anatomy #48 Neuron cell body

Neuron cell body

Also called the perikaryon or soma

• Biosynthetic center of neuron

– Synthesizes proteins, membranes, chemicals

– Rough ER (chromatophilic substance, or Nissl bodies)

• Contains spherical nucleus with nucleolus

• Some contain pigments

• In most, plasma membrane is part of receptive region that receives input info from other neurons

Most neuron cell bodies are located in

CNS

-– Nuclei: clusters of neuron cell bodies in CNS

– Ganglia: clusters of neuron cell bodies in PNS

– CNS contains

both neuron cell bodies and their processes

PNS contains

chiefly neuron processes

Tracts

Bundles of neuron processes in CNS

Nerves

Bundles of neuron processes in PNS

Two trypes of processes

-Dendrites

– Axon

Dendrites

Motor neurons can contain 100s of these short, tapering, diffusely branched processes

-Contain same organelles as in cell body

– Receptive (input) region of neuron

– Convey incoming messages toward cell body as graded potentials (short distance signals)

– In many brain areas, finer dendrites are highly specialized to collect information § Contain dendritic spines, appendages with bulbous or spiky ends

The axon: structure

Each neuron has one axon that starts at cone-shaped area called axon hillock

– In some neurons, axons are short or absent; in others, axon comprises almost entire length of cell

- Some axons can be over 1 meter long

– Long axons are called

nerve fibers

Distal endings are called

axon terminals or terminal boutons

The axon: functional characteristics

– Axon is the conducting region of neuron

– Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal

Axons have efficient internal transport mechanisms

- Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements

– Movement occurs in both directions

Terminal:

region that secretes neurotransmitters, which are released into extracellular space

Anterograde

away from cell body

– Examples: mitochondria, cytoskeletal elements, membrane components, enzymes

retrograde

toward cell body

– Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins

Myelin sheath –

It is composed of myelin, a whitish, protein-lipid substance

– Function of myelin § Protect and electrically insulate axon

- Increase speed of nerve impulse transmission

Myelinated fibers:

segmented sheath surrounds most long or large-diameter axons

Nonmyelinated fibers:

do not contain sheath

- Conduct impulses more slowly

Myelination in the PNS

Formed by Schwann cells

- Wraps around axon in jelly roll fashion

-One cell forms one segment of myelin sheath

Myelin sheath gaps

Gaps between adjacent Schwann cells

Sites where axon collaterals can emerge

Formerly called nodes of Ranvier

Nonmyelinated fibers

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 long extensions of adjacent neuroglia

White matter:

regions of brain and spinal cord with dense collections of myelinated fibers § Usually fiber tracts

– Gray matter:

mostly neuron cell bodies and nonmyelinated fibers