The Central Nervous System

The Brain

Coverings of the Brain

I. The Cranium – the bony vault of the skull.

II. Meninges – tough, moist membranes surrounding the brain and spinal cord. Infection of meninges is called meningitis.

A. Dura mater – outermost membrane. Very tough, protective layer consisting of:

1. The Periosteal layer – thick, outer portion, continuous with the periosteum of the cranial bone.

2. Meningeal layer- thin, inner portion. The spinal dura mater consists only of this layer.

3. Subdural space – only a potential space which may fill with fluid or blood (subdural hematoma).

B. Arachnoid layer – a fibrous, delicate middle zone. Below this layer is the subarachnoid space filled with cerebrospinal fluid.

C. Pia mater – innermost of the meningeal layers. It is closely adherent to the surface of the brain and spinal cord.

Cerebrospinal Fluid and the Ventricles of the Brain

Cerebrospinal fluid circulates through the subarachnoid space and the ventricles of the brain, as well as, around the spinal cord. CSF is manufactured by specialized blood vessels, the choroid plexi. These vessels are located within spaces of the brain called the ventricles. The brain contains four ventricles:

1. The Lateral ventricles – Each cerebral hemisphere contains a lateral ventricle (ventricles I and II). These ventricles are located immediately below the corpus callosum. They are separated from one another by a thin partition called the septum pellucidum.

2. The Third Ventricle – This ventricle is found below the lateral ventricles in the diencephalon. Each lateral ventricle is connected to ventricle III by a narrow opening – the Foramen of Monro.

3. The Fourth Ventricle – This ventricle is found between the brain stem and the cerebellum. The III and IV ventricles are connected by a channel called the Aqueduct of Sylvius. The floor of the IV ventricle has three small holes, called the Foramen of Magendie (medial opening) and the Foramina of Luschka (lateral openings) which open into the subarachnoid space.

Characteristics of CSF

1. It is a clear, colorless liquid.

2. Its total volume is only about 125 ml at any given time.

3. CSF contains proteins, glucose, urea, salts and leukocytes. Erythrocytes are absent unless there has been some trauma causing bleeding into the ventricles or subarachnoid space.

4. It acts as a shock absorbing fluid and distributes nutrients to the brain and meningeal tissue.

5. After being released from within the brain, CSF is absorbed by blood vessels on the surface of the brain and spinal cord.

The Cerebrum

The cerebrum is the largest and most highly developed portion of the human brain. The surface of the cerebrum shows raised ridges called gyri. The gyri are separated from each other by sulci, indentations in the surface.

Cerebral Hemispheres – The cerebrum is divided into two identical cerebral hemispheres. A deep indentation – the sagittal fissure – separates the hemispheres.

Cerebral Lobes – Each hemisphere is divided physically and functionally into four lobes. The lobes are named after the overlying cranial bones. The cerebral lobes comprise most of the cerebral cortex.

Lobe Functional Areas Function(s)

Frontal Primary Motor Cortex Initiates conscious motor control. Broca’s
area is located here on the left side only. It
coordinates the movements of the mouth,
tongue and larynx involved in speech.

Premotor Area Regulates skilled motor activities. This area

“instructs” the Primary Motor cortex.

Prefrontal Area Controls the elaboration of thought and

emotion; where personality and the intellect
reside. Site of the “prefrontal lobotomy”.

Parietal Primary Somatosensory Cortex Involved with the interpretation of sensory

and somatosensory association area information from various regions of the body,

especially the skin, i.e., General Sensation.

Occipital Primary Visual Cortex Responsible for the interpretation of visual

and visual association area input – Sight. It is the destination of the optic

nerve (CN II). Visual messages pass through
the thalamus on their way to the primary visual
cortex.

Temporal Primary Auditory Cortex Perception and interpretation of auditory input
and auditory association area .

Wernicke’s Area Lies at the border of the temporal, occipital and
parietal lobes on the left side only. Allows us to
put words to auditory, visual or even touch
sensations (Braille). This area turns these
sensations into meaningful ideas. Other visual
forms of communication include body language
and facial expressions.

Olfactory Area Receives and interprets odors. Is

the destination of the olfactory tract and CN I.

Basal Cerebral Ganglia (actually nuclei)

1. Nuclei buried deep in the cerebrum just superior to the thalamus.

2. The basal ganglia control the background, gross, intentional body movements which are always associated with the more precise movements of arms, hands, fingers and feet. For example, when you are picking up a pencil, the conscious reach and grasp actions are performed by the arm, hand and fingers. The cerebral nuclei position your shoulder and stabilize the arm to make the grasp more precise.

3. The basal nuclei help maintain body posture and muscle tone.

4. Component structures of the basal nuclei are the caudate nucleus, the globus pallidus and the putamen.

Cerebral Tracts – The principal tracts of white matter passing through the cerebrum:

1. Corpus callosum – a flat, sheet-like nerve tract connecting the right and left cerebral hemispheres. Contains more than 200 million axons.

2. Optic radiation – consists of bundles of axons connecting the thalamus to the occipital lobe. It relays visual signals from the eyes to the occipital lobe. As nerve fibers in the left optic nerve reach the optic chiasm, about one half of them cross over to the right side of the brain. The remaining fibers continue to the left side. From here, all visual nerve fibers enter the optic radiation and pass into the left occipital lobe. The same pattern is repeated on the other side of the brain.

3. Internal capsule – The internal capsule serves as a relay between the cerebral cortex and the lower brain and spinal cord.

Limbic System – The limbic system includes nuclei and nerve tracts along the border between the cerebrum and the diencephalon.

Components of the Limbic System

1. Hippocampus is a sea horse shaped region responsible for establishing new memories, especially those that are emotionally charges. Altzheimer’s affects this region first. In severe mental illness (schizophrenia) the hippocampus is reduced in size. Estrogen increases the synaptic connections between neurons here. This is thought to be responsible for greater emotional expression in women.

2. Fornix is a tract of white matter that connects the hippocampus with the hypothalamus.

3. Mamillary body is a nipple-like protrusion from the hypothalamus which receives fibers from the fornix. It processes sensory information and movements associated with eating, e.g., chewing, licking and swallowing.

4. Amygdala is crucial in the formation of emotions. It is highly connected to our learning and memory centers. The amygdala is central to most brain events involving fear.

The Diencephalon

The diencephalon is located directly under the corpus callosum. The third ventricle is found in the diencephalon. The diencephalon is composed of three major regions:

A. Thalamus – is a motor relay station for sensory information from the spinal cord to the cerebral cortex. All sensory information going to the brain (except for olfactory) has to make a pit stop in the thalamus.

1. All somatosensory signals (touch, pressure and pain) are relayed through the thalamus to the Somatic Sensory area of the parietal lobe.

2. Visual signals are relayed to the occipital lobe via the thalamus.

3. Auditory signals are relayed to the temporal lobe via the thalamus.

4. Signals for the coordination of muscle activity pass through the thalamus from the cerebellum and brain stem to the cerebral motor cortex and basal ganglia.

5. The thalamus is the largest portion of the diencephalon. It is Yo – yo shaped, i.e, has two large lateral portions connected in the center by the intermediate mass (massa intermedia). The space around the intermediate mass is the third ventricle.

B. Epithalamus – is located in the most superior and anterior portion of the diencephalon.

1. The choroid plexus of the third ventricle is found in the epithalamus.

2. The posterior commissure is found here. This is another nerve tract uniting the two sides of the brain.

3. The epithalamus contains the pineal body. The pineal’s function remains unclear. It produces melatonin, a substance that may affect the wake-sleep cycle, biorhythms, the onset of puberty and emotional states such as Seasonal Affective Disorder (SAD).

C. Hypothalamus is located just below the thalamus. It is a major center for the control of internal body functions through the activities of several nuclei:

1. Preoptic nucleus – controls body temperature.

2. Supraoptic nucleus – lies just above the optic nerves. It regulates the secretion of anti-diuretic hormone. This hormone helps to regulate the balance between fluids and electrolytes in the body (osmoregulation), as well as, blood pressure.

3. Paraventricular nucleus – manufactures the hormone oxytocin which stimulates the smooth muscle of the uterus, mammary glands and prostate gland to contract.

4. Medial nuclei – produce a sense of satiety or satisfaction from food intake when blood sugar rises.

5. Lateral regions of the hypothalamus – Stimulation of this area produces the feeling of hunger of thirst depending on the precise location stimulated.

6. The hypothalamus exerts a great deal of control over the function of the pituitary gland. Hormones produced in the hypothalamus regulate the release of hormones from the anterior pituitary. Oxytocin and ADH are actually manufactured in the hypothalamic nuclei. These hormones then move through nerve tracts in the infundibulum, a stalk-like structure, into the posterior pituitary for storage and release.

The Cerebellum

The cerebellum is located posterior to the pons and medulla oblongata. Through its nerve connections to the cerebrum and spinal cord, it coordinates all motor activity of the body.

Connecting Structures

1. Superior cerebellar peduncle – connects the cerebellum to the midbrain, diencephalon and cerebrum.

2. Middle cerebellar peduncle – connects the cerebellum to sensory and motor nuclei in the pons.

3. Inferior cerebellar peduncle – connects the cerebellum to the medulla oblongata.

Anatomy

1. Externally, the cerebellum is composed of a middle structure called the vermis and two cerebellar hemispheres.

2. Internally, the cerebellum exhibits a cortex and a subcortical white matter:

a. Cerebellar cortex – a 3 to 5 mm thick sheet of 30 billion nerve cells which covers the highly convoluted cerebellar surface. Very large Purkinje cells are located here. Each cell can synapse with several thousand incoming axons.

b. White matter – consists of bundles of axons forming tracts. This white matter shows a distinct branching configuration when seen in sagittal section. Early anatomists referred to this arrangement as the “arbor vitae” – the tree of life.

c. Deep nuclei are found in the center of each hemisphere. Axons arising from these cells carry messages out of the cerebellum.

d. The folds in the surface of the cerebellum are called Folia.

e. Damage to the cerebellum (stroke) will produce ataxia, a disturbance in balance.

The Brain Stem

The brain stem is the last major portion of the brain. It contains the centers for regulating physiological processes, e.g., respiration, blood pressure, heart rate and wake-sleep cycles.

A. Midbrain – is the uppermost region of the brain stem.

1. It is located at the junction of the cerebellum and pons.

2. It is a relay station for motor and sensory messages between parts of the brain and between the brain and spinal cord.

3. It contains the cerebral peduncles (little feet). These are ventrolateral bulges that contain sensory fibers going to the thalamic nuclei and motor fibers headed for the pyramidal tract of the medulla oblongata.

4. The tectum is the posterior region of the midbrain. Two pairs of rounded bulges, the corpora quadrigemina are found here. The quadrigemina consist of two superior colliculi which coordinate motor reflexes initiated by visual stimuli, e.g., turning the head towards something in our visual field. The two inferior colliculi coordinate motor reflexes due to auditory input, turning the head toward a sound.

5. The center of the reticular formation is located in the midbrain. The reticular formation consists of a network of interconnected nuclei throughout the brain stem.

a. The reticular formation constitutes a system producing activation and arousal of the entire brain.

6. The midbrain contains the red nucleus which provides unconscious regulation of motor activities such as muscle tone and posture. The red nucleus has a very rich blood supply giving it a deep red color.

7. The substantia negra is located lateral to the red nucleus. The substantia negra controls motor tracts form the cerebrum by releasing an inhibitory neurotransmitter, dopamine. Damage to this area leads to Parkinson’s disease.

B. The Pons – is located just superior to the medulla oblongata and inferior to the midbrain.

1. It contains many of the longitudinal nerve tracts passing through the brain stem:

a. Corticospinal tracts – The axons of these tracts originate at the cerebral cortex and pass directly to the motor neurons of the spinal cord.

b. Spinothalamic tracts – are sensory tracts carrying nerve messages of crude touch, pain and temperature from various levels of the spinal cord to the thalamus and then to the cerebral cortex.

2. The pons contains nuclei for regulating respiratory movements:

a. The apneustic center stimulates the respiratory rhythmicity center (RRC) in the medulla oblongata to produce inspiration.

b. The pneumotaxic center inhibits the RRC producing a passive expiration.

3. The pons contains tracts that connect the cerebellum with the brain stem, cerebrum and spinal cord through the cerebellar peduncles.

C. Medulla oblongata – is the last region of the brain stem before the spinal cord.

1. The Pyramids are two protruding, longitudinal columns on the anterior surface of the medulla. They contain the corticospinal tracts.

a. Corticospinal tracts contain descending nerve fibers for the control of conscious motor activity.

b. In the lower medulla, these tracts decussate (cross over). As a result, the left cerebral cortex initiates motor activity on the right side of the body and vice versa. This is referred to as Decussation of the pyramids. 85% of the corticospinal tracts decussate.

2. Medullary nuclei are clusters of cell bodies within the interior of the medulla that carry out a variety of regulatory functions:

a. Autonomic centers:

i. Cardiovascular centers regulate heart rate, and vasomotion.

ii. Respiratory rhythmicity center sets the breathing pace with input from the apneustic and pneumotaxic centers of the pons.

b. Relay stations for sensory information are found in the. The sensory messages of fine touch and proprioception decussate and are then carried to the thalamus.

3. The Spinothalamic pathway passes through the medulla and converges on the thalamus. It carries sensory information of crude touch, pressure and pain, as well as, thermal sensation. There is no decussation of these messages in the medulla. This has already occurred in the spinal cord at the level of entry to the medulla.