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
