neuroanatomy for the speech-language pathologist

NEUROANATOMY FOR THE SPEECH-LANGUAGE

PATHOLOGIST

I. Organization of the Nervous System

The nervous system is the body’s principal control and integrating center.

It serves three broad functions: sensory, integrative, and motor: It senses changes within the body and in the

outside environment; It interprets the change.; and It responds to the interpretation by initiating

action by muscular contraction or glandular secretion.

Through sensation, integration, and response, the nervous system rapidly maintains the body’s homeostasis.

A. Divisions of the Nervous System

The nervous system has two principal divisions:

the central nervous system (CNS); and

the peripheral nervous system (PNS).

1. The Central Nervous System

The CNS is command central for the entire nervous system.

It consists of the brain and the spinal cord.

The term “brain” comes from the old Anglo-Saxon word braegen which means “the center of the nervous system.”

In Greek, the word enkephalos pertains to the mass of nerve tissue housed within the bony confines of the head and provides the root word “encephalon.”

1. The Central Nervous System: Brain

Your brain is three pounds of tofu-like tissue containing 1.1 trillion cells including 100 billion neurons.

The brain is responsible for higher level human functions such as reasoning and language.


It is mushroom-shaped and is divided into four principal parts: the brainstem, the diencephalon, the cerebrum, and the cerebellum.

The brainstem is like the stalk of the mushroom.

The lower end is continuous with the spinal cord.


As it ascends from the spinal cord, it has three divisions: the medulla oblongata, the pons, and the midbrain.

Below the cerebrum and behind the brainstem is the cerebellum.


Above the brainstem, the diencephalon consists primarily of the thalamus and hypothalamus.

The cerebrum spreads over the diencephalon, constituting about 7/8 of the total brain, and occupying most to the cranium.

1.The Central Nervous System: Spinal Cord

The spinal cord begins as a continuation of the medulla from where it exits the cranium at the foramen magnum of the occipital bone.

It continues for approximately 16-18” to end at the level of the upper border of the second lumbar vertebra.


The cord serves as a conduit for the ascending and descending fiber tracts that connect the peripheral and spinal nerves with the brain.

It is functionally segmented into 31 sections, which each give rise to a pair of spinal nerves.


The adult spinal cord is about 1” in circumference, except in the mid-cervical and mid-lumbar regions where it is slightly larger.

The superior enlargement is termed the cervical enlargement; it extends from C4 to T1.

Nerves serving the upper extremities arise from this area of the spinal cord.

The inferior enlargement is termed the lumbar enlargement, extending from T9-T12.

Nerves serving the lower extremities arise from this area of the spinal cord.


Below the lumbar enlargement, the spinal cord tapers off between L1 and L2 into a conical portion known as the conus medullaris.

From the conus medullaris, the filum terminale, a non-nervous fibrous tissue, extends to attach to the coccyx.


Nerves arising from the lower portion of the cord must angle inferiorly in the vertebral canal before leaving it.

Because it looks likes wisps of coarse hair flowing from the end of the cord, it is aptly named cauda equina—horse’s tail.

2.The Peripheral Nervous System

The PNS consists of all nervous tissue found outside of the bony confines of the skull and vertebral column.

The PNS serves the communication link between the body and the CNS.

The PNS connects the brain and spinal cord with receptors, muscles, and glands.

The PNS is broadly divided into spinal and cranial nerves.

2.The Peripheral Nervous System

There are direct connections between the brain and the cranial nerves, and the spinal cord and the spinal nerves.

The PNS is also divided into afferent and efferent systems.

The afferent system consists of nerve cells that convey information from receptors in the periphery of the body to the CNS.

The efferent system consists of nerve cells that convey information from the CNS to skeletal muscles and glands.

2.The PNS: Spinal Nerves

There are 31 pairs of spinal nerves named and numbered according to the region and the level of the spinal cord from which they emerge.

There are eight pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 coccygeal spinal nerve pair.


C1 emerges between the occipital bone and the atlas.

All other spinal nerves leave the vertebral column from the intervertebral foramina between the adjoining vertebrae.


Each spinal nerve has two points of attachments or roots.

The posterior root contains the sensory fibers.

The anterior root contains the motor fibers.

Peripheral nerves segregate their sensory and motor neurons.


Sensory neurons enter the cord dorsally, through the dorsal or posterior root.

Motor neurons leave the cord ventrally, through the ventral or anterior root.

Then the sensory and motor components fuse together to form the peripheral nerve.

This principle—that the dorsal nerve root is only sensory and the ventral nerve root only motor—is named the Bell-Magendie Law for Sir Charles Bell and Francois Magendie who first stated it.


The spinal nerves act on visceral (gut) or somatic (body) structures.

General somatic afferent nerves convey information about pain, temperature, and mechanical stimuli from receptors in the skin, muscles, and joints.

General visceral afferent nerves convey information from receptors in visceral structures, e.g., walls of the digestive tract.

General visceral efferent nerves convey information to an autonomic nerve fiber.

General somatic efferent nerves convey information to a skeletal muscle.

2.The PNSystem: Cranial Nerves

There are 12 pairs of cranial nerves.

All pairs leave the cranium through the foramina of the skull.

They are designated with Roman numerals and names.

The Roman numerals indicate the level at which the nerve arises from the brain.

The name indicates the nerve distribution or function.

2.The PNS: Cranial Nerves

I-OlfactoryII-OpticIIIOculomotorIV-TrochlearV-TrigeminalVI-AbducensVII-FacialVIII-VestibulocochlearIX-GlossopharyngealX-VagusXI-AccessoryXII-Hypoglossal

2.The PNS: Cranial Nerves

Based on the functional components of each nerve, there are three types of cranial nerves:

Somatic efferent nerves (III, IV, VI, XII) contain mostly fibers innervating skeletal musculature.

Special sensory nerves (I, II, VIII) contain fibers relating to the special senses of sight, smell and taste, and hearing and equilibrium.

Branchiomeric nerves (V, VII, IX, X, XI) contain special visceral efferents that innervate the striated muscles of the larynx, pharynx, and face.

3.The Autonomic Nervous System (ANS)

The ANS is a unique component of the PNS.It regulates the activities of smooth muscles, cardiac muscle, and certain glands.

Through its afferents and efferents, the ANS automatically and involuntarily regulates visceral activities, such as pupillary size change, lens accommodation, dilation of blood vessels, adjustments to the rate and force of heartbeat, movements of the GI tract, and most glandular secretions.

There are three subdivisions of the ANS.


The enteric nervous system consists of two interconnected plexuses of sensory neurons, interneurons, and visceral motor neurons, in the walls of the alimentary canal.

The enteric nervous system coordinates gut motility.


This little brain in our innards, in connection with the big one in our skulls, partly determines our mental state and plays key roles in certain diseases throughout the body.

The enteric system consists of sheaths of neurons embedded in the walls of the long tube of our gut, or alimentary canal, which extends from the esophagus to our anus.


It contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system.

This multitude of neurons in the enteric nervous system enables us to "feel" the inner world of our gut and its contents.

Much of this neural firepower comes to bear in the elaborate daily grind of digestion.


Breaking down food, absorbing nutrients, and expelling of waste requires chemical processing, and mechanical mixing and rhythmic muscle contractions that move everything on down the line.

Thus equipped with its own reflexes and senses, the “enteric” brain can control gut behavior independently of the brain.


In other words, the brain in the head doesn't need to get its hands dirty with the messy business of digestion, which is delegated to the brain in the gut.

We likely evolved this intricate web of nerves to perform digestion and excretion "on site," rather than remotely from our brains through the middleman of the spinal cord.

The enteric brain informs our state of mind in other more obscure ways, as well.


A big part of our emotions are probably influenced by the nerves in our gut.

Butterflies in the stomach—signaling in the gut as part of our physiological stress response—is but one example.

Although gastrointestinal (GI) turmoil can sour one's moods, everyday emotional well-being may rely on messages from the lower enteric brain to the brain above.


Irritable bowel syndrome—which afflicts more than two million Americans—also arises in part from too much serotonin in our entrails, and could perhaps be regarded as a "mental illness" of the second brain.

Scientists are learning that the serotonin made by the enteric nervous system might also play a role in more surprising diseases, including the bone-deteriorating disease osteoporosis in post menopause.


The sympathetic nervous system prepares us for situations in which energy needs to be expended.

It’s the body’s alerting system.It increases our heart rate, decreases peristaltic actions, and diverts blood from the gut to the skeletal muscles.

The parasympathetic nervous system has a calming effect on bodily function by decreasing heart rate and blood pressure, increasing intestinal peristalsis and salivation, and opening sphincters.


The parasympathetic nervous system enhances energy storage through conservation and restoration.

Both the sympathetic and parasympathetic nervous systems work with the endocrine system to maintain the stability of the body’s internal environment.

B. Embryological Levels of the Brain

The brain develops very rapidly during the first few years of life.

Growth is mainly due to an increase in the size of cells already present, proliferation and growth of neuroglia, development of synaptic contacts and dendritic branching, and myelination of various fiber tracts.

1. Development of the CNS and PNS

The development of the NS begins at about the third week of life with a thickening of the ectoderm of the neural plate.

The plate folds inward and forms a longitudinal groove, called the neural groove.


The raised edges of the neural plate are called neural folds.

As development continues, the folds increase in height, meet, and form a tube, the neural tube.

The neural tube forms first in the region that will become the cervical area and then closes like a zipper with progression passing both caudally and cranially.


The opening at the cranial end is called the anterior neuropore.

The opening at the caudal end is called the posterior neuropore.

Closure of the anterior neuropore takes place by day 24 and the posterior neuropore by day 26.

The primitive CNS is now a hollow tubular structure closed at both ends.


The fluid filled cavity of this tube is called the neural canal which will develop into the ventricular system of the brain and the central canal of the spinal cord.

If the cranial portion of the neural tube fails to close, the result is anacephaly, and the overall structure of the brain is grossly disturbed.

If the posterior neuropore fails to close, the result is spina bifida.

2. Development of the Cerebral Vesicles

By the fourth week, three distinct bulges, the primary vesicles, appear in the anterior neuropore.

From the top down, we have the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain).


By the fifth week, five secondary vesicles develop.

The prosencephalon divides into the two telecephalon, which become the cerebral hemispheres of the brain, and the single diencephalon, which gives rise to the thalamus and hypothalamus.


The mesencephalon remains unchanged and becomes the midbrain.

The rhombencephalon divides into the metencephalon, which becomes the pons and cerebellum, and the myelencephalon, which becomes the medulla oblongata.


In addition to the development of cerebral structures, the vesicles also give rise to the ventricular system.

Specifically, the prosencephalon develops the two lateral ventricles contained within the two cerebral hemispheres.


The third ventricle develops within the diencephalon.

The mesencephalon gives rise to the cerebral aqueduct.

The rhombencephalon gives rise to the fourth ventricle.


The ventricles are a continuous series of fluid-filled spaces extending through all major divisions of the CNS.

Each lateral ventricle communicates with the 3rd ventricle through the intraventricular foramen.

The 3rd ventricle communicates with the 4th ventricle through the cerebral aqueduct of the midbrain.


Production of the cerebral spinal fluid filling the ventricles is manufactured by small vascular tufts called the choroid plexus.

The choroid plexus are found in the roof of the 3rd and 4th ventricles.

The choroid plexus of the 3rd ventricle protrude into the lateral ventricle.

Neurulation

Neurulation is the process that establishes the central nervous system.

As seen in cross section, the embryonic neural tube forms three layers.

From the neurocoel (neural canal) outward, there are the ependymal layer, the mantle layer, and the marginal layer.

Neurulation

Some of the cells of the ependymal layer remain in place to become the thin, ciliated lining of the adult central canal, but most migrate outward to join mantle cells in forming both neurons and neuroglia.

These will be the gray matter of the adult.

Neurulation

With cell migration, the mantle layer develops the characteristic “butterfly” shape.

The lateral walls of the tube thicken and maturing neurons clump into two different plates.

Neurulation

The two plates are divided by a shallow, longitudinal groove called the sulcus limitans.

The sulcus limitans separates the developing gray matter into a dorsal alar plate and a ventral basal plate.

Neurulation

These plates signal the future locations of sensory and motor functions, respectively.

Alar and basal plates become dorsal and ventral horns, respectively, while intermediate regions develop interneurons, mixed nerves.

Neurulation

Finally, cells from the marginal layer mature by growing out their processes (axons and dendrites).

This layer is penetrated by nerve fibers growing out of the deeper layers.

It becomes the white matter of the adult cord.

Neurulation

The brainstem develops in a manner similar to the spinal cord.

From the medulla through the midbrain, alar and basal plates form sensory and motor columns of cells that supply cranial nerves.

However, the organization of alar and basal plates in the brainstem differ from those of the spinal cord.

Neurulation

In the 6 mm embryo, the thin ependymal roof of the neural tube, the spinal cord, becomes even thinner as the ventricle of the neural tube begins to widen in the early stages of the development of the 4th ventricle.

With continued development, alar and basal plates shift laterally and become located in the floor of the ventricle.

Neurulation

The sulcus limitans continues to be identifiable helping to mark the boundary between sensory and motor areas.

In the medulla and pons, the alar plate comes to lie lateral to the basal plate, not dorsal to it.

The basal plate forms the motor nuclei of the cranial nerves, medial to the sulcus limitans in the ventricular floor.

Neurulation

Lateral to the sulcus, the alar plate forms sensory relay nuclei.

Rostral to the midbrain, the diencephalon and cerebral hemispheres develop from the alar plate.

The cerebellum also develops from alar plate.

Portions of the alar plate migrate ventrally and form the inferior olivary nucleus, a cerebellar relay nucleus.

Differences in Male & Female Brains

The majority of brain development that determines sex-specific circuits happens during the first 18 weeks of pregnancy.

Until eight weeks old, every fetal brain looks female—female is nature’s default gender setting.

If you were to watch a female and male brain developing via time-lapse photography, you would see that some of the neural connections are being laid out according to a blueprint drafted by both genes and sex hormones (Brizendine, 2010).


A huge testosterone surge beginning in the eighth week will turn this unisex brain male by killing off some cells in the communication centers and growing more cells in the sex and aggression centers.

If the testosterone surge doesn’t occur, the female brain continues to grow unperturbed.

Scientists agrees that when cells in various areas of the male and female brains are stimulated by hormones such as testosterone and estrogen, they turn on and off different genes.


For boys, the genes that turn on will trigger the urge to track and chase moving objects, hit targets, test their own strength, and play at fighting off enemies.

For girls, the genes that turn on will enhance female brain circuits and centers for observation, gut feelings, even tending and caring and her fetal brain cells will continue to sprout more connections in the communication centers and areas that process emotion.

neuroanatomy for the speech-language pathologist

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