Histo - Nervous Tissue Flashcards
List and describe the major functions of the nervous system.
- gathers information or sensory input via sensory receptors
- through integration, it processes and interprets the sensory input and decides what, if any, action should be taken
- produces a response or motor output, activating effector organs
List and describe the two divisions of the nervous system. What are some of the structures within each division?
- central nervous system (CNS) - integrative and control centers of the body; composed of the brain and the spinal cord
- peripheral nervous system (PNS) - communication lines between the CNS and the rest of the body; composed of all nervous structures outside the brain and spinal cord; composed of structures such as cranial nerves (arise from brain), spinal nerves (arise from spinal cord), ganglia (groups of neuron cell bodies not in CNS), and plexuses (intermingling of neuron cell process from ventral rami of different cord levels)
- a nerve is a collection of axons (i.e. a process of neurons) found in the PNS
List and describe the two types of cells found in nervous tissue. What are the major function(s) of each?
- nerve cells or neurons - functional unit of nervous system; main active and excitable cells; generate and transmit action potentials; cannot go through mitosis
- neuroglia, glia, or supporting cells - supporting cells; outnumber neurons; can provide structural support, electrical insulation (myelin sheath), and metabolic exchanges between vascular system and neurons; do NOT generate action potentials; can go through mitosis
List and describe the special characteristics of neurons.
- extreme longevity - most neurons survive over 100 years
- most are amitotic - lose the ability to divide (typically but there are some exceptions)
- high metabolic rate - they need oxygen and glucose; dead in a few minutes without oxygen
List and describe the basic structural components of neurons. What are the major function(s) of each part?
- neuron cell body/perikaryon - man function is that it acts as the biosynthetic center of the neuron; always unmyelinated; no action potentials (nerve impulses)
- processes:
- dendrites - main function is that it acts as main receptive or main input regions of neuron; always unmyelinated; no action potentials; a neuron can have one or more dendrites
- axon/nerve fiber - main function is that it acts as conducting region of neuron as it can generate action potentials and transmit them; can be myelinated; a neuron can only have 1 axon; at the end, axon terminal releases chemical neurotransmitters when impulse reaches it
What organelles are located in each component of a neuron?
- neuron cell body/perikaryon - nucleus with a prominent nucleolus; regular organelles except there is no centrioles (because they are only needed for mitosis), many rough ER and ribosomes (Nissl bodies) for production of proteins, many microtubules for structure and internal trafficking
- dendrites - no golgi apparatus
- axon/nerve fiber - no Nissl bodies
What is a Nissl body?
clumps of rough ER and ribosomes that stain purple with hematoxin
List and define the classification of neurons based on structure.
- multipolar neurons - 3 or more processes
- bipolar neurons - 2 processes (very rare)
- unipolar neurons - 1 process (cell body is attached like a “T”)
List and define the classification of neurons based on function.
- sensory or afferent neurons - carry impulse toward the CNS
- motor or efferent neurons - carry impulse away from the CNS
- interneurons or association neurons - found between motor and sensory neurons
How do the 2 classification schemes for neurons (based on structure and based on function) overlap?
- multipolar neurons = motor (efferent), or interneurons
- bipolar = special sensation
- unipolar = sensory (general)
List and describe the classification of synapses based on which neuronal components are involved.
- axodendritic synapse - axon terminal and dendrite
- axosomatic synapse - axon terminal and cell body
- axoaxonic synapse - between axons
- dendrodendritic synapse - between dendrites
- dendrosomatic synapse - between dendrite and cell body
Which two types of synapses are the most common?
axodendritic and axosomatic synapses
Define presynaptic neuron.
neuron carrying impulse/signal toward synapse
Define postsynaptic neuron.
neuron carrying impulse/signal away from synapse (note: this can also be an effector cell)
List and describe the 2 varieties of synapses based on the signal used to transmit information between cells.
- electrical synapses - action potentials conducted directly between adjacent cells through gap junctions
- chemical synapses - signals from cell to cell across interstitial fluid
Where are the prominent locations for electrical synapses in the human body?
allows cells to synchronize so can be found in places like the heart
What are the steps involved with information transfer at a chemical synapse?
- nerve impulse arrives at synaptic end bulb of a presynaptic axon
- depolarization causes voltage-gated calcium channels to open
- calcium ions signal for the release of neurotransmitter (stored in synaptic vesicles) via exocytosis into the synaptic cleft
- neurotransmitter diffuses across synaptic cleft and binds to neurotransmitter receptors at the postsynaptic membrane; each receptor is part of a ligand-gated channel
- the binding of the neurotransmitter opens the associated ligand-gated channel allowing ions to flow across the postsynaptic membrane
- this produces local changes in membrane potential called the postsynaptic potential, a type of graded potential (depolarizing or hyperpolarizing)
What cellular components are found at a chemical synapse?
- presynaptic knob (synaptic end bulb or presynaptic axon terminal)
- synaptic cleft
- postsynaptic membrane
What are the ways a neurotransmitter can be removed from the postsynaptic receptor?
- degradation by enzymes
- reuptake by astrocytes or presynaptic terminal
- diffusion away from synapse
List the 6 types of neuroglia and where they are located (CNS or PNS).
- astrocyte - CNS
- microglial cell - CNS
- ependymal cell - CNS
- oligodendrocyte - CNS
- Schwann cell - PNS
- satellite cells - PNS
Describe oligodendrocytes.
- function is to produce myelin sheaths in CNS for MORE THAN 1 neuron
- predominate glial cell in the brain
- mostly in white matter
Describe astrocytes.
- perivascular feet wrap around blood vessels and cover endothelial cells of the capillaries to contribute to the blood-brain barrier and regulate vasodilation
- transfers ions, etc.
- embryonic and fetal development uses this scaffolding
- can get rid of excess neurotransmitters
- largest of the neuroglia with large radiating processes
- more commonly in gray matter than white matter
- connect with one another through gap junctions to form large networks of cells in CNS
Describe ependymal cells.
- surround the central canal of the spinal cord and lines spaces in the brain
- joined to one another
- have cilia which extend into the CSF and helps circulate CSF
- transport and secrete materials derived from body’s plasma
Describe microglia cells.
- originate from blood monocytes
- when activated, proliferate and become phagocytotic and are APCs in areas of injury
- nucleus is very elongated and dense-staining
Describe Schwann cells.
- produce myelin in the PNS; wrap around the axon
- one Schwann cell makes up one internodal segment on one axon
Describe satellite cells.
- found around the cell bodies in ganglion
- job is not well understood but may be protecting the cell body
Define white matter.
composed of primarily myelinated axons and some neuroglia; white due to myelin
Define gray matter.
composed of neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia; gray due to Nissl bodies and lack of myelin
Define ganglion.
clusters of neuronal cell bodies in the PNS
Define nucleus.
clusters of neuronal cell bodies in the CNS
Define nerve.
bundle of neuronal axons in PNS
Define tract.
bundle of neuronal axons in CNS
List and describe the connective tissue coverings associated with nerves.
- endoneurium - around individual axons
- perineurium - forms fascicles
- epineurium - around the whole nerve
List and describe the components of a nerve.
- neuron process
- Schwann cells
- connective tissue - keeps it organized
- blood vessels - brings in oxygen/nutrients and takes away waste
- lymphatic vessels
List and describe the classification of nerves based on function (which direction they are transmitting their nerve impulses).
- sensory (afferent) - conduct action potentials toward the CNS
- motor (efferent) - conduct action potentials away from the CNS
- mixed nerves - contain both sensory and motor fibers
List and describe the classification of nerves based on their origination point.
- cranial nerves - originate at brain; 12 pairs
- spinal nerves - originate at spinal cord; 31 pairs
Describe the interaction of Schwann cells with unmyelinated nerve fibers.
unmyelinated axons are still associated with Schwann cells and snuggle up next to them but they aren’t insulated because the Schwann cells don’t wrap around them; Schwann cells just provide structure for the axons
What are the functions of myelin sheaths?
- protect and electrically insulate axons from other axons
- increases speed and nerve impulse transmission
How does the myelin sheath form?
Schwann cell wraps around and around the axon; multiple layers of plasma membrane fuse together leaving a bunched up section of the Schwann cell on the outside of the sheath called the neurilemma
Define and describe continuous vs. saltatory conduction.
- continuous conduction - unmyelinated axons; the electrical signal travels slowly since action potential needs to be triggered at each sodium channel along the length of the axon
- saltatory conduction - myelinated axons; the electrical signal travels fast since the action potential only needs to be triggered at the unmyelinated nodes of Ranvier along the length of the axon
What is the rate of impulse propagation in an axon dependent upon?
- axon diameter - impacts ion flow and therefore, impacts propagation rate
- myelination - continuous vs. saltatory conduction
