MODULE 15A - [Anatomy 1.0] GENERAL ORGANIZATION OF THE NERVOUS SYSTEM

NEURON

• Excitable cells that are specialized for the reception of stimuli and the conduction of the nerve impulse

CELL BODY

• Consists essentially of a mass of cytoplasm in which a nucleus is embedded

• Bounded externally by a plasma membrane

  • Surface projects one or more processes called neurites (e.g. dendrites and axons)

Dendrites

neurites responsible for receiving information

Axons

a single, long tubular neurite that conducts impulses away from the cell body

nerve fibers

Dendrites and axons are often referred to as ______ ______

Nissl Bodies (Nissl Substance)

• Clumps of rough endoplasmic reticulum that are found throughout the cytoplasm of the cell body

• synthesizes proteins within the cell body

Axon Hillock; Axon

Nissl Bodies are absent in _____ and within the _____

Axon Hillock

• A small conical elevation on the cell body that gives rise to the axon

• Region of the cell body close to the axon

DENDRITES

• Short processes of the cell body

• Often branch profusely to increase the surface area of the reception of axons from other neurons.

• Cytoplasm resembles that of the cell body

• Function: receive nerve impulse toward the cell body.

AXON

• Longest process of the cell body

• Arises from axon hillock

• branches profusely before their termination

• Function: always conducts impulses away from the cell

  body

  • Except for axons of unipolar neurons which may also carry an impulse toward the body

Myelin Sheath

• Outer covering of the axon

• Multi-layered phospholipid

• Function: increases the conduction velocity of the nerve impulses along the axon

• Unipolar

• Bipolar

• Multipolar

Enumerate the morphological classification of neurons based on the number of its neurites

UNIPOLAR NEURONS

• Cell body has a single neurite (or axon) that branches within a short distance from the cell body

• One end is preceding to some peripheral structure and the other end enters the central nervous system (CNS)

• The fine terminal branches at the peripheral end of the axon are often referred to as the dendrites.

• Single neurite divides a short distance from cell body

• Location: Posterior root ganglion

BIPOLAR NEURONS

• Have an elongated cell body

• From each end emerges a single neurite

• A total of 2 neurites, which are both axons, with the fine terminal branches at the peripheral end of one of the axons, also being referred to as dendrites

• Single neurite emerges from either end of cell body

• Location: Retina, sensory cochlea, and vestibular ganglia

MULTIPOLAR NEURONS

• Most common kind

• Have several neurites arising from the cell body, which are mostly dendrites and one long axon

• Many dendrites and one long axon

• Location: Fiber tracts of the brain and spinal cord, peripheral nerves, and motor cells of spinal cord

• Golgi Type I

• Golgi Type II

Enumerate the morphological classification of neurons based on the number of its size

Golgi Type I

• Size: Medium to large

• Arrangement of Neurites: Single long axon

• Location: Fiber tracts of brain and spinal cord, peripheral nerves, and motor cells of spinal cord

Golgi Type II

• Size: Small to Medium

• Arrangement of Neurites: Short axon that often resemble its dendrites

• Location: Cerebral and cerebellar cortex

• CNS

  • Ependymal Cells

  • Oligodendrocytes

  • Astrocytes

  • Microglia

• PNS

  • Satelite Cells

  • Schwann Cells

Enumerate the types of Neuroglia in the CNS and PNS

EPENDYMAL CELLS

• Line the fluid filled cavities of the brain and the central

  canal of the spinal cord.

• Made up of:

  • Ependymocytes

  • Choroidal epithelial cells

Ependymocytes

Assist in the circulation of the cerebrospinal fluid (CSF) within the cavities by the movements of their cilia

Choroidal epithelial cells

Involved in the production and secretion of the CSF from the choroid plexuses

MICROGLIAL CELLS

• Phagocytes that arise from macrophages

• Aid in removal of damaged neurons and infectious

  agents within the CNS

MACROGLIAL CELLS

• Further divided into 4 cell types

• Two are found within the CNS:

  • Astrocytes and Oligodendrocytes

• The other two are found in the peripheral nervous system (PNS):

  • Schwann cells and Satellite (or capsular) cells

Astrocytes

• Most numerous cells in the CNS

• Have small cell bodies with branching processes

  that extend in all directions.

• Has two types

  • Fibrous

  • Protoplasmic

• Contains Perivascular Feet

• Functions:

  • Serve as a supporting framework for neurons and nerve fibers

  • Serve as “electrical insulators” between neurons → prevent axon terminals from influencing neighboring and unrelated neurons

• Serve as phagocytes by taking up degenerating

  synaptic axon terminals.

Perivascular feet

• Expanded processes of astrocytes on blood vessels

• Form an almost complete covering on the external surface of the capillaries

• Important for the blood-brain barrier

blood-brain barrier

selectively allow and block the passage of materials from the blood to the CNS.

• Covering the synaptic contacts between neurons; forming barriers

• Taking up neurotransmitter substances

• Controlling the electrolyte balance of the

  CNS

Mechanisms of how astrocytes serves as electrical insulators between neurons

replacement gliosis

Process where astrocytes fill in the spaces previously occupied by the neurons following the death of neurons due to disease,

Oligodendrocytes

Forms the myelin sheath of axons of neurons in the CNS

myelin sheath

Provides axons with an insulating coat and greatly increases the speed of nerve conduction

60 nerve fibers (axons)

How many nerve fibers (axons) can 1 oligodendrocyte myelinate

Schwann Cells

Produce myelin for the nerves of the

1 segment of an axon

How many nerve fibers (axons) can 1 Schwann cell myelinate

Nodes of Ranvier

Areas of interruption or gaps along the myelin sheath that covers the axons of neurons

• Essential in the speed and timing of delivery of impulses from 1 neuron to another

Satellite Cells (Capsular Cells)

Glial cells that envelope the peripheral and central processes of each neuron from the autonomic ganglia

Gliomas

Tumors of Neuroglia are also called?

40% to 50% ; ependymomas

Tumors of Neuroglia account for _____ to _____ of intracranial tumors and are highly invasive except for _____

Tumors of astrocytes (astrocytomas and glioblastomas)

Most common tumors of neuroglia

Meninges

The brain and spinal cord are surrounded by supporting membranes called _____

Dura mater → Arachnoid mater → Pia mater

PAD = Padding of the Brains

Enumerate the meninges from the outermost to the innermost membrane

DURA MATER

• Outermost meninx

• A strong fibrous membrane that consists of two layers:

  • Endosteal layer

  • Meningeal layer

Endosteal layer (Dura Mater)

a periosteum surrounding the inner surface of the cranial bones

Meningeal layer (Dura Mater)

Forms 4 folds within the cranium, which are the Falx Cerebri, Falx Cerebelli, Tentorium Cerebelli, and Diaphragma Sellae

• Falx Cerebri

• Falx Cerebelli

• Tentorium Cerebelli

• Diaphragma Sellae

Enumerate the folds of the meningeal layers of the Dura Mater

Falx Cerebri

Located at the midline between the two cerebral hemispheres

Falx Cerebelli

Separates the two cerebellar hemispheres and lies inferior to the tentorium cerebelli

Tentorium Cerebelli

Forms a roof over the posterior cranial fossa, shielding the superior surface of the cerebellum, and supports the occipital lobes of the cerebral hemispheres

Diaphragma Sellae

• Small circular fold of dura that forms the roof of the sella turcica, protecting the superior surface of the pituitary gland.

• It has a tiny opening in the middle segment that allows the passage of the stalk of the pituitary gland.

ARACHNOID MATER

• A thin, delicate membrane that loosely surrounds the

  brain and the spinal cord.

• Lies between the dura mater and the pia mater.

PIA MATER

• A membrane that closely invests the brain, covering

  the gyri and descending into the deepest sulci.

• Highly vascular and contains the cerebral arteries

  entering the substance of the brain and spinal cord.

• EPIDURAL SPACE

• SUBDURAL SPACE

• SUBARACHNOID SPACE

Enumerate the Meningeal spaces

EPIDURAL SPACE

A potential space located between the bone and the dura mater.

• Potential spaces that may be filled with blood due to traumatic tearing of blood vessels located in these spaces.

Epidural Hemorrhage

The presence of blood inside the epidural space

SUBDURAL SPACE

Potential space located between the dura and the arachnoid.

• Potential spaces that may be filled with blood due to traumatic tearing of blood vessels located in these spaces.

Subdural hemorrhage

The presence of blood inside the Subdural space

SUBARACHNOID SPACE

• Between the arachnoid and the pia mater

• Contains CSF and communicates with the ventricles of the brain where CSF is formed.

Subarachnoid hemorrhage

The presence of blood inside the subarachnoid space

CENTRAL NERVOUS SYSTEM

• Consists of the brain and spinal cord

• Main centers for correlation and integration of nervous information

• Covered by meninges and suspended in the CSF

• Protected by the skull and vertebral column

• Interior of the CNS is organized into gray and white

  matter

GRAY MATTER

Consists of nerve cells (neural body) embedded in neuroglia, thus the gray color

WHITE MATTER

• Consists of nerve fibers (neural axons and dendrites) embedded in neuroglia

• The white color is due to the presence of lipid material in the myelin sheath

BRAIN

• Lies in the cranial cavity and is continuous with the spinal cord through the foramen magnum

• It is conventionally divided into three major divisions

• Forebrain

• Midbrain

• Hindbrain

Enumerate the three divisions of the brain

• Cerebrum

• Diencephalon

Enumerate the two parts of the forebrain

• Pons

• Medulla Oblongata

• Cerebellum

Enumerate the parts of the Hindbrain

• Midbrain

• Pons

• Medulla Oblongata

Enumerate the parts of the Brainstem (Note this is different from the three major divisions of the brain)

CEREBRUM

• The largest part of the brain

• Consists of two cerebral hemispheres (left and right)

• Each hemisphere extends from the frontal to the occipital bones in the skull, superior to the anterior and middle cranial fossae, and posteriorly it lies above the tentorium cerebelli.

• The two cerebral hemispheres are separated by a deep

  medial longitudinal fissure, into which projects the falx cerebri.

corpus callosum

The white matter that connects the two hemispheres of the brain

Cortex

• Surface layer of the cerebral hemispheres

• Composed of gray matter.

• It has folds called gyri or gyrus

• Several large sulci are used to subdivide the cerebral hemispheres into lobes, which are named from the bones under which they lie.

• Part of the cerebrum

gyri or gyrus

• This greatly increases the surface area of the cortex.

• Part of the cerebrum

Sulcus or sulci

• The gyri or gyrus are separated by folds called

• Part of the cerebrum

Inner Core of the Brain

• Consist of white matter which contains several masses of gray matter, called basal nuclei or ganglia.

• Part of the cerebrum

Corona Radiata

• Fan shaped collection of nerve fibers passing in the

  white matter to and from the cerebral cortex to the brain stem.

• Converges on the basal nuclei

• Part of the cerebrum

Internal Capsule

• Convergence of corona radiata on the basal nuclei which passes in between the basal nuclei

• Part of the cerebrum

Caudate Nucleus

• Tailed nucleus on the medial side of the internal capsule

• Part of the cerebrum

Lentiform Nucleus

• Lens-shaped nucleus on the lateral side of the internal capsule

• Part of the cerebrum

Lateral Ventricles

• The cavities within each cerebral hemisphere (anterior horn and inferior horn).

• Part of the cerebrum

DIENCEPHALON

Lies below the cerebral hemispheres and consists of a dorsal thalamus and a ventral hypothalamus.

Thalamus

• A large egg-shaped mass of gray matter that lies on either side of the third ventricle.

• Part of the Diencephalon

Hypothalamus

• Forms the lower part of the lateral wall and floor of

  the third ventricle.

• Part of the Diencephalon

MIDBRAIN

A narrow part of the brain that connects the forebrain to the hindbrain.

cerebral aqueduct

• narrow cavity which connects the 3rd and the 4th ventricles.

• Part of the Midbrain

PONS

• Part of the hindbrain

• Situated between the midbrain and the medulla oblongata and anterior to the cerebellum.

• Its name means “bridge”

  • Comes from the large number of transverse fibers that connect the two cerebellar hemispheres on its anterior aspect.

MEDULLA OBLONGATA

• Conical in shape

• Connects the pons to the spinal cord

  • Conduit for ascending and descending nerve

    fibers

CEREBELLUM

• Lies within the posterior cranial fossa, posterior to the pons and medulla oblongata.

• It consists of two hemispheres

• Also known as the “little brain”

• It also has a cortex composed of gray matter and an inner core of white matter, with several masses of gray matter or nuclei.

vermis

The two hemispheres of the cerebellum is connected by a median portion called

4th ventricle

The medulla pons and cerebellum surround the cavity filled with cerebrospinal fluid called the

FOURTH VENTRICLE

• Connected superiorly to the 3rd ventricle through the

  cerebral aqueduct

• Inferiorly, it is continuous with the central canal of the spinal cord.

*No Answer*

Parts of the brain seen in MRI T1 weighted sagittal section of the head

SPINAL CORD

• Lies below the brain

• Situated within the vertebral canal and is also surrounded by three meninges, the dura, arachnoid, and pia mater.

Foramen Magnum; Medulla Oblongata; Lumbar Region; Conus Medullaris

The Spinal Cord begins superiorly at the _____ below the _____ and terminates inferiorly in the _____ into the _____.

Phylum terminal

Prolongation of the pia mater that attaches the conus medullaris to the back of the coccyx.

gray matter; white matter

Unlike the brain's structure, the spinal cord has an inner core of _____ and an outer covering of _____.

GRAY MATTER

seen as an H-shaped pillar with anterior and posterior gray horns or columns.

Gray commissure

• The Anterior and Posterior gray horn of the gray matter of the spinal cord is connected by

• This part also contains the central canal

WHITE MATTER

• Divided into anterior, lateral, and posterior columns

31 pairs

Along the entire length of the spinal cord are attached _____ of spinal nerves by the anterior roots and posterior roots.

Anterior Roots

• Efferent/Motor fibers

• Carry nerve impulses away from the CNS

Posterior Roots

• Afferent/Sensory fibers

• Carry nerve impulses toward the CNS

• Has a posterior root ganglion

  • A swelling that contains cell bodies of sensory nerve fibers.

spinal nerve; intervertebral foramen

Anterior and Posterior roots unite to form a _____ that exits through its respective ____.