Neural_Tissue_notes.doc WCR 2016
(See Marieb & Hoehn, Human Anatomy Physiology, chapter 11)
Nervous system organization: CNS & PNS
CNS: brain & spinal cord
PNS: afferent (sensory) & efferent (motor)
Motor part of PNS: somatic & autonomic
Somatic: innervates skeletal muscle; mostly conscious control
Autonomic*: innervates smooth muscle, cardiac muscle, glands; mostly subconscious
* This traditional conception of the autonomic nervous system does not reflect its true complexity.
Cells in nervous system
Neurons
Long lived, excitable, high metabolic rate amitotic (non-mitotic)
Neuron parts
Dendrites (inputs), soma, axon (output).
Neuron shapes: multipolar, bipolar, unipolar (unipolar are also called pseudounipolar)
Where would you find each shape neuron?
Glial cells
Know the several types & what they are and do.
In CNS: astrocytes, oligodendrocytes, microglia, ependymal cells
In PNS: Schwann cells, satellite cells
Myelination (by oligodendrocytes in CNS, by Schwann cells in PNS); myelin sheath, nodes (of Ranvier), gray vs. white matter.
Neurophysiology
Channel types
Chemically-gated (ligand-gated): open when neurotransmitter binds
Voltage-gated: open or close in response to change in membrane voltage. Voltage gated Na, K channels are essential for action potentials.
Mechanically-gated: open when local membrane is stretched or distorted by pressure or touch. Mechanically gated channels are essential for sense of touch, for proprioception (i,e. joint position sense), sense of muscle force, barorecepors, fullness of bladder and GI tract, hearing.
Resting membrane potential
Due to uneven distribution of Na & K, especially K, across the cell membrane: more K inside than outside. (Na is opposite: more Na outside). Membrane at rest is mainly permeable to K (although it can & does become more K-permeable during the late phase of an action potential). At rest, the transmembrane voltage assumes a value that roughly balances the electrical & chemical (diffusive) forces on K: electrical force pulls K+in to the negative cell, diffusive force drives K+out of the cell, to where it is less concentrated. The force balance for K isn’t perfect (because the membrane permeability to Na is not zero, it is a very little bit more than zero), so there’s a background leak of K (out) and Na (in), which is counteracted by the Na-K pump, which pumps Na out and K in.
Hyperpolarization, depolarization.
Graded potentials
Small, die away with time and distance from the initial stimulus.
Action potentials
Electrical events of the action potential include resting potential, rising phase of act. pot., falling phase of act. pot., period of hyperpolarization. What properties of the nerve cell membrane cause or account for each of the phases? Voltage-gated Na and K channelsplay key roles. APs are self-propagating – meaning what? They have an all-or-none quality – meaning what? Relationship between stimulus intensity and action potential behavior of a neuron: bigger stimulus means more action potentials per second, but the individual APs are not bigger when the stimulus is bigger (see the all-or-none principle).
AP propagation
AP in one spot causes adjacent membrane to depolarize until it reaches threshold; then that patch has an AP, and the AP moves down an axon.
Continuous (in non-myelinated axons)
By saltatory conduction (in myelinated axons)
Threshold
All-or-none behavior of APs
Coding of stimulus intensity
More intense stimulus does not make bigger APs. More intense stimulus increasesAP frequency, and increases number of active neurons.
Refractory period – time after an AP when it is impossible to have another AP, or it takes a bigger-than-normal stimulus.
Conduction velocity = how fast an AP moves along an axon
Larger if axon diameter is greater and/or if axon is myelinated
Synapses
Chemical synapses. Know the difference between pre- and post-synaptic. Know the basic steps described in transmission at a chemical synapse and their correct order. We studied this in skeletal neuromuscular junction, which is one particular type of synapse. Know the difference between an IPSP and an EPSP.
Electrical synapses
Formed by gap junctions. In adults: eye saccade generation region in brain
Neurotransmitters
Can be classified by chemical structure (representative examples given)
ACh (sui generis)
Amines (norepinephrine, dopamine, serotonin)
Amino acids (GABA, glutamate, glycine)
Peptides (polymers of a few to a dozen or more amino acids) (endorphins)
Purines (ATP)
Gases & lipids (NO)
Can be classified by function, i.e. what they do and how they act.
What they do: excite or inhibit (a few can be either, depending on location)
Excitatory/inhibitory neurotransmitters cause EPSPs/IPSPs
How they act: direct or indirect
Direct-acting neurotransmitters bind directly to channels (ligand-gated channels). Thus the channel itself is a receptor for the neurotransmitter. (Example: ACh binding to ACh receptor-channel in skeletal muscle.)
Indirect-acting neurotransmitters bind to G-protein-coupled receptors and act through second messengers. The receptor for the neurotransmitter is coupled to a G protein. When the transmitter binds to the receptor, the G-protein is activated, kicking off a cascade of events leading eventually to a change in channel properties. Indirect-acting transmitters take longer to act than direct acting neurotransmitters.
Examples of 2nd messengers (all are intracellular, since that is the nature of a 2nd messenger): calcium, cyclic AMP, etc.
Neurotransmitter examples (in parentheses: chemical class and where found)
ACh(its own class; found in CNS & PNS)
Excitatory when it acts at skeletal neuromuscular junction; where it acts via direct action onnicotinic ACh receptor channels.
Excitatory or inhibitory when it acts at the muscarinic ACh receptor, where it acts indirectly.
Norepinephrine (amine, found in CNS & PNS)
Excitatory or inhibitory, key transmitter in sympathetic division of autonomic nervous system, acts indirectly via G proteins & 2nd messengers.
Serotonin (amine, found in CNS)
Helps regulate mood. SSRIs (e.g. Prozac) are antidepressant drug that block serotonin reuptake, acts mostly indirectly via G proteins & 2nd messengers.
Glutamate (amino acid, found in CNS, both brain & spinal cord)
Main excitatory transmitter in brain; key role in memory & learning, acts directly & indirectly.
GABA (amino acid, found in brain portion of CNS)
Main inhibitory neurotransmitter in brain, acts directly (opens Cl channels) or indirectly.
Glycine (amino acid, found in spinal cord & brain stem portions of CNS)
Main inhibitory transmitter in spinal cord.
Endorphins (peptides, found in CNS)
Endogenous opiates (endo-morphine=endorphin). Inhibit pain by blocking substance P effects.
Substance P (peptide, CNS & PNS)
Transmits pain signals in PNS
Nitric oxide(NO) (gas, found in CNS & PNS)
Causes local relaxation of blood vessels in peripheral tissues, leading to greater local blood flow, including in erectile tissues.
Above list not 100% complete but a good start.
Neural Integration
Circuit types
Converging
Diverging
“Reverberating” (feedback)
Patterns of processing
Serial
Parallel
Student request: Explain the difference between voltage, potential, voltage difference, and potential difference.
These four terms are equivalent. Voltage (or electrical potential) is always measured between two points.One may say “voltage difference” or “potential difference” to remind the listener that one is measuring a voltage between two points. If the "second point" is not specified, one assumes the measurement is “with respect to ground”*. The volt is the unit used to measure voltage (or potential).
The following five statements are equivalent.
1."The transmembrane voltage is -70 millivolts."
2. "The voltage difference between the inside and the outside of the cell is -70 millivolts."
3. "The potential difference between the inside and the outside of the cell is -70 millivolts."
4."The intracellular voltage is -70 millivolts."
5. "The intracellular electrical potential is -70 millivolts."
Statements 4 and 5 do not specify where the "other side" of the voltage measurement is. Any physiologist reading 4 or 5 will understand it to mean the same as 1, 2, and 3.
*In modern cellular physiology research, "ground" usually means "outside the cell". The definition of "ground" for electromyogram (EMG), electrocardiogram (EKG, ECG), and electroencephalogram (EEG) is complicated.