Anatomy - General Organization of the Nervous System

Neuron

Excitable cells that are specialized for the reception of stimuli and the conduction of the nerve impulse. Comprises the nerve cell and all its processes

Cell Body

Consists essentially of a mass of cytoplasm in which a nucleus is embedded. Bounded externally by a plasma membrane

Neurites

The cell body surface projects one or more processes called?

Dendrites

Neurites responsible for receiving information

Axons

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

Nissl Bodies (Nissl Substance)

Clumps of rough endoplasmic reticulum throughout the cytoplasm of the cell body. Absent in axon hillock and within the axon. Function: synthesize proteins within the cell body

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. Devoid of Nissl bodies

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 impulses toward the cell body

Axon

Longest process of the cell body. Devoid of Nissl bodies. Arises from axon hillock. Usually do not branch close to the cell body

Axon Terminal

What do you call the distal ends of the axon branches?

Axon

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 conduction velocity of nerve impulse along the axon

Unipolar Neurons

Pure sensory. Cell body has a single neurite (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 dendrites.

Bipolar Neurons

Pure sensory. Have an elongated cell body. From each end emerges a single neutrite

Multipolar Neurons

Most common kind. Have a number of neurites arising from the cell body. Mostly dendrites and one long axon

Golgi Type I

Medium to large. Pyramidal, and commonly multipolar. Has a single long axon and short branches of dendrites

Golgi Type II

Small to medium. Interneuron, resembles a star, and is found in the cerebral and cerebellar cortex. Has a single short axon that often resembles its dendrites

Unipolar Neuron

Single neurite divides a short distance from cell body. Posterior root ganglion

Bipolar Neuron

Single neurite emerges from either end of cell body. Retina, sensory cochlea, and vestibular ganglia

Multipolar Neuron

Many dendrites and one long axon. Fiber tracts of brain and spinal cord, peripheral nerves, and motor cells of spinal cord

Golgi Type I

Single long axon. Fiber tracts of brain and spinal cord, peripheral nerves, and motor cells of spinal cord

Golgi Type II

Short axon that with dendrites resembles a star. Cerebral and cerebellar cortex

Neuroglia

Non-excitable supporting cells. Dividing into 3 basic types

Ependymal Cells

Line the fluid filled cavities of the brain and the central canal of the spinal cord.

Ependymocytes

Assist in the circulation of the cerebral spinal 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 the removal of damaged neurons and infectious agents within the CNS.

Macroglial Cells

Further divided into four cell types. Two found in CNS: Astrocytes and Oligodendroctyes. Two found in PNS: Schwann cells and Satellite cells

Astrocytes

Most numerous cells in the CNS. They have small cell bodies within branching processes that extend in all directions.

Fibrous

Astrocytes found in the white matter

Protoplasmic

Astrocytes found in the gray matter

Perivascular feet

Many of the processes of astrocytes end in expansions on blood vessels, which are called _____________, where they form an almost complete covering on the external surface of capillaries. These are important for the blood-brain barrier to selectively allow and block the passage of materials from the blood to the CNS

Astrocytes

Serve as supporting framework for neurons and nerve fibers. Serve as “electrical insulators” between neurons → prevents axon terminals from influencing neighboring and unrelated neurons

Replacement Gliosis

Following the death of neurons due to disease, astrocytes fill in the spaces previously occupied by the neurons in a process called

Oligodendrocytes

Forms the myelin sheath of axons of neurons in the CNS. Provides axons with insulating coat and greatly increases the speed of nerve conduction

True

T/F: 1 oligodendrocyte → up to 60 nerve fibers (or axons)

Schwann Cells

Produce myelin for the nerves of the PNS

False

T/F: 1 Schwann cell → multiple segment of an axon

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

Tumors of Neuroglia

Also called gliomas. Account for 40 to 50% of intracranial tumors and are highly invasive except for ependymomas. Most common: Tumors of astrocytes (astrocytomas and glioblastomas)

Meninges

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

Dura mater → Arachnoid mater → Pia mater

From the outermost to the innermost membrane, arrange the meningeal layers.

Dura Mater

A strong fibrous membrane that consists of two layers: Endosteal and Meningeal Layer

Endosteal Layer

A periosteum surrounding the inner surface of the cranial bones

Meningeal Layer

Layer of the dura mater which forms 4 folds within the cranium

Falx Cerebri

Folds of the meningeal layer located at the midline between the two cerebral hemispheres

Falx Cerebelli

Folds of the meningeal layer that 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

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

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

Subdural Space

Potential space located between the dura and the arachnoid.

Epidural or Subdural Hemorrhage

Epidural and subdural spaces are potential spaces that may be filled with blood due to traumatic tearing of blood vessels located in these spaces, such as in cases of?

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

Presence of blood inside the subarachnoid space is termed

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 embedded in neuroglia, thus the gray color

White Matter

Consists of nerve fibers 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

Brainstem

A collective term for the midbrain, pons, and medulla oblongata.

Cerebrum

The largest part of the brain

Corpus callosum

The cerebrum consists of two cerebral hemispheres (left and right) which are connected by a mass of white matter called the?

Cortex

Surface layer of the cerebral hemispheres. Composed of gray matter. It has folds called gyri or gyrus, which are separated by fissures called sulci or sulcus.

Sulci (Sulcus)

This greatly increases the surface area of the cortex.

Cortex

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

Inner Core of the Brain

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

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

Internal Capsule

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

Caudate Nucleus

Tailed nucleus on the medial side of the internal capsule

Lentiform Nucleus

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

Lateral Ventricles

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

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.

Hypothalamus

Forms the lower part of the lateral wall and floor of the third ventricle.

Midbrain

A narrow part of the brain that connects the forebrain to the hindbrain. Its narrow cavity is the cerebral aqueduct, which connects the 3rd and the 4th ventricles.

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 connected by a median portion, which is the vermis. 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.

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.

3rd Ventricle

What is the superior connection of the 4th ventricle?

Central Canal

What is the inferior connection of the 4th ventricle?

Spinal Cord

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

Spinal Cord

Begins superiorly at the Foramen Magnum below the medulla oblongata and terminates inferiorly in the lumbar region into the Conus medullaris.

Phylum terminal

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

Gray matter

On cross section, this is seen as an H shaped pillar with anterior and posterior gray horns or columns, united by a thin Gray commissure containing the central canal.

White Matter

Divided into anterior, lateral, and posterior columns. Along the entire length of the spinal cord are attached 31 pairs 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.

Intervertebral Foramen

Anterior and Posterior roots unite to form a spinal nerve that exits through its respective?

Peripheral Nervous System

Consists of cranial nerves, spinal nerves and their associated ganglia.

Cranial Nerves

Consists of 12 pairs which leave the brain and pass through different skull foramina.

Spinal Nerves

Consists of 31 pairs which leave the spinal cord and pass through intervertebral foramina in the vertebral column. Named according to regions of the vertebral column they are associated. Length of the roots increase progressively from above downward.

Cervical region spinal nerve

Roots are short. Run and exit the vertebral column almost horizontally along their respective foramina as compared with roots of lumbar and sacral nerves.

Cauda Equina

Spinal nerves (or lower nerve roots) that exit the vertebral column below the conus medullaris, forming a vertical leash of nerves around the phylum terminal.

Spinal Tap (Lumbar Puncture)

ADULT: Spinal cord terminates at the level of the lower border of the first lumbar vertebra. INFANT: Spinal cord may reach inferiorly to the third lumbar vertebra and the subarachnoid space extends inferiorly as far as the lower border of the second sacral vertebra. Done above or below the L4 vertebra to avoid damage to the spinal cord.

Autonomic Nervous System

Part of the nervous system concerned with the innervation of the involuntary structures. Distributed throughout the CNS and PNS

Sympathetic Nervous System

Triggers the fight-or-flight response