BIO210 Weekly Guide #9
CENTRAL NERVOUS
SYSTEM
After completing this laboratory you should be able to:
1) identify the major divisions of the brain, as well as the major structures within each division
2) provide a simple statement of the major known function(s) of each brain region
3) recognize and describe the structure of the spinal cord in toto and in cross sections
4) recognize neural tissue in histological section and be able to state which from region of the central nervous system each representative sample was taken
5) describe the major functional parts of the neuron and synapse
6) describe the fundamental cellular, electrochemical and neurochemical mechanisms underlying the resting potential, action potential initiation and propagation, neurotransmitter release, postsynaptic potentials, and somatic generator potentials
7) Relate these basic mechanisms to the function of nerve cells in processing information in the nervous system
Guide to Gross Anatomy Guide to Histology Guide to Physiology
Outline
I. Nervous Tissue
A. Tissue organization {FAP 12-1}
4th basic tissue type
cell body aggregates -- nuclei, ganglia
cell process aggregates -- fiber tracts, nerves
complex structures -- laminae, cortex, neuropil
B. Neurons {FAP 12-2}
function - specialization for excitation and information transfer/integration
cell body (soma, perikaryon)
function
structure: nucleus, nucleolus, Nissl substance (rough ER), Golgi, mitochondria
dendrites
function
branches, spines
axon
function - specialization for propagation of information without degradation
structure : axon hillock, myelination, nodes of Ranvier, terminal arborization,
boutons, collaterals, neurofilaments, synapses
synapses (structural and functional unit of the nervous system)
Function - chemical transmission
structure - presynaptic and postsynaptic
types of neurons
unipolar - embryologic
pseudounipolar - e.g. DRG cells
bipolar - e.g. retina, spiral ganglion
multipolar - e.g. anterior horn cells, pyramidal cells, Purkinje cells,
most projection neurons and interneurons
C. Glial cells {FAP 12-3}
functions - support, protection, defense, nourishment, insolation
types of glial cells
macroglia
astrocytes - nourishment
oligodendrocytes - myelination of CNS fiber tracts
Schwann cells - myelination of PNS nerves
microglia - macrophages
ependymal cells - line ventricles
D. Nervous system divisions {FAP 12-1}
central vs. peripheral
somatic vs. visceral
II. Neuronal Function
A. Basic electrochemistry {FAP 12-4}
electrical potentials, conductance, current, and "mobile ions"
electrical properties of the cell membrane
gradients - electrical, chemical, and electrochemical
B. The resting potential {FAP 12-4}
intracellular proteins - Gibbs-Donnan Equilibrium
sodium and potassium
pumps - electrogenic potential
gradients - ionic equilibrium potentials
conductances - Ohmic/Nernstian potential
C. Action potentials {FAP 12-5, 12-6}
generation
fundamental properties
shape and timing
all-or-none
threshold
refractory periods
frequency coding
propagation
size and myelination
D. Synaptic transmission {FAP 12-7}
presynaptic sequence and neurotransmitter release
postsynaptic events
receptor/channel complexes
postsynaptic potentials
the synaptic cleft
E. Somatic processes {FAP 12-9}
summation and integration
generator potentials and passive spread
the axon hillock
F. Neurotransmitters vs. neuromodulators {FAP 12-8}
III. Central Nervous System - Brain {FAP14-1 to 14-9}
A. Development
neural tube
3 primitive vesicles - pros- , mes- , rhombencephalon
gray vs. white matter
B. Prosencephalon (forebrain)
Telencephalon
lateral ventricles (I & II)
anterior, posterior, and inferior horns
cerebral cortex
lobes
frontal, parietal, occipital, temporal, cingulate,
insular, limbic,
fissures
midsagittal, temporal (lateral)
gyri
precentral and postcentral
sulci
central
parieto-occipital
limbic system
olfactory bulbs and tracts, hippocampus, amygdala,
cingulate gyrus, septal nuclei, fornix, diencephalic structures
(mammilary bodies, thalamus)
subcortical white matter
corona radiata, corpus callosum, ant. & post. commisures,
internal capsule, fornix
basal ganglia (striatum)
caudate nucleus
lentiform nucleus
putamen & globus pallidus
Diencephalon
ventricle III
interventricular foramina of Monro
thalamus
sulcus limitans, massa intermedia
hypothalamus
mammilary bodies, pituitary stalk
epithalamus (pineal gland)
pituitary - neurohypophysis vs. adenohypophysis
optic chiasm
C. Mesencephalon (midbrain)
cerebral aqueduct
cerebral peduncles
corpora quadrigemina
superior colliculus
inferior colliculus
D. Rhombencephalon (hindbrain)
ventricle IV
foramina of Magendie and Luschka
Metencephalon
cerebellum
hemispheres, vermis
folia
cerebellar peduncles
arbor vitae
pons
Myelencephalon (medulla oblongata)
pyramids
IV. Central Nervous System - Spinal Cord {FAP 13-1 to 13-3}
A. Meninges
B. Central canal
C. Gray matter
anterior (ventral), posterior (dorsal), and lateral horns
D. White matter
anterior (ventral), posterior (dorsal), and lateral funiculi
ventral and dorsal roots
dorsal root ganglia
E. Conus medullaris
F. Spinal nerve roots, cauda equina, filum terminale
Gross Anatomy List
Special Note: Due to the complexity of the CNS and the difficulty in doing justice to this complexity using just bad drawings on the board, the instructor will probably choose to present most of the gross anatomy lecture as an extended demonstration using models and brain slices, as well as class dissection of preserved sheep brains. Therefore, all of the gross anatomy structures in the preceding lecture outline (sections II and III) should be considered part of the Gross Anatomy List.
The Brain: see lecture outline above and the guide below
The Spinal Cord: see lecture outline above and the guide below
Guide to Gross Anatomy
Central Nervous System Organization
For each brain structure mentioned below, in the lecture outline, or in the gross anatomy list you should be able to do the following:
Identify the structure in prepared brain specimens, and models
Know in which of the 5 major brain regions it is located
Know with which of the 3 primitive brain vesicles it is associated
Know its spatial relationship to the ventricle of that region
Know its basic function
The importance of understanding that the central nervous system develops from a neural tube cannot be stressed strongly enough. It is the key to understanding the final adult structure of the brain.
a) The three primitive brain vesicles which develop from the neural tube, the associated five major brain regions, the major structures in each region, and the corresponding ventricular structures are: {FAP Fig. 14-5; APL Fig. 13.9}
Prosencephalon (forebrain)
Telencephalon Ventricle I&II (lateral ventricles)
cerebral cortex
subcortical white matter
basal ganglia
Diencephalon Ventricle III
thalamus (& epithalamus)
hypothalamus (& pituitary)
Mesencephalon (midbrain) Cerebral Aqueduct
cerebral peduncles
superior colliculi
inferior colliculi
Rhombencephalon (hindbrain) Ventricle IV
Metencephalon
pons
cerebellar peduncles
cerebellum
Myelencephalon
medulla oblongata
- Study the cast of the ventricles. Locate the following: {FAP Fig. 14-2; APL Fig. 13.6}
lateral ventricles (I&II) cerebral aqueduct
anterior horn ventricle IV
posterior horn foramina of Luschka
inferior horn foramen of Magendie
interventricular foramina of Monro central canal of the spinal cord
ventricle III
- Study the ventricular model again, long and hard, until you can draw it accurately from memory. No kidding. The ventricular system reflects the development of the lumen of the neural tube, and it is your best guide to understanding the structure of the brain. As you learn the location of each brain structure, pay particular attention to its relationship to the ventricular system.
b) When you study a brain slice you should always begin by identifying the ventricular structure(s) in the slice. This should tell you the location and orientation of the slice,what major regions of the brain are represented, and what brain structures you should expect to see.
Prosencephalon {FAP Figs. 141-10 to 14-16; APL Fig. 13.7. 13.8, 13.9, 13.12}
The prosencephalon, or forebrain represents the anterior end or the neural tube. It develops into the telencephalon and diencephalon. In the region of the telencephalon, the growing neural tube splits laterally. The two tube ends grow in an outwardly curving spiral, like a ram's horns, to form the lateral ventricles. The tissue surrounding the lateral ventricles undergoes tremendous development to form the cerebral hemispheres. The cerebral cortex develops from the outer margin of this spiral. The tissue on the inner margin of the spiral becomes the striatum, or basal ganglia. The diencephalon develops around the third ventricle. As the name suggests, there are two principal regions of the diencephalon, each a collection of nuclei and fiber tracts. These are the thalamus and the hypothalamus.
a) Locate the lateral ventricles in the midsagittal sections and brain slices. Follow the paths of the ventricles as they curve from the inferior horns to the anterior horns. Notice that the curvature of the ventricles causes them to appear twice (superior curvatures and inferior horns) in some coronal sections.
b) On the models, locate the following cortical lobes of each cerebral hemisphere:
frontal temporal insular
parietal cingulate limbic
occipital
Locate the also following structures:
midsagittal fissure calcarine sulcus precentral gyrus
lateral sulcus (Sylvian fissure) central sulcus postcentral gyrus
parieto-occipital sulcus
- What lobes do each of the above fissures and sulci separate? Note that the superficial lobes of the cerebral cortex are named for the bones under which they lie, not vice-versa.
- What is the function of the postcentral gyrus? What is the function of the precentral gyrus? How does the body "map" onto each of these gyri? With which sensory systems are the occipital and temporal lobes most closely associated?
- Which horns of the lateral ventricles correspond to the frontal, occipital, and temporal lobes?
- The limbic lobe is particularly hard to understand in an introductory class. This primitive lobe is part of a system of deep cortical and diencephalic structures - the limbic system. In most vertebrates this system is most closely associated with the sense of smell. In mammals the most direct sensory input is still from the olfactory bulbs, however, the limbic system seems to be primarily involved in motivational and emotional control. Its major pathways follow a circular circuit - the "Papèz circuit".
- Find the hippocampus on the limbic system model. This is a deep, primitive cortical structure which is strongly involved in memory. The location of the hippocampus is difficult to visualize. It closely follows the outer curvature of each lateral ventricle. Along the superior curvature of the ventricle the corpus callosum (see below) has grown through the hippocampus and obliterated it, but it is still present in the regions of the posterior curvature and inferior horns of the ventricle.
c) The subcortical white matter consists of masses of nerve fibers which connect the cortex with lower brain and spinal cord structures, and connect widely separated regions of cortex with each other.
- In the prepared coronal brain slices and brain stem model, study the pattern of fibers which radiate outward from the brain stem to reach the cerebral cortex. This is the corona radiata. It may also be seen clearly in the prepared brain which has the cortex stripped away. You should recognize that some of these fibers are descending from the cortex, while the rest are ascending into it.
- Locate the corpus callosum in the midsagittal and coronal brain sections and the models. It is the principal fiber tract connecting the two cerebral hemispheres. Locate the genu (knee) and splenium (tail) regions in sagittal section.
- Locate the anterior and posterior commissures in the sagittal sections of brains and models. These carry cortical, diencephalic, and midbrain fibers across the midline. Locate also the fornix, a major subcortical fiber tract which connects cortical and thalamic structures with the mammilary nuclei of the hypothalamus.
- The septum pellucidum is a thin sheet of limbic gray matter which lies along the midline of the anterior telencephalon, between the superior curvatures of the lateral ventricles and bounded by the genu of the corpus callosum. It is part of the limbic system.
d) Locate the basal ganglia, just lateral to the lateral ventricles in the brainstem model and brain sections. The three paired nuclei are the globus pallidus, the putamen, and the caudate.
- What is the primary collective function of the basal ganglia?
- Note the fibers of the internal capsule which separate the caudate from the lentiform (globus pallidus + putamen). These are fibers ascending and descending between the corona radiata and the cerebral peduncles, i.e. to and from the cortex.
e) The diencephalon surrounds the third ventricle. Its primary divisions are the thalamus and the hypothalamus.
- The sulcus limitans is longitudinal groove in the primitive neural tube wall which separates the dorsal sensory cells from the ventral motor cells. This groove is still apparent in the lateral wall of the 3rd ventricle, where it separates the dorsal thalamus (sensory) and ventral hypothalamus (visceral motor, sort of). Locate the sulcus limitans in midsagittal section models and prepared brains.
- Locate the interventricular foramina of Monroe which connect the lateral ventricles to the 3rd ventricle.
- The optic chiasm, ventral to the ventricle floor, marks the partial decussation (crossing) of optic nerve fibers running from the retina to the thalamus and superior colliculi. Locate this structure in the models, brains, and slices.
f) Locate the hypothalamus on the inferiolateral wall of the third ventricle. Some hypothalamic nuclei are involved in homeostatic visceral control of such things as body temperature, blood osmolarity, and blood glucose levels. Other hypothalamic nuclei are involved in regulating the hormones of the anterior pituitary. Still others contain neurosecretory cells which release their hormones from long processes extending into the posterior pituitary. Several fiber tracts from deeper brainstem nuclei to telencephalic structures pass through the hypothalamus.
- Trace the hypothalamus back through the brain in coronal sections. Two handy landmarks on the ventral surface of the brain are the optic chiasm and the mammilary bodies, which mark the anterior and posterior borders of the hypothalamus.
- Study the pituitary in the models. Locate the anterior and posterior lobes and the infundibulum (pituitary stalk). Locate the infundibulum also on the prepared brains and brain sections. We will deal more with the pituitary during the our study of the endocrine system.
g) Locate the thalamus on the superiolateral wall of the 3rd ventricle of the midsagittal sections. The various thalamic nuclei lie in the primary sensory pathways of all of the senses except olfaction. Fibers ascending from the sense organs (vision) or lower brainstem nuclei (other senses) synapse with thalamic neurons which project to the primary sensory cortical areas.
- Trace the thalamus back through the brain in the coronal sections. Notice that it is always just lateral to the 3rd ventricle. Notice also that the bulk of the thalamus is posterior to, as well as dorsal to the hypothalamus. Locate the massa intermedia which bridges across the midline near the center of the third ventricle.
- Locate the epithalamus at the posteriosuperior margin of the thalamus. The primary specialized structure here is the endocrine pineal gland.
Mesencephalon {FAP Fig. 14-9; APL Fig. 13.9, 13.10}
The mesencephalon, or midbrain, surrounds the cerebral aqueduct. In coronal sections it lies ventral to the posterior thalamic nuclei. The dominant structures of the aqueduct roof are the corpora quadrigemina. The dominant structures of the floor are the cerebral peduncles. The midbrain also contains scattered nuclei involved with oculomotor control (NIII & NIV) and some visceral functions.
a) Locate the cerebral aqueduct in midsagittal and coronal sections. Because of the upright stance of humans, the ventricular system must undergo a right angle bend between ventricle III and the central canal of the spinal cord. Hence, the cerebral aqueduct cuts obliquely through coronal sections of the brain.
b) Locate the paired superior and inferior colliculi, collectively called the corpora quadrigemina. The superior colliculi are involved in visual reflexes and control of eye movements. The inferior colliculi lie in the primary auditory pathway.
c) Locate the massive cerebral peduncles. These carry fibers through the midbrain to and from the cerebral hemispheres. Inferiorly they arise from the medulla, while superiorly they cut through the basal ganglia to fan out as the corona radiata.
Rhombencephalon {FAP Fig. 14-6 to 14.8; APL Fig. 13.9, 13.10}
The rhombencephalon, or hindbrain, is named for the rhomboidal (diamond) shape of the fourth ventricle. The roof of the ventricle is an extremely thin sheet in the adult; all of the important neural structures of this region are derived from the floor. In the adult brain the ventricle runs almost vertically from its rostral to its caudal end. The two major regions than develop from the rhombencephalon are the metencephalon and the myelencephalon.
The major structures of the metencephalon are the pons, cerebellar peduncles, and the cerebellum. The major structure of the myelencephalon is the medulla oblongata.
a) The pons is a broad region of horizontal fibers and nuclei which lies on the ventral surface of the metencephalon. The superficial fibers radiate laterally and dorsally to form the middle cerebellar peduncle. Longitudinal fibers running from the medulla into the cerebral peduncles run deeper. The nuclei of the pons are involved in respiratory control, cardiac control, and the functions of cranial nerves V-VIII. Locate the pons on the prepared brains and models.
b) The cerebellum is a cortical structure involved in motor control and coordination. Each hemisphere arises from the cerebellar peduncles, extends laterally, then dorsally to meet and fuse over the dorsal midline, having completely surrounded the real roof of the 4th ventricle in the process. Deep to the cortical mass of the cerebellum are the cerebellar nuclei which serve as relay stations for cerebellar input and output. Locate the following structures of the cerebellum in the models and brain preparations:
hemispheres inferior cerebellar peduncle
vermis middle cerebellar peduncle
folia superior cerebellar peduncle
arbor vitae
- Note that the bulk of the cortical mass of the cerebellum lies in the two lateral hemispheres. The vermis is a small ridge of cortical tissue along the midline.
- The superior, middle, and inferior cerebellar peduncles carry fibers which connect the cerebellum to higher brain structures, the pons, and the medulla & spinal cord, respectively.
- The useful space of cortical structures is surface area. The folia of the cerebellum are long, parallel, leaf-like folds which greatly increase the cerebellar surface area. What are the analagous folds of the cerebrum called?
- Study the arbor vitae, the pattern of radiating white matter (fibers) seen in sagittal sections of the cerebellum. What is the analagous pattern of white matter in the cerebrum called?
c) The medulla oblongata of the myelencephalon floor is essentially a transition between the organizational schemes of the spinal cord and the brain. It consists of fibers of passage, and scattered nuclei. The dominant structures which you should locate are the pyramids. These bulges mark the level at which many ascending and descending fibers decussate, or cross over to the contralateral side. The medulla also contains nuclei associated with visceral functions and cranial nerves IX-XII.
Spinal Cord {FAP Fig. 13-2 to 13-5}
The spinal cord develops from the caudal remainder of the neural tube, and maintains a more obvious tube-like architecture.
a) Locate the following external structures on the cat spinal cord and models:
{APL Fig. 13.16}
ventral median fissure ventral roots cauda equina
ventral lateral sulci dorsal roots conus medullaris
dorsal median sulcus dorsal root ganglia filum terminale
dorsal lateral sulci spinal nerves anterior spinal artery
- Identify the dorsal (posterior) and ventral (anterior) surfaces of the spinal cord. Note that the ventral median fissure is a broader groove than the dorsal median sulcus. Note also that the anterior spinal artery runs in the ventral median sulcus.
- Note that the spinal cord is considerably shorter than the spinal column. At what
vertebral level is the conus medullaris? Why do you suppose that spinal taps (CSF samples) are taken below this level? The spinal nerves fan out below this point in the shape of a horse's tail, hence the name cauda equina. What is the filum terminale?
- Note the enlargements of the cord in the cervical and lumbar regions. What is the functional significance of these enlargements?
- Do the dorsal root ganglia contain sensory or motor neurons? Note that the dorsal or ventral root of each spinal nerve exits the cord as a row of rootlets in the dorsal or ventral lateral sulcus. We will return to the spinal roots, ganglia, rami, and nerves again next week.
b) Locate the following regions in the spinal cord cross-section models:
{APL Fig. 13.17, 13.18}
white matter: gray matter: central canal
dorsal funiculi dorsal (posterior) horns dorsal medial septum
lateral funiculi ventral (anterior) horns
ventral funiculi
- The central canal is continuous with the fourth ventricle and is filled with CSF.
- The dorsal funiculi carry ascending sensory fibers while the lateral and ventral funiculi carry both ascending sensory (superficial) and descending motor (deep) fibers.
- In which of the horns of the gray matter would you expect to find motor neurons? What kinds of neurons would you expect to find in the dorsal horns? In what regions of the spinal cord would you expect to find lateral horns?
c) The spinal cord, like the brain, in invested in three layers of meninges. The pia is adherent to the surface of the cord and carries the blood vessels. The arachnoid surrounds the subarachnoid space which is filled with CSF. The dura is only a single layer which is attached at specific points to the periosteum of the vertebral canal by the denticulate ligaments. The dura is separated from the periosteum by a layer of adipose tissue. Identify the dura in the spinal cord cross-section model.
Guide to Histology
In spite of the fact that there are only two basic cell types, neurons and neuroglia, central nervous system functional histology is dizzyingly complex. For example, by conservative estimate there are perhaps 5,000,000,000 neurons, each of which makes synaptic contact with an average of 1,000 others. From the lecture and readings you are expected to get no more than a general appreciation of this complexity.
However, you should specifically be able to:
1) Define the following neuron cytology terms: soma (perikaryon), Nissl substance, dendrite, axon, axon hillock, terminal arborization, synapse, myelin.
2) Define the following CNS histological terms: gray matter, white matter, nucleus, fiber tract, lamina, cortex, dorsal and ventral horns, dorsal and ventral roots.
3) State the function of the following glial types: astroglia, oligodendrocytes, microglia, ependyma.
4) Define and give examples of the following neuron types: unipolar, pseudounipolar, bipolar, multipolar, principal (projection) neuron, interneuron.
5) Recognize the following three principal neuron types and state in what region of the CNS each occurs: pyramidal cell, Purkinje cell, ventral horn cell (anterior horn cell).
Work through the following CNS histology preparations: {APL Section 5-4}
a) Brain
Examine the slides of the whole rat brain (sagittal section, cerebral cortex, and cerebellum).
- In the outer layers of the brain, try to distinguish gray and white matter. Identify the choroid plexus in the lateral ventricle. What is its function?
- Examine the cells of the cortical gray matter under high power. Identify the pyramidal neuron nuclei and glial nuclei. Note the prominent nucleolus in the neurons. In which direction do the axons project? How would you classify pyramidal cells by neuron type?
- Golgi silver stains are silver impregnation methods that apply a black or golden precipitate to a few (1% to 10%) of the neurons present. For those cells which do stain, both the perikarya and the processes take up the stain, providing one method of tracing neuronal processes. In the cerebral cortex slides, find a large, multipolar neuron and try to follow their dendrites and axons until they pass out of the plane of section.
- Identify the large teardrop-shaped Purkinje cells in the cerebellum slides. Note that they form a distinct layer between the outer "molecular" and inner "granule" layers of the cerebellar cortex. In which direction do their axons project? How would you classify these neurons? Compare them to the pyramidal cells of thecerebrum.
b) Spinal Cord
Distinguish the white matter and gray matter in the spinal cord. Identify the dorsal and ventral horns, as well as the entering and exiting fibers of the dorsal and ventral roots.
- Study one of the large multipolar neurons of the gray matter. Note the large pale nucleus and prominent darker nucleolus. Note also the dark granules in the cytoplasm, called Nissl substance. These granules are actually rough endoplasmic reticulum. What is their function? Why must a nerve have large numbers of these organelles?
- The largest neurons of the ventral horn are called ventral horn cells. What is their function? Where do their axons project?
- The small central canal of the spinal cord is lined by simple cuboidal to columnar cells, the ependyma. These have been variously classified either as epithelial cells or as neuroglia. What fluid fills the central canal?
Guide to Physiology
For physiology this week review tha basic structure and function of nerve cells, electrical potentials, synaptic transmission, and neuronal integration in APL Unit 12.