Chapter 15- The Brain and Cranial Nerves

Yay!  We made it to the brain! Let’s start by looking at how it’s formed.

 

The central nervous system starts as a hollow neural tube, with a fluid-filled internal cavity called the neurocoel. In the fourth week of development, three areas in the cephalic portion of the neural tube enlarge rapidly through expansion of the neurocoel. This enlargement creates three prominent primary brain vesicles, which are named for their relative positions.

 

1. The prosencephalon (or forebrain)
2. The mesencephalon (or midbrain)
3. The rhombencephalon (or hindbrain).

 

These divisions actually divide further. The prosencephalon (forebrain) and rhombencephalon (hindbrain) are subdivided further, forming secondary brain vesicles. The prosencephalon (forebrain) forms the telencephalon and the diencephalon. The telencephalon forms the cerebrum.

 

The hollow diencephalon is kind of like a house. It has a roof (the epithalamus), walls (the left and right thalamus) and a floor (the hypothalamus). The portion of the rhombencephalon (hindbrain) closest to the mesencephalon (midbrain) forms the metencephalon. The ventral portion of the metencephalon develops into the pons, and the dorsal portion becomes the cerebellum. The portion of the rhombencephalon (hindbrain) closer to the spinal cord becomes the myelencephalon, which will form the medulla oblongata.

 

Now, the major regions and landmarks.

 

There are six major divisions in the adult brain:

 

1. The cerebrum
2. The diencephalon
3. The mesencephalon
4. The pons
5. The cerebellum
6. The medulla oblongata

 

 
The cerebrum is divided into large, paired cerebral hemispheres, separated by the longitudinal fissure. This is where the conscious thought processes, intellectual functions, memory storage and retrieval and complex motor patters originate.

 
The diencephalon is the deep portion of the brain attached to the cerebrum. It has three subdivisions (which I mentioned earlier, but now we’re going to learn what they do).

 

1. The epithalamus contains the hormone-secreting pineal gland (which is an endocrine structure)
2. The thalamus (both right and left) are sensory information relay and processing centers.
3. The hypothalamus is a visceral control center. This division contains centers involved with emotions, autonomic functions, and hormone production. It’s the primary link between the nervous and endocrine systems. 

 

The remaining regions of the brain are collectively known as the brain stem. This consists of the mesencephalon, pons, and medulla oblongata. The brain stem contains important processing centers and also relays info to or from the cerebrum or cerebellum.

 

The mesencephalon (midbrain) process visual and auditory information and coordinate and direct reflexive somatic motor responses to these stimuli. It also contains centers involved with the maintenance of consciousness.

 

The pons is immediately inferior to the mesencephalon. It contains nuclei involved with both somatic and visceral motor control. The term pons refers to a bridge, and the pons connects the cerebellum to the brain stem. The cerebellum automatically adjusts motor activities on the basis of sensory information and memories of learned patterns of movement.

 

The medulla oblongata relays sensory information to the thalamus and to other brain stem centers. It also contains major centers concerned with the regulation of autonomic function, such as heart rate, blood pressure, and digestive activities. The superior portion of the medulla oblongata has a thin, membranous roof, and the inferior portion resembles the spinal cord.

 

Organization of gray and white matter in the brain stem:

 

The general distribution of the gray matter in the brain stem resembles that of the spinal cord, in that there is an inner region of gray matter surrounded by tracts of white matter. The gray matter surrounds the fluid filled ventricles and passageways that correspond to the central canal of the spinal cord. Although tracts of white matter are present, the arrangement is not as predictable as it is in the spinal cord. In the cerebrum and cerebellum, the white matter is covered by neural cortex, which is a superficial layer of gray matter.

 

Ventricles are fluid-filled cavities (like the oceans) within the brain. They are filled with CSF and lined with ependymal cells. There are four ventricles in the brain, one within each cerebral hemisphere, a third in the diencephalon, and the fourth lies between the pons and cerebellum and extends into the superior portion of the medulla oblongata.The lateral ventricles in the cerebral hemisphere are separated by the septum pellucidum. The cavity within the diencephalon is known as the third ventricle. The third and fourth ventricles are connected by a slender canal known as the aqueduct of the midbrain. The fourth ventricle goes into cerebral canal of the spinal cord.

 

MENINGES! I just love that word. The cranial meninges provide protection; act as shock absorbers that prevent contact with surrounding bones. These are like the spinal meninges, because they have the same three layers: the dura mater, arachnoid mater, and pia mater.

 

The cranial dura mater is made of up two fibrous layers. The outermost layer (or endosteal layer) is fused to the periosteum lining the cranial bones. The innermost layer is called the meningeal layer.

 
There are four locations in the meningeal layer of the cranial dura mater where they extend deep into the cranial cavity.

 

1. The falx cerebri is a fold of dura mater that projects between the cerebral hemispheres in the longitudinal fissure. Two large venous sinuses, the superior saggittal sinus and inferior saggittal sinus travel within this fold.
2. The tentorium cerebelli separates and protects the cerebellar hemispheres from those of the cerebrum. The transverse sinus lies within the tentorium cerebelli.
3. The falx cerebelli extends in the midsaggittal line inferior to the tentorium cerebelli, dividing the two cerebellar hemispheres.
4. The diaphragma sellae is a continuation of the dural sheet that lines the sella turcica of the sphenoid.

 

Arachnoid Mater- just a couple quick things:

 Deep to the arachnoid mater is the subarachnoid space, which contains a delicate, web like meshwork of collagen and elastic fibers that link the arachnoid mater to the underlying pia mater. There are also arachnoid granulations, which are where fingerlike extensions of the cranial arachnoid mater penetrate the dura mater. At these projections, cerebrospinal fluid flows past bundles of fibers (the arachnoid trabeculae) crosses the arachnoid mater, and enters the venous circulation.

 

The cranial pia mater is tightly attached to the surface contours of the brain, following its contours and lining the sulci. The pia is anchored to the surface of the brain by the processes of astrocytes.

We’re now going to look at the Blood-Brain Barrier a little more closely:

The barrier provides a means to maintain a constant environment, which is necessary for both control and proper functioning of CNS neurons. The BBB remains intact throughout the CNS, except in three places (check out page 385 for the full description).

 

Cerebrospinal Fluid

 Cerebrospinal fluid completely surrounds and bathes the exposed surfaces of the central nervous system. As I mentioned earlier, it has several important functions.

 

1. Cushioning delicate neural structures
2. Supporting the brain
3. Transporting nutrients, chemical messengers, and waste products.

 

The choroids plexus (which consist of a combination of specialized ependymal cells and permeable capillaries) is responsible for the production of cerebrospinal fluid.

 

Cerebrum

 The cerebrum is the largest region of the brain. Remember that it consist of the paired cerebral hemispheres, which rest on the diencephalon and brain stem. A thick blanket of neural cortex covers the cerebral hemispheres that form the superior and lateral surfaces of the cerebrum. The cortical surface forms a series of elevated ridges, or gyri, separated by shallow depressions, called sulci or deeper grooves, called fissures.

 

The Cerebral Lobes:

 The book tells us to remember these three facts about the lobes

 

1. Each cerebral hemisphere receives sensory information from and generates motor commands to the opposite side of the body.
2. The two hemispheres have some functional differences, although anatomically they appear to be identical.
3. The assignment of a specific function to a specific region of the cerebral cortex is imprecise.

 

 Ok, a quick breakdown of the lobes and what they do (look at table 15.2)

 Frontal lobe- conscious control of skeletal muscles

Parietal lobe- conscious perception of touch, pressure, vibration, pain, temperature and taste

Occipital lobe- conscious perception of visual stimuli

Temporal lobe- conscious perception of auditory and olfactory stimuli

All lobes- integration and processing of sensory data, processing and initiation of motor activities.

 

Central White Matter

 The central white matter is covered by the gray matter of the cerebral cortex. It contains myelinated fibers that form bundles that extend from one cortical area to another that connect areas of the cortex to other regions of the brain. There are three kinds:  

 

1. Association fibers
2. Commissural fibers
3. Projection fibers

 
Association fibers interconnect portions of the cerebral cortex within the same cerebral hemisphere. Remember that the arcuate fibers are the shortest association fibers, and they curve in an arc. The longer association fibers are the longitudinal fasciculi, and are used for long distance communication. Table 15.3 has a pretty good description of these.

 Basal Nuclei are paired masses of gray matter within the cerebral hemispheres. There’s more, but Chapter 15 is so long that it’s already going to be two blogs, so I’m going to move on to….

 
The Limbic System

 The function of the limbic system include

1. Establishment of emotional states and related behavioral drives
2. The linking of conscious, intellectual functions of the cerebral cortex with the unconscious and autonomic functions of other portions of the brain
3. Facilitating memory storage and retrieval

 

Check out table 15.5 for a breakdown of all the components.

 
Diencephalon

 The diencephalon connects the cerebral hemispheres to the brain stem. It consists of the epithalamus, thalamus (left and right) and the hypothalamus. The posterior portion of the epithalamus contains the pineal gland, which is an endocrine structure that secretes the hormone melatonin. For those of you who were not already aware of this, melatonin is involved in the regulation of the day-night cycles.

 
The thalamus processes and relays sensory information. Check out table 15.6 for more info.

 
Hypothalamus contains centers involved with emotions and visceral processes that affect the cerebrum as well as other components of the brain stem. It also controls a variety of autonomic functions and forms a link between the nervous and endocrine systems.

 
The functions of the hypothalamus:

1. Subconscious control of skeletal muscle contractions
2. Control of autonomic functions
3. Coordination of activities of the nervous and endocrine systems
4. Secretions of hormones: antidiuretic (restricts water loss at the kidneys) and oxytocin (stimulates smooth muscle contractions in uterus and prostate gland, and myoepithelial cell contractions in mammary glands)
5. Production of emotions and behavioral drives
6. Coordination between voluntary and autonomic functions
7. Regulation of body temperature
8. Control of circadian rhythms

 

The Pons

 This extends inferior from the mesencephalon to the medulla oblongata. It contains:

1. Sensory and motor nuclei for four cranial nerves
2. Nuclei concerned with the involuntary control of respiration
3. Nuclei that process and relay cerebellar commands arriving over the middle cerebellar peduncles
4. Ascending, descending, and transverse tracts

 

 Keep in mind that the pneumotaxic center and apneustic center are important for breathing.

 
The Cerebellum

This has two primary functions: adjusting the postural muscles of the body and programming and fine tuning voluntary and involuntary movements.

The Medulla Oblongata

 The medulla oblongata physically connects the brain with the spinal cord, and all communication between the brain and the spinal cord involves tracts that ascend or descend through the medulla oblongata.

 
Nuclei in the medulla oblongata may be:

 

1. Relay stations
2. Nuclei of cranial nerves
3. Autonomic nuclei

 
Ok, I know that the cranial nerves are a part of this chapter. However, because they are so important, they are actually getting their own blog.

As mentioned earlier, if you have any compliments, questions, suggestions (but not complaints, I don’t have time to read any bitching, time is of the essence here!) please e-mail me at hcha0243@postoffice.uri.edu.