ANATOMY OF THE NERVOUS SYSTEM

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ANATOMY OF THE NERVOUS SYSTEM

• Lecture 9 10

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Functions of the nervous system

• 1. Initiate and/or regulate movement of body
  parts
• 2. Regulate secretions from glands
• 3. Gather information about the external
  environment and the internal environment of the
  body
• using senses (sight, hearing, touch, balance,
  taste) and mechanisms to detect pain,
  temperature, pressure, and certain chemicals,
  such as carbon dioxide, hydrogen, and oxygen
• 4. Maintain an appropriate state of consciousness
• 5. Stimulate thirst, hunger, fear, rage, and
  sexual behaviors appropriate for survival

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• The entire nervous system can be divided into two
  parts
• 1- Central nervous system (CNS)
• includes the brain and spinal cord
• 2- Peripheral nervous system (PNS), which
  consists of
• Cranial nerves
• spinal nerves

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• A further distinction is often made
• the autonomic nervous system (ANS)
• which integrates activity of visceral structures
• smooth muscle, cardiac muscle, and glands
• The ANS has elements in both
• Central nervous system.
• Peripheral nervous system.

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• In the PNS
• 1- sensory (afferent) nerves
• - gather information about the external and
  internal environments and relay this information
  to the CNS.
• - The specialized organs or cells that
  detect specific stimuli are sensory receptors.

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• CNS interprets information arriving via the PNS,
  integrates that information, and initiates
• appropriate movement of body parts
• glandular secretion
• behavior response.
• 2- Motor efferent
• - Communication between the CNS and muscles and
  accomplished via nerves of the PNS

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Microscopic Neuroanatomy

• The individual nerve cell is called a neuron.
• Each neuronal cell body gives rise to one or
  more nerve processes and cytoplasmic extensions
  of the cell.

• The nerve processes are called dendrites if they
  transmit electrical signals toward the cell
  bodies
• They are called axons if they conduct electrical
  signals away from the cell bodies.

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• Neurons may be classified morphologically
  according to their number of nerve processes
• Unipolar neurons have one process
• Bipolar neurons have one dendrite and one axon
• Multipolar neurons have a number of dendrites in
  addition to their single axon.
• We do not have true unipolar neurons, but many
  sensory neurons have their single dendrite and
  axon fused so as to give the appearance of a
  single process
• This configuration is pseudounipolar.

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• Nervous tissue consists not only of neurons but
  also of supportive cells.
• In the CNS, these supportive cells are the
  neuroglia, comprising a variety of glial cells
• Groups of nerve cell bodies within the CNS are
  generally called nuclei, while groups of nerve
  cell bodies in the PNS are called ganglia.
• In general terms
• Aggregates of neuronal cell bodies form the gray
  matter of the CNS
• Regions characterized primarily by tracts are
  white matter.

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Development of CNS

• Shortly after gastrulation, ectodermal cells on
  the dorsum just cranial to the primitive streak
  begin to proliferate and differentiate into a
  neural plate.
• The neural plate proliferates faster along its
  lateral margins than on the midline, creating the
  neural groove

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Development of CNS

• The edges of which (the neural folds) ultimately
  meet dorsally to form the neural tube.
• The entire CNS is formed from the cells of the
  neural tube.
• The lumen of the neural tube persists in the
  adult as the central canal of the spinal cord and
  as the ventricles of the brain.

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• Development of the spinal cord continues by an
  increase in the thickness of the wall of the
  neural tube.
• As cells divide and differentiate, three
  concentric layers of the neural tube emerge
• an inner (ventricular zone)
• a middle (intermediate zone)
• a superficial (marginal zone)

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• The thin ventricular zone of cells (also called
  ependymal zone) surrounds the lumen of the neural
  tube
• The site of mitosis of neuronal and glial
  precursors in the developing nervous system.
• It will ultimately form the ependyma of the
  central canal of the spinal cord and of the
  ventricles of the brain.

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• As cells are born in the germinal layer, they
  migrate outward to form the intermediate zone
  (also called mantle zone).
• The intermediate zone comprises neurons and
  neuroglia and becomes the gray matter near the
  center of the cord.
• The dorsal parts of the intermediate zone develop
  into the dorsal horns.
• - The ventral intermediate zone becomes the
  ventral horns

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• The marginal zone, which is most superficial,
  consists of nerve processes that make up the
  white matter of the spinal cord.
• The spinal cord white matter is divided into
  dorsal, lateral, and ventral funiculi, which are
  delimited by the dorsal and ventral horns of gray
  matter.

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• Development of the brain
• The first gross subdivisions of the brain create
  the three-vesicle stage.
• These subdivisions, which consist of three
  dilations of the presumptive brain, are
• prosencephalon, or forebrain
• mesencephalon, or midbrain
• rhombencephalon, or hindbrain.
• In the five-vesicle stage of development
• the prosencephalon further subdivides to form the
  telencephalon (future cerebrum) and the
  diencephalons ,
• the rhombencephalons divides into the
  Metencephalon (future pons and cerebellum) and
  the myelencephalon (future medulla oblongata).
• The mesencephalon does not subdivide.

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Central Nervous System

• Brain
• The gross subdivisions of the adult brain
  include
• cerebrum
• cerebellum
• brainstem
• The cerebrum develops from the embryonic
  Telencephalon.
• The components of the brainstem are defined in a
  number of ways
• include the diencephalon, midbrain, pons, medulla
  oblongata

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• Cerebrum comprises
• the two cerebral hemispheres, including the
  cerebral cortex, the basal nuclei
• The surface area of the cerebrum in domestic
  mammals is increased by numerous foldings to form
  convex ridges, called gyri (singular gyrus),
  which are separated by furrows called fissures or
  sulci.
• A particularly prominent fissure, the
  longitudinal fissure, lies on the median plane
  and separates the cerebrum into its right and
  left hemispheres.
• Deep to the cerebral cortex are aggregates of
  subcortical gray matter called the basal nuclei

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Diencephalon

• Is a derivative of the prosencephalon.
• thalamus
• epithalamus
• hypothalamus
• the third ventricle are included in the
  diencephalon.
• The thalamus is an important relay center for
  nerve fibers connecting the cerebral hemispheres
  to the brainstem and spinal cord.
• The epithalamus, dorsal to the thalamus,
  includes a number of structures, the pineal
  gland, which is an endocrine organ in mammals.
• The hypothalamus, ventral to the thalamus,
  surrounds the ventral part of the third ventricle
  and comprises many nuclei that function in
  autonomic activities and behavior.
• Attached to the ventral part of the hypothalamus
  is the hypophysis, or pituitary gland

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Mesencephalon

• The mesencephalon, or midbrain
• lies between the diencephalon rostrally and the
  pons caudally.
• The two cerebral peduncles
• four colliculi are the most prominent features
  of the midbrain.
• The cerebral peduncles, also called crura
  cerebrii, are large bundles of nerve fibers
  connecting the spinal cord and brainstem to the
  cerebral hemispheres.
• These peduncles consist of both sensory and
  motor fiber tracts.
• The colliculi are four small bumps (colliculus is
  Latin for little hill) on the dorsal side of the
  midbrain.
• They consist of right and left rostral colliculi
  and right and left caudal colliculi.
• The rostral colliculi coordinate certain visual
  reflexes,
• The caudal colliculi are relay nuclei for
  audition (hearing).

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Metencephalon.

• The metencephalon includes
• the cerebellum dorsally and the pons ventrally.
• The cerebellum features two lateral hemispheres
  and a median ridge called the vermis.
• The surface of the cerebellum consists of many
  laminae called folia. In the cerebellum, like the
  cerebrum, the white matter is central, and the
  gray matter is peripheral in the cerebellar
  cortex.
• The cerebellum is critical to the accurate
  timing and execution of movements it acts to
  smooth and coordinate muscle activity.
• The pons is ventral to the cerebellum, and its
  surface possesses visible transverse fibers that
  form a bridge from one hemisphere of the
  cerebellum to the other.

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Myelencephalon

• The myelencephalon becomes the medulla oblongata
  in the adult.
• It is the cranial continuation of the spinal
  cord
• The medulla oblongata (often simply called the
  medulla)
• contains a number of important autonomic centers
  and nuclei for cranial nerves.

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Ventricular System

• The ventricles of the brain are the remnants of
  the lumen of the embryonic neural tube.
• Right and left lateral ventricles lie within the
  respective cerebral hemispheres.
• They communicate with the midline third ventricle
  by way of the interventricular foramina.
• Most of the third ventricle is surrounded by the
  diencephalon.
• The third ventricle connects with the fourth
  ventricle by way of the mesencephalic aqueduct
  (cerebral aqueduct) passing through the midbrain.
• The fourth ventricle, between the cerebellum
  above and pons and medulla below, communicates
  with the subarachnoid space surrounding the CNS
  through two lateral apertures.

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• Each ventricle features a choroid plexus
• a tuft of blood capillaries that protrudes into
  the lumen of the ventricle.
• The plexus of capillaries is covered by a layer
  of ependymal cells that are continuous with the
  lining membrane of the ventricles.
• Cerebrospinal fluid (CSF), filling the
  ventricular system and surrounding the CNS, is
  formed primarily by the choroid plexuses, with a
  smaller contribution made by the ependyma lining
  the ventricles.
• CSF is a modified transudate, formed primarily
  through active secretion by the ependymal cells,
  especially those of the choroid plexuses.

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Meninges

• The coverings of the brain and spinal cord are
  the meninges (singular meninx).
• They include, from deep to superficial
• the pia mater
• the arachnoid
• the dura mater.
• The pia mater, the deepest of the meninges, is a
  delicate membrane that invests the brain and
  spinal cord, following the grooves and
  depressions closely.
• The pia mater forms a sheath around the blood
  vessels and follows them into the substance of
  the CNS.

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Meninges

• The arachnoid
• arises embryologically from the same layer as the
  pia mater but separates from it during
  development so that a space forms between them.
• Because of the weblike appearance of these
  filaments, this middle layer is called the
  arachnoid (arachnoidea, arachnoid mater).
• Together, the pia mater and arachnoid constitute
  the Ieptomeninges (from the Latin word lepto,
  delicate), reflecting their fine, delicate
  nature.
• The space between the two layers is the
  subarachnoid space.
• It is filled with CSF.

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Meninges

• The Dura mater is the tough fibrous outer
  covering of the CNS.
• Within the cranial cavity the dura mater is
  intimately attached to the inside of the cranial
  bones and so fulfills the role of periosteum.
• It also forms the falx cerebri, a median
  sickle-shaped fold that lies in the longitudinal
  fissure and partially separates the cerebral
  hemispheres.
• Another fold of dura mater, the tentorium
  cerebelli, runs transversely between the
  cerebellum and the cerebrum.
• In some locations within the skull, the dura
  mater splits into two layers divided by channels
  filled with blood. These dural sinuses receive
  blood from the veins of the brain and empty into
  the jugular veins.
• They are also the site of reabsorption of CSF
  back into the circulation.

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Spinal Cord

• The spinal cord is the caudal continuation of the
  medulla oblongata.
• Unlike that of the cerebrum, the spinal cords
  gray matter is found at the center of the cord,
  forming a butterfly shape on cross-section.
• Myelinated fiber tracts, the white matter,
  surround this core of gray matter.
• A spinal cord segment is defined by the presence
  of a pair of spinal nerves. Spinal nerves are
  formed by the conjoining of dorsal and ventral
  roots.

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Spinal Cord

• Sensory neurons of neural crest origin are
  present in aggregates, called dorsal root
  ganglia, lateral to the spinal cord.
• The neurons within these ganglia are
  pseudounipolar
• they give rise to axons that enter the dorsal
  horn of the spinal cord and other fibers that
  join with motor fibers from the ventral horn
  neurons to become a spinal nerve extending into
  the periphery.
• The ventral root of the spinal nerve consists
  largely of motor fibers that arise from the nerve
  cells in the ventral horn of the spinal cord.
• The dorsal and ventral roots unite to form the
  spinal nerve close to the intervertebral foramen
  between adjacent vertebrae.

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