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Nervous System

Nervous Tissue: is composed of:

neurons and supporting cells and extracellular matrix

monitors the environment (internal and external) = AFFERENT

integrates (sensory, motor, alertness, desire…etc).

controls movement (EFFERENT)

controls the state of the viscera (EFFERENT)

affects body systems through neurotransmitters, neuromodulators, electrical synapses and endocrine mechanisms

some neurons are secretory, they release a substance and use the blood as a pathway to reach their target (neuroendocrine


Stepping on a Nail

1. Sensory information about the nail in his foot is sent to the dorsal spinal cord via sensory neurons (afferent)

2. Sensory neuron synapse with interneurons to communicate with motor neurons in the ventral spinal cord

3. Motor neurons are inhibited in the leg to allow the leg to bend and motor neurons are activated to muscles that will pull his foot away from the nail (efferent)


Neurons & Supportive Cells

Neurons: are excitable cells, they “fire” action potentials function to send and receive information via synapses (or gap junctions in some neurons) are postmitotic (they are highly specialized and do not divide)

Supporting cells (greatly outnumber neurons, ~10:1) are “non-conducting”, and provide: physical support electrical insulation metabolic exchange blood-brain barrier


Neuron Structure

is an elaborate cell
has a cell body (soma), two types of specialized processes (dendrites and an axon)
dendrites (in a general sense) are the receiving end

information is conveyed through the soma along the axon to the target
receives excitatory and inhibitory innervation (input)

Neurons are plastic: If they receive a lot of enrichment they will make more dendrites, but if they are deprived, then they pull back


Types of Neuronal Structures

Neurons come in many different morphologies, but we can classify them into three basic categories:

1.Bipolar – retina and CN VIII; one axon one dendrite

2.Uni/Pseudounipolar – sensory (afferent) neurons [in sensory ganglia]; one process (the axon)

3.Multipolar – motor and interneurons; one axon with many dendrites


Bipolar Neuron

in the retina

one axon, one dendrite


Unipolar Neuron

H & E stain

1 process - the axon


Propagation of Action Potentials

Input goes through dendrites to the cell body/soma through the hillock to the axon terminals and the information is carried to other cells via the axon branches



are utilized to collect information from other neurons or about a stimulus (sensory neurons)

typically have a greater diameter than axons, are unmyelinated and form dendritic trees (arbors)
contain all organelles, except Golgi
often give rise to spines, which are used to increase the receptive surface area of a neuron in order to make connections with other neurons or sample the environment



is usually large, with a euchromatic nucleus and prominent nucleolus
contains Nïssl substance (stains with basic dyes and correspond to stacks of rough endoplasmic reticulum)
contains numerous organelles, which extend into the dendrites but not the axon (there are some minor exceptions)



Each neuron has only one axon, but this axon can split an innervate multiple targets.
axons can range in diameter from 0.5 to 20 microns in diameter
axons can be relatively short or travel more than a meter to their target


axon hillock
initial segment – where action potentials are generated
Action potentials run away from the cell body toward terminals
Conduction velocity depends on axon size (diameter) and myelination state



Axons can be myelinated or unmyelinated
A myelin sheath (lipid rich layers) functions to insulate the axon from the extracellular space
the axon hillock, initial segment and terminal arborizations are devoid of myelin
The myelin sheath is composed of concentric layers of either:
Oligodendrocytes in the CNS (several axons per oligo).
Schwann cell membrane (one axon per Schwann cell) in the PNS


Schwann Cell Myelination


Demyelination Diseases

Multiple Sclerosis: mycroglia/ WBCs will attack the myelin sheaths via phagocytosis and lysosomal degradation causing degeneration of axons and loss of signals/sensory

degenerative disease that attacks the CNS

Vitamin B12 deficiency- affects spinal cord, optic nerves and peripheral nerves

Tabes dorsalis- side effect of untreated syphilis, affects discriminative touch, vibration, and proprioception; demyelinate the dorsal column losing touch, proprioreception, etc.



Neurons communicate with each other through synapses.  A synapse is a junction between two cells to facilitate transmission of impulses. There are two types:

1.Electrical – in reality these are gap junctions, between interneurons and excitatory cells in the cortex, common in invertebrates
2.Chemical – presynaptic and postsynaptic elements, synaptic cleft of 20 – 30 nm

Different types of synapses:

  Axodendritic – axon to dendrite

  Axosomatic- axon to soma

  Axoaxonic- axon to axon


Classes of Synapses (2)

The synaptic densities can have different morphologies; they can be:

•symmetrical= associated with inhibitory synapses; the vesicles are elongated and contain an inhibitory neurotransmitter (GABA and/or glycine)
•asymmetrical= associated with excitatory synapses; the vesicles are round and contain excitatory neurotransmitter (glutamate)


A number of different SMALL molecules are used as neurotransmitters

•GABA + glycine
•ACh + NE


Kiss & Run

when an action potential reaches the bouton, it depolarizes the membrane → there are voltage sensitive Ca++ channels here

the influx of Ca++ permits the binding of vesicles with fusion machinery

neurotransmitter is released → traverses the cleft → binds to a receptor → cellular event



CNS Organization

The CNS is divided into:

A.white matter – myelinated axons (lighter)
B.gray matter – cell bodies (darker)

Both of which contain:

1.Neurons and/or processes [somata, dendrites + axons]
3.Blood vessels
4.Connective tissue and ECM
•A cluster of neuronal cell bodies in the CNS is a “nucleus”


CNS Axon

White and Gray Matter

Myelin surrounding the outside


PNS Organization

The PNS includes: 

A.Nerves + nerve terminals/endings
B.Ganglia – collections of neuronal cell bodies in the PNS

Both of which contain:

1.Axons (or axons of passage)
2.Supporting cells
        i.Satellite cells
        ii.Schwann cells
3.Blood vessels
4.Connective tissue


Connective Tissue of Nerves

The CT in a nerve is arranged in three layers:

1.epineurium – dense CT, binds fascicles together
2.perineurium – Surrounds the nerve fascicle
3.endoneurium – loose CT; surrounds axons within a fasicle



There are two types of ganglia (clumps of neuronal cell bodies outside the CNS):
1.Sensory ganglia
•contain pseudounipolar neurons
•no synapses within the ganglia
•central projection into CNS

2.Autonomic ganglia
•visceral motor ganglia
•sympathetic or parasympathetic
•contain multipolar neurons
•synapses from other motor neurons


Supporting Cells – CNS

Within the CNS, supporting cells out number neurons 10:1, supporting cells are termed neuroglia; there are 3 types of neuroglia:




NOTE: Only nuclei are seen in routine sections.


Satellite Cells

are found mainly in ganglia and only nuclei are seen with routine stains
processes form a complete layer around the neuron, axons must penetrate the satellite cells to reach the soma of the neuron
supporting cells maintain a controlled environment around the neuron
Note the different arrangement in sensory vs. sympathetic ganglia.

Small cuboidal cells surrounding cell bodies in ganglia

Sensory- nucleus is in the center

Sympathetic- nucleus is not centrally located

Found in ganglia and round nuclei

When signal comes through it must break through the satellite cells to get out


Glial Cell Morphology

Fibrous Astrocyte


Protoplasmic Astrocyte

Microglial Cell



Astrocytes are the largest of the neuroglial cells. Two main types of astrocytes:

a. protoplasmic – gray matter

b. fibrous – white matter

Astrocytes have numerous tentacle-like processes with end-feet associated with:
2.blood vessels
1.Maintain the microenvironment of the neuron by mobilizing wastes and nutrients
2.Contribute to the blood brain barrier
3.Confine or metabolize neurotransmitters at the synapse
4.Synaptogenesis – hold other proceses and endure they will connect a new element?

When time to clear out Ach for recycling, astrocytes will do this



are phagocytic cells, present only in small numbers
proliferate and are active in regions of injury (they can migrate) and disease
are small cells with darkly staining nuclei and typically have a few short, spiny processes
Originate outside of the brain during development
More closely related to macrophages than other neuronal cells
supportive cell


Ependymal Cells

lines the internal ventricular chamber of the CNS
is composed of epithelial cells (cuboidal or columnar) that often contain cilia

Line the ventricles in CNS and provide CSF to the ventricles


Choroid Plexus

The ependyma is specialized in regions, where it exists as small tufts of small vascular elements covered with modified ependymal cells= choroid plexus
The choroid plexus is responsible for the formation of cerebrospinal fluid within the ventricles.


Responses to Injury (PNS): Degeneration

in the PNS, the axon distal to the injury undergoes Wallerian degeneration and the axon breaks down into fragments which are digested by Schwann cells
if a motor axon is cut, the muscle fiber innervated by that axon undergoes degeneration
the cell body of the damaged neuron undergoes chromatolysis (loss of Nissl substance)