A&P Chapter 12: Neural Tissue
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What are the 2 kinds of cells found in neural tissue?
- 1. Neurons: cells that send and receive signals (the communicators)
- 2. Neuroglia: cells that support and protect neurons (support, feed, nourish, protect, clean up after)
What makes up the CNS?
- - The brain
- - The spinal cord
What makes up the PSN?
- - Everything that isn't the brain or the spinal cord
- - The cranial and spinal nerves
What is the relationship between receptors/effectors and afferent/efferent fibers?
- - The receptors are sensory receptors of the PNS that send their information to the brain on afferent fibers
- - The effectors are motor neurons that are following the commands of the CNS based on the information from the sensory receptors; the information travels to the muscles and glands on efferent nerve fibers
What are the structures that make up a neuron?
- 1. Dendrites: short, highly branched projections that receive information from other neurons
- 2. Soma: the cell body that contains the nucleus
- 3. Axon: Long projection from the soma that carries an electrical signal; can be myelinated or unmyelinated
What is a synapse?
The area where a neuron communicates with another cell. A presynaptic axon meets a postsynaptic receptor. Neurotransmitters/signals are released from the presynaptic membrane and travel across the synaptic cleft to the postsynaptic receptors
What are the 3 functional classifications of neurons?
- 1. Sensory neurons: Afferent neurons of the PNS
- 2. Motor neurons: Efferent neurons of the PNS
- 3. Interneurons: The in between neurons interpreting sensory information and making decisions in the CNS
What are the 4 neuroglia in the CNS?
- 1. Oligodendrocytes: support cells that myelinate neurons to allow faster conduction of electrical pulses; one cell can myelinate multiple neurons
- 2. Astrocytes: create the BBB; controls interstitial environment
- 3. Ependymal cells: produce CSF, found lining the ventricles and the central canal of the spinal cord
- 4. Microglia: immune response cells of the brain; clean up debris, waste and pathogens
What are the 2 neuroglia of the PNS?
- 1. Schwann cells: myelinate the nerves of the PNS; one cell myelinates a group of axons
- 2. Satellite cells: regulate the environment around the neuron; adjust the gases and neurotransmitters in the interstitial environment
What is myelin?
Myelin is an insulating sheath that surrounds axons in multiple layers of neuroglial membrane; significantly increases the rate of electrical impulses
What are the steps in an action potential?
- 1. Chemically gated Na channels open allowing Na cause a depolarizing graded potential
- 2. If threshold of -55mV is reached, voltage gated Na channels open and flood the axon, raising the membrane potential causing a chain reaction of voltage gated Na channels to open down the axon
- 3. The membrane potential reaches 30mV, VG Na channels slam closed and VG K channels open, allowing a rush of K into the cell to repolarize the membrane
- 4. VG K channels are open long enough to hyperpolarize to -90mV when they close; Na/K ATPase reestablishes the resting membrane potential
How is the transmembrane potential established?
Na/K ATPase pumps 3 Na out of the cell and 2 K into the cell causing the extracellular fluid to be more positive and the intracellular fluid to less positive
What types of channels cause changes in the membrane potential?
- 1. Chemically gated channels: open when a ligand/neurotransmitter binds and changes the structure of the protein to open and allow a specific molecule across the membrane
- 2. Voltage gated channels: open when the membrane reaches a certain electrical charge, when this is reached the gate opens
- 3. Mechanically gated channels: open when membrane is physically distorted
How do graded potentials differ from action potentials?
- 1. Graded potentials: local, can be big or small, can depolarize or hyperpolarize the membrane, many can accumulate to allow the membrane to reach threshold, chemically gated channels
- 2. Action potentials: travel, all or nothing, an action potential WILL occur if the membrane reaches threshold, voltage gated channels
What is the difference between depolarization, repolarization, and hyperpolarization?
- 1. Depolarization: moves the membrane potential more positive away from -70mV (chemically gated and VG Na channels)
- 2. Repolarization: moves the membrane potential closer to -70mV (VG Na channels close, VG K channels open)
- 3. Hyperpolarization: moves the membrane potential more negative (chemically gated channels and VG K channels)
What are the 2 types of refractory periods?
- 1. Absolute refractory period: Na channels are open or inactivated, no possibility for action potential, lasts 0.4-1.0 msec
- 2. Relative refractory period: Membrane potential is almost normal, a very large stimulus could initiate an action potential
What are the 2 types of conduction of action potentials?
- 1.Continuous propagation: unmyelinated axons, requires more energy and is slower
- 2. Saltatory propagation: myelinated axons, the AP travels from node to node, much faster than continuous propagation, requires less energy
What are the different classifications of nerve fibers?
- Type A: Largest, myelinated, 140m/s, carry rapid info to/from the CNS that is the most important information
- Type B: Medium, myelinated, 18 m/s, carry intermediate signals
- Type C: Smallest, unmyelinated, 1 m/s, carry slower information short distances
What are the 2 types of synapses?
- 1. Electrical synapses: Direct physical contact between cells, connected by gap junctions
- 2. Chemical synapses: Signal transmitted across synaptic gap by chemical neurotransmitters
How are neurotransmitters released in a chemical synapse?
- 1. AP reaches the telodendria and VG Ca channels open
- 2. Ca triggers exocytosis of ACh
- 3. ACh binds to receptors and depolarizes the postsynaptic membrane
- 4. Excess ACh is removed from the synaptic cleft by AChE
What are the 2 types of summation?
- 1. Temporal summation: rapid, repeated stimuli at one synpase, multiple times
- 2. Spatial summation: many stimuli arrive at multiple synapses, multiple locations
What are EPSPs and IPSPs?
- 1. EPSP: Excitatory PostSynaptic Potential, depolarizes the postsynaptic membrane
- 2. IPSP: Inhibitory PostSynaptic Potential, hyper polarizes the postsynaptic membrane
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