Human Physiology Unit 2 Exam
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OpenStax_Physiology_CH12.pptxPHYSIOLOGY
Chapter 12 THE NERVOUS SYSTEM AND NERVOUS TISSUE
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College Physics
Chapter # Chapter Title
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Structure of the nervous system
Structure of the Nervous system
Neuron is the communicator.
Glial cells are the associated cells. Astrocytes, microglia, oligodendrocytes
Structure of the nervous system
Gray Matter and White Matter
Gray matter consists of a large concentration of cell bodies, while white matter consists of a large concentration of axons. Called “white” because of the myelin around the axon.
(credit: modification of work by “Suseno”/Wikimedia Commons)
Structure of the nervous system
Central versus Peripheral
Collections of cell bodies that function together; CNS – nucleus PNS – ganglion
Collections of axons; CNS – tract PNS – nerve
Optic nerve v. optic tract
Function of the nervous system
Sensation, Integration and Response
Sensation – Registering a change from homeostasis known as a stimulus – taste, touch, sight, smell, hearing These are external stimuli. Internal stimuli can be the stretch of the stomach after eating, a change in blood ions.
Integration – Stimuli are presented to the part of the nervous system that processes that type of information. E.g. Sight goes to the eyes then to the brain. Some information is integrated with previous known information such as memories and classified accordingly through higher brain function.
Response – After integration a response occurs and can be voluntary or involuntary. E.g. Someone you are attracted to walks into the room
Sensation is sight
Integration is I see and recognize this person, I like them, I want to talk to them, I want to make a good impression
Voluntary response is making eye contact, walking to them, saying “hello”
Involuntary response is increased heart rate, sweating.
Function of the nervous system
Somatic nervous system controls voluntary responses.
Voluntary = move intentionally
Reflex = move unintentionally
Automatic = habitual movement (like driving)
Autonomic nervous system controls involuntary responses.
Regulates organ systems e.g. Heart rate, blood sugar
Enteric nervous system controls the smooth muscle and glandular tissue of the digestive system.
Can operate independently from the CNS
Structure of a neuron
Parts of a Neuron
The major parts of the neuron are labeled on a multipolar neuron from the CNS.
Types of neurons
Neuron Classification by Shape
Unipolar cells have one process that includes both the axon and dendrite and are typically sensory. Bipolar cells have two processes, the axon and a dendrite. Bipolar are “one-way” systems found in the retina and olfactory system. All other neurons are multipolar cells that have more than two processes, the axon and two or more dendrites.
Glial cells of the CNS
Glial Cells of the CNS
The CNS has astrocytes for support, oligodendrocytes for myelination, microglia act as phagocytes, and ependymal cells that create CSF.
Glial cells of the PNS
Glial Cells of the PNS
The PNS has satellite cells for support and Schwann cells for myelination.
Sensation, Integration and Response
Testing the Water
(1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) Stimulus from the sensory endings, if strong enough, will initiate an action potential on a sensory neuron.(3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. (4) An action potential at this neuron and travels up the sensory pathway to a region of the brain called the thalamus. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent (7) An action potential is sent down the spinal cord. (8) The axon of the neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.
Function of a neuron
The function of a neuron is based on changes in the concentration of intra- and extracellular ions. Membrane Potential is the difference in the concentrations of these charges.
Ions move across the cell membrane through protein channels. Some channels require energy to open and close.
Function of a neuron
Ligand-Gated Channels
Some channels need a “ligand” to bind to the protein to activate. When the ligand, the neurotransmitter acetylcholine, binds to the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are sodium, calcium, and potassium.
Function of a neuron
Mechanically Gated Channels
When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.
Function of a neuron
Voltage-Gated Channels
Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.
Function of neuron
Leakage Channels
In certain situations, the channel opens and closes randomly allowing ions to move across the membrane.
Membrane Potential
Measuring Charge across a Membrane with a Voltmeter
The difference between the charge intracellularly and extracellularly is measured in millivolts (mV). Membrane Potential assumes the extracellular space is 0mV therefore the Resting Membrane Potential for the cell is -70mV.
It is more negative inside than outside because there are 10x more Na+ ions outside than K+ inside.
Action Potential
Stages of an Action Potential
(1) At rest, the membrane voltage is -70 mV. (2) External stimulus is applied and a Na+ channel opens. If enough Na+ moves in to raise voltage from -70mV to -55mV voltage-gated channels open and more Na+ comes in. (3) As Na+ rushes in the cell begins to depolarize and the membrane voltage begins a rapid rise toward+30 mV. (4) Once it reaches +30mV, voltage-gated K+ channels open and K+ moves outside the cell. The membrane voltage starts to return to a negative value. (5) K+ channels take longer to close so repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) Through the action of non-gated channels and the Na+/K+ pump the membrane voltage returns to the resting value.
Action potential
The whole thing takes about 2 milliseconds.
Once started can’t start another until the membrane potential reaches < -55mV.
Absolute refractory if membrane potential is > -55mV
Refractory Period once membrane potential reaches < -55mV. At this point need a stronger stimulus that the first one. (Why we grab onto and put pressure on boo-boos)
Action potential
Starts at the dendritic end of the neuron.
Na+ moves into the cell and travels along just inside the cell membrane. Raises voltage to above threshold level (-55mV) thus opening voltage-gated channels down the line…..
Absolute refractory period keeps it from going backwards so only travels in one direction.
Saltatory conduction – voltage-gated channels are located at the Nodes of Ranvier. Change in voltage starts to decrease between the nodes but gets a “boost” at each node when the channels open and more Na+ comes in.
Neuron communication
Changes in the membrane potential are determined by the size of the stimulus. (warm water versus hot water) Graded potentials are temporary changes in the membrane voltage that determine if an action potential occurs. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane.
Depolarizing is usually excitatory while hyperpolarization is often inhibitory.
Neuron communication
Synapse – connection between electrically active cells
Electrical synapse – AP passes from one cell to another as if it was one cell
Chemical synapse – messenger is a neurotransmitter
Chemical synapse
AP reaches the end of the axon where voltage-gated Ca++ channels open. Ca2+ enters the bulb helping vesicle-bound neurotransmitter to fuse with the cell membrane and be released through exocytosis. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor as a ligand activating ligand-gated Na+ channels. Na+ comes in, AP is generated. The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake.
Neurotransmitter systems
Cholinergic system – neurotransmitter is Acetylcholine.
1. Nicotinic receptors – nicotine can bind to this as well as acetylcholine
2. Muscarinic receptors – muscarine is product of certain mushrooms
Amino Acids – glutamate, GABA gamma-aminobutyric acid, glycine
Biogenic amine – made from amino acids; Serotonin from tryptophan; dopamine, norepinephrine and epinephrine from tyrosine
Neuropeptides – short chains of amino acids; Met-enkephalin and beta-endorphin
Effects of neurotransmitters
An ionotropic receptor is a channel that opens when the neurotransmitter binds to it.
A metabotropic receptor is a complex that causes metabolic changes in the cell when the neurotransmitter binds to it (1). After binding, the G protein binds to the effector protein (2). When the G protein contacts the effector protein, a second messenger is generated (3). The second messenger can then go on to cause changes in the neuron, such as opening or closing ion channels, metabolic changes, and changes in gene transcription.
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