 40

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.

Question 41

reticular formation

Answer 41

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.

Question 42

medulla anatomy

Answer 42

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.

Question 43

pyramidal decussation

Answer 43

The site, located at the spinomedullary junction, at which most fibers in each pyramid cross the midline to form the contralateral lateral corticospinal tract.

Question 44

anterior median fissure

Answer 44

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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.

Question 45

olive

Answer 45

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.

Question 46

spinal tract of the trigeminal nerve

Answer 46

the spinal tract of the trigeminal nerve is the head’s equivalent of Lissauer’s tract.

Question 47

cuneate tubercle

Answer 47

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.

Question 48

gracile tubercle

Answer 48

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.

Question 49

vestibular area

Answer 49

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.

Question 50

vagal trigones

Answer 50

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.

Question 51

hypoglossal trigone

Answer 51

A triangular elevation in the floor of the caudal fourth ventricle formed by the underlying hypoglossal nucleus.

Question 52

facial colliculus

Answer 52

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.

Question 53

The Pons Anatomy

Answer 53

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.

Question 54

middle cerebellar peduncle

Answer 54

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”).

Question 55

basal pons

Answer 55

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.

Question 56

brachium conjunctivum

Answer 56

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.

Question 57

superior cerebellar peduncle (brachium conjunctivum)

Answer 57

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.

Question 58

lateral lemniscus

Answer 58

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.

Question 59

The Midbrain Includes the Superior and Inferior Colliculi, the Cerebral Peduncles, and the Cerebral Aqueduct

Answer 59

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.

Question 60

brachium of the inferior colliculus

Answer 60

(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).