Ch 7 Flashcards
CNS
brain + spinal cord
PNS
cranial/spinal nerves
neurons
- -> conduct impulses but generally cannot divide 9but CAN repair)
- respond to chemical/physical stimuli (ex. pain/pressure/heat)
- conduct electrochemical impulses (ex. an action potential)
- release chemical regulators (at synapse)
- enable perception of sensory stimuli, learning, memory, and control of muscles/glands
glial cells (neuroglia)
support the neurons, can NOT conduct impulses, but CAN divide
Neuron Structure
- cell body
- dendrites
- axon
Cell body
contains the nucleus/other organelles
cluster in groups = nuclei/ganglia
nuclei
cell body in CNS
ganglia
cell body in PNS
dendrites
receive impulses and conducts a graded impulses toward the cell body
-shorter than axon
axon
conducts action potentials away from the cell body
axon hillock
where action potential is generated –> then propagated down the axon
Axonal Transport
an active process (needs energy) needed tome organelles and proteins from the cell body –> axon terminals
- fast component moves vesicles (neurotransmitters)
- slow components move microfilaments, microtubules, and proteins (a.k.a. cytoskeleton)
- -> anterograde/retrograde transport
anterograde transport
cell body –> dendrites/axon
retrograde transport
dendrites/axon –> cell body
Functional classification of neurons
- -> based on direction impulses are conducted
1. sensory
2. motor
sensory neurons
conduct impulses from sensory receptors to CNS
motor neurons
conduct impulses from CNS to target organs
-not just voluntary: somatic (ex. skeletal muscle) vs. autonomic (ex. HR)
Categories of motor neurons
- Somatic
- Autonomic
- sympathetic
- parasympathetic
somatic motor neurons
responsible for reflexes and VOLUNTARY control of skeletal muscles
autonomic motor neurons
innervate INVOLUNTARY targets such as smooth muscles, cardiac muscle, and glands
- sympathetic: emergency situations/”fight or flight”
- parasympathetic: normal functions/”rest and digest”
Nerves
bundles of axons located outside the CNS
- most composed of sensory + motor neurons (“mixed nerves”)
- some have sensory only
tract
bundle of axons in CNS
Types of neuroglia in PNS
- Schwann Cells
2. Satellite Cells
Types of neuroglia in CNS
- Oligodendrocytes
- Microglia
- Astrocytes
- Ependymal Cells
Schwann Cells
PNS neuroganglia
form myelin sheaths around peripheral axons
Satellite Cells
PNS neuroganglia
support cell bodies within the ganglia of PNS
-ex. secreting growth factors
Oligodendrocytes
CNS neuroganglia
form myelin sheaths around the axons of CNS neurons
-analogous to schwann cells
Microglia
CNS neuroganglia
migrate around CNS tissue and phagocytize foreign and degenerated material
Astrocytes
CNS neuroganglia
regulate the external environment of the neurons
-regulate tight junctions to regulate material movement in endothelial cells –> create BBB
Ependymal Cells
CNS neuroganglia
line the ventricles and secrete cerebrospinal fluid
BBB
- capillaries in the brain do not have pores between adjacent cells but are joined by tight junctions
- substances can only be moved by very selective processes of diffusion through endothelial cells, active transport, and bulk transport
- movement is transcellular, not paracellular
- astrocytes: support cell bodies/regulate formation of BBB
Resting Membrane Potential
neurons have a resting potential of -70mV
- established by large negative molecules inside the cell
- Na+/K+ pumps
- permeability of the membrane to positively charged, inorganic ions
–> at rest there is a high concentration of K+ inside the cell and Na+ on the outside
Depolarization
occurs when positive ions enter the cell (usually Na+)
- -> membrane potential moves towards 0/more positive
- EXCITATORY
Hyperpolarization
occurs when positive ions leave the cell (usually K+)
-INHIBITORY
Ion Gating
Two types of channels
- K+ leakage channels: not gated (always open), increases permeability to K
- Voltage-gated K+ channel: open when a particular membrane potential is reached, closed at resting potential
-Na+ voltage-gated channels are closed at rest, the membrane is less permeable to Na+ at rest
Voltage Gated K+ Channel
- -> these channels open if the membrane potential depolarizes to -55mV (a.k.a. threshold)
1. Channels open –> sodium rushes in due to electrochemical gradient
2. Membrane potential increases toward Na+ equilibrium potential
3. Channels deactivated at +30mV
4. Voltage-Gated K+ Channels open and K+ rushes out of cell following electrochemical gradient
5. This makes the cell depolarize back toward K+ equilibrium potential
Action Potentials
- At threshold membrane potential (-55mV), voltage-gated Na+ channels open and Na+ rushes in
- As cell depolarizes, more Na+ channels are open, and the cell becomes more and more permeable to Na+
- POSITIVE feedback loop
- causes an overshoot of membrane potential –> reaches +30mV - At +30mV, Na+ channels close and K+ channels open
- results in repolarization of membrane potential
- NEGATIVE feedback loop
All-or-None
-threshold is reached –> action potential occurs
- size of stimulus will NOT:
1. affect size of action potential (will always reach +30mV - may recruit more neurons)
2. affect action potential duration, but will make them happen more frequently
Refractory Period
action potentials can only increase in freq to a certain point
- there is a refractory period after action potential when neuron can NOT become excited again (for milliseconds)
- absolute vs. refractory periods
- each action potential remains a separate, all-or-none event
Absolute Refractory Period
occurs during the action potentials
-Na+ channels are inactive (not just closed)
Relative Refractory Period
- K+ channels are still open (still in hyper polarization phase)
- only a very strong stimulus can overcome this
Conduction of Nerve Impulses
- action potential occurs at neuron membrane
- voltage gated Na channels open as a wave down the axon
- axon potential at one location serves as depolarization stimulus for next region of axon
Conduction: Unmyelinated
- chaotic/unorganized
- axon potentials produced down entire length of axon
- slow conduction rate b/c so many action potentials are generated
- amplitude of each action potential is the same (conducted w/o reduction)
Conduction: Myelinated
- much more organized
- myelin = insulation
- Nodes of Ranvier allow Na and K to cross membrane every 1-2mm
- -> Na ion channels concentrated at the nodes
- action potentials “leap” from node to node
- –> called “saltatory conduction”
What is the resting potential in a myelinated neuron?
-70mV
What is axon potential conduction speed increased by?
- diameter: reduces resistance to spread of charges via cable properties
- Myelination b/c of saltatory conduction
- -> thin unmyelinated= 1 m/sec
- -> thick myelinated= 100 m/sec
Synapse
- ->the functional connection between a neuron and the cell it’s signaling
- electrical or chemical
- in CNS, second cell = another neuron
- in PNS, second cell= muscle/gland (neuromuscular junction)
Electrical Synapses
- can occur in mostly in smooth/cardiac muscle, between neurons of brain or glial cells
- cells joined by gap junctions
- stimulation causes phosphorylation or dephosporylation of connexion proteins to open or close the channels
Chemical Synapses
- most involve the release of a neurotransmitter from axon terminal
- synaptic cleft: very small, released neurotransmitter can readily diffuse across this space
Release of a Neurotransmitter
Neurotransmitter is enclosed in synaptic vesicles in the axon terminal
- when action potential reaches the end of the axon –> voltage gated Ca channels open
- Ca stimulates the fusing of synaptic vesicles to the plasma membrane and exocytosis of neurotransmitter
Actions of Neurotransmitter
Neurotransmitter diffuses across the synapse, where it binds to a specific receptor protein.
- neurotransmitter is referred to as the ligand
- results in opening of chemically regulated ion channels (a.k.a. ligand-gated ion channels)
Acetylcholine (ACh)
Neurotransmitter: directly opens ion channels when it binds to a receptor
- Excitatory in some areas of CNS, in some autonomic motor neurons, and in all somatic motor neurons
- Inhibitory in some autonomic neurons
Nicotinic ACh Receptors
- can be stimulated by nicotine
- found on motor end plate of skeletal muscle cells, in autonomic ganglia, and in some parts of the CNS
Muscarinic ACh Receptors
- can be stimulated by muscarine (from poisonous mushrooms)
- found in CNS and plasma membrane of smooth/cardiac muscles and glands innervated by autonomic motor neurons
Agonists
drugs that can stimulate a receptor
Antagonists
drugs that inhibit a receptor (ex. beta-blockers)
Acetylcholinesterase (AChE)
- enxyme that inactivates AChE activity shortly after it binds to the receptor
- hydrolyzes ACh into acetate and choline, which are taken back into the presynaptic cell of reuse (recycles to rebuild ACh)
Monoamines
Regulatory molecules derived from AAs
- Catecholamines: derived from tyrosine (ex. dopamine, norepinephrine, and epinephrine)
- Serotonin: derived from L-tryptophan
- Histamine: derived from histidine
Monoamine Action/Inactivation
- made from presynaptic axon, released via exocytosis, diffuse across synapse, and bind to specific receptors
- quickly taken back into the presynaptic cell (reuptake) and degraded by monoamine oxidase (MAO)
