11.3 Neurons

How many special characteristics do neurons have and what are they?

3 1. Extreme longevity (last a person's lifetime) 2. Amitotic (do not divide) 3. Highly metabolic (requires continuous supply of oxygen and glucose)

Functional characteristics of Neurons

3 types of neurons grouped by direction in which nerve impulses travels relative to CNS 1. Sensory 2. Motor 3. Interneurons

Neuron cell body function

Biosynthetic center of neuron; synthesizes proteins, membranes, and channels; Rough ER (chromatophilic substance, or Nissl bodies)

What can axons do with other neurons?

Carries on many conversations with different neurons at the same time.

Motor

Carry impulses from CNS to effectors; multipolar, most cell bodies are located in CNS (except some autonomic neurons)

What do axons rely on?

Cell bodies to renew proteins and membranes because they do not have the synthesizing machinery.

Myelination in the PNS

Formed by Schwann cells; wraps around axon in jelly roll fashion,; one cell forms one segment of myelin sheath

How does movement occur in axons?

In both directions 1. Anterograde 2. Retrograde

Length of Axons

In some neurons, axons are short or absent; in others, axon compromises almost entire length of cell; some axons can be over 1 meter long

Where are most neuron cell bodies located?

In the CNS

What happens if an Axon is cut or damaged?

It will quickly decay if cut or damaged.

What do plasma membranes of myelinated fibers in the PNS have less of?

Less protein; no channels or carrier, so good electrical insulators; interlocking proteins bind adjacent myelin membranes

Nerve fibers

Long axons

Dendrites

Motor neurons can contain 100s of these short, tapering, diffusely branched processes; they contain some organelles as in the cell body

Axon collaterals

Occasional branches on axons

Function of Myelin

Protect and electrically insulate axon; increase speed of nerve impulse transmission

Dendrite function

Receptive (input) region of a neuron; covey incoming messages towards cell body as graded potentials (short distance signals); in many brain areas, finer dendrites are highly specialized to collect information.

What happens at the axon terminal of a neuron?

Region that secretes neurotransmitters, which are released into extracellular space; can excite or inhibit neuron it contacts.

How many types of neuron processes are there and what are they?

There 2 types 1. Dendrites 2. Axons

Retrograde

Toward cell body examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins

Sensory Neurons

Transmit impulses from sensory receptors toward CNS; almost all are unipolar; cell bodies are located in ganglia in PNS

Terminus

Where axons branch profusely at their end; can number as many 10,000 terminal braches

Bipolar

a neuron with 2 processes (1 axon and 1 dendrite); rare (ex. retina and olfactory mucosa)

Multipolar Neurons

a neuron with 3 or more processes (1 axon, others dendrites); most common and major neuron type in the CNS

Unipolar

a neuron with one T-like process; also called psuedounipolar: 1. Peripheral (distal) process: associated with sensory receptor 2. Proximal (central) process: enters CNS

Interneurons

also called association neurons; lies between motor neurons and sensory neurons; shuttle signals through CNS pathways; most are entirely in the CNS; 99% of body's neurons are interneurons

Neuron Processes

armlike processes that extend from the cell body

Anterograde

away from cell body examples: mitochondria, cytoskeletal elements, membrane components, enzymes

Tracts

bundles of neuron processes in CNS

Nerves

bundles of neuron processes in the PNS

Retrograde axonal transport

certain viruses and bacterial toxins damage neural tissues by using this example: polio, rabies, and herpes simplex viruses, and tetanus toxin

Nuclei

clusters of neuron cell bodies in the CNS

Ganglia

clusters of neuron cell bodies in the PNS

Myelin sheath

composed of myelin, a whitish, protein-lipid substance

Axon

conducting portion of a cell; neuron processes that carry impulses away from the cell body, efferent pathways. Each neuron has one axon

Axon Function

conducting region of a neuron; generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal

Axon Hillock

cone shaped area where axon starts

Neuron Cell Body

contains spherical nucleus with nucleolus; some contain pigments; in most plasma membrane is part of the receptive region that receives input from other neurons.

Nonmyelinated fibers

do not contain a sheath thus impulses are conducted more slowly; thin fibers not wrapped in myelin; surrounded by schwann cells but no coiling; one cell may surround 15 different fibers.

Myelin sheaths in the CNS

formed by processes of oligodendrocytes not whole cells; each cell can wrap up to 60 axons at once; myelin sheath gap is present; no outer collar of perinuclear cytoplasm, thinnest fibers are unmyelinated but covered by extensions of adjacent neuroglia.

Myelin sheath gaps (PNS)

gaps between adjacent schwann cells; sites where axon collaterals can emerge; formerly called nodes of Ranvier

Structural Classification of Neurons

grouped by number of processes: 1. Multipolar 2. Bipolar 3. Unipolar

What does an axon have regarding transport?

has efficient internal transport mechanisms; molecules and organelles are moved along axons by motor proteins and cytoskeletal elements.

Gray matter (myelin sheaths in the CNS)

mostly neuron cell bodies and nonmyelinated fibers

Outer collar of perinuclear cytoplasm

peripheral bulge containing nucleus and most of cytoplasm

White matter (myelin sheaths in the CNS)

regions of brain and spinal cord with dense collections of myelinated fibers (usually fiber tracts)

Myelinated fibers

segmented sheath surrounds most long or large diameter axons.

Neurons

structural units of the nervous system; large highly specialized cells that conduct impulses; all have a cell body and one or ore processes.

Which nervous system contains both neuron cell bodies and processes?

the CNS

Which nervous system contains chiefly neuron processes?

the PNS

Axon Terminals or Terminal Boutons

the distal endings of a axon

Dendritic spines

thorny appendages with bulbous or spiky ends that bristle on dendrites; with represents points of close contact (synapses) with other neurons.

Retrograde transport

viruses containing "corrected" genes or microRNA to suppress defective genes can enter cell