Sistema nervoso

DIENCEPHALON

The diencephalon is the one region of the adult brain that retains its name from

embryologic development. It is the connection between the cerebrum and the rest of

the nervous system, with one exception.

The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum

through the diencephalon. Output from the cerebrum passes through the

diencephalon. The single exception is the system associated with olfaction, which

connects directly with the cerebrum. The two major regions of the diencephalon are

the thalamus itself and the hypothalamus.

The thalamus is a collection of nuclei that relay information between the cerebral

cortex and the periphery, spinal cord, or brain stem. All sensory information, except

for the sense of smell, passes through the thalamus before processing by the cortex.

Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the

thalamus, and thalamic neurons project directly to the cerebrum. The thalamus

processes that information. For example, the portion of the thalamus that receives

visual information will influence what visual stimuli are important, or what receives

attention.

The hypothalamus is a collection of nuclei that are largely involved in regulating

homeostasis. The hypothalamus is the executive region in charge of the autonomic

nervous system and the endocrine system through its regulation of the posterior

pituitary gland. Other parts of the hypothalamus are involved in memory and emotion

as part of the limbic system.

BRAIN STEM

The midbrain (mesencefalo), the pons and the medulla are collectively referred to as

the brain stem. The structure connects the brain to the spinal cord. Attached to the

brain stem, but considered a separate region of the adult brain, is the cerebellum. The

midbrain coordinates sensory representations of the visual, auditory, and

somatosensory perceptual spaces. The pons is the main connection with the

cerebellum. The pons and the medulla regulate several crucial functions, including the

cardiovascular and respiratory systems and rates.

• The cranial nerves connect through the brain stem and provide the brain with

the sensory input and motor output associated with the head and neck,

including most of the special senses.

• The major ascending and descending pathways between the spinal cord and

brain, specifically the cerebrum, pass through the brain stem.

CEREBELLUM

The cerebellum is the “little brain.” It accounts for approximately 10 percent of the

mass of the brain. It is covered in gyri and sulci like the cerebrum, and looks like a

miniature version of that part of the brain. The cerebellum is largely responsible for

comparing information from the cerebrum with sensory feedback from the periphery

through the spinal cord. Descending fibers from the cerebrum have branches that

connect to neurons in the pons. Those neurons project into the cerebellum, providing a

copy of motor commands sent to the spinal cord. Sensory information from the

periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the

medulla known as the inferior olive. Fibers from this nucleus enter the cerebellum and

are compared with the descending commands from the cerebrum.

CNS Arterial supply

CNS blood supply is from internal carotid arteries (which originate from common

carotid arteries) and from vertebral arteries (which originate from subclavian arteries).

CNS has a privileged blood supply:

• To protect CNS from the toxins and pathogens that may be traveling through the

blood stream, there is strict control over what can move out of the general

systems and into the brain and spinal cord thanks to the blood-brain barrier.

• The CNS filters blood into cerebrospinal fluid (CSF), which is then circulated

through the cavities of the brain and spinal cord called ventricles

• As a lack of oxygen to the CNS can be devastating, the cardiovascular system

has specific regulatory reflexes to ensure that the blood supply is not

interrupted.

reflex: The bases of the common carotids contain stretch receptors

Orthostatic

that immediately respond to the drop in blood pressure upon standing. The orthostatic

reflex is a reaction to this change in body position, so that blood pressure is

maintained against the increasing effect of gravity. Heart rate increases—a reflex of

the sympathetic division of the autonomic nervous system—and this raises blood

pressure

• The internal carotid artery enters the cranium through the carotid canal in the

temporal bone.

• The vertebral arteries pass through the neck region by the transverse foramina

of the cervical vertebrae. The vertebral arteries enter the cranium through the

foramen magnum of the occipital bone.

Branches off the left and right vertebral arteries merge into the anterior spinal artery

supplying the anterior aspect of the spinal cord, found along the anterior median

fissure. The two vertebral arteries then merge into the basilar artery, which gives rise

to branches to the brain stem and cerebellum. The left and right internal carotid

arteries and branches of the basilar artery all become the circle of Willis, a confluence

of arteries that can maintain perfusion of the brain even if narrowing or a blockage

limits flow through one part.

CNS Venous Return

After passing through the CNS, blood returns to the circulation through a series of

dural sinuses and veins. The superior sagittal sinus runs in the groove of the

longitudinal fissure, where it absorbs CSF from the meninges.

The superior sagittal sinus drains to the confluence of sinuses, along with the occipital

sinuses and straight sinus, to then drain into the transverse sinuses. The transverse

sinuses connect to the jugular veins. From there, the blood continues toward the heart

to be pumped to the lungs for reoxygenation.

Meninges

The outer surface of the CNS is covered by a series of membranes composed of

connective tissue called the meninges, which protect the brain.

• The dura mater is a thick fibrous layer and a strong protective sheath over the

entire brain and spinal cord. It is anchored to the inner surface of the cranium

and vertebral cavity. The dura also surrounds and supports the venous sinuses

• The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac

around the CNS. Beneath the arachnoid (in the subarachnoid space) is a thin,

filamentous mesh called the arachnoid trabeculae, which looks like a spider

web, giving this layer its name. The subarachnoid space is filled with circulating

CSF. The arachnoid emerges into the dural sinuses as the arachnoid

granulations, where the CSF is filtered back into the blood for drainage from the

NS.

• Directly adjacent to the surface of the CNS is the pia mater, a thin fibrous

membrane that follows the convolutions of gyri and sulci in the cerebral cortex

and fits into other grooves and indentations.

The Ventricular System

Cerebrospinal fluid (CSF) circulates throughout and around the CNS to remove

metabolic wastes from the interstitial fluids of nervous tissues and return them to the

blood stream. The ventricles are the open spaces within the brain where CSF

circulates. CSF is produced by filtering of the blood by a specialized membrane known

as a choroid plexus. The CSF circulates through all of the ventricles to eventually

emerge into the subarachnoid space where it will be reabsorbed into the blood.

There are 4 ventricles, all of which developed from the original hollow space within the

neural tube, the central canal. The first two are named the lateral ventricles and are

deep within the cerebrum. These ventricles are connected to the third ventricle by two

openings called the interventricular foramina. The third ventricle is the space between

the left and right sides of the diencephalon, which opens into the cerebral aqueduct

that passes through the midbrain. The aqueduct opens into the fourth ventricle, which

is the space between the cerebellum and the pons and upper medulla.

Cerebrospinal fluid is produced within the ventricles by a type of specialized

membrane called a choroid plexus. Ependymal cells surround blood capillaries and

filter the blood to make CSF. The fluid is a clear solution with a limited amount of the

constituents of blood. It is essentially water, small molecules, electrolytes, oxygen and

carbon dioxide.

Within the subarachnoid space, the CSF flows around all of the CNS, providing two

important functions:

• picks up metabolic wastes from the nervous tissue and moves it out of the CNS.

• acts as a liquid cushion for the brain and spinal cord.

The Peripheral Nervous System

The PNS is not as contained as the CNS because it is defined as everything that is not

the CNS. Some peripheral structures are incorporated into the other organs of the

body. In describing the anatomy of the PNS, it is necessary to describe the common

structures, the nerves and the ganglia, as they are found in various parts of the body.

Ganglia

A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be

categorized, for the most part, as either sensory ganglia or autonomic ganglia,

referring to their primary functions. The most common type of sensory ganglion is a

dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with

axons that are sensory endings in the periphery, such as in the skin. The ganglion is

an enlargement of the nerve root.

The cells of the dorsal root ganglion are unipolar cells with associated satellite cells.

Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the

dorsal root ganglion, except that it is associated with a cranial nerve instead of a

spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia

are outside the skull. The neurons of cranial nerve ganglia are also unipolar in shape

with associated satellite cells. The other major category of ganglia are those of the

autonomic nervous system, which is divided into

• sympathetic chain ganglia and prevertebral ganglia : constitute a row of ganglia

along the vertebral column (sympathetic chain ganglia) and anterior to the

vertebral column (prevertebral ganglia). The neurons of these autonomic

ganglia are multipolar in shape. The neurons of the chain, paravertebral, and

prevertebral ganglia then project to organs in the head and neck, thoracic,

abdominal, and pelvic cavities to regulate the sympathetic aspect of

homeostatic mechanisms

• Parasympathetic terminal ganglia that receive input from cranial nerves or

sacral spinal nerves and are responsible for regulating the parasympathetic

aspect of homeostatic mechanisms. Terminal ganglia below the head and neck

are often incorporated into the wall of the target organ as a plexus. A plexus, in

a general sense, is a network of fibers or vessels. For example, the enteric

plexus is the extensive network of axons and neurons in the wall of the small

and large intestines. The