Histology of Nervous Tissue

Two types of neuroglia produce myelin sheaths:

Schwann cells (in the PNS) and oligodendrocytes (in the CNS). Schwann cells begin to form myelin sheaths around axons during fetal development

Structural Classification

STRUCTURAL CLASSIFICATION Structurally, neurons are classified according to the number of processes extending from the cell body

Neurons

A nerve cell, consisting of a cell body, dendrites, and an axon. neurons provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions.

Action Potential (nerve impulse)

An electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

Stimulus

Any change in the environment that is strong enough to initiate an action potential.

Ependymal cells

Appearance: Epithelial cells arranged in a single layer Range in shape from cuboidal to columnar Many are ciliated Function: Line ventricles of the brain-spaces filled with cerebrospinal fluid-and the central canal or spinal cord Form cerebrospinal fluid and assist in its circulation

Satellite cells

Appearance: Flattened cells arranged around cell bodies of neurons in ganglia Function: Support neurons in PNS ganglia

Schwann cell

Appearance: Flattened cells that encircle PNS axons Function: Produce part of the myelin sheath around a single axon of a PNS neuron Participate in regeneration of PNS axons

Microglia

Appearance: Small cells with few processes Derived from mesodermal cells that also give rise to monocytes and macrophages Function: Protect the CNS cells from disease by engulfing invading microbes Clear away the debris of dead cells Migrate to areas of injured nerve tissue

Oligodendrocytes

Appearance: Smaller than astrocytes, with fewer processes Round or oval cell body Function: Form supporting network around the CNS neurons Produce myelin sheath around several adjacent axons of CNS neurons

Astrocytes

Appearance: Star shaped with many processes Function: Help maintain appropriate chemical environment for the generation of neuron action potentials Provide nutrients to neurons Take up excess neurotransmitters Participate in the metabolism of neurotransmitters Maintain proper balance of calcium and potassium ions Assist with the migration of neurons during brain development Help form the blood-brain barrier

Dendrites

Are the receiving or input portions of a neuron. The plasma membranes of dendrites (and cell bodies) contain numerous receptor sites for binding chemical messengers from other cells. Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a treeshaped array of processes extending from the cell body.

Classification of Neurons

Both structural and functional features are used to classify the various neurons in the body.

Neuroglia of the CNS

Can be classified on the basis of size, cytoplasmic processes, and intracellular organization into four types: astrocytes, oligodendrocytes, microglia, and ependymal cells

Functional Classification

Functionally, neurons are classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS.

Neuroglia

In contrast to neurons, do not generate or propagate action potentials, and they can multiply and divide in the mature nervous system. In cases of injury or disease, multiply to fill in the spaces formerly occupied by neurons.

Neuroglia of the PNS

Neuroglia of the PNS completely surround axons and cell bodies. The two types of glial cells in the PNS are Schwann cells and satellite cells

Electrical Excitability

Like muscle cells, neurons possess the ability to respond to a stimulus and convert it into an action potential.

Parts of a Neuron

Most neurons have three parts: (1) a cell body, (2) dendrites, and (3) an axon

Myelination

The axons of most mammalian neurons are surrounded by a multilayered lipid and protein covering, called the myelin sheath. This sheath is produced by the neuroglia. The amount of myelin increases from birth to maturity. The myelin sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. Axons with such a covering are said to be myelinated, whereas those without it are unmyelinated.

Synapse

The site of communication between two neurons or between a neuron and an effector cell. The tips of some axon terminals swell into bulb-shaped structures called synaptic end bulbs. Synaptic end bulbs contain many minute membrane enclosed sacs called synaptic vesicles that store a chemical neurotransmitter.

Cell Body

Where a neuron synthesizes new cell products or recycles old ones. also known as the perikaryon or soma The cell body contains a nucleus surrounded by cytoplasm that includes typical organelles such as lysosomes, mitochondria, and a golgi complex. The cell body also contains prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components. The cytoskeleton includes neurofibrils and microtubules. Neurofibrils are composed of bundles of intermediate filaments that provide the cell shape and support. Microtubules assist in moving materials to and from the cell body and axon.

Interneurons or association neurons

are mainly located within the CNS between sensory and motor neurons. Integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure.

Schwann cells in the PNS

begin to form myelin sheaths around axons during fetal development. The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the neurolemma (sheath of Schwann). Gaps in the myelin sheath, called nodes of Ranvier, appear at intervals along the axon

Motor or efferent neurons

convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves Motor neurons are multipolar in structure.

Sensory or afferent neurons

either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure.

Unipolar Neurons

have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body These neurons are more appropriately called pseudounipolar neurons because they begin in the embryo as bipolar neurons.

Bipolar Neurons

have one main dendrite and one axon They are found in the retina of the eye, the inner ear, and the olfactory

Oligodendrocytes in the CNS

myelinate parts of many axons as Schwann cells myelinate part of a single PNS axon.

Axon

propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a long thin cylindrical projection that often joins the cell body at a cone-shaped elevation called the axon hillock. The first part of the axon is called the initial segment. A membrane known as the axolemma surrounds the cytoplasm or axoplasm. The axon and its collaterals end by dividing into many fine processes called axon terminals

Multipolar Neurons

usually have several dendrites and one axon Most neurons in the brain and spinal cord are of this type, as well as all motor neurons