click
on this image for an audio messageB. Association (integration of input for decision-making)
C. Motor (stimulates muscles and gland effectors)
2. Spinal cord
2. Motor neurons and effectors (muscle and gland tissues)
sends information to effectors from CNS
click
on this image for an audio message
A. Neuroglia (non-conducting nervous tissue)
1. Structure
smaller than neurons but more abundant -varied shapes (elongate, star, dendritic, flat)2. Functions
physically support neurons
phagocytotic
enhance speed of conduction via myelin sheath formation
produce Cerebrospinal fluid
B. Neurons (excitable cell)
1. General cell structure
a. Cell body and organelles- responsible for providing normal cellular functions (gray matter)
b. Membranous extensions off cell body
1) Dendrites - responsible for conducting impulses to cell body (often many off one cell body)
2) Axon - responsible for conducting impulses away from cell body (usually long and branched)
end in axon terminal and synaptic bulbs , originate in axon hillock
Signals always travel from dendrites along axons to ends of axons.
c. No mitotic spindle so no replacement of cell
d. myelination -fatty membranous sheath around neuron extensions. (white matter)
fromed by neuroglia (oligodendrocytes in CNS; neurolemmocytes or Schwann cells
in PNS) to enhance impulse conduction rate
Describe neuron structures and their functions.
2. Neuron types (structure)
a) multipolar -many dendrites, one axon
most cells in brain & spinal cord (CNS) but also includes all motor neurons (PNS)
b) bipolar -one dendrite, one axon
cells in retina, nasal epithelium
c unipolar -one extension splitting into two forming axon & dendrite
most sensory neurons (PNS)
3. Neuron types (functional)
1. Sensory neurons -input from receptors in skin, muscles, organs to CNS
2. Motor neurons -from CNS to effectors (muscles and glands)
3. Associative neurons- provide integration of input and decision-making prior to forming motor output
Compare and contrast neuron types, their functions and locations.
click
on this image for an audio message2. Sensory neuron cell
3. Association (integrator) neuron cell
4. Motor neuron cell
5. Effector (muscle or gland) cell
*synapses between all cells, cells communicate with other cells via neurotransmitters
click
on this image for an audio message2. Control centers in arc control effectors
b. Smooth & cardiac muscle plus exocrine and endocrine glands (autonomic
reflex arc), e.g., control of heart rate, blood pressure, respiration rate
c. smooth muscle and glands of gastrointestinal tract (enteric reflex arc),
e.g., control of digestive muscular and secretory activities.
Diagram the reflex arc. Label all cells, synapses and effectors.
VI. Reviews required for understanding neurophysiology
2. Distribution of ions across plasma membrane is not in equilibrium
b. Gated ion channels that open or close to alter ion flow across membrane.
2) Voltage - gated protein channels open when voltage of membrane (potential)
changes.
3) Mechanical-gated protein channels open when membrane is deformed.
3. This difference in voltage across membrane results in a "polarized"
membrane. Resting membrane potential voltage is generally –70mV (minus sign
indicates inside is negative relative to outside)
Describe how the resting membrane potential is generated
in an excitable cell.
2). Hyperpolarization -more polarized membrane potential (–70 mV to –90 mV)
Results from K+ ions being allowed out of (or Cl– ions being allowed into )
cell by opening K+ (out) or Cl– (in) gated ion channels
d . Local/graded membrane potential voltages decrease with distance traveled
along membrane so used to communicate over short distances. Hence "local"
. The greater the distance the smaller the voltage gets.
e. Local/graded membrane potential voltages vary with intensity of stimuli. Hence "graded" potentials increase in voltage with more neurotransmitter or mechanical stimulation available.
f. Important in generating (triggering) action potentials.
Compare and contrast the types of local/graded potentials.
d. Generation of action potentials at trigger zones
trigger zone defined as location on neuron membrane with many Na voltage gated
channels just prior to long membrane extension (usually axon) of neuron.
c) if hyperpolarization is "sensed" then no Na voltage gates
open. No action potential is generated.
When membrane potential reaches +30 mV, then voltage gated Na+ ions channels
close, stopping depolarization phase of action potential.
2) Repolarization phase (returning voltage and ion concentrations to normal)
Also at +30 mV, voltage-gated K+ ion channels open so K+ ions leave cell making
interior less positive eventually taking membrane potential voltage to normal.
ATP-driven Na+/K+ drives ions back to normal ionic concentrations.
3) Once repolarized, trigger zone is ready to generate another action potential.
Frequency of action potential generation varies up to 1000 per second.
Describe the events of action potential generation and
repolarization at a trigger zone.
e. Propagation of action potential down axons
2) Repolarization follows reversal to allow patch of membrane to become ready
for next action potential generated at trigger zone.
So... Waves of depolarization followed by wave of repolarization
Action potential moves one way because Na+ ion channels are closed on previous
patch so they can't be opened soon (refractory period)
Describe the processes of action potential propagation.
2) Myelinated membranes
myelin insulates membrane so voltage change affects only nodes of Ranvier
therefore AP is forced to move to next node in a leaping (saltatory) conduction
method
saltatory conduction much faster than continuous conduction
Construct a reflex arc. Diagram the five different components.
Indicate the location of synapses, chemically gated channels and voltage gated
channels.
b. Electrically coupled cells (bridged by gap junctions) act in synchronized manner (e.g., smooth muscles and cardiac muscles) and in CNS neurons that do same operation all the time (some eye movements, some posture)
2. Chemical synapses
no physical connection between cells so neurotransmitter chemicals are used to cross synaptic cleft and trigger action potential on postsynaptic membrane
a. Presynapse cell membrane (secretes neurotransmitter) on axon terminal
b. Synaptic cleft (filled with extracellular fluid and usually an enzyme to remove neurotransmitter, e.g., Acetylcholinesterase or Monamine oxidase)
c. Postsynapse cell membrane (contains chemically gated channel =neurotransmitter receptor) on dendrite
d. Overall, the purpose of the neurotransmitter is to be a chemical signal that passes between cells (carries on the action potential)
B. Events allowing information transfer at a single synapse
b)if open K+ gates then K+ go out of cell therefore inside of cell more negative (graded hyperpolarization) therefore inhibition of following neuron (Inhibitory PostSynaptic Potential, IPSP)
c) if open Cl– gates then Cl– comes in therefore hyperpolarization (IPSP) therefore inhibition of neuron
5. so a single portion of the membrane of a postsynaptic neuron (dendrite)
can be excited (Depolarization graded potential ) or inhibited (Hyperpolarization
graded potential) depending on neurotransmitter produced by presynaptic cell
(axon of previous cell) at that portion. Remember, an excited membrane (EPSP
or graded depolarization) will tend to generate action potentials.
6. therefore synapse structure allows information to flow only one way (presynaptic
cell to postsynaptic cell)
Fun
website demonstrating role of neurotransmitters in drug addictions
Relate the events that occur to generate a graded
potential at a single synapse (neurotransmission)
C. Synaptic integration (result of summing the information from many synapses )
1) A single neuron may have 100-1000's of synapses -each with a different type, size and freqency of
postsynaptic potentials (i.e., large and small EPSPs and large and small EPSPs) that are "sensed" at the neuron's trigger zone
2) Generation of action potential results depends on sum (integration) of the two types of postsynaptic potentials at trigger zone.
3) Postsynaptic potentials can be spatially (synapses at different locations) or temporally summed (different synapses are creating potentials at different frequencies).
4) Process of summation and possible triggering of action potential
a. Graded potentials from many postsynaptic membranes (dendrites of neuron) travel short distance across cell body to affect trigger zone.
b) voltage gated Na channels at trigger zone measure amount of change in membrane potential by summing all graded potentials. The sum of all IPSPs + sum of all EPSPs = final voltage value at trigger zone.
1) if final summed voltage at trigger zone is above threshold depolarization, then action potential is generated
2) if final summed voltage at trigger zone is NOT above threshold depolarization, then action potential is NOT generated
3) large sum of graded potentials DOES NOT cause larger action potentials but does cause generation of more frequent action potentials
Describe the process of synaptic integration and its role in generation of an action potential.
Ok, musical video describing neurotransmission across a NMJ. Just have fun with this!
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Professor Thomas M. Lancraft
Human Anatomy and Physiology Courses
at St. Petersburg College
St. Petersburg/Gibbs Campus
7/2009