The Nervous System

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The Nervous System
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The Central Nervous System

• The central nervous system (CNS) can naturally be
  divided into two gross divisions the brain and
  the spinal cord.
• Taken together, the brain and spinal cord serve
  as the neuraxis.
• The CNS is surrounded and protected by the
  cranial bones and the vertebral column.

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The Brain

• The brain is shaped like an oval melon slight
  soft to touch.
• The average brain weighs about 3 lbs.
• It is housed in the bony skull or cranium.

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The Spinal Cord

• The spinal cord, on the other hand, is a long
  pigtail of flesh extending from the base of the
  cranium from a large opening in the skull called
  the foramen magnum.

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The Spinal Cord

• It is about 18 long and is normally found in the
  center of the vertebral column.
• Like the brain is protected by the skull, the
  spinal cord is well protected by the bones of the
  spine.

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The Meninges

• In addition to bony protection, the CNS is
  surrounded by three layers of non-nervous
  connective tissue known collectively as the
  meninges.

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Cerebrospinal Fluid

• Space between two layers of the meninges contain
  cerebrospinal fluid (CSF) that moistens,
  lubricates, and protects the brain and spinal
  cord.

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The Meninges

• The three layers of meninges from most external
  to the most internal are the dura mater (green),
  the arachnoid mater (purple tendrils), and the
  pia mater (red).

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The Dura Mater

• The dura mater is the outermost covering and
  functions as a protective sheath for the brain
  and spinal cord.

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The Arachnoid Mater

• The arachnoid mater is a delicate layer of woven
  fibers that loosely covers the brain and spinal
  cord.
• In the space that is left, CSF, arteries, veins,
  and cranial nerves pass.

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The Pia Mater

• The pia mater closely follows the dips of the
  surface of the brain and adheres to the spinal
  cord.
• It is highly vascular, giving the brain its pink
  color.

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The Cerebrum

• The largest mass of brain tissue is called the
  cerebrum.
• The surface of the cerebrum, composed of grey
  matter, is called the cortex.

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The Cerebral Cortex

• The cortex is about 2 to 4 mm thick and contains
  billions of nerve cells.
• In most areas, there are six layers of nerve cell
  bodies.

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The Cerebral Hemispheres

• The cerebrum fills the upper part of the cranium
  and consists of two equal portions called the
  cerebral hemispheres.
• Each hemisphere has three surfaces.
• This is its convex, lateral surface.

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The Cerebral Hemispheres

• This is its flat medial surface.

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The Cerebral Hemispheres

• This is its irregular inferior surface.

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The Cortical Surface

• The cortical surface of the cerebrum is
  characterized by a maze of folds and depressions.
• Each fold is known as a gyrus or convolution.

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The Cerebral Surface

• Each depression is termed a sulcus, if shallow or
  a fissure, if particularly deep.

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Median Longitudinal Fissure

• There are a number of important fissures which
  serve as reference landmarks.
• The longitudinal fissure separates the two
  cerebral hemispheres.

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The Corpus Callosum

• The separation of the two hemispheres is only
  partial because a prominent band of white matter
  fibers, the corpus callosum, unites the them.

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Central Sulcus

• On the lateral surface, the central sulcus
  (fissure) begins midway between the front and the
  back of the cerebrum.
• It course downward terminating at the midpoint
  of the superior and inferior borders.

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Lateral Sulcus

• The lateral sulcus begins at the inferior border
  of the lateral surface of each hemisphere and
  course obliquely upward to terminate slightly
  more than halfway back.

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Lobes

• The central and lateral fissures help to divide
  the cerebral hemispheres into four lobes
  frontal, temporal, parietal, and occipital.
• They are named after the cranial bones to which
  they are immediately adjacent.

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Frontal Lobe

• The frontal lobe (green) is bounded inferiorly by
  the lateral sulcus and posteriorly by the central
  fissure.
• The frontal lobe makes important contributions to
  movement.

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Primary Motor Area

• Just immediately in front of the central sulcus,
  there is a prominent gyrusthe precentral gyrus.
• It is also known as the motor strip or primary
  motor area.

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Primary Motor Area

• The cells in this area are responsible for
  executing voluntary movements of the skeletal
  muscles on the opposite, or contralateral, side
  of the body.

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Premotor Area

• Immediately anterior to M1 is the premotor area.
• The premotor area is thought to be involved in
  planning and programming of voluntary movements.

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Supplemental Motor Area

• On the medial surface of the frontal cortex,
  under the premotor area, on the superior gyrus,
  the supplemental motor area.

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Supplemental Motor Area

• This area is important in temporal (time-ordered)
  organization of movements, especially in
  sequential performance of multiple movements. 
• It is also important in the initiation of
  voluntary movements.

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Brocas Area

• Finally, Brocas area, of the inferior frontal
  gyrus, is thought to control voluntary speech
  output.

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Parietal Lobe

• The parietal lobe (pink) is bounded anteriorly
  by the central fissure and inferiorly by the
  posterior end of the lateral sulcus.

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The Parietal Lobe

• Posteriorly, there is an imaginary boundary
  between the parietal and occipital lobes.

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The Parietal Lobe

• Like the frontal lobe, there is a prominent gyrus
  just posterior to the central fissure called the
  postcentral gyrus.

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Primary Sensory Area

• The postcentral gyrus is the sight of the sensory
  strip, or the primary sensory (S1) area.
• Sensations of pain, temperature, and light
  touch/pressure are sent to this area from the
  opposite side of the body for perception.

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Supramarginal Gyrus

• Another important gyrus is the supramarginal
  gyrus.
• It can be found curving around the posterior end
  of the lateral sulcus.
• This area is important for writing.

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Occipital Lobe

• The occipital lobe (yellow) is found behind the
  parietal and temporal lobes.
• It is the seat of vision.
• At the occipital pole we find the primary visual
  (V1) area.

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The Temporal Lobe

• The temporal lobe (blue) is found inferior to the
  lateral sulcus.
• Much of the cortical center for hearing is
  located on the superior temporal gyrus.

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Primary Auditory Area

• The primary auditory area (A1) is found on the
  anterior portion of this gyrus in an area known
  as Heschls gyrus.

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Wernickes Area

• Posterior to Heschls gyrus is an auditory
  association area known as Wernickes area.
• This area is believed to integrate auditory
  stimuli.

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The Angular Gyrus

• Finally, curving around the posterior end of the
  first temporal gyrus, the angular gyrus extends
  into the parietal lobe, behind the supramarginal
  gyrus.
• This area is important to comprehension of the
  written word.

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Cortical Connections

• There are extensive myelinated nerve fibers
  leading to and from the cortex.
• Projection fibers convey impulses to and from
  remote regions of the brain and spinal cord.
• There are both afferent and efferent fibers
  traveling to and from the cerebral cortex.

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Cortical Connections

• Association fibers interconnect cortical regions
  in the same hemisphere.
• They can be long or short.
• Short fibers connect cells in adjacent gyri.
• Long fibers connect cortical regions in the same
  hemisphere.

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Cortical Connections

• Commissural fibers interconnect corresponding
  cortical regions of the two hemispheres.
• The corpus callosum interconnects regions of the
  cortex in all lobes with corresponding regions of
  the opposite hemisphere.

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The Brainstem

• Supporting the cerebrum, like the stalk of a
  mushroom, is the brainstem.

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The Brainstem

• The brainstem appears as a series of structures
  that seem to be an upward extension of the spinal
  cord thrust between the two cerebral hemispheres.

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The Brainstem

• The three brainstem structures from bottom to top
  are the medulla oblongata, the pons, and the
  midbrain.

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Cerebellum

• Extending off the back of the pons is the
  cerebellum, or little brain.
• It is connected to the pons by three sets of
  feet, or peduncles.

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The Medulla

• The medulla is the most inferior portion of the
  brainstem.
• It is continuous with the spinal cord.
• It is about 1 long.

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The Medulla

• It contains ascending and descending nerve tracts
  plus the nerve cell bodies that control
  phonation, velopharyngeal closure, swallowing,
  and articulation.

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The Medulla

• In the anterior portion of the medulla, there are
  descending motor tracts from M1 from the cerebral
  cortex called the pyramidal tracts.

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The Medulla

• These tracts cross overdecussateto the other
  side of the medulla on their way to the spinal
  cord.

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The Medulla

• On the posterior side of the medulla, there are
  ascending nerve tracts coming from the spinal
  cord with information about the bodys periphery.

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The Medulla

• They also cross over to the opposite side in the
  medulla before continuing up stream to the S1 in
  the parietal lobe.

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The Pons

• The pons is located between the medulla and the
  midbrain in front of the cerebellum.
• Information from the cerebellum flows into and
  out of the pons.

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The Midbrain

• The midbrain is the narrowest portion of the
  brainstem above the pons.
• Often it is difficult to see because it is thrust
  into the cerebrum.

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The Midbrain

• On the posterior side of the midbrain are four
  little swellings called the colliculi.
• There are two superior and two inferior colliculi.

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TheColliculi

• The inferior colliculi serve as relay stations
  for the auditory nervous system.
• The superior colliculi serve as relay stations
  for the visual nervous system.

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The Colliculi

• In response to either visual or auditory
  stimulation, these colliculi respond with a
  localization reflex or the head toward the
  stimulus.

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The Spinal Cord

• The spinal cord is the lower extension off the
  medulla that exits the cranium at the foramen
  magnum.
• It extends from the upper border of the 1st
  cervical vertebra to the lower border of the 1st
  lumbar vertebra.

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The Spinal Cord

• The spinal cord is protected by the meninges,
  cerebrospinal fluid, and the individual vertebra
  that comprise the vertebral column.

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The Spinal Cord

• A transverse section through the spinal cord
  reveals a central core of grey matter surrounded
  by white matter.
• The grey matter has the appearance of the letter
  H.

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Grey Horns

• On the basis of two prominent sulci, the anterior
  median fissure and the posterior median sulcus,
  the grey matter can further be divided into
  anterior and posterior grey horns.

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White Columns

• The white matter also has separate columns.
• The anterior and posterior white columns contain
  ascending nerve tracts.
• The lateral white columns contain descending
  tracts.

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Spinal Nervs of the PNS

• Comprising part of the peripheral nervous system,
  31 total pairs of spinal nerves extend from each
  segment of the spinal cord.

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Spinal Nerve Dorsal Root

• Each spinal nerve has two portions.
• The posterior or dorsal portion is called the
  dorsal root and it brings sensory information
  into the posterior grey horn.

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Spinal Nerve Ventral Root

• The anterior or ventral portion is called the
  ventral root and it takes motor information out
  of the spinal cord through the anterior grey horn.

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Cranial Nerves of the PNS

• The peripheral nervous system also includes the
  12 pairs of cranial nerves.
• The cranial nerves exit from various level of the
  brainstem to reach the sense organs and muscles
  of the head and neck.

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Cranial Nerves

• The cranial nerves directly associated with
  speech, language, and hearing functions include
  cranial nerves V, VII, VIII, IX, X, and XII.

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Trigeminal Nerve (V)

• Cranial nerve V is the Trigeminal Nerve.
• The branches of this nerve supply the skin and
  mucosa of the face, nose, and mouth regions, as
  well as the muscles of mastication.

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Facial Nerve (VII)

• Cranial nerve VII is the facial nerve.
• It serves the muscles of facial expression and
  salivary secretion.
• It serves taste function for the anterior 2/3 of
  the tongue.

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Vestibulocochlear Nerve (VIII)

• Cranial nerve VIII is the vestibulocochlear
  nerve.
• The vestibular division serves the function of
  equilibrium, especially the orientation of the
  head in space.

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Glossopharyngeal Nerve (IX)

• Cranial nerve IX is the glossopharyngeal nerve.
• It supplies the muscles of the pharynx as well as
  sensations of the faucial pillars, soft palate,
  and posterior 1/3 of the tongue for taste.

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Vagus Nerve (X)

• Cranial nerve X is the vagus nerve.
• It wanders widely, serving both motor and sensory
  functions.
• Two important divisions of this nerve are the
  superior laryngeal nerve and the inferior
  (recurrent) laryngeal nerve.

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Superior Laryngeal Nerve

• The sensory portion of the superior laryngeal
  nerve helps initiate the cough response to
  foreign material penetration.
• The motor portion of the superior nerve
  innervates a glottal tensor.

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Inferior (Recurrent) Laryngeal Nerve

• The motor portion of the inferior laryngeal nerve
  innervates muscles of abduction and adduction.
• The sensory portion mediates taste sensation on
  the tongue root, epiglottis, and pharynx.

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Hypoglossal Nerve (XII)

• Cranial nerve XII is the hypoglossal nerve.
• It is the nerve that innervates both intrinsic
  and extrinsic muscles of the tongue.
• It is particularly important for speech
  articulation and the oral preparatory and oral
  stages of swallowing.

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Cranial Nerve Damage

• Lesions to cranial nerves produce many and varied
  symptoms and signs.
• For example, facial paralysis results when
  cranial nerve VII is involved.
• Hearing disorders result when cranial nerve VIII
  is involved.
• Resonance and articulation disorders result when
  cranial nerve IX is affected.