A&P - 12.2 Neural Tissue (parts of a neuron)

cell body (soma)

enlarged metabolic region of the cell where the nucleus and most the major organelles are located

• FUNCTION: control center of neuron

• has a single, centrally located nucleus with large nucleolus

• cytoplasm contains mitochondria, lysosomes, Golgi complex, inclusions, entensive rough ER, and cytoskeleton

• mature neurons have no centrioles, no mitosis after adolescence

nucleus

localized collection of neuron cell bodies that are functionally related; a “center” of neural function

• FUNCTION: controls the cell’s activities and stores genetic information

cell membrane

• FUNCTION: protect the cell and control what enters and leaves it, also plays a key role in sending electrical signals

• what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes

cell processes

called tracts within the CNS while they are called nerves within the PNS

• dendrites

• axon

tract

bundle of axons in the central nervous system having the same function and point of origin

nerve

cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system

dendrites

one of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells (branches that come off the soma)

• FUNCTION: receive chemical and electrical signals and conduct them towards the cell body (primary site for receiving signals from other neurons)

  • these electrical signals are not considered nerve impulses; they are called graded potentials rather than action potentials

• the more dendrites the neuron has, the more information it can receive

graded potentional

change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it

axon hillock

tapering of the neuron cell body that gives rise to the axon (axon forms at a tapered area called the axon hillock)

• within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm

• serves as the “trigger zone” because graded potentials must reach this area of the neuron before they can be converted into action potentials

axon (nerve fiber)

single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell - originates from one side of the soma called the axon hillock

• FUNCTION: capable of generating action potentials and transmit them as nerve impulses away from the cell body towards the telodendria and axon terminals

• only one axon per neuron

• schwann cells and myelin sheath enclose axon

• branches extensively on distal end

• synaptic knob

• specialized for rapid conduction of nerve signals from soma to remote points in the body

action potential

change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers

axolemma

the plasma membrane surrounding the axon is called the axolemma

axoplasm

cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body

initial segment

first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated

myelin

lipid-rich insulating substance surrounding protein that is characterized by also binding to muscarine and is a metabotropic receptor

• many axons are wrapped an insulating substance called myelin

• made from glial cells

myelinated fibers

conduct impulses rapidly

• these form the white matter of the nervous tissue

white matter

regions of the nervous system containing mostly myelinated axons

• making the tissue appear white because of the high lipid content of myelin

unmyelinated fibers

conduct impulse relatively slow compared to myelinated fibers

• this type forms the gray matter of the nervous tissue

gray matter

regions of the nervous system containing cell bodies of neurons with few not no myelinated axons

• actually may be more pink or tan in color, but called gray in contrast to white matter

myelin sheath

axons are often covered with a whitish, fatty (protein-lipoid) material, myelin sheath

• protects and electrically insulates neurons from one another

• acts as insulation much like the plastic or rubber that is used to insulate electrical wires

• there are gaps in the myelin covering of an axon

• formed by glial cells - the linner layers of the glial cell form the insulating myelin sheath while the outer layer is called the neurolemma

• in the PNS, Schwann cells envelop and then rotate around the axons forming the sheath while in the PNS, the myelin sheath is form by oligodendrocytes

nodes of Ranvier

gapes within the myelin sheath are called nodes of Ranvier

• gapes between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon

axon segment

the length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment

• single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the intital segment, and the last, which is followed by the axon terminal)

loss of myelination is a characteristic of which neurodegenerative disorders

• multiple sclerosis (MS)

• Guillain-Barre syndrome

multiple sclerosis

• autoimmune disease

• antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body, this causes inflammation and the destruction of the myelin in the CNS

• somatic and automic deficits, control of the musculature is compromised, as is control of organs such as the bladder

Guillain-Barre syndrome

• demyelinating disease of the PNS

• result of an autoimmune reaction but the inflammation is the the peripheral nerves

• sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness

telodenria

smaller end branches of the axon

• at the end of the telodenria are the axon terminals which have the synatic vesicles

axon terminal

end of the axon, where there are usually several branches extending toward the target cell and posses many synaptic vesicles

synaptic end bulb

swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse

neurotransmitter

chemical signal that is released from the synaptic end bulb of a neuron to causes a change in the target cell

synaptic knob

(terminal button) - little swelling that forms a junction (synapse) with the next cell (nerve, muscle, gland)

• contains synaptic vesicles full of neurotransmitters

synaptic vesicles

store and release neurotransmitters at the synapse

• act like messenger packages that store, transport, and release neurotransmitters to help neurons communicate with each other

synapse

narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell

synaptic cleft

small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element

pre-synaptic neuron

nerve cell that sends a signal to another neuron across a synapse

• before the synapse

• responsible for transmitting information to the next cell

• ends in a structure called the axon terminal

post-synaptic neuron

nerve cell that receives the signal from another neuron across a synapse

• after the synapse

• responsible for receiving and responding to the message sent by the presynaptic neuron

• on the dendrites or sometimes the cell body

confusing terminology between the structures of the CNS and PNS

CNS

• group of neuron cells bodies (i.e., gray matter) = nucleus

• bundle of axons (i.e., white matter) = tract

PNS

• group of neuron cells bodies (i.e., gray matter) = ganglion

• bundle of axons (i.e., white matter) = nerve

ganglion

localized collection of neuron cell bodies in the PNS