Nervous System Basic Structure and function

amyotrophic lateral sclerosis

ALS, or Lou Gehrig's disease), astrocytes release a toxin that destroys motor neurons, causing progressive weakness. In Huntington disease (HD), which causes uncontrollable movements and cognitive impairment, microglia in the brain release a toxin that damages neurons. ALS and HD affect specific sets of neurons. Identifying the roles of neuroglia in nervous system disorders suggests new targets for treatments

Neuroglia of the PNS

Schwann cells produce the myelin on peripheral myelinated neurons, as described earlier. Satellite cells provide nutritional support and help regulate the concentrations of ions around neuron cell bodies within ganglia.

dendrites

Small cellular processes receive the input typically highly branched, providing receptive surfaces with which processes from other neurons communicate. Some dendrites have tiny, thornlike spines (dendritic spines) on their surfaces, which are contact points for other neurons short processes branching off cell body - may have one or many receive input and transfer it to cell body more dendrites = more input possible

Resting Membrane potential The role of Na+

Typical neuron RMP -70 mV Difference between -90 mV of K+ movement only due to Na+ movement Na+ enters cell through Na+ leak channels - these channels are not as leaky as K+ leak channels and there are less of them Na+ moves down concentration gradient, and also "pulled" by electrical gradient Combination of some K+ leaking out and some (less) Na+ leaking in is typical RMP of -70mV

Functional organization Motor nervous system

also known as efferent nervous system initiates and transmits motor output from CNS transmits information to the effectors may be further divided into the somatic and visceral parts

saltatory conduction

appear to jump from node to node the greater the axon diameter, the faster the impulse Occurs in myelinated axons Action potentials propagated only at neurofibril nodes Myelinated regions with limited numbers of voltage gated Na+ and K+ channels well insulated, preventing ion movement Neurofibril nodes with large number of voltage-gated Na+ and K+ channels lack myelin insulation

enkephalins

are present throughout the brain and spinal cord molecule is a chain of five amino acids Synthesis of enkephalins increases during periods of painful stress, and they bind to the same receptors in the brain as the narcotic morphine acts longer than enkephalins and is a much more potent pain reliever

multipolar neuron

many processes arising from its cell body one is an axon; the rest are dendrites Most neurons whose cell bodies lie within the brain or spinal cord are of this type associated with the autonomic nervous system neurons aggregate in specialized masses of nerve tissue most common type, have many dendrites and a single axon

oligodendrocyte

myelin is produced by a type of neuroglia rather than by a Schwann cell (CNS) Can myelinate 1 mm of many axons Extensions wrapping around axons No neurilemma formed Neurofibril nodes between adjacent "wraps"

Impulse Conduction

myelinated axon functions differently. Myelin contains a high proportion of lipid that excludes water and water-soluble substances. Thus, myelin prevents almost all flow of ions through the membrane that it encloses and serves as an electrical insulator nodes of Ranvier between Schwann cells or oligodendrocytes interrupt the sheath nodes, the axon membrane has channels for sodium and potassium ions that open during a threshold depolarization. stimulated to threshold, an action potential occurs at the trigger zone. This causes a bioelectric current to flow away from the trigger zone through the cytoplasm of the axon. As this local current reaches the first node, it stimulates the membrane to its threshold level, and an action potential occurs there, sending a bioelectric current to the next node downstream action potentials occur only at the nodes.

Classification of Neuroglia

neural function, providing scaffolding and controlling the sites at which neurons contact one another embryo, neuroglia guide neurons to their positions and may stimulate them to specialize also produce the growth factors that nourish neurons and remove excess ions and neurotransmitters that accumulate between neurons four types Astrocytes oligodendrocytes microglia Ependyma comprise more than half of the volume of the brain and outnumber neurons 10 to 1, are critical to neuron function

postsynaptic neuron

neuron receiving input at the synapse

central nervous system (CNS)

brain and spinal cord,

somatic nervous system

communicates voluntary (conscious) instructions originating in the CNS to skeletal muscles, causing contraction

Mixed nerves

contain both sensory and motor neurons most named nerves in this category individual neurons transmitting one type of information

Autonomic reflexes

enable ANS to control visceral function Controls smooth muscle contractions, cardiac muscle contractions, and/or secretions of glands Mediated by autonomic reflex arcs in response to stimulus Autonomic reflex examples Cardiovascular reflex slows heart rate and decreases volume ejected Gastrointestinal reflex control proximal GI tract - stimulates secretion of gastric glands by parasympathetic stimulation, stimulated by sight or smell of food Micturition reflex mechanism leading to bladder emptying

axon hillock

neurons the axon arises from the cell body as a cone-shaped thickening

Substance P

neuropeptide that consists of eleven amino acids and is widely distributed. It functions as a neurotransmitter (or perhaps as a neuromodulator) in the neurons that conduct impulses associated with pain into the spinal cord and on to the brain. Enkephalins and endorphins may relieve pain by inhibiting the release of substance P from these neurons.

Motor neurons

efferent neurons multipolar and conduct impulses out of the brain or spinal cord to effectors control skeletal muscle contraction are under voluntary (conscious) control. Those that control cardiac and smooth muscle contraction and the secretions of glands are part of the autonomic nervous system and are largely under involuntary control neurons of the motor nervous system conduct motor output to somatic and visceral effectors all multipolar most cell bodies located in CNS

Cell body (or soma)

enclosed by plasma membrane and contains cytoplasm surrounding a nucleus conducts electrical signals to axon free and bound ribosomes termed chromatophilic substance due to dark staining with basic dyes gray color of gray matter due to chromatophilic substance and lack of myelin

axon terminal

end, an axon may have many fine extensions, each with a specialized ending

Local Potential Changes

excitable; that is, they can respond to changes in their surroundings detect changes in temperature, light, or pressure outside the body, whereas others respond to signals from inside the body, often from other neurons environmental change affects the membrane potential by opening a gated ion channel

Neuroglia

found throughout the nervous system, and in the brain they greatly outnumber neurons including nourishing neurons and sending and receiving chemical messages assemble in a special, protective way in the brain. The blood-brain barrier done by astrocytes

Anaxonic neurons

have dendrites and no axons produce local electrical changes but no action potentials

Bipolar neurons

have two processes extending from cell body, one dendrite and one axon limited, e.g., in retina of the eye

monoamine oxidase

inactivates the monoamine neurotransmitters epinephrine and norepinephrine after reuptake. This enzyme is found in mitochondria in the synaptic knob. Destruction or removal of neurotransmitter prevents continuous stimulation of the postsynaptic neuron.

excitatory

increase postsynaptic membrane permeability to sodium ions will bring the postsynaptic membrane closer to threshold and may trigger impulses.

Hypothalamus

integration and command center for autonomic functions communicates with association areas of cortex affected by sensory processing in thalamus impacted by emotional states in limbic system also communicates with brainstem, cerebellum, and spinal cord central structure involved in drives that act through ANS

action potential 2

key to understanding how this happens is a rapid change in the membrane potential, first in a positive direction, then in a negative direction, returning to the resting potential

absolute refractory period

lasts about 1/1,000 of a second (1 millisecond), the axon's voltage-gated sodium channels are temporarily not responsive at all, and the axon cannot be stimulated no amount of stimulus able to generate a second action potential Na+ channels opened then closed in inactivated state remain closed until potential almost to resting potential ensures that action potential moves in one direction only

myelin

layers are composed consists of several types of lipids and proteins gives the cell membranes of Schwann cells a higher proportion of lipid than other cell membranes coating is called a myelin sheath Process by which part of an axon wrapped in myelin Myelin, insulating covering around axon consists of repeating layers of glial cell plasma membrane has high proportion of lipids gives glossy appearance and insulates axon Completed by neurolemmocytes (PNS) Completed by oligodendrocytes (CNS)

Neurotransmitters

least 100 different types of neurotransmitters in the brain alone acetylcholine, which stimulates skeletal muscle contractions Other neurotransmitters are synthesized in the cytoplasm of the nerve cell terminal and stored in vesicles increases the membrane's permeability to calcium ions by opening calcium ion channels. Calcium ions diffuse inward, and in response, some of the synaptic vesicles fuse with the presynaptic membrane and release their contents by exocytosis into the synaptic cleft

threshold potential

only if neurons are sufficiently depolarized, the membrane potential reaches a level is approximately -55 millivolts in a neuron

synaptic transmission

process by which the impulse in the presynaptic neuron signals the postsynaptic cell result of synaptic transmission, the presynaptic neuron stimulates or inhibits a postsynaptic cell one-way process carried out by neurotransmitters impulse travels along the axon of the pre-synaptic neuron to the axon terminal. Most axons have several rounded synaptic knobs at their terminals, which dendrites do not have. These knobs have arrays of membranous sacs, called synaptic vesicles, that contain neurotransmitter molecules. When an impulse reaches a synaptic knob, voltage-sensitive calcium channels open and calcium diffuses inward from the extracellular fluid. The increased calcium concentration inside the cell initiates a series of events that fuses the synaptic vesicles with the cell membrane, where they release their neurotransmitter by exocytosis.

three general functions of the nervous system

receiving information, deciding what to do, and acting on those decisions sensory, integrative, and motor

Ion Movements During Action Potentials

release more neurotransmitter, or if other neurons that synapse with the same cell have an additive effect on depolarization, threshold may be reached, and an action potential result. The mechanism uses another type of ion channel, a voltage-gated sodium channel that opens when threshold is reached

resting potential

resting neuron, one that is not sending impulses or responding to other neurons, the membrane potential has a value of -70 millivolts Distribution of substances inside and outside neuron Essential for neuron function More prevalent within cytosol negatively charged phosphate ions (e.g., in ATP) negatively charged proteins K+ More prevalent in interstitial fluid Na+ Cl-

axon

Nerve Fiber carries the information away from the cell in the form of bioelectric signals within the nervous system are not isolated, but bundled in groups peripheral nervous system, such bundles of axons are called nerves. central nervous system (brain and spinal cord) they are called tracts. longer process emanating from cell body makes contact with other neurons, muscle cells, or glands first part, a triangular region, axon hillock cytoplasm here termed axoplasm plasma membrane here termed axolemma devoid of chromatophilic substance gives rise to side branches, axon collaterals branch extensively at distal end into telodendria (axon terminals) at extreme tips, expanded regions, synaptic knobs knobs containing numerous synaptic vesicles contain neurotransmitter

Neurotransmitters - chemicals released by presynaptic neurons

Bind to specific receptors on postsynaptic cell Cause stimulation or inhibition, depending on neurotransmitter/receptor combination a specific neurotransmitter can be either stimulatory or inhibitory with different blends of receptors Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters Acetylcholine - Synthesized and released by cholinergic neurons Bind to cholinergic receptors (two types)... Nicotinic receptors Muscarinic receptors

Nerve

Cablelike bundle of parallel axons Macroscopic structure (essentially small organs) Epineurium thick layer of dense irregular connective tissue encloses the entire nerve, provides support and protection Perineurium layer of dense irregular connective tissue wraps bundles of axons, fascicles, and supports blood vessels Endoneurium delicate layer of areolar connective tissue separates and electrically insulates each axon has capillaries that supply the axon

synaptic potentials

Changes in chemically gated ion channels create local potentials

Nervous system activities

Collects information specialized nervous structures, receptors monitor changes in external and internal environment, stimuli e.g., receptors in the skin detecting information about touch Processes, integrates and evaluates information then determines if response required Initiates response to information initiate response via nerves to effectors include muscle tissue and glands e.g., muscle contraction or change in gland secretion

Nervous System controls

Controls and interprets all these sensations and muscle movements Body's primary communication and control system Integrates and regulates body functions Uses electrical activity transmitted along specialized nervous system cells

Cell body

Cytoplasmic inclusions in neurons include glycogen, lipids, and pigments such as melanin. Near the center of the neuron cell body is a large, spherical nucleus with a conspicuous nucleolus

neurofilaments

fine filaments extends into the axon and supports it

Beta adrenergic receptors

have two major subtypes with opposite effects β1 receptors are primarily stimulatory found in heart (increase heart rate and force) found in kidney (stimulate renin secretion) β2 receptors have primarily inhibitory effects in smooth muscle of vessels to heart, liver, and skeletal muscle cause smooth muscle relaxation and vessel dilation lung (bronchodilation) uterine and GI tract smooth muscle (relaxation) detrusor muscle of bladder (relaxation)

All-or-None Response

if a neuron responds at all, it responds completely conducted whenever a stimulus of threshold intensity or above is applied to an axon and all impulses conducted on that axon are the same strength. A greater intensity of stimulation produces more impulses per second, not a stronger impulse.

Refractory Period

number of impulses per second that an axon can generate is limited, because during an impulse, that part of the axon becomes unresponsive to another normal threshold stimulus two parts

Brainstem nuclei

mediate major ANS reflexes control changes in blood pressure, blood vessel diameter, and digestion control changes in heart rate, pupil size, and eye lens shape for focusing

Postsynaptic potentials

Graded potentials that occur in postsynaptic neurons Occur after release of neurotransmitter from presynaptic neuron and subsequent opening of gated channels after binding of neurotransmitters Results in small local potential Postsynaptic neuron able to bind many neurotransmitters at once numerous postsynaptic potentials generated at once Type of graded potential formed depends on neurotransmitter (and type of receptor/chemically gated channel) excitatory (EPSP) or inhibitory (IPSP) neurotransmitter

Electrical synapse

Much less common Presynaptic and postsynaptic neuron physically bound together Gap junctions present No delay in passing electrical signal In limited regions of brain and eyes

nodes of Ranvier

Narrow gaps in the myelin sheath between Schwann cells

Process of myelination

Neurolemmocyte starts to wrap around axon Its cytoplasm and plasma membrane begin to form layers Wrapping continues Layers of plasma membrane form the myelin sheath Its cytoplasm and nucleus is pushed to the periphery termed neurilemma

reuptake

Other neurotransmitters are transported back into the synaptic knob of the presynaptic neuron or into nearby neurons or neuroglia

Axon Regeneration Factors influencing axon regeneration

PNS axons vulnerable to cuts, trauma Regeneration possible if cell body intact enough neurilemma remains Regeneration success more likely if amount of damage less extensive smaller distance between site of damage and structure it innervates

Resting Membrane potential The role of Na+/K+ Pumps

Play relatively small role in establishing RMP Three Na+ pumped out for two K+ pumped in More significant role in maintaining gradients of K+ and Na+ following diffusion as part of neuron's electric current Ions pumped back up concentration gradient by pump Two-thirds of a neuron's energy expenditure

Neurons can also be classified by functional differences

Sensory Neurons Interneurons Motor Neurons

Removal of neurotransmitters from synaptic cleft

Temporary association between neurotransmitter and receptor Necessary to eliminate molecule after stimulation Can occur by degradation neurotransmitter chemically inactivated in synaptic cleft e.g., breakdown of ACh by acetylcholinesterase Can occur by reuptake neurotransmitter reabsorbed by transport protein in presynaptic neuron "recycled" into another synaptic vesicle for reuse e.g., drugs, selective serotonin reuptake inhibitors block reuptake of serotonin and used in treatment of depression

synaptic cleft

separated only by a space general pattern is that neurons receive input through the dendrites and the cell body, and send output in the form of an impulse conducted away from the cell body, down the axon

acetylcholinesterase

To keep signal duration short, enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose some neurotransmitters decomposes acetylcholine on postsynaptic membranes Other neurotransmitters are transported back into the synaptic knob of the presynaptic neuron or into nearby neurons or neuroglia

Norepinephrine

a type of biogenic amine Synthesized and released by adrenergic neurons NE released along length of axon Epinephrine, released by the adrenal medulla, is almost the same (both are a type of catecholamines) Both bind to adrenergic receptors Bind to adrenergic receptors (two types)... Alpha Beta

Functional organization Sensory nervous system

also known as afferent nervous system responsible for receiving sensory information from receptors and transmits information to the CNS further divided into somatic and visceral sensory

neuromodulators

are substances that alter a neuron's response to a neurotransmitter or block the release of a neurotransmitter.

synaptic knob

axon terminal ends close to the receptive surface of another cell

Amino acids

building blocks of proteins some also neurotransmitters e.g., glutamate, glycine, aspartate

Neuropeptides

chains of amino acids include enkephalins and somatostatin

gated channels

channels are always open, whereas others may be either open or closed, somewhat like a gate. Both chemical and electrical factors can affect the opening and closing

autonomic nervous system

communicates instructions from the CNS that control viscera, such as the heart and various glands, and thus causes involuntary subconscious actions.

peripheral nervous system (PNS)

connect the central nervous system to other body parts composed of the nerves

Functional organization Somatic sensory

: detects stimuli that we consciously perceive receptors include: eyes and nose, tongue and ears, skin proprioceptors (receptors detecting body position)

Functional organization Visceral sensory

: detects stimuli we do not consciously perceive receptors include structures within blood vessels and internal organs e.g., detecting stretch of organ wall

Functional organization Somatic motor

: transmits motor output from CNS to voluntary skeletal muscles effector consciously controlled e.g., pressing on accelerator of your car

Functional organization Autonomic motor

: transmits output from CNS without conscious control transmits to cardiac muscle, smooth muscle, glands

polarized

A cell membrane is usually electrically that the inside is negatively charged with respect to the outside due to an unequal distribution of positive and negative ions across the membrane. It is important in the conduction of impulses in both muscle fibers and neurons.

divergence

A neuron has a single axon, but axons may branch at several points. Therefore, impulses conducted by a neuron of a neuronal pool by reaching several other neurons One neuron stimulates two others, each of which stimulates several others, and so forth

Distribution of pumps and channels

Entire plasma membrane of neuron Na+ leak channels K+ leak channels present in greater numbers than Na+ leak channels easier for K+ to move through Na+/K+ pumps important in maintaining resting membrane potential Membrane of functional segments in a neuron Receptive segment: includes dendrites and cell body chemically gated channels here (cation channels, K+, Cl-), no significant voltage-gated channels Initial segment and Conductive segment : axon hillock and axon contains voltage-gated Na+ and K+ channels Transmissive segment: includes synaptic knobs contains voltage-gated Ca2+ channels and pumps

neuronal pools

Interneurons, which are the neurons completely within the CNS groups of neurons that synapse with each other and perform a common function, even though their cell bodies may be in different parts of the CNS receives input from neurons and each pool generates output excitatory or inhibitory effects on other pools or on peripheral effectors

chemically gated

Ion channels that respond to neurotransmitter molecules

monoamines

a group of compounds such as epinephrine, norepinephrine, dopamine, and serotonin are modified amino acids

peptides

a large group which are short chains of amino acids synthesized in the rough endoplasmic reticulum of a neuron cell body and transported in vesicles down the axon to the nerve cell terminal

excitatory postsynaptic potential (EPSP)

if a neurotransmitter binds to a postsynaptic receptor and opens sodium ion channels, the ions diffuse inward, depolarizing the membrane, possibly triggering an action potential Sequence of events Excitatory neurotransmitter crosses synaptic cleft. binds to receptor which is also a chemically gated cation channel More Na+ moves into neuron than K+ moves out. Inside becomes slightly more positive. less negative state called excitatory postsynaptic potential (EPSP) Local current of Na+ becomes weaker decreases in intensity with distance traveled Degree of change in RMP dependent on amount of neurotransmitter bound per unit time More excitatory neurotransmitter released, more cation channels open, greater change in the positive direction

Axon

includes many mitochondria, microtubules, and neurofibrils (ribosomes are found only in the cell body) conveys biochemicals and organelles, which can be quite a task in these long cells

neurilemma

neurilemmal sheath parts of the Schwann cells that contain most of the cytoplasm and the nuclei remain outside the myelin sheath and comprise surrounds the myelin sheath In the brain and spinal cord, myelinated axons do not have neurilemmae.

presynaptic neuron

neuron conducting an impulse to the synapse is the sender

Repolarization

propagation Return to resting membrane potential Voltage-gated K+ channels normally closed Stimulated to open by threshold Not open until depolarization has ended Exit of K+, making inside of axon negative Return to RMP (-70 mV) Triggers voltage-gated Na+ channels to return to resting state Propagation of repolarization Opening of voltage-gated K+ channels adjacent Open sequentially along length of axon

unmodified amino acids

such as glycine, glutamic acid, aspartic acid, and gamma-aminobutyric acid—GABA

neurotransmitters

Biological messenger molecules send and receive electrochemical messages across synapses. Released from synaptic vesicles into cleft Diffusion of neurotransmitter across cleft Binding of some neurotransmitters to receptors Synaptic delay time between neurotransmitter release and binding Single postsynaptic neuron often stimulated by more than one neuron

chromatophilic substance

(Nissl bodies) Scattered throughout the cytoplasm are many membranous packets consist mainly of rough endoplasmic reticulum

impulses

(action potentials) bioelectric signals allow the neuron to communicate with other neurons and with cells outside the nervous system

Sensory neurons

(afferent neurons) impulses from peripheral body parts into the brain or spinal cord distal ends, the dendrites of these neurons or specialized structures associated with them act as sensory receptors, detecting changes in the outside world or in the body Most sensory neurons are unipolar neurons of the sensory nervous system conduct input from somatic and visceral receptors most unipolar, few bipolar cell bodies usually located in posterior root ganglia, outside CNS

myelinated axons

(medullated) Axons that have myelin sheaths axons conduct impulses rapidly compared to unmyelinated axons Groups of myelinated axons appear white. The white matter in the brain and spinal cord gets its color from masses of myelinated axons

Steps of axon regeneration

1) Axon severed by trauma 2) Sealing off and swelling of proximal portion of severed axon disintegration of distal axon and myelin sheath termed Wallerian degeneration survival of neurilemma 3) Formation of regeneration tube neurilemma and remaining endoneurium 4) Axon regeneration and remyelination guided by regeneration tube nerve growth factor released by neurolemmocytes 5) Innervation restored

convergence

Any single neuron in a neuronal pool may receive input from two or more other neurons. Axons originating from different neurons leading to the same postsynaptic neuron exhibit

Satellite cells

Arranged around neuronal cell bodies in a ganglion Physically separate cell bodies in ganglion from surrounding fluid Regulate the exchange of nutrients and waste products e.g., surrounding bodies of sensory neurons in a posterior root ganglion

Structural organization

Central nervous system anatomic division of the nervous system includes brain and spinal cord brain protected in the skull spinal cord protected in the vertebral canal Peripheral nervous system other anatomic division includes nerves, bundles of neuron processes includes ganglia, clusters of neuron cell bodies

Special characteristics of neurons

Excitability: responsive to stimulation type dependent on its location most respond only to binding of molecules, neurotransmitters Conductivity: electrical charges propagated along membrane can be local and short-lived or self-propagating Secretion: release neurotransmitters in response to electrical charges given neuron releasing only one type of neurotransmitter may have excitatory or inhibitory effect on target Extreme longevity: most formed before birth still present in advanced age Amitotic: mitotic activity lost in most neurons not always the case (e.g., occasionally in hippocampus)

CNS axon regeneration

Extremely limited growth-inhibiting molecules secreted by oligodendrocytes larger number of axons crowded within the CNS regrowth obstructed by scars from astrocytes and connective tissue

Nicotinic receptors

Found on all ganglionic neurons and adrenal medulla When neurotransmitter bound to receptor, ion channel open (movement of Na+ into cell) and an excitatory postsynaptic potential produced Nicotinic receptors always produce a stimulatory response

All or none

If threshold reached, action potential propagated If threshold not reached, not propagated Same intensity of response to values greater than threshold Similar to what occurs with a gun with sufficient pressure on trigger, gun fired with insufficient pressure on trigger, not fired travels at same velocity even if pressure is greater than needed

Action potentials summary

Initially, the membrane is polarized (-70 mV) Threshold stimulus reached (-55 mV) Sodium channels open and membrane depolarizes (0 to +30 mV) Potassium leaves cytoplasm and membrane repolarizes (+30 mV) Brief period of hyperpolarization (-90 mV)

Neuroglia and Axonal Regeneration

Injury to the cell body usually kills the neuron, and because mature neurons do not divide, the destroyed cell is not replaced unless neural stem cells are stimulated to proliferate. a damaged peripheral axon may regenerate. For example, if injury or disease separates an axon in a peripheral nerve from its cell body, the distal portion of the axon and its myelin sheath deteriorate within a few weeks, although the Schwann cells and neurilemma remain. Macrophages remove the fragments of myelin and other cellular debris. The proximal end of the injured axon develops sprouts shortly after the injury. Influenced by nerve growth factors that nearby neuroglia secrete, one of these sprouts may grow into a tube formed by the remaining Schwann cells. At the same time, Schwann cells along the length of the regenerating portion form new myelin around the growing axon. Growth of a regenerating axon is slow (up to 4 millimeters per day), eventually the new axon may reestablish the former connection. Nerve growth factors, secreted by neuroglia, may help direct the growing axon. However, the regenerating axon may still end up in the wrong place, so full function often does not return. If an axon of a neuron within the CNS is separated from its cell body, the distal portion of the axon will degenerate, but more slowly than a separated axon in the PNS. axons in the CNS lack neurilemmae, and the myelin-producing oligodendrocytes do not proliferate following injury. Consequently, the proximal end of a damaged axon that begins to grow has no tube of sheath cells to guide it. Therefore, regeneration is unlikely.

CNS Control of the ANS

Involvement of CNS ANS is a regulated nervous system, not independent Influenced by four CNS regions: cerebrum, hypothalamus, brainstem, and spinal cord Cerebrum - ANS affected by conscious activities hereh

Resting Membrane potential The role of K+

K+ diffusion (from inside to outside the cell) is the most important factor in determining RMP Dependent on the electrochemical gradient and leak channels Outward movement through leak channels facilitated by steep concentration gradient - leaves relatively more negatively charged structures inside, so the inside of the cell is negative Outward movement opposed by electrical gradient attraction of negative inside of cell for K+ becomes greater as more K+ diffuses out At equilibrium, chemical concentration gradient equal to electrical gradient opposing movement RMP would be -90 mV if only K+ channels were present

Graded potentials

Local potentials that only occur in the receptive segment of a neuron due to opening of chemically gated channels Temporarily allow passage of small amount of specific ion, so a local current established ions moving parallel to plasma membrane experience resistance from contents of cytosol, so the current eventually becomes weaker and ceases May result in depolarization or hyperpolarization depending on channel that opens Degree dependent on stimulus magnitude larger stimulus opening more chemically gated channels and flow of more ions Decreases in intensity with distance along the membrane Short-lived, lasts until local ion current ceases

Chemical synapse

Most common Composed of presynaptic neuron, signal producer Composed of postsynaptic neuron, signal receiver Between axon and any portion of postsynaptic neuron most commonly with a dendrite Knob almost touches the postsynaptic neuron narrow fluid filled gap, the synaptic cleft

Three states of voltage-gated Na+ channels

Most gated channels either closed or open Voltage gated Na+ channels are unique - have two gates (activation gate and inactivation gate) and three states Resting state inactivation gate open; activation gate closed entry of Na+ prevented Activation state inactivation gate open; activation gate open (in response to voltage change) Na+ moving through the channel Inactivation state inactivation gate temporarily closed; activation gate open entry of Na+ prevented Resting state must be reestablished

Autonomic Tone

Most organs are innervated by both divisions of ANS, termed dual innervation Also, both divisions are always active, stimulating target tissue continuously to varying degrees This is referred to as autonomic tone - tissue response depends on which division is "on" more at a particular time E.g., diameter of most blood vessels always in a partially constricted state due to sympathetic tone Decrease in stimulation below tone causes vessel dilation Increase above sympathetic tone causes greater vessel constriction

Channels

Move substances down concentration gradient Leak channels always open for continuous diffusion Chemically gated channels normally closed allow specific type of ion to diffuse when open Voltage-gated channels normally closed open in response to changes in electrical charge across membrane and allow specific type of ion to diffuse Voltage-gated channels normally closed open in response to changes in electrical charge across membrane and allow specific type of ion to diffuse e.g., voltage gated Na+ channels

Nerve fiber groups

Nerve fiber: an axon and its myelin sheath Classified into three major groups Group A conduction velocity as fast as 150 m/sec large diameter myelinated fibers e.g., most somatic sensory neurons, somatic motor neurons Group B and Group C small in diameter, unmyelinated, or both e.g., sensory and motor visceral neurons group B: 15 m/sec group C: 1 m/sec

Schwann cells

Neurolemmocytes Neuroglia encase the large axons of peripheral neurons in lipid-rich sheaths tight coverings form as Schwann cell membranes wind and wrap around axons enclose, but do not wind around, the smallest axons of peripheral neurons. Consequently, these axons do not have myelin sheaths. Instead, the axon or a group of axons may lie partially or completely in a longitudinal groove of a Schwann cell Ensheathe PNS axons to form myelin sheath Allows for faster action potential propagation (PNS) Can myelinate only 1 mm of single axon Takes many to myelinate entire axon Gaps between neurolemmocytes neurofibril nodes, or nodes of Ranvier

Two types of cells overall

Neuron basic structural unit of the nervous system excitable cells that transmit electrical signals Neuroglia nonexcitable cells that primarily support and protect neurons

neuropeptides

Neurons in the brain or spinal cord synthesize act as neurotransmitters or as neuromodulators

effectors

Neurons that conduct impulses from the CNS to responsive structures carry out the motor functions of the nervous system outside the nervous system and include muscles and glands

inhibitory

Neurotransmitters that make reaching threshold less likely they decrease the chance that an impulse will occur.

Classes of neurotransmitters

Neurotransmitters, various small organic compounds Released at synaptic cleft Approximately 100 known Classified into major groups Acetylcholine excitatory or inhibitory neurotransmitter released in both CNS and PNS molecule released from motor neuron at neuromuscular junction

Glial cells

Nonexcitable cells found in CNS and PNS Smaller than neurons Capable of mitosis Far outnumber neurons Half volume of nervous system Physically protect and nourish neurons Provide physical scaffolding for nervous tissue help guide migrating neurons to their destination Critical for normal function at neural synapses

Continuous conduction

Occurs in unmyelinated axons Sequential opening of voltage-gated Na+ and K+ channels

Distribution of Ions

Potassium ions (K+) are the major intracellular positive ion (cation), and sodium ions (Na+) are the major extracellular cation. The distribution is created largely by the sodium/potassium pump (Na+/K+pump), which actively transports sodium ions out of the cell and potassium ions into the cell These channels, formed by membrane proteins, can be selective; that is, a particular channel may allow only one type of ion to pass through and exclude all other ions of different size and charge. Thus, even though concentration gradients are present for sodium and potassium, the ability of these ions to diffuse across the cell membrane depends on the presence of channels.

facilitation

Repeated impulses on excitatory presynaptic neurons may result in the increased release of neurotransmitter in response to an impulse, making it more likely to bring the postsynaptic cell to threshold

Parasympathetic (Ach)

Response are usually localized because the long preganglionic neurons have limited numbers of branches "rest-and-digest" division Preganglionic neuron in brainstem or S2-S4 spinal cord Termed craniosacral division Ganglionic neuron innervating muscles or glands Preganglionic axons longer Postganglionic axons shorter Few preganglionic axons Ganglia close to or within effector

Muscarinic receptors

Responsive to muscarine, a mushroom toxin Found in all target membranes in parasympathetic division and selected sympathetic cells Different subtypes with different effects, either stimulated or inhibited by binding ACh depending on target tissue function e.g., binding of ACh in GI tract results in stimulation and increased motility binding on cardiac muscle decreases heartbeat rate

Parasympathetic

Sacral components Target organs Distal portion of large intestine, rectum Bladder, distal ureter Most reproductive organs, and others Causes: smooth muscle motility secretory activity in digestive tract contraction in bladder wall erection of clitoris and penis

Summation of EPSP and IPSP

Summation of the excitatory and inhibitory effects of the postsynaptic potentials commonly takes place at the trigger zone. This is usually in a proximal region of the axon, but in some sensory neurons it may be in the distal peripheral process. This region has an especially low threshold for triggering an action potential. In this way, the trigger zone, as its name implies, serves as a decision-making part of the neuron. Addition of graded postsynaptic potentials (IPSPs and EPSPs) occurs at the initial segment Determines if threshold membrane potential is reached -55 mV, +15 mV from RMP If threshold reached voltage-gated channels open action potential generated that travels along axon IPSPs negate effects of EPSPs Thousands of EPSPs required to reach threshold must arrive at nearly the same time Spatial summation release of neurotransmitter from multiple presynaptic neurons action potential initiated if enough EPSPs generated Temporal summation repeated release of excitatory neurotransmitter at same location effects added if occur within small timeframe action potential initiated if threshold reached

collaterals

The axon may give off branches

Resting Potential

Under resting conditions, non-gated (always open) channels determine the membrane permeability to sodium and potassium ions. resting cell membrane is only slightly permeable to these ions, but the membrane is more permeable to potassium ions than to sodium ions cytoplasm of these cells has many negatively charged ions (anions), which include phosphate (PO4-2), sulfate (SO4-2), and proteins, that are synthesized inside the cell and cannot diffuse through cell membranes

Sympathetic (NE and Epi)

Usually many structures activated simultaneously, termed mass activation Short preganglionic neurons have many branches Especially important in response to stress e.g., multiple changes during exercising increased heart rate, blood pressure, breathing rate, pupil dilation, etc. Also due to differences in neurotransmitters and effectors (adrenal medulla in particular) "fight-or-flight" division Preganglionic neuron in lateral horns of T1-L2 Termed thoracolumbar division Ganglionic neuron innervating muscles or glands Preganglionic axons shorter Postganglionic axons longer Many preganglionic axons Ganglia relatively close to spinal cord (in sympathetic trunk ganglia or prevertebral ganglia)

hyperpolarized

as a result, the membrane potential becomes more negative than the resting potential inside of cell becomes more negative e.g., from -70 mV to -80 mV may result from opening of gated K+ channels or from opening of gated Cl- channels loss of positive ion (K+) or gain of negatively charged ion (Cl-) return to resting membrane potential Voltage-gated K+ channels open longer than time needed to reestablish RMP Inside of neuron briefly more negative than RMP hyperpolarized Closure of K+ channels RMP reestablished by Na+/K+ pumps

ganglia

associated with the autonomic nervous system neurons aggregate in specialized masses of nerve tissue

Interneurons

association or internuncial neurons lie within the brain or spinal cord multipolar and form links with other neurons relay information from one part of the brain or spinal cord to another they may conduct incoming sensory information to appropriate regions for processing and interpreting cell bodies of some interneurons aggregate in specialized masses of nervous tissue called nuclei entirely within the CNS receive stimulation from many other neurons receive, process, and store information "decide" how body responds to stimuli facilitate communication between sensory and motor neurons 99% of neurons generally multipolar

Oligodendrocytes

astrocytes but are smaller and have fewer processes. They form in rows along axons (nerve fibers), and myelinate these axons in the brain and spinal cord. Unlike the Schwann cells of the PNS, oligodendrocytes can send out a number of processes, each of which forms a myelin sheath around a nearby axon. In this way, a single oligodendrocyte may myelinate many axons. However, these cells do not form neurilemmae. Large cells with slender extensions Processes ensheathing portions of axons of different neurons Processes repeatedly wrapping around axon Insulate axons in a myelin sheath Prevent passage of ions through axonal membrane Allow for faster action potential propagation through CNS

sensory receptors

at the ends of neurons in the peripheral nervous system provide the sensory function of the nervous system gather information by detecting changes inside and outside the body monitor external environmental factors convert (or transduce) their information into impulses, which are then conducted along peripheral nerves to the CNS

Ependyma

cuboidal or columnar cells in shape and may have cilia form the inner lining of the central canal that extends downward through the spinal cord form a one-cell-thick epithelial-like membrane that covers the inside of spaces in the brain called ventricles gap junctions join ependymal cells, allowing free exchange between cells. The ependymal layer itself is porous, allowing substances to diffuse freely between the interstitial fluid of the brain tissues and the fluid (cerebrospinal fluid) in the ventricles Ependymal cells also cover the specialized capillaries called choroid plexuses associated with the ventricles of the brain. Here they help regulate the composition of the cerebrospinal fluid. Line internal cavities of brain and spinal cord Ciliated simple cuboidal or simple columnar epithelial cells Slender processes with extensive branching Form choroid plexus with nearby blood capillaries helps produce cerebrospinal fluid liquid that bathes external CNS and fills internal cavities cilia helping to circulate CSF

Parasympathetic Cranial components

decreased airway diameter decrease in heart rate and force increased smooth muscle motility and secretory activity increased secretion of lacrimal and salivary glands increased secretion of salivary gland (parotid) increased mucus production eye muscles ciliary muscle - control focusing of the lens constrictor muscle of the iris - allow less light into the eye

action potential

depolarizing stimulus is not sufficient to bring the postsynaptic cell to threshold Generated within the initial segment and propagated along axon Due to opening of voltage-gated channels Threshold value minimum voltage change to open voltage-gated channel any value below this, a subthreshold value If threshold value reached channels open and membrane potential reversed Na+ channel opens, enters the neuron making inside relatively positive flow of local current to adjacent areas results in opening of voltage-gated channels in these areas successive opening down the axon followed by sequential opening of voltage-gated K+ channels movement of K+ out of neuron returns membrane to RMP involve temporary reversal of polarity across plasma membrane inside becomes relatively positive followed by a return to RMP are self-propagated maintain intensity as move to synaptic knob obey the "all or none law" if threshold reached, action potential sent if not reached, no action potential sent

Monoamines

derived from certain amino acids carboxyl group removed and functional group added subgroup added determines type includes subgroup, catecholamines (norepinephrine, epinephrine, dopamine)

inhibitory postsynaptic potential (IPSP)

different neurotransmitter binds other receptors and increases membrane permeability to potassium ions, these ions diffuse outward, hyperpolarizing the membrane. Because an action potential is now less likely to occur Sequence of events Inhibitory neurotransmitter crosses synaptic cleft. binds to chemically gated K+ channel or Cl- channel depends on neurotransmitter and channels present If neurotransmitter binds K+ channel, K+ moves out of neuron. If neurotransmitter binds Cl-channel, Cl- flows into neuron. Inside of the cell becomes slightly more negative more negative state termed inhibitory postsynaptic potential (IPSP) Local current of ions becomes weaker. decreases in intensity with distance traveled toward initial segment

depolarized

membrane becomes less negative (more positive) than the resting potential, the membrane Propagation of depolarization Sequential opening of voltage-gated Na+ channels along the axon Flow of Na+ into cell causes adjacent regions to also reach threshold triggers voltage-gated Na+ channels in these areas Process repeated rapidly down synaptic knob does not go backwards voltage-gated Na+ channels here in inactivated state (they are in absolute refractory period) Local anesthetics Lidocaine Inhibit action of voltage-gated Na+ channels Block nerve signal Pain signal blocked from reaching CNS Application of ice reduces pain sensation slows transmission of sensory action potentials

relative refractory period

membrane reestablishes its resting potential. During this time, even though repolarization is incomplete, a stimulus of higher than usual intensity may trigger an impulse. limits how many action potentials may be generated in a neuron in a given period maximum theoretical frequency of impulses in a neuron is about 700 per second. In the body, this limit is rarely achieved—frequencies of about 100 impulses per second are common with greater stimulation, action potential possible Na+ returned to resting state neuron hyperpolarized due to extended time K+ channels remain open

axonal transport

movement occurs in both directions between the cell body and the ends of the axon movement occurs in both directions between the cell body and the ends of the axon. For example, enzymes required for neurotransmitter synthesis are produced in the cell body and transported to the axon terminals. Old organelles and other cellular components may be transported in the reverse direction to be recycled. It is a highly regulated process.

trigger zone

multipolar neuron, the first part of the axon, the cone-shaped axon hillock or initial segment it contains many such voltage-gated sodium channels resting membrane potential, these sodium channels remain closed, but when threshold is reached, they open for an instant, briefly increasing sodium permeability diffuse inward through the open sodium channels, down their concentration gradient, aided by the attraction of the sodium ions to the negative electrical state on the inside of the membrane membrane potential at the trigger zone changes from its resting value and momentarily becomes positive on the inside (still considered depolarization). At the peak of the action potential, the membrane potential may reach +30 mV or more Axons are capable of action potentials, but the cell body and dendrites are not. An action potential at the trigger zone causes an electric current to flow a short distance down the axon, which stimulates the adjacent membrane to reach its threshold level, triggering another action potential. The second action potential causes another electric current to flow farther down the axon. This sequence of events results in a series of action potentials occurring sequentially all the way to the end of the axon without decreasing in amplitude, even if the axon branches. The propagation of action potentials (or impulse conduction) continues along the axon

unipolar neuron

single process extending from its cell body pseudounipolar, because they start out with two processes that merge into one during development short distance from the cell body, this process divides into two branches, which really function as a single axon One branch (peripheral process) has dendrites near a peripheral body part. The other branch (central process) enters the brain or spinal cord. The cell bodies of most unipolar neurons are found in ganglia. have single short neuron process that emerges from cell and branches like a T

Microglia

small cells and have fewer processes support neurons and phagocytize bacterial cells and cellular debris. They usually proliferate whenever the brain or spinal cord is inflamed because of injury or disease Small cells with slender branches Smallest percentage of CNS glial cells Phagocytic cells of the immune system Wander CNS and replicate during infections Engulf infectious agents Remove debris from dead or damaged tissue

synapse

small space between a neuron and the cell(s) with which it communicates Where neuron functionally connected to neuron or effector Two types: chemical and electrical

cell body

soma or perikaryon contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules, neurofilaments

Spinal cord

some autonomic responses controlled at level of spinal cord e.g., defecation and urination (in children) may be inhibited by higher centers

Astrocytes

star-shaped cells commonly found between neurons and blood vessels provide support and hold structures together with abundant cellular processes aid metabolism of certain substances, such as glucose, and they may help regulate the concentrations of important ions, such as potassium ions, in the interstitial space of nervous tissue respond to injury of brain tissue and form a special type of scar tissue, which fills spaces and closes gaps in the CNS nutritive function, regulating movement of substances from blood vessels to neurons and bathing nearby neurons in growth factors important role in the blood-brain barrier, which restricts movement of substances between the blood and the CNS. Gap junctions link astrocytes to one another, forming protein-lined channels through which calcium ions travel, possibly stimulating neurons. Processes touching capillary walls and neurons ends termed perivascular feet Most abundant glial cell in CNS feet wrap around capillaries in the brain, and together: strictly control substances entering brain nervous tissue from blood (what can and what cannot cross barrier?) protects neurons from toxins allows nutrients to pass Regulate tissue fluid composition control movement of substances between blood and interstitial fluid Form a structural network cytoskeleton strengthening and organizing nervous tissue Assist neuronal development direct development of neurons in fetal brain secrete chemicals regulating formation of connections Occupy the space of dying neurons space formerly occupied by dead neurons filled by cells produced by astrocyte division

unmyelinated axons

those that do not have these sheaths appears gray. Thus, the gray matter Associated with neurolemmocytes No myelin sheath covers them Axon in depressed portion of neurolemmocyte Not wrapped in repeated layers In CNS, unmyelinated axons not associated with oligodendrocytes

membrane potential

transmembrane potential called a potential difference because it represents stored electrical energy that can be used to do work at some future time Measure of volts

Alpha adrenergic receptors

typically stimulatory and there are several subtypes α1 receptors are located in most smooth muscle cells and stimulate smooth muscle contraction found in most blood vessels (vasoconstriction) arrector pili muscle (contraction) uterus (contraction) ureters, internal urethral sphincter (closing) α2 receptors are located in the pancreas inhibit insulin secretion involved with contraction of GI tract sphincters facilitate blood clotting

Depolarization more

voltage-gated sodium channels quickly close, but at almost the same time, slower voltage-gated potassium channels open and briefly increase potassium permeability potassium ions diffuse outward through the open potassium channels, the inside becomes negatively charged once more. The membrane is thus repolarized voltage-gated potassium channels then close as well

Role of sodium/potassium pump

we would expect potassium to diffuse out of the cell more rapidly than sodium could diffuse in. This means that every millisecond (as the membrane potential is being established in our hypothetical cell), a few more positive ions leave the cell than enter it. As a result, the outside of the membrane gains a slight surplus of positive charges, and the inside reflects a surplus of the impermeant negatively charged ions. This situation separates positive and negative electrical charges between the inside and outside surfaces of the cell membrane. All this time, the cell continues to expend metabolic energy in the form of ATP to actively transport sodium and potassium ions in opposite directions, thus maintaining the concentration gradients for those ions responsible for their diffusion in the first place.