Nervous Tissue I

BIO 408

Histology

Dr. D. L. Daley

Cells of the Nervous System

n Neurons - cells responsible for receptive, integrative and motor functions

n Perhaps 100 billion neurons

n Neuroglia - cells responsible for support and protection of neurons

n Ten times more glial cells -1000 billion (Trillion)

Neurons

n Range from 5 to 150 mm in diameter and are both the smallest cells of the body and the largest

n Soma - cell body

n Dendrites - input processes

n May show extensive arborization

n Axon - output process may branch

n May be up to 100cm long varying diameters

n Axon terminal - expanded areas or end bulbs (terminal boutons)

Neurons

n Cell body (soma or perikaryon)

n Nucleus large, centrally located with well defined nucleolus

n Cytoplasm with abundant RER - stained with basic dyes - get Nissl bodies

n Axon hillock - expanded area where axon arises & APs begin

n May have abundant SER - may extend into dendrites as hypolemmal cisternae

n Juxtanuclear Golgi complex

n Numerous mitochondria & only one centriole

n some neurons have inclusions such as melanin granules (e,g. substantia nigra cells)

Neurons

n Dendrites

n Receptive portion

n Most have multiple dendrites that branch like a tree (dendritic tree or arbor)

n Spines are located on the surface of some dendrites where synaptic connections are made

n Spines are know to diminish with age and poor nutrition

n Structural changes can occur in the spines of individuals with trisomy of chromosome 13 or 2

n Some dendrities contain synaptic vesicles and make synaptic contact with dendrites of other neurons

Axons

n The long distance conduction portion of a nerve cell

n Conducts action potentials

n All mature neurons have a single axon that may travel a significant distance from the soma

n Distances can be one meter or more

n Axon thickness is related to conduction velocity

n Conduction velocity increases with axon diameter

Axons

n Some axons have collateral branches, arising at right angles to the trunk of the axon

n As the axon terminates, it may branch forming a terminal arbor

n The axon hillock is a pyramid-shaped region of the soma, usually opposite the dendrites

n The portion of the axon from the its origin to its myelin sheath is called the initial segment

n This region is called the spike trigger zone since this id the point where action potentials (spikes) are initiated

Axons

n Some axons have no thick wrappings - unmyelinated axons

n The myelin sheath is a thick insulating layer on the outside of many axon

n The myelin sheath is formed by neuroglial (glial) cells that wrap themselves around the axon forming multiple layers of membrane insulation

n The myelination of an axon acts to increase the conduction velocity of the axon as compare to a similar diameter unmyelinated axon

n The presence of myelin is responsible for the subdivision of the CNS into white matter and gray matter

Axons

n Axonal transport is the movement of materials between the soma and the axon terminals

n Anterograde transport - transport from the soma to the axon terminals

n Retrograde transport - transport from the axon terminals to the soma

n Axonal transport is as essential to normal trophic relationships within the axon as it is between neurons and and muscles and glands

Axonal Transport

n Three velocities - fast, intermediate and slow

n Fast - up to 400 mm/day - in anterograde transport of organelles

n In retrorgade transport the fastest speed is less than half the anterograde speed and the slowest is about 0.2 mm/day

Axonal Transport

n Anterograde transport is used in the translocation of organelles and vesicles as well as macromolecules like actin, myosin, clathrin and some enzymes necessary for neurotransmitter synthesis

n Radiolabeled amino acids injected into the soma can be used to study anterograde transport

Axonal Transport

n Retrograde transport - protein building block of neurofilaments, subunits of microtubules, soluble enzymes and material taken up by endocytosis (toxins and viruses)

n Horseradish peroxidase is an enzyme long used to study retrograde axonal transport - detected by histochemical techniques

Classification of Neurons

Neurons Classified by Function

n Sensory (afferent) - receive sensory input at their dendrites & conduct impulses into the CNS

n Motor (efferent) - originate within the CNS & conduct impulses to muscles, glands and other neurons

n Interneurons - located completely within the CNS - interconnect sensory, motor and other interneurons - function in the integration of inputs

Neuroglial Cells

n Support the function of neurons, mechanically and metabolically

n May be as many as 10 times the number of neuroglial cells than neurons in the nervous system

n In the CNS - astrocytes, oligodendrocytes, microglia and ependymal cells

n Peripherally - Schwann cells and Satellite cells

Astrocytes

n Largest of the neuroglial cells

n Protoplasmic astrocytes in gray matter

n Many short branching processes

n End in pedicels that come into contact with blood vessels

Astrocytes

n Fibrous astrocytes in white matter

n Processes are long and mostly unbranched

n Closely associate with blood vessels and pia mater - separated from these structures by their own basal lamina

n Astrocyes located at the periphery of the CNS form a continuous layer over blood vessels - part of the blood-brain barrier

Oligodendrocytes

n Interfascicular oligodendrocytes

n Located in rows besides bundles of axons

n Produce the myelin sheath of central neurons

n A single oligodendrocyte wraps several axons and forms only one piece of the myelin sheath

n Satellite oligodendrocytes

n Closely applied to the soma of large neurons

n Function is unclear

Microglial Cells

n Scattered throughout the CNS - resemble oligodendrocytes

n Spines cover the cell bodies and processes

n Function as phagocytes in clearing debris and damaged structures in the CNS

n When activated - serve as antigen-presenting cells & secrete cytokines

n Originate from bone marrow

Ependymal Cells

n Low columnar to cuboidal epithelial cells lining the ventricles of the brain and the central canal of the spinal cord

n Some are ciliated and aid in CSF circulation

n Modification of some of these cells in the ventricles of the brain contribute to the formation of the choroid plexus that secretes CSF

Satellite Cells

n Cell bodies of neurons located in ganglia outside the CNS are surrounded by satellite cells

n These cells provide structural and metabolic support

n Embryonic origin similar to Schwann cells - from neural crest cells

Schwann Cells

n Located in the peripheral NS

n They form the concentric layer of cell membrane that form the myelin sheath of axons of the peripheral NS

n The myelin sheath of an axon is composed of many Schwann cell that each wrap a limited section of axon

n Between Schwann cell wrappings are the Nodes of Ranvier - breaks between successive Schwann cells

Synaptic Transmission

n A synapse is the functional site of communication between two neurons or a neuron and muscle or gland

n Electrical synapse - not common in spinal cord, brain stem. retina, and cerebral cortex

n Gap junctions - with typically bidirectional communication

Synaptic Transmission

n Chemical synapse - most common

n Presynaptic membrane releases one and sometime two neurotransmitters into a synaptic cleft (20 - 30 nm)

n Typically the transmitter is released via exocytosis of synaptic vesicles

n The postsynaptic membrane has outward facing receptors - these are directly or indirectly couple to ion channels

n EPSPs or IPSPs

Types of Synaptic Contacts

n Axodendritic - axon to dendrite

n Axosomatic - axon to soma

n Axoaxonic - axon to axon

n Dendrodendritic - dendrite to dendrite

Peripheral Nerves

n Bundles of axons surrounded by a connective tissue sheaths

n Bundles are called fascicles

n Epineurium - outermost sheath - dense irregular connective tissue

n Perineurium - middle layer of dense connective tissue - thinner that epineurium

n Endoneurium - innermost layer - surrounds individual axons - loose connective tissue

Peripheral Nerve