NAME COURSE BIO 210: Anatomy and Physiology Text: Fundamentals of Anatomy and Physiology 9ed. Chapter 12 NEURAL TISSUE Like a telephone switchboard, the nervous system directs a countless number of incoming and outgoing calls to make order in the complex world of the human body. Neurophysiology: DIVISIONS: Central Nervous System: Peripheral Nervous System: Cranial nerves: nerves connected to the brain Spinal nerves: nerves connected to the spinal cord PNS DIVISIONS: Afferent division: o Receptors Efferent division: 1
o Effector! Somatic Nervous System (SNS): controls skeletal muscle contractions on a voluntary conscious level and a subconscious level (reflex)! Autonomic Nervous System (ANS): controls smooth muscle, cardiac muscle, adipose tissue, and glands; a visceral control center. Sub divisions are Sympathetic and Parasympathetic divisions NERVE TISSUE Neurons (Figure 12-2) o Cell Body contains the surrounded by ; found mostly in the for protection by the skeleton; life span of over 100 years, but cannot divide because these cells lack.! Perikaryon surrounding the nucleus; present for ATP production (for busy neurons), and present for protein synthesis; clusters of RER and ribosomes stain darkly and are called which make up in the cerebral cortex o Dendrites processes increase surface area for cell bodies to ; processes that transmit impulses the cell body 2
o Axon process of the cell body that transmits impulses from the cell body; can be very! Axoplasm of the axon! Axolemma specialized plasma membrane surrounding the! Initial segment base of the! Axon hillock thickened area where the axon joins the! Telodendria extensions on the end of the axon; terminal branches o Axoplasmic transport movement of materials between the cell body and the synaptic terminals; two-way transport! Anterograde flow materials carried toward the (neurotransmitters)! Retrograde flow materials carried toward the o Synaptic terminals (synaptic knobs) point of with another cell 3
SYNAPSES (Figure 12-3) Synapse o Presynaptic neuron the neuron conducting impulses the synapse; information giver o Postsynaptic neuron or cell the neuron that transmits the impulse the synapse; information receiver o Synaptic cleft space separating the two cells o Neurotransmitter chemical that is released into the synapse by the arrival of an o Synaptic vesicles within the, neurotransmitters are found within small sacs Neuromuscular Junction a synapse between a neuron and a muscle Neuroglandular junction synapse between a neuron and a gland 4
CLASSIFICATION OF NEURONS (Figure 12-4) Anaxonic found in brain and special sense organs; more than two processes and they are all ; functions poorly understood Bipolar sense receptors like smell, sight or hearing; rare otherwise; processes separated by the Unipolar sensory neurons of the PNS; longest extends from toes to spinal cord; and are continuous with cell body off to the Multipolar most common; all motor neurons in PNS; or more dendrites and a axon Functional classification of NEURONS Sensory Neurons o Somatic sensory neurons: receive information about the outside world by way of receptors in the, and o Visceral sensory neurons: monitor internal conditions within the SENSORY RECEPTORS Interoceptors monitor digestive, respiratory, cardiovascular, urinary and reproductive systems; distension, deep pressure, pain Exteroceptors monitor the external environment with touch, temperature, pressure, taste, smell, sight, equilibrium, and hearing Proprioceptors monitor the position and movement of skeletal muscles and joints 5
Motor neurons o Somatic motor neurons: linked to ; voluntary o Visceral motor neurons: linked to,, and ; involuntary Effectors the organs or tissues that respond to information from the CNS; muscles and glands Interneurons NEUrOGLIAL CELLS of the CNS Figure 12-5 and 12-6 provide support and protection to the neural tissues Ependymal cells line the central canal of the spinal cord and ventricles of the brain forming an epithelium called ; secrete and circulate the CSF; o Cerebral Spinal Fluid Astrocyte star-shaped; largest and most numerous supporting cell Functions include: Maintaining the blood brain barrier, provides a structural framework for the CNS, repairs damaged neural tissue, guide embryonic neural development, controls the interstitial fluid environment Blood-Brain Barrier Astrocytes aid in the brain to of substances to the brain tissue. Examples:,, and other unnecessary or harmful substances. Astrocytes send to the capillaries to form between their epithelial cells. CNS capillaries are less than others in the body so that chemicals do not cause the brain to unnecessary. 6
Oligodendrocytes neuroglial cells that form the in the CNS; processes extend into pad shape and wrap around the axon forming concentric layers of ; this the axons from contact with extracellular fluid o Myelin sheath o Internode o Nodes of Ranvier Microglia cells of the CNS; garbage collectors NEUrOGLIAL CELLS of the PNS Figure 12-5, 12-7 provide support and protection to the ganglia clusters of cell bodies outside of the CNS Satellite cells (amphicytes) surround neuron in regulating surrounding environment Schwann cells (neurilemma cells) coil their around the of nerves to create the ; protect axons from interstitial fluid; can enclose segments of several axons o Neurilemma Response To injury (Figure 12-8) Discussion: What happens to an axon when it is damaged? 7
WALLERIAN DEGENERATION repairing of damaged nerves; follow steps on Figure 12-8 1. 2. 3. 4. Transmembrane potential (Figure 12-9) Resting Potential transmembrane potential of a ; a change in this potential starts Graded Potential stimulus produces a temporary, change in transmembrane potential; decreases with from the stimulus Action Potential graded potential can trigger an that spreads along the surface of the axon and maintains the potential despite distance from the ; travels to a synapse Synaptic Activity arrival of the to a synapse causes the release of from the which bind to receptors on the allowing permeability and graded potentials Information Processing response of the cell PASSIVE FORCES ACTING ACROSS THE PLAMSA MEMBRANE (Figure 12-10) Chemical Gradient: Intracellular K + concentration is, ions move out of the cell through open K + channels Extracellular Na + concentration is, ions move into the cell through open Na + channels 8
Electrical Gradient: (Figure 12-10) ions leave cytoplasm at a faster rate than enters (membrane more permeable to ). Because of this, cytosol exhibits a charge along the interior of the cell membrane and the extracellular fluid outside of the cell membrane exhibits a charge. These charges are separated by the which restricts their movement causing a measured in Average resting plasma membrane potential is ; the negative sign is representative that the interior of the cell is negative with respect to the exterior Current movement of charges to eliminate a Resistance how much a barrier movement of those charges o Resistance high: o Resistance low: Electrochemical Gradient: (Figure 12-10) Potassium Ion Gradient an electrical gradient opposes the chemical gradient for potassium ions; the chemical gradient (high inside, low outside) pulls potassium out of the cell but the electrical gradient opposes this movement because positive ions are attracted to the negative inner membrane. Because of these opposing forces, the net gradient is a decreased exit from the cell Sodium Ion Gradient chemical gradient (high outside, low inside) plus the electrical gradient (positive charge attracted to the negative interior of the cell) combine to give a net gradient driving sodium into the cell The electrochemical gradient is a form of. Any stimulus that increases the of the plasma membrane to or ions causes sudden and dramatic ion movement. If a sodium channel opens, sodium will flow in regardless of what the stimulus was. ACTIVE FORCES ACTING ACROSS THE PLAMSA MEMBRANE 9
Sodium-Potassium Exchange Pump (Figure 12-9) carrier proteins called ATPase pump out Na + molecules from the cell and carries K + molecules into the cell to per ATP molecule, maintaining intracellular CHANGES IN THE TRANSMEMBRANE POTENTIAL cells respond to stimuli which brings them out of a resting potential in order to modify their activities Passive or leak channels Active channels Chemically gated channel (Figure 12-11a) open or close when they bind specific chemicals; receptors that bind ACh are chemically gated channels; most common on dendrites and cell bodies Voltage-gated channel (Figure 12-11b) opens or closes based on changes in the transmembrane potential; an excitable membrane, or one capable of generating and conduction an action potential is commonly found in an axon of unipolar and multipolar neurons and the sarcolemma Mechanically gated channels (Figure 12-11c) opens or closes based on a distortion in the plasma membrane; found in sensory receptors responding to touch, pressure, or vibration GRADED POTENTIALS (Figure 12-12 and 12-13) local potentials that cannot spread far from the site of stimulation; a stimulus that opens a gated channel creates a graded potential which can trigger specific cell functions or initiate an action potential Look at diagram, Figure 12-12: 1. Depolarization enters cell causing a shift in charges within the cell (more on the inside of the cell) 2. Spread of sodium ions within the cell produces a that depolarizes adjacent portions of the membrane 3. Degree of depolarization decreases with away from the stimulation from resistance of the 4. The bigger the, the greater the local depolarization 10
Action potentials An action potential is a nerve impulse that is propagated along an axon like dominoes reaching the synaptic terminals; dependant on voltage-gated channels All or Nothing Principle All or Nothing Principle a stimulus will either trigger an action potential that will reach a synapse, or not. If the required threshold is met, an action potential will occur regardless of the stimulus strength. Generation of an Action Potential (Figure 12-14, SPOTLIGHT) 1. Resting potential sodium and potassium gated channels closed 2. Depolarization to threshold stimulus initiates a graded depolarization large enough to pass the threshold and open volted-gated sodium channels 3. Sodium channels activated and rapid depolarization sodium rushes into the open channels causing rapid depolarization; inner membrane more positive than negative 4. Inactivation of sodium channels and activation of potassium channels transmembrane potential reaches +30mV which closes the voltage-gated sodium channels and opens the voltage gated potassium channels moving them out and shifting the transmembrane potential back to resting levels 5. Repolarization the membrane continues to move toward resting levels, sodium channels stay closed, potassium channels stay open 6. Hyperpolarization K + channels remain open and the inside of the cell becomes more negative than resting potential until the voltage reaches -90mV 7. Resting potential voltage gated potassium channels close and transmembrane potential returns to normal due to the sodium/potassium exchange pump Refractory period each section of membrane will not respond to additional stimuli during the time that an action potential begins until the normal resting potential has stabilized 11
Changes in the Transmembrane Potential (this process occurs in each non insulated part of an axon) PHASE Na + Channels Cell at Rest Na + Flow K + Channels K + Flow Voltage range Transmembrane Potential Depolarization Repolarization Hyperpolarization PROPAGATION OF ACTION POTENTIALS Propagation flow of charge with the message repeated over and over as it flows down the pathway of an axon Continuous Propagation (Figure 12-15) occurs in an axons; an action potential at the initial segment spreads the information one segment of membrane at a time; flows in direction because the previous section is in a and cannot be depolarized yet DISCUSSION: If the initial current spreads in all directions, why doesn t the action potential travel toward the cell body? Saltatory Propagation (Figure 12-15) only nodes respond to depolarization because myelin insulates and creates to ion flow; action potential jumps from to allowing the impulse to move more rapidly and uses less than continuous propagation Axons can move an action potential at a speed as fast as 268 mph to as slow as 2 mph depending on the diameter (bigger = faster) and myelination; urgent news gets priority and travels the fastest 12
THE PROCESS OF IMPULSE TRANSMISSION ACROSS THE SYNAPSE Electrical synapse pre and post synaptic membranes are locked together at junctions allowing flow of between cells. This allows local to flow from one cell to another; rare (found in the eye) Chemical Synapse uses a to send information between pre and postsynaptic membranes Excitatory neurotransmitter action potentials o Acetylcholine:! Inhibitory neurotransmitter causes hyperpolarization to an action potential o Acetylcholinesterase: CHOLINERGENIC SYNAPSES synapses that release acetylcholine (Figure 12-16) 1. 2. 3. 4. 5. 13
6. 7. 8. Neuromodulators alter the rate of neurotransmitter release by the presynaptic neuron or change the postsynaptic cell s response to neurotransmitters Opioids pain relieving neuromodulators o Endorphins o Enkephalins o Endomorphins o Dynorphins Common Neurotransmitters: Acetylcholine (ACh) synapses that release ACh are referred to as ; this is the most common of all neurotransmitters. It is released at all junctions and synapses throughout the CNS and PNS Norepinephrine ; synapses that release this neurotransmitter are referred to as Dopamine and (dopamine inhibits unwanted muscle tension and contraction. is the result of low dopamine levels) Seratonin affects,, and ; SSRI s (selective serotonin reuptake inhibitors) are commonly taken to increase the amount of serotonin at the to relieve symptoms of Nitric oxide stimulate in vessel walls Carbon monoxide functions in the brain as a neurotransmitter (functions poorly understood) 14