Sheet: microscopic structures

of the ns

Done by: Rasha Rakan and

Nisreen Obiedat

Lecture Objectives

• Classify the types of neurons.

• Describe the structure of the different parts of neurons.

• Describe the types of glia cells and their functions.

• Describe the process of myelination of myelinated axons.

• Describe the structure of peripheral nerves.

• Describe the structure of ganglia (sensory and autonomic).

Nervous Tissue

• Relatively little intercellular space

• Cells (neurons or neuroglia) rest on a continuous

basal lamina

• Some cells have microvilli, cilia

• Strong tendency for cells to be bound to one

another

• Clasping structures between connected cells

called synapses

Neurons

• Functional unit of nervous

system

• Have capacity to produce action

potentials and transmit it

– electrical excitability

• Cell body

• Cell processes = dendrites (transmit change in voltage towards the cell body)& axons (transmit change in voltage away from the cell body)

Parts of a Neuron

Axons or

Dendrites

Cell body

Neuroglial cells

Nucleus with

Nucleolus

Diversity in Neurons

• Both structural and functional features are used to

classify the various neurons in the body.

• On the basis of the number of processes extending

from the cell body (structure), neurons are classified

as multipolar and bipolar

• Most neurons in the body are interneurons and are

often named for the histologist who first described

them or for an aspect of their shape or appearance.

Structural Classification of Neurons

• Based on number of processes found on cell body

– multipolar = several dendrites & one axon

• most common cell type

– bipolar neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory

– unipolar neurons = one process only(develops from a bipolar)

• are always sensory neurons

Functional Classification of Neurons

Peripherally :

• Sensory (afferent) neurons

– transport sensory information from skin, muscles, joints, sense organs & viscera to CNS

• Motor (efferent) neurons

– send motor nerve impulses to muscles & glands

Inside the CNS regardless of their function:

• Interneurons (association) neurons

– connect sensory to motor neurons

– 90% of neurons in the body

Association or Interneurons All interneurons and motor neurons are bipolar if it is preganglionic or somatic motor neuron their cell body is inside the CNS and the rest is outside If the motor neuron was postganglionic then all of its parts are outside the CNS Sensory neurons are either bipolar (only special sensors, because the receptors are relatively close to the cell body like retina where the dendrites are in the external layer and the cell body in internal layer) or unipolar ( somatic or visceral sensory neurons, because their cell bodies are in dorsal root ganglia while their receptors are in periphery) The explanation is: that the receptor is far from the cell

body, if that space between the cell body and the receptors was dendrites it isn’t be able to generate AP, as soon as it reaches the trigger zone it will disappear.

Neuronal Structure & Function

Cell bodies of sensory cells in general are outside the CNS with few exceptions that we will mention later

Cell body • Also called perikaryon

• A trophic center

• Receives a great number of nerve endings that convey excitatory or inhibitory stimuli

• Could be small (4-5μm) or large (150 μm) that is visible by naked eye

it has nucleus and nucleolus, so it is responsible for all process and nourishment of other parts of the neuron. If we cut the cell body the axon will degenerate because it lost its supply.

• Cell organelles: – Single, large, euchromatic (pale-staining) nucleus with prominent nucleolus

– Nissl bodies (chromatophilic substance) it appears as dark dots, and it is the reason of grayish colors of neurons (that’s why we call it gray matter)

• rough ER & free ribosomes for protein synthesis it synthesizes neurotransmitters( all neurons are active means the are productive and are abundant with rough ER, Golgi apparatus and mitochondria)

– neurofilaments (10nm) give cell shape and support. When stained by silver form

neurofibrils that can be seen by light microscope intermediate fibers of cytoskeleton, they are well developed, supports the cell body and helps the body with its inside processes

– microtubules (22nm) move material inside cell very well developed because it is responsible for transporting materials from cell bodies towards periphery

– lipofuscin granules (harmless aging .)They are residual lysosomal products. Indicates that that the cell is aging cell, brownish in color

Neuronal processes

• The dendrites are receiving or input

portions of a neuron. It can’t generate AP but it can

transmit change of voltage .The membrane of cell body is able of

summation of voltage changes to generate AP.

• Theaxonconducts nerve impulses from the

neuron to the dendrites or cell body of

another neuron or to an effector organ of the

body (muscle or gland).

Dendrites

• Extend from cell body

• In proximal portion, continues cytoplasmic character of perikaryon

• Usually irregular contour and spines (microvilli-like projections)

• Usually multiple

• Rarely myelinated, usually unsheathed

• Ramifies by branching at acute angles

• If cell body is in CNS then dendrites remain in CNS

• Conducts impulses towards the cell body

Axons • Conduct impulses away from cell body

• Long, thin cylindrical process of cell

• One per neuron

• Do not contain ribosomes

– Dependence on perikaryon

• If axon is severed, its peripheral parts degenerate and die

• Arises at axon hillock (its contents are not similar to cell body contents) Impulses arise from initial segment (trigger zone)

• Branches to form collaterals at obtuse angles they are the base of networking inside CNS. Any info inside a neuron has to be copied to other neurons. It can reach hundreds in number

• end in fine processes called axon terminals

• Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters

Axons are different in sizes and according to that not all of them are mylinated. A and B fibers are large mylinated and thus AP are quickly transmitted. ( 100 time more than small unmylinated fibers like type C.

How to differentiate between dendrites and axons

1. Dendrites are multiple, axons are only one for each neuron 2. Dendrites proximal part is conical in shape and the part that connects them to the cell body has the same contents found inside the cell body (like Nissl bodies) this doesn’t exist in axons 3. Dendrites start to branch with sharp-angled branches, while axons branches have 90 degree angle. 4. Dendrites are unmylinated and shorter than axons

A summry

• Half of the volume of the CNS

• Smaller cells than neurons

• 50X more numerous

• Cells can divide

– rapid mitosis in tumor formation (gliomas)

• 4 cell types in CNS

– astrocytes, oligodendrocytes, microglia & ependymal

• 2 cell types in PNS

– schwann and satellite cells

Neuroglial Cells

Astrocytes

• Star-shaped cells

• Form blood-brain barrier by covering blood capillaries

• Metabolize neurotransmitters ;forming part of the recycle process

• Regulate K+ balance

• Provide structural support

• Have many types

Microglia

• Small cells found near

blood vessels

• Phagocytic role -- clear

away dead cells

• Derived from cells

”WBC’s” that also

gave rise to

macrophages &

monocytes

Ependymal cells

(CSF)

•

They’re low columnar epithelial cells lining the

ventricles and they form part of the choroid plexus

*choroid plexus: blood capillaries +epyndymal cells.

Satellite Cells

• Flat cells surrounding neuronal cell bodies in peripheral ganglia

• Support neurons in the PNS ganglia

• Mainly in the dorsal root ganglia

Oligodendrocytes

• Most common glial

cell type

• Each forms myelin

sheath around more

than one axons in CNS

• Analogous to Schwann

cells of PNS

• Huge cells with many

branches so myelinate more

than one axon

Myelination

• A multilayered lipid and protein covering called the

myelin sheath and produced by Schwann cells and

oligodendrocytes surrounds the axons of most

neurons

• The sheath electrically insulates the axon and increases

the speed of nerve impulse conduction

• Myelination works as a capacitor which means that the

action potential travels from node of ranvier to another

without dropping below the threshold

• *cell membrane is the most structure in the cell rich in

lipid.

Schwann

Cell

• Cells encircling PNS axons

• Each cell produces part of the myelin sheath surrounding an axon in the PNS

• Participate in myelination of one axon only unlike oligodendrocytes which participate in myelination of more than one axon

Axon Coverings in PNS • All axons surrounded by a lipid &

protein covering (myelin sheath)

produced by Schwann cells

• Neurilemma is cytoplasm & nucleus of

Schwann cell

– gaps called nodes of Ranvier

• Myelinated fibers appear white

– jelly-roll like wrappings made of

lipoprotein = myelin

– acts as electrical insulator

– speeds conduction of nerve impulses

• Unmyelinated fibers

– slow, small diameter fibers

– only surrounded by neurilemma but no

myelin sheath wrapping

Axolemma: cell membrane of an axon

Myelination in

• Schwann cells myelinate (wrap around) axons in the PNS

during fetal development

• Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath

• Tube guides growing axons that are repairing themselves :

the neurolemma help in preserving the axon pathway so if

any axon is damaged the neuroleema help it in

regenerating

Myelination in the CNS

• Oligodendrocytes myelinate axons in the CNS

• Broad, flat cell processes wrap about CNS axons, but

the cell bodies do not surround the axons

• No neurilemma is formed so there is Little regrowth

after injury is possible due to the lack of a distinct tube

or neurilemma

Gray and

White

Matter

• White matter = myelinated processes (white in color)

• Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color)

– In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter

– In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS

• A nucleus is a mass of nerve cell bodies and dendrites inside the CNS.

Spinal Nerves

Connective tissue coverings of spinal nerves:

• Epineurium :around the entire nerve

perineurium :around each fascicle

• Endoneurium : around each axon

• Fascicles : bundles of axons

Spinal Nerves

Ganglia

• Dorsal root (sensory) ganglia

– Nuclei are large, prominent and central

– Neurons arranged in rows

– Two population of neurons based on size

• Large 100 μm diameter

• Small 10-15 μm diameter

– Surrounded by Satellite (nurse) cells and well developed capsule

– Fixation causes perikaryon to shrink away from capsule

Ganglia • Autonomic ganglia

– Nuclei are eccentric

– No neuronal clusters

– One size neuron-small

– Poorly defined capsule, fewer satellite cells

– Lipofscin granules

– multipolar

Autonomic ganglia Dorsal root ganglia ”sensory”

eccentric Centric

nucleus

absent Present : arrows Neuronal clusters

Small size only Small (10-15)micro m Large (100)micro m

Size of neuron

fewer More Satellite cells

Poor defined Well defined Capsule(C.T)

Multipolar ”autonomic motor neurons & post ganglionic neurons”

Unipolar ”general sensory neurons”

Type of neurons

present Lipofuscin granules

Spinal Cord

• Anterior median fissure

• Posterior median sulcus

• Gray and white commissures

• Central canal

• Anterior, posterior & lateral gray horns

– Anterior horns contain motor neurons

– Posterior horns receive sensory fibers from neurons in the spinal ganglia

• Anterior, posterior & lateral white columns

Internal Anatomy of Spinal Cord

Internal Anatomy of Spinal Cord