[BIO 430] Ch.12 - Nervous Tissue

Factors That Affect the Speed of Propagation

(1) amount of myelination (2) axon diameter - Larger-diameter axons propagate action potentials faster (3) temperature - Axons propagate action potentials at lower speeds when cooled

Action potential - Voltage Gated Channels

- 2 types of voltage gated channels open and close during an action potential. Voltage-gated Na channels open, allowing Na to rush into the cell, which causes the depolarizing phase. Then voltage-gated K channels open, allowing K to flow out, which produces the repolarizing phase. - After-Hyperpolarizing Phase - occurs when voltage-gated K channels remain open after the repolarizing phase ends - action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the threshold (about 55 mV in many neurons) - Diff neurons may have diff thresholds, but the threshold in a particular neuron usually is constant.

CNS and PNS - Basic

- Central nervous system (CNS) - consists of the brain and spinal cord - spinal cord is connected to the brain through the foramen magnum - Peripheral nervous system (PNS) - consists of all nervous tissue outside the CNS

Classification of Nerve Fibers (B and C Functions, Coding of Stimulus Intensity)

- Examples of motor functions of B and C fibers: - - - constricting and dilating the pupils - - - increasing and decreasing the heart rate - - - contracting and relaxing the urinary bladder. Encoding of Stimulus Intensity - frequency of action potentials is responsible for detecting stimuli of differing intensities - light touch generates a low frequency of action potentials while firmer pressure elicits action potentials that pass down the axon at a higher frequency - 2nd factor is the number of sensory neurons recruited (activated) by the stimulus. A firm pressure stimulates a larger number of pressure-sensitive neurons than does a light touch

Structural Diversity in Neurons

- Neurons diverse in size & shape - pattern of dendritic branching is varied and distinctive for neurons in different parts of the nervous system; a few small neurons lack an axon, and many others have very short axons - the longest axons are from the toes to lowest part of the brain

Neuroglia of PNS

- Schwann cells - encircle PNS axons; each myelinates a single axon; participate in axon regeneration, which is more easily accomplished in the PNS -Satellite cells - surround the cell bodies of neurons of PNS ganglia; provides structural support & regulates the exchanges of materials between neuronal cell bodies and interstitial fluid

Summation of Postsynaptic Potentials

- Summation - process by which graded potentials add together; greater the summation of EPSPs, the greater the chance that threshold will be reached. - 2 types of summation: spatial summation & temporal summation - Spatial summation - summation of postsynaptic potentials in response to stimuli that occur at diff locations in the membrane of a postsynaptic cell at the same time - - - results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs - Temporal summation - summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times - - - results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession - - - b/c a typical EPSP lasts about 15 msec, the second release of neurotransmitter must occur soon after the first one - Most of the time, spatial and temporal summations act together to influence the chance that a neuron fires an action potential - - - A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some of which release inhibitory neurotransmitters

Propagation - Continuous Conduction

= involves depolarization, repolarization, & flow through voltage-gated channels of each adjacent segment of the plasma membrane - action potential propagates only a relatively short distance in a few milliseconds - occurs in unmyelinated axons and in muscle fibers

Parts of a Neuron - Dendrites

= receiving or input portions of a neuron - plasma membranes of dendrites (and cell bodies) contain numerous receptor sites for binding chemical messengers from other cells - usually are short, tapering, highly branched

Process of Chemical Synapse Transmitting a Signal

(1) A nerve impulse arrives at a synaptic end bulb (2) The depolarizing phase of the nerve impulse opens voltage gated Ca2 channels; Ca2 flows inward through the opened channels. (3) increase concentration of Ca2 inside presynaptic neuron triggers exocytosis of the synaptic vesicles; as vesicle membranes merge with the plasma membrane, neurotransmitter molecules are released into the synaptic cleft. (4) neurotransmitter diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron's plasma membrane (5) Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane (6) As ions flow through the opened channels, the voltage across the membrane changes. This change is a postsynaptic potential - - - Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition) - - - Ex. opening of Na channels allows inflow of Na, which causes depolarization. But, opening of Cl- or K channels causes hyperpolarization. - - - Opening Cl- channels permits Cl- to move into the cell, while opening the K channels allows K to move out—in either event, the inside of the cell becomes more negative (7) When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron - - - At most chemical synapses, only one-way information transfer can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell.

Excitatory and Inhibitory - Postsynaptic Potentials

- A neurotransmitter causes either an excitatory or an inhibitory graded potential - A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold; - - - Excitatory postsynaptic potential (EPSP) - A depolarizing postsynaptic potential - - - Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable; it is more likely to reach threshold when the next EPSP occurs - neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory - - - During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential inside becomes more negative and farther from threshold than in its resting state - - - Inhibitory postsynaptic potential (IPSP) - A hyperpolarizing postsynaptic potential

Damage and Repair in the PNS - Process

- About 24 to 48 hours after injury to a process of a normal peripheral neuron, the Nissl bodies break up into fine granular masses (process called Chromatolysis Desuctruction) - By the third to fifth day, the axon distal to the damaged region becomes swollen and breaks into fragments; myelin sheath also deteriorates; the neurolemma remains. - - - Degeneration of the distal portion of the axon and myelin sheath is called Wallerian degeneration - Following chromatolysis, signs of recovery in the cell body become evident: - - - Macrophages phagocytize the debris. - - - Synthesis of RNA and protein accelerates, which favors rebuilding or regeneration of the axon - Schwann cells on either side of the injured site multiply by mitosis, grow toward each other, and may form a regeneration tube across the injured area - - - tube guides growth of a new axon across the injured area into the distal area previously occupied by the original axon; New axons cannot grow if the gap at the site of injury is too large or if the gap becomes filled with collagen fibers. - During the first few days following damage, buds of regenerating axons begin to invade the tube formed by the Schwann cells - - - Axons from the proximal area grow at a rate of about 1.5 mm per day across the area of damage, find their way into the distal regeneration tubes, and grow toward the distally located receptors and effectors - - - some sensory and motor connections are reestablished and some functions restored. In time, the Schwann cells form a new myelin sheath

Small Molecule Neurotransmitters

- Acetylcholine - best-studied neurotransmitter; is an excitatory neurotransmitter at some synapses & an inhibitory neurotransmitter at other synapses - Amino Acids - Several are neurotransmitters in CNS. Glutamate and aspartate have powerful excitatory effects - - - Gamma aminobutyric acid and glycine are important inhibitory neurotransmitters. GABA is found only in the CNS, where it is the most common inhibitory neurotransmitter - Biogenic Amines - Prevalent in nervous system; may cause excitation or inhibition; Norepinephrine, dopamine, and epinephrine are classified chemically as catecholamines - - - Norepinephrine - plays roles in arousal (waking from deep sleep), dreaming, and regulating mood; smaller # of neurons in the brain use it as a neurotransmitter - - - Dopamine - active during emotional responses, addictive behaviors, and pleasurable experiences - - - - - - muscular stiffness in Parkinson disease is due to degeneration of neurons that release dopamine - - - Serotonin - concentrated in part of the brain called the raphe nucleus; Thought to be involved in sensory perception, temperature regulation, control of mood, appetite, induction of sleep, & depression - ATP and Other Purines - characteristic ring structure of the adenosine portion of ATP is called a purine ring. Adenosine itself, as well as its triphosphate, diphosphate, and monophosphate derivatives (ATP, ADP, and AMP), is an excitatory neurotransmitter in both the CNS and the PNS - Nitric Oxide - important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal glands, and nerves to the penis and has widespread effects throughout the body - Carbon Monoxide - an excitatory neurotransmitter produced in the brain and in response to some neuromuscular and neuro glandular functions

Electrical Synapses

- At an electrical synapse, action potentials conduct directly between the plasma membranes of adjacent neurons through gap junctions - - - Each gap junction contains hundreds of tubular connexons, which act like tunnels to connect the cytosol of the two cells directly - - - As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell - - - Gap junctions are common in visceral smooth muscle, cardiac muscle, developing embryo, & brain - Electrical synapses have 2 main advantages: (1) Faster communication - Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses (2) Synchronization - Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions - - - value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract

Damage and Repair in the PNS

- Axons and dendrites that are associated with a neurolemma may undergo repair: - - - if the cell body is intact - - - if the Schwann cells are functional - - - if scar tissue formation doesn't occur rapidly - Most nerves in the PNS consist of processes that are covered with a neurolemma. A person who injures axons of a nerve in an upper limb, for example, has a good chance of regaining nerve function. - When there is damage to an axon, changes usually occur both in the cell body of the affected neuron and in the portion of the axon that is proximal and distal to the site of injury

Classification of Nerve Fibers (A, B, C)

- Axons can be classified based on: amount of myelination, their diameters, and their propagation speeds - A fibers - largest-diameter axons; myelinated ; are axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and thermal and pain sensations; axons of motor neurons that conduct impulses to skeletal muscles are also A fibers - B fibers - myelinated; exhibit saltatory conduction; have a longer absolute refractory period than A fibers; conduct sensory nerve impulses from the viscera to the brain and spinal cord & constitute all the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia - C fibers - smallest-diameter axons; unmyelinated; exhibit the longest absolute refractory periods; Conducts sensory impulses from the viscera; Autonomic motor fibers that extend from autonomic ganglia are C fibers

PNS Components

- Components: nerves, ganglia, enteric plexuses, sensory receptors - Nerve - bundle of 100-1000 of axons plus connective tissue and blood vessels that lies outside brain and spinal cord - Ganglia - small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord; closely associated with cranial and spinal nerves - Enteric plexuses - extensive networks of neurons located in walls of organs of GI tract; neurons of plexuses help regulate the digestive system - Sensory receptor - structure of the nervous system that monitors changes in the external or internal environment

Neural Circuits

- In a simple series circuit, a presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural circuits are more complex. - A single presynaptic neuron may synapse with several postsynaptic neurons. - Divergence - arrangement that permits one presynaptic neuron to influence several postsynaptic neurons at the same time; amplifies the signals - Convergence - arrangement where several presynaptic neurons synapse with a single postsynaptic neuron; permits more effective stimulation or inhibition of the postsynaptic neuron - Reverberating Circuit - arrangement where incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on - - - Branches from later neurons synapse with earlier ones; impulses are sent back through the circuit again anda gain; output signal differ depending on # of synapses and the arrangement of neurons in the circuit - - - Inhibitory neurons may turn off a reverberating circuit after a period of time - - - Body responses thought to be the result of output signals from reverberating circus: breathing, coordinated muscular activities, waking up, & short-term memory - Parallel After-Discharge circuit - a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell - - - differing # of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs - - - If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. Parallel after-discharge circuits may be involved in precise activities such as mathematical calculations.

Electrical Signals In Neurons

- Like muscle fibers, neurons are electrically excitable. Communicates through 2 types of electrical signals: (1) Graded potentials used for short distance communication (2) Action potentials allow communication over long distances within body. - Muscle Action Potential - action potential in a muscle fiber - Nerve Action Potential (Nerve Impulse) - action potential in a neuron (nerve cell) - Resting Membrane Potential - exists b/c of small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane - greater the diff in charge across the membrane, the larger the membrane potential (voltage); buildup of charge occurs only very close to the membrane - cytosol elsewhere in the cell is electrically neutral - In neurons, resting membrane potential ranges from 40-90 mV (typical 70 mV) - A cell that exhibits a membrane potential is polarized. Most body cells are polarized; membrane potential varies from 5 mV to 100 mV in diff types of cells

Disorders

- Multiple sclerosis - disease that causes a progressive destruction of myelin sheaths surrounding neurons in the CNS; usually appears between ages 20-40, affecting females twice as often as males - Epilepsy - characterized by short, recurrent attacks of motor, sensory, or psychological malfunction; epileptic seizures afflict about 1% of the world's population. - Guillain-Barré syndrome - disorder in which macrophages strip myelin from axons in the PNS; may result from the immune system's response to a bacterial infection. Most patients recover completely or partially, but about 15% remain paralyzed - Neuroblastoma - tumor that consists of immature nerve cells; commonly occurs in the abdomen and most frequently in the adrenal glands; most common tumor in infants - Neuropathy - Any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve - Rabies - fatal disease caused by a virus that reaches the CNS via fast axonal transport; usually transmitted by the bite of an infected dog or other meat-eating animal; symptoms are excitement, aggressiveness, & madness, followed by paralysis & death

Nervous Tissue - Neurons and Neuroglia

- Nervous tissue comprises two types of cells: neurons and neuroglia - cells combine in various ways in diff regions of nervous system. Forms the complex processing networks within the brain and spinal cord & connects all regions of the body to the brain and spinal cord - Neurons - Functions - sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions ; As a result of specialization, no ability to undergo mitotic divisions. - Neuroglia - smaller cells & outnumber neurons ; support, nourish, protect neurons, & maintain the interstitial fluid - Unlike neurons, neuroglia continues to divide throughout an individ's lifetime. - Neurons and neuroglia differ structurally & functionally depending on location (CNS or PNS); Some neurons are tiny and propagate impulses over a short distance (less than 1 mm) within the CNS. Others are the longest cells in the body

Neurogenesis in the CNS

- Neurogenesis - the birth of new neurons from undifferentiated stem cells—occurs regularly in some animals - The lack of neurogenesis in other regions of the brain and spinal cord seems to result from two factors: (1) inhibitory influences from neuroglia, particularly oligodendrocytes (2) absence of growth-stimulating cues that were present during fetal development. - Axons in the CNS are myelinated by oligodendrocytes rather than Schwann cells, and this CNS myelin is one of the factors inhibiting regeneration of neurons - after axonal damage, nearby astrocytes proliferate rapidly, forming a type of scar tissue that acts as a physical barrier to regeneration; Thus, injury of the brain or spinal cord usually is permanent

Neuroglia - Base, Function, Injury, Types

- Neuroglia make up half the volume of CNS; smaller and 5-25 times more numerous than neurons - don't generate action potentials, and can multiply and divide in the mature nervous system - In cases of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons. - - - Brain tumors derived from glia, called gliomas, tend to be highly malignant and to grow rapidly. 6 types - 4 in CNS (astrocytes, oligodendrocytes, microglia, & ependymal cells) ; 2 in PNS (Schwann cells & Satellite cells)

Collections of Nervous Tissue

- Neuronal Cell Bodies - grouped together in clusters - - - a nucleus is a cluster of neuronal cell bodies located in the CNS - Axons - grouped in bundles - - - Nerve is a bundle of axons in PNS; cranial nerves connect brain to periphery & spinal nerves connect spinal cord to periphery - - - Tract is a bundle of axons located in CNS; interconnect neurons in spinal cord and brain - Regions of nervous tissue - grouped as gray or white matter - - - White matter - composed primarily of myelinated axons - - - Gray matter - contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia; appears gray b/c Nissl bodies impart a gray color and there is little or no myelin - - - Blood vessels are present in both white and gray matter. - - - In spinal cord, white matter surrounds an inner core of gray matter; in brain, a thin shell of gray matter covers the surface of the largest portions of the brain (cerebrum & cerebellum)

Parts of a Neuron - Axons (Neurotransmitter, Slow Axonal Transport, Fast Axonal Transport)

- Neurotransmitter - molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell - neurons contain 2-3 types of neurotransmitters, ea. w/ diff effects on the postsynaptic cell. - b/c some substances synthesized or recycled in the neuron cell body are needed in the axon or at the axon terminals, 2 types of transport systems carry materials from the cell body to the axon terminals and back. - Slow Axonal Transport - conveys axoplasm in one direction only—from the cell body toward the axon terminals - supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons - Fast Axonal Transport - moves materials in both directions—away from and toward the cell body - uses motor proteins to move materials along the surfaces of microtubules of the neuron's cytoskeleton - Fast axonal transport that occurs in an anterograde (forward) direction moves organelles and synaptic vesicles from the cell body to the axon terminals. - Fast axonal transport that occurs in a retrograde (backward) direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. - Substances that enter the neuron at the axon terminals are also moved to the cell body by fast retrograde transport. - - - Substances: trophic chemicals (nerve growth factor), harmful agents (tetanus toxin), & viruses that cause rabies, herpes simplex, and polio.

Myelination - CNS

- Oligodendrocyte - myelinates parts of several axons; ea. puts forth about 15 broad, flat processes that spiral around CNS axons, forming a myelin sheath; neurolemma not present b/c oligodendrocyte cell body and nucleus don't envelop the axon; Nodes of Ranvier are present, but fewer in number - - - Axons in CNS display little regrowth after injury. This is thought to be due to the absence of a neurolemma, and due to an inhibitory influence exerted by the oligodendrocytes on axon regrowth - amount of myelin increases from birth to maturity; presence increases speed of nerve impulse conduction - - - infant's responses to stimuli aren't as rapid or coordinated as an older child or adult b/c myelination is still in progress during infancy

Neuroglia of CNS - Oligodendrocytes, Microglia, Ependymal cells

- Oligodendrocytes - resemble astrocytes but are smaller and contain fewer processes; may also play a role in learning and memory - - - responsible for forming and maintaining the myelin sheath around CNS axons - - - myelin sheath is a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be myelinated. - Microglia - small cells with slender processes that give off numerous spine like projections; function as phagocytes - Ependymal cells - cuboidal to columnar cells arranged in a single layer that process microvilli and cilia - - - cells line the ventricles of the brain and central canal of the spinal cord - - - produce, possibly monitor, and assist in the circulation of cerebrospinal fluid; also form the blood-cerebrospinal fluid barrier

Resting Membrane Potential

- Resting Membrane Potential produced by 3 factors: - Unequal distribution of ions in the ECF and cytosol - - - Extracellular fluid rich in Na and Cl ions - - - In cytosol, the main cation is K, and the 2 dominant anions are phosphates and amino acids in proteins. - - - As K ions exit, inside of the membrane becomes negative and outside becomes positive - Inability of most anions to leave the cell - - - Most anions inside cell are not free to leave b/c they can't follow the K out of the cell b/c they're attached to non-diffusible molecules (ex. as ATP and large proteins) - Electrogenic nature of the Na-K ATPases. - - - Membrane permeability to Na is low b/c few amount of sodium leak channels; small inward Na leak and outward K leak are offset by the Na/K ATPases (sodium-potassium pumps) - - - Na-K pumps help maintain the resting membrane potential by pumping out Na as fast as it leaks in while the Na/K ATPases bring in K - - - K ions eventually leak back out of the cell as they move down their concentration gradient - - - Since pumps remove more positive charges from the cell than they bring into the cell, they are electrogenic, which means they contribute to the negativity of the resting membrane potential; total contribution is small

Myelination - PNS

- Schwann cells form myelin sheaths around axons during fetal development - - - Neurolemma - outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath - - - Neurolemma found only around axons in the PNS. Neurolemma aids neuron regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon - - - Nodes of Ranvier - gaps in myelin sheath; appears at intervals along axon; ea. Schwann cell wraps one axon segment between 2 nodes

Functions of the Nervous System

- Sensory function - Sensory receptors detect internal or external stimuli; sensory info is carried into the brain and spinal cord through cranial and spinal nerves - Integrative function - integration: processes sensory info by analyzing it and making decisions for appropriate responses - Motor function - Once sensory info is integrated, nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

Neurotransmitters

- Some neurotransmitters bind to their receptors and act quickly to open or close ion channels in the membrane. Others act more slowly via second-messenger systems to influence chemical reactions inside cells. The result of either process can be excitation or inhibition of postsynaptic neurons - Many neurotransmitters are also hormones released into the blood stream by endocrine cells in organs throughout the body. - Neurotransmitters can be divided into 2classes based on size: Small molecule neurotransmitters & neuropeptides

Signal Transmission at Synapses

- Synpase - region where communication occurs between 2 neurons or between a neuron and an effector cell (muscle cell or glandular cell) - Presynaptic neuron - a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal - Postsynaptic cell - cell that receives a signal - - - may be a nerve cell called a postsynaptic neuron - carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse - Most synapses are either: - - - axodendritic (from axon to dendrits) - - - axosomatic (from axon to cell body) - - - axoaxonic (from axon to axon) - synapses may be electrical or chemical and they differ both structurally and functionally

Different Postsynaptic Effects for the Same Neurotransmitter

- The same neurotransmitter can be excitatory at some synapses and inhibitory at other synapses, depending on the structure of the neurotransmitter receptor to which it binds. - Ex. at some excitatory synapses, acetylcholine (ACh) binds to ionotropic receptors that contain cation channels; at some inhibitory synapses, ACh binds to metabotropic receptors coupled to G proteins that open K channels

Propagation of Action Potentials

- To communicate info from one part of the body to another, action potentials in a neuron travels from the trigger zone of the axon to the axon terminals - Propagation - an action potential keeps its strength as it spreads along the membrane - 2 types of propagation: continuous conduction & saltatory conduction

Structure of Neurotransmitter Receptors

- When a neurotransmitter binds to the correct neurotransmitter receptor, an ion channel opens and a postsynaptic potential (either an EPSP or IPSP) forms in the membrane of the postsynaptic cell. - Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors - - - based on whether the neurotransmitter binding site and the ion channel are components of the same protein or are components of different proteins

Propagation - Along Myelinated Axon, Consequences of Flow Across Nodes of Ranvier

- When an action potential propagates along a myelinated axon, an electric current (carried by ions) flows through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next - action potential at the first node generates ionic currents in the cytosol and extracellular fluid that depolarize the membrane to threshold, opening voltage-gated Na channels at the second node - resulting ionic flow through the opened channels constitutes an action potential at the second node. Then, the action potential at the second node generates an ionic current that opens voltage-gated Na channels at the third node, and so on - Each node repolarizes after it depolarizes. - flow of current across the membrane only at the nodes of Ranvier has 2 consequences: (1) action potential appears to "leap" from node to node as each nodal area depolarizes to threshold, thus the name "saltatory." ; it travels much faster than it would in an unmyelinated axon of the same diameter (2) Opening a smaller number of channels only at the nodes

Action potential - After-Hyperpolarizing Phase

- While the voltage-gated K channels are open, outflow of K may be large enough to cause an after-hyperpolarizing phase of the action potential - the voltage gated K channels remain open and the membrane potential becomes even more negative (about 90 mV) - As the voltage-gated K channels close, the membrane potential returns to the resting level of 70 mV. most voltage- gated K channels do not exhibit an inactivated state. Instead, they alternate between closed (resting) and open (activated) states

Graded Potentials - Summation & Diff Names

- a graded potential can become stronger and last longer by summating with other graded potentials - - - Summation - process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential. If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), then they cancel each other out and the overall graded potential disappears. - Graded potentials have different names depending on which type of stimulus causes them and where they occur. - - - graded potential in response to a neurotransmitter is called a postsynaptic potential - - - graded potentials in sensory receptors and sensory neurons are called receptor potentials and generator potentials

Action potential - Thresholds

- action potential will not occur in response to a subthreshold stimulus, a stimulus that is a weak depolarization that cannot bring the membrane potential to threshold - action potential will occur in response to a threshold stimulus, a stimulus that is strong enough to depolarize the membrane to threshold - Several action potentials will form in response to a supra threshold stimulus, a stimulus that is strong enough to depolarize the membrane above threshold - the greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period - All-or-None Principle - an action potential either occurs completely or it does not occur at all

Graded Potentials - Process

- graded potential occurs when a stimulus causes mechanically gated or ligand-gated channels to open or close in an excitable cell's plasma membrane; occur mainly in the dendrites and cell body of a neuron - graded means that the electrical signals vary in amplitude, depending on the strength of the stimulus - larger or smaller depending on how many ligand-gated or mechanically gated channels have opened (or closed) and how long each remains open - opening or closing of ion channels alters flow of ions across the membrane, producing a flow that is localized (it spreads to adjacent regions along the plasma membrane from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels) - - - This mode of travel by which graded potentials die out as they spread along the membrane is known as decremental conduction - Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only

Regeneration and Repair of Nervous Tissue

- nervous system exhibits plasticity (capability to change based on experience) - chemical and electrical signals drive the changes that can occur: - - - the sprouting of new dendrites - - - synthesis of new proteins - - - changes in synaptic contacts with other neurons - mammalian neurons have very limited powers of regeneration - In PNS, damage to dendrites and myelinated axons may be repaired if the cell body remains intact and if the Schwann cells that produce myelination remain active - In CNS, little or no repair occurs. Even when the cell body remains intact, a severed axon cannot be repaired or regrown

Action potential - Repolarizing Phase

- opening occurs at the same time the voltage-gated Na channels are closing b/c voltage-gated K channels open more slowly - slower opening of voltage-gated K channels and the closing of previously open voltage-gated Na channels produce the repolarizing phase of the action potential - Slowing of Na inflow and acceleration of K outflow cause the membrane potential to change from 30 mV to 70 mV. Repolarization also allows inactivated Na channels to revert to the resting state.

Chemical Synapses

- plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close but they don't touch; separated by the synaptic cleft, a space of 20-50 nm* that is filled with interstitial fluid - In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron - The postsynaptic neuron receives the chemical signal and produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter) - - - postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential) - - - chemical synapses relay signals more slowly than electrical synapses

Comparison of Electrical Signals

- produce 2 types of electrical signals: graded potentials & action potentials - propagation of a muscle action potential along the sarcolemma into the T tubule system initiates muscle contraction - Diffs between action potentials in muscle fibers and in neurons - - - typical resting membrane potential of a neuron is 70 mV, but 90 mV in skeletal and cardiac muscle fibers - - - duration of a nerve impulse is shorter than skeletal, cardiac and smooth muscle fibers - - - propagation speed of action potentials along the largest-diameter, myelinated axons is about 18 times faster than the propagation speed along the sarcolemma of a skeletal muscle fiber

Parts of a Neuron - Axons (Parts)

- single axon (axis) of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell - axon is a thin, cylindrical projection that joins to cell body at a cone-shaped elevation called the axon hillock - part of the axon closest to the axon hillock is the initial segment. Nerve impulses arise at the junction of the axon hillock and the initial segment, an area called the trigger zone, and travel along the axon to destination - contains mitochondria, microtubules, neurofibrils - no rough ER, therefore protein synthesis doesn't occur in axons - Axoplasm - cytoplasm of axon; surrounded by plasma membrane (axolemma) - Axon collaterals - side branches of axons; may branch off typically at right angle - Axon terminals - fine division of axons and its collaterals - Synapse - site of communication between 2 neurons OR between a neuron and effector cell - Synaptic end bulbs - tips of some axon terminals that swell into bulb-shaped structures - Varicosities - tips of axon terminals that have a string of swollen bumps - - - synaptic end bulbs and varicosities contain synaptic vesicles that store neurotransmitter

Effects of Excitatory and Inhibitory Effects on the Postsynaptic Neuron

- sum of all the excitatory and inhibitory effects determines the effect on the postsynaptic neuron, which may respond in the following ways (1) EPSP - If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. - - - Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized (2) Nerve impulse(s) - If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level (3) IPSP - If the total inhibitory effects are much greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse

Parts of a Neuron - Cell Body

3 parts: cell body, dendrites, axon - Cell Body - contains a nucleus surrounded by cytoplasm that has cellular organelles such as lysosomes, mitochondria, and a Golgi complex - Neuronal cell bodies also contain free ribosomes and prominent clusters of rough ER (Nissl bodies) - Ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS - cytoskeleton includes both neurofibrils composed of bundles of: - - - intermediate filaments that provide the cell shape and support - - - microtubules, which assist in moving materials between the cell body and axon - Lipofuscin - product of neuronal lysosomes; accumulates as the neuron ages, but doesn't harm the neuron - Nerve fiber - any neuronal process (extension) that emerges from the cell body of a neuron

Myelination - General

= axons surrounded by a multilayered lipid and protein covering (myelin sheath) are myelinated - sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction; Axons w/o sheath are unmyelinated - 2 types of neuroglia produce myelin sheaths: Schwann cells (in the PNS) & oligodendrocytes (in the CNS).

Functional Classification of Neurons

= classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS - Sensory or afferent neurons - contains sensory receptors at their distal ends (dendrites) or are located just after sensory receptors - - - Once stimulus activates a sensory receptor, sensory neuron forms an action potential in its axon that is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar - Motor or efferent neurons - convey action potentials away from the CNS to effectors (muscles and glands) in PNS through cranial or spinal nerves; Motor neurons are multipolar in structure. - Interneurons or association neurons - mainly in CNS between sensory and motor neurons; integrate (process) incoming sensory info from sensory neurons and elicit a motor response by activating motor neurons; Most interneurons are multipolar in structure

Structural Classification of Neurons

= classified by # of processes extending from the cell body - Multipolar neurons - several dendrites & one axon; most neurons in the brain and spinal cord are of this type, as well as all motor neurons - Bipolar neurons - one main dendrite and one axon; in the retina, inner ear, & olfactory - Unipolar neurons - have dendrites and one axon fused to form a continuous process that emerges from the cell body; called pseudounipolar neurons because they begin in the embryo as bipolar neurons - - - unipolar dendrites function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, & thermal stimuli - - - trigger zone for nerve impulses in a unipolar is at the junction of the dendrites and axon. The impulses propagate toward the synaptic end bulbs - - - cell bodies of unipolar are in the ganglia of spinal and cranial nerves - Purkinje cells - in the cerebellum - Pyramidal cells - found in the cerebral cortex of the brain, which have pyramid-shaped cell bodies

Neuroglia of CNS - Astrocytes

= classified by size, cytoplasmic processes, and intracellular organization - Astrocytes - star shaped cells, have many processes, largest & most numerous of the neuroglia - 2 types - Protoplasmic astrocytes (short branching processes; in gray matter) ; Fibrous astrocytes (long unbranched processes; in white matter) - processes make contact with blood capillaries, neurons, and the pia mater - Functions: - - - Contain microfilaments that give them strength, which enables them to support neurons - - - Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood, create a blood-brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS. - - - In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the brain. - - - Help to maintain the appropriate chemical environment for the generation of nerve impulses. May also play a role in learning and memory

Neuropeptides

= neurotransmitters widespread in CNS and PNS & consists of 3 to 40 amino acids linked by neuropeptides; Neuropeptides bind to metabotropic receptors and have excitatory or inhibitory actions - opioid peptides include the endorphins and dynorphins; opioid peptides possibly the body's natural painkillers - - -linked to improved memory and learning; feelings of pleasure or euphoria; control of body temperature; regulation of hormones that affect the onset of puberty, sexual drive, and reproduction; and mental illnesses such as depression and schizophrenia - - - Acupuncture may produce analgesia (loss of pain sensation) by increasing the release of opioids. - Substance P - neuropeptide that enhances the perception of pain; useful as a treatment for nerve degeneration - - - Enkephalin and endorphin suppress the release of substance P, decreasing the #of nerve impulses being relayed to the brain for pain sensations

Action potential

= sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state - 2 main phases: (1) Depolarizing phase - negative membrane potential becomes less negative, reaches zero, and then becomes positive (2) Repolarizing phase - membrane potential is restored to the resting state of 70 mV - - - After repolarizing phase there may be an after-hyperpolarizing phase, more negative than the resting level

Graded Potentials - Hyperpolarizing, Depolarizing

= small deviation from the membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative) - Hyperpolarizing Graded Potential - when response makes membrane more polarized - Depolarizing Graded Potential - when response makes membrane less polarized

Propagation - Saltatory Conduction

= special mode of action potential propagation that occurs along myelinated axons; occurs b/c of the uneven distribution of voltage-gated channels - Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma. - at the nodes of Ranvier (where there is no myelin sheath), the axolemma has many voltage-gated channels. Hence, current carried by Na and K flows across the membrane mainly at the nodes

Action potential - Depolarizing Phase

= the electrical and the chemical gradients favor inward movement of Na; resulting in rush of Na causes the depolarizing phase of the action potential - inflow of Na changes the membrane potential from 55 mV to 30 mV. At the peak of the action potential, the inside of the membrane is 30 mV more positive than the outside - Each voltage-gated Na channel has two separate gates, an activation gate and an inactivation gate - - - In the resting state of a voltage-gated Na channel, the inactivation gate is open, but the activation gate is closed. As a result, Na cannot move into the cell through these channels - - - In the activated state of a voltage-gated Na channel, both the activation and inactivation gates in the channel are open and Na inflow begins

Refractory Period

= time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus - During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential - - - graded potentials do not exhibit a refractory period - - - Large-diameter axons have a larger surface area and have a brief absolute refractory period - - - Small-diameter axons have absolute refractory periods Under normal body conditions - Relative Refractory Period - a second action potential can be initiated, but only by a larger-than-normal stimulus - - - coincides with the period when the voltage-gated K channels are still open after inactivated Na channels have returned to their resting state

Ionotropic Receptors

= type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel - the neurotransmitter binding site and the ion channel are components of the same protein - ionotropic receptor is a type of ligand-gated channel - In absence of neurotransmitter, the ion channel of the ionotropic receptor is closed. When correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell. - Many excitatory neurotransmitters bind to ionotropic receptors that contain cation channels. EPSPs result from opening these cation channels. - When cation channels open, they allow passage of the 3 most plentiful cations (Na, K, and Ca2) through the postsynaptic cell membrane, but Na inflow is greater than either Ca2 inflow or K outflow and the inside of the postsynaptic cell becomes less negative (depolarized). - Many inhibitory neurotransmitters bind to ionotropic receptors that contain chloride channels. IPSPs result from opening these Cl- channels. - When Cl- channels open, a larger number of chloride ions diffuse inward. The inward flow of Cl- ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized).

Metabotropic Receptors

= type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel - Metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein - When a neurotransmitter binds to a metabotropic receptor, the G protein either directly opens (or closes) the ion channel or it may act indirectly by activating another molecule, a "second messenger," in the cytosol, which in turn opens (or closes) the ion channel. - neurotransmitter binding site and the ion channel are components of different proteins

Removal of Neurotransmitter

Neurotransmitter is removed in 3 ways: (1) Diffusion - Some of the released neurotransmitter molecules diffuse away from the synaptic cleft (2) Enzymatic degradation - Certain neurotransmitters are inactivated through enzymatic degradation (3) Uptake by cells - Many neurotransmitters are actively transported back into the neuron that released them (reuptake) - Others are transported into neighboring neuroglia (uptake). The neurons that release norepinephrine, rapidly take up the norepinephrine and recycle it into new synaptic vesicles. - neurotransmitter transporters - membrane proteins that accomplish such uptake

PNS Division

Somatic Nervous System (SNS) - Sensory neurons - convey info from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell to the CNS - Motor neurons - conduct impulses from the CNS to skeletal muscles only; voluntary only Autonomic Nervous System (ANS) - Sensory neurons - convey info from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, to the CNS - Motor neurons - conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands; involuntary - - - consists of 2 branches: Sympathetic division (increases heart rate - "fight-or-flight") & Parasympathetic division (decreases heart rate - "rest-and-digest") - Enteric Nervous System (ENS) - part of autonomic; sensory neurons monitors: - - - chemical changes within the GI tract, secretions of the GI tract organs such as acid from the stomach, and activity of GI tract endocrine cells, which secrete hormones. - - - stretching of its walls. contraction of GI tract smooth muscle to propel food through the GI tract