2. Nerve tissue

neuron

A nerve cell

receive, analyze, conduct and transmit information

nerve tissue contains

neurons

non-neuronal cells called glial cells or glia

Characteristic cell and matrix components of the peripheral and central nervous systems

Neurons consists of

dendrite(s), a cell body (soma), and an axon that may or may not be myelinated

CNS

brain + spinal cord

Interneurons

connect and send information between sensory neurons and motor neurons.

located within the central nervous system

PNS

nerves & ganglions

PNS Carry sensory

sensory (afferent) inputs to the CNS

Sensory or afferent neurons send information from sensory receptors in skin, eyes, nose, tongue, ears, etc. toward the central nervous system

PNS Carry motor

Carry motor (efferent) outputs from CNS to skeletal, cardiac, smooth muscles of blood vessels, organs and glands

Motor or efferent neurons send information away from the central nervous system to muscles or glands. In response to impulses, muscles contract and glands secrete

2 main functional subdivisions of nervous system

Somatic (voluntary/conscious) control

Autonomic (involuntary/unconscious) control

• parasympathetic 

• sympathetic

• enteric digestive

Parasympathetic

located between spinal cord and medulla

Rest and digest/Feed and breed

Sympathetic

located near thoracic and lumbar regions of spinal cord

Fight or flight response (HR, rate of respiration, pupillary response,etc

in PNS: clusters of neurons with associated nerve fibers and supporting cells are referred to as

ganglia

In CNS: clusters of neurons are referred to as

"nuclei”

Supporting Cells (names in CNS vs PNS)

non-conducting cells that are in intimate positions with neurons

CNS: Neuroglia aka Glia

PNS:

• Schwann cells: surround processes of nerve cells separating them from ECM

• Satellite cells: surround nerve cell bodies (analogous to Schwann)

Functional Components of neuron

Cell body (perikaryon)

Axon

Synapse

Dendrites

Axon

long process (usually one) extending from the cell transmitting impulses

Synapse

specialized terminal → another neuron or effector cell

Dendrites

shorter processes that transmit impulses from the periphery (other neurons) towards the cell body

cell processes that receive action potentials from other neurons

One neuron may have many dendrites each with numerous branches

Are never covered with myelin

Nissl substance (Nissl body)

Ribosomes and rER are seen in clumps 

CYTOSKELETON IN NEURONS

Microfilaments (actin) 

Microtubules

neurofilaments

Microtubules

move materials throughout the neuron, especially in the axon

neurofilaments

function to support the dendrites and axons

Intermediate filaments

silver stain

cell body (soma)

Contains a nucleus and cytoplasm called perikaryon (‘peri-kar-ee-in’)

No myelin associated with the soma

Nissl substance is present

Responsible for the maintenance of the neuron because most protein synthesis occurs here

receives information from other neurons via synapses

soma morphology

reflects its high metabolic activity

Little heterochromatin in the nucleus

A very prominent nucleolus

Nissl substance (Nissl bodies) are observed in the perikaryon

• indications of a lot of protein synthesis

Axon

Each neuron has only ONE AXON

Action potentials travel along axons from the cell body → down axon → and toward synapses at the axon terminal

Axons make synaptic contacts with other neurons or with target organs such as muscle and glands

Depending on the diameter of the axon, it may be myelinated

do not have Nissl substance and must receive all proteins from the cell body

Myelin is made by

Schwann cells (supporting cell in PNS)

staining neurons in the CNS

The nucleus and perikaryon of the soma are basophilic with H/E

You can usually see both axons and dendrites and their connection to the soma of neurons in the central nervous system

staining neurons in the PNS

You don’t usually see the axons or dendrites or their connection to the
soma in neurons of the peripheral nervous system

NEURON CLASSIFICATION: PSEUDO-UNIPOLAR

do not have separate dendrites and an axonal process, but rather one
branched process that serves both functions

Dorsal root ganglia contain cell bodies of sensory neurons (located in DR ganglion near but outside spinal cord)

Types: Sensory neurons with cell bodies in spinal and cranial nerve ganglia

Examples: dorsal root ganglia and cranial nerve ganglia

T-shaped process

A central process (axon) extends toward the spinal cord.

Called the central axon in the diagram

peripheral process

A afferent axon that carries sensory information from periphery

respond to touch, temperature, pain, stretch)

Called the peripheral axon in the diagram

NEURON CLASSIFICATION: BIPOLAR

One true dendrite with variable branching

One axon

Types: Sensory neurons of taste, smell, hearing, and sight

NEURON CLASSIFIC ATION: MULTIPOLAR

Many dendrites

The extent of branching and the shape of the dendritic tree is characteristic of different neuron types

One axon

Example: Spinal motor neurons

Functionally, these neurons either conduct impulses that will cause activity
such as the contraction of muscles or conduct impulses that permit
communication between neurons within the central nervous system

Myelin + cells that make it (PNS + CNS)

surrounds axons only

an extension of a cell membrane

In PNS, the cells that form myelin are called Schwann cells

In CNS, the cells that form myelin are called oligodendrocytes

internode/internodal segment

Schwann cells or oligodendrocytes form myelin around 1 or 2 mm of the length of an axon.

node of Ranvier or a node

The region between myelin

histology of myelin (light microscope)

Axons are eosinophilic (red arrows)

Schwann cell nuclei are basophilic (blue arrows)

Myelin is unstained (X)

Nodes of Ranvier (black arrows) are “pinches” along the axons

histology of myelin (TEM)

Myelin is osmiophilic (binds to osmium tetroxide)

used in the preparation of tissues for TEM

Myelin appears very electron dense using TEM

MYELIN ASSOCIATION WITH AXONS

In PNS: One Schwann cell forms myelin around only one axon 

Schwann cells can surround many axons with cytoplasm but do not form any myelin

In CNS: One oligodendrocyte myelinates many axons

PERIPHERAL NERVOUS SYSTEM (PNS) ORGANIZATION FOR HISTOLOGY PURPOSES

Neuron cell bodies (soma)\

Groups of soma in the PNS are called ganglia

Soma sometimes called ganglion cells

Satellite cells surround ganglion cells as glia

Axons

Groups of axons in the PNS are called nerves

Schwann cells form myelin around axons or surround axons without forming myelin

Blood vessels

Connective tissue investments

ganglion (s), ganglia (pl)

any group of neuron cell bodies in PNS 

Histology of ganglia

Histology: Soma are round, have a large nucleus with prominent nucleolus, Nissl substance in cytoplasm (basophilic) and possibly lipofuscin (brown, represents waste pigment from breakdown of blood cells)

Satellite cells surround the soma as a protective covering

Loose connective tissue surrounds the soma

PNS NERVE

a bundle of axons held together by three types of connective tissue investments

The axons are also called nerve fibers

Peripheral nerves are mixtures of sensory and motor axons, and unmyelinated and myelinated axons

HISTOLOGY OF PERIPHERAL NERVES: LM

In longitudinal section peripheral nerve appears wavy

The paler staining regions represent areas of myelin. The basophilic dashes are the nuclei of Schwann cells

In cross section peripheral nerve appears like a group of small circles

HISTOLOGY OF PERIPHERAL NERVES: TEM

Myelinated axons have an electron dense band around them (M)

Unmyelinated axons are groups of axons surrounded by Schwann cell cytoplasm (NM)

endoneurium

a layer of collagen type I and reticular fibers (collagen type III) that surrounds individual axons

peripheral nerves

fascicles + perineurium

Bundles of axons = fascicles

Connective tissue that surrounds fascicles = perineurium

peripheral nerves

epineurium

surrounds the entire nerve

Blood vessels that supply the nerve are located here.

peripheral nerves

Receptors

terminals of sensory nerves that receive and respond to stimuli

transducers that convert energy in the environment into impulses that are conducted along axons

classified based on the type of stimulus that they detect

mechanoreceptor

a sensory receptor that responds to mechanical pressure or distortion

thermoreceptor

responds to heat and cold

nociceptor

responds to pain

nerves in dental pulp are only nociceptors