chapter 11 exam 4

and compare and contrast continuous conduction and saltatory conduction.

There are two ways action potentials are propagated along a neuron • Continuous conduction • Action potentials

Describe how an action potential is conducted

Action potentials are formed when a stimulus causes the cell membrane to depolarize past the threshold of excitation, causing all sodium ion channels to open.

Motor or efferent neuron

Motor or efferent neurons carry nerve impulses away from the CNS • Interneurons or association neurons are located within the CNS between sensory and motor neurons • Interneurons integrate or process sensory information from sensory neurons, and elicit a motor response by activating the appropriate motor neurons

peripheral nervous system

• The peripheral nervous system is composed of all the nervous tissue outside of the CNS • Consists of: • Nerves: a nerve is a bundle of hundreds to thousands of axon plus their associated connective tissue and blood vessels • Each nerve follows a defined path and services a specific region of the body • Ganglia: ganglia are small masses of nervous tissue, consisting primarily of nerve cell bodies located outside of the CNS • Enteric plexuses are extensive networks of neurons located in the walls of organs of the gastrointestinal tract • Help to regulate the digestive system • Sensory receptors: these are structures of the nervous system that monitor changes in the external or internal environment • Examples include touch receptors in the skin, photoreceptors in the eye and olfactory (smell) receptors in the nose • The PNS is divided into the somatic nervous system (SNS), the autonomic nervous system (ANS) and the enteric nervous system(ENS)

Describe the basic functions of the human nervous system

• The primary function of the nervous system is to keep controlled conditions within the limits that maintain life • It will accomplish this by rapidly regulating body activities through nerve impulses • The nervous system carries out a complex array of tasks • A few examples are: we can sense many things like smells and touch, produce speech and recall past events; it also controls body movements and regulates the internal organs • The activities of the nervous system can be divided into three basic functions: sensory (input), integrative (processing) and motor (output) • Sensory function: occurs when sensory receptors detect either external or internal stimuli and carry that information to the CNS through cranial and spinal nerves • Integrative function: occurs when the nervous system processes sensory information, analyzing it and making decisions for an appropriate response • Motor function: once sensory information has been integrated, the nervous system may produce a motor response by activating effectors (muscles and glands) through cranial and spinal nerves • As an example, think of a mosquito landing on your arm, and what your response is

the synapse

• The site of communication between two neurons or a neuron and an effector cell is called the synapse • The ends of some neurons will swell into bulbs called synaptic end bulbs • Within the synaptic end bulbs are tiny membrane enclosed sacs called synaptic vesicles • These synaptic vesicles contain neurotransmitters • Neurotransmitters are chemical messengers that are released by the neuron into the synapse between a neuron and the cell it communicates with (the postsynaptic cell) • Neurotransmitter can either excite or inhibit the postsynaptic cell • Most neurons contain two or three different neurotransmitters, each having a different effect on the postsynaptic cell

There are 6 types of neuroglia cells

Astrocytes, oligodendrocytes, microglia and ependymal cells are found only in the CNS Schwann cells and satellite cells are present in the PNS

Depression

Depression • Depression is a condition that affects 18 million in the US yearly • Depressed people feel sad and helpless, have a lack of interest in activities they once enjoyed and experience suicidal thoughts • There are various types of depression that exist • People with major depression experience symptoms that last for more than two weeks • People with dysthymia experience cycles of depression followed by periods of feeling normal • People with bipolar disorder experience recurrent episodes of depression followed by extreme elation • People with seasonal affective disorder (SAD) experience depression during the winter months, when days are short • The exact cause of depression is unknown • Research suggests that depression is linked to an imbalance of neurotransmitters in the brain • Serotonin, norepinephrine and dopamine • Additionally, there appears to be other factors that contribute to the condition • Stress, chronic illness, familial history, hormonal changes and personality traits like low self esteem • Medications are the most common treatment for depression • Selective serotonin reuptake inhibitors provide relief from some forms of depression • Function by inhibiting reuptake of serotonin, prolonging its activity in the brain • Fluoxetine (Prozac), paroxetine (Praxil) and sertaline (Zoloft) are all common examples

Excitotoxicity

Excitotoxicity • High levels of glutamate in the interstitial fluid of the CNS causes excitotoxicity • This is destruction of the neurons through prolonged activation of excitatory synaptic transmission • The most common cause is from lack of oxygen to the brain, as occurs during a stroke • Loss of oxygen causes glutamate transporters to fail • Glutamate then accumulates in the interstitial spaces between neurons, literally stimulating them to death • Clinical trials are being done on antiglutamate drugs to determine if their administration after a stroke can help protect against excitotoxicity

Grey matter

Grey matter contains cell bodies, dendrites, unmyelinated axons axon terminals and neuroglia

Describe in detail the structure of a neuron

Most neurons have three parts, a cell body, dendrites and an axon

Describe the ways that a nervous impulse travels across a synapse,

Nerve impulses travel directly across connected synapses via electricity, while the impulses use special chemicals to cross non-touching synapses. These chemicals are called neurotransmitters, and they can change the way nerves communicate with each other in the brain. The neuron that sends the message is usually called the presynaptic neuron, while the receiving neuron is called the postsynaptic neuron

Damage and repair in the PNS

Neurogenesis is the formation of new neurons from undifferentiated stem cells Until the early 1990's, it was believed that no new neurons were produced in the adult brain Then some research showed that epidermal growth factor could stimulate both neurons and astrocytes taken from adult mice A few years later, science showed that new neurons do arise in the adult human hippocampus, an area of the brain crucial for learning However, there is nearly a complete lack of neurogenesis in the other regions of the brain and spinal cord This seems to be a result of two factors Inhibitory influences from neuroglia The absence of growth stimulating cues that are present during fetal development Thus, injury to the brain and spinal column is usually permanent

cortex

Neuronal cell bodies are often grouped together in clusters • These clusters are called ganglia in the PNS • They are called a nucleus in the CNS

Oligodendrocytes

Oligodendrocytes resemble astrocytes but are smaller and have fewer processes The processes of the oligodendrocytes are responsible for the forming and maintaining the myelin sheath around axons in the CNS Each oligodendrocyte myelinates parts of several axons Each oligodendrocyte extends about 15 broad, flat processes that spiral around CNS axons This forms the myelin sheath The cell body of the oligodendrocyte does not envelop the axon Thus there is no neurolemma This is believed to be part of the reason axons in the CNS display little regrowth after injury There are nodes of Ranvier present, but they are fewer in number as compared to the PNS

satellite cells

Satellite cells are flat cells that surround the cell bodies of neurons in the PNS ganglia Provide structural support Regulate exchange of materials between the cell bodies and the interstitial fluid

List the various neurotransmitters and neuromodulators indicated by your instructor, where they operate (CNS or PNS), and define each as generally excitatory, inhibitory or both

Small molecule neurotransmitters include acetylcholine, amino acids, biogenic amines, ATP and other purines and nitric oxide

Synapses can be either electrical in nature

Synapses can be either electrical or chemical in nature At an electrical synapse, action potentials are conducted directly between the plasma membranes of adjacent neurons This will occur through gap junctions Remember that gap junctions form little tunnels that connect the cytoplasm of adjacent cells Ions will flow from one cell through the gap junctions to the next cell This will spread the action potential from one cell to another Gap junctions are common in visceral smooth muscle cardiac muscle and the developing embryo; they also occur in the brain There are two main advantages of electrical synapses They transmit the action potential faster then chemical synapses Transmission is direct, not slowed by the events that take place in a chemical synapse Electrical synapses can synchronize a group of neurons or muscle fibers A large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions Important in visceral or cardiac muscle, where coordinated contraction will result in movement of food or generation of a heartbeat

The ENS is the

The ENS is the "Brain of the gut" • Consists of over 100 million neurons in enteric plexuses that extend most of the length of the GI tract • Many of the neurons in the enteric plexuses function independently of the CNS or ANS to some extent • They do communicate with the CNS via sympathetic and parasympathetic neurons • Enteric motor neurons govern contraction of the smooth muscle in the GI tract, secretions of the GI tract and activities of endocrine cells • Sensory motor neurons of the ENS monitor chemical changes in the GI tract and stretching of the walls of the organs of the GI tract

Synapses can be either chemical in nature

The membrane of the pre and post synaptic neurons are close, but they do not touch This gap is called the synaptic cleft Is roughly 20-50nm across and is filled with interstitial fluid Nerve impulses can not cross this space directly, but rely on an indirect form of communication In response to the arrival of an action potential, the presynaptic neuron will release a neurotransmitter This chemical will diffuse across the synaptic cleft The post synaptic neuron will receive the chemical signal and produce a postsynaptic potential This is a graded potential The time it takes for the release of the neurotransmitter, the diffusion across the synaptic cleft and generation of the postsynaptic potential is about 0.5msec This is why electrical synapses are faster than chemical synapses

List the subdivisions and organization of the human nervous system. (Fig. 12.1)

The nervous system is organized into two main subdivisions The central nervous system peripheral nervous system

White matter

White matter is composed primarily of myelinated axons, and appears white

Neurotransmitters

are chemical messengers that are released by the neuron into the synapse between a neuron and the cell it communicates with (the postsynaptic cell)

anterograde transport

moves materials like organelles and synaptic vesicles from the cell body towards the axon terminals

Retrograde transport

moves membrane vesicles and other materials from the axon to the cell body to be degraded or recycled

dendrites

• A nerve fiber is a general term for any neuronal process (extension) that emerges from the cell body • There are typically two types of processes, multiple dendrites and a single axon • Dendrites are the receiving portion of the neuron • The plasma membranes of dendrites contain numerous receptors for chemical messengers from other cells • Structurally, dendrites are short, tapered and highly branched • Form an array of processes that extend from the cell body • They contain cytoplasm which includes Nissl bodies, mitochondria and other typical organelles

Diagram the structure of a typical synapse.

• A synapse is a region where communication occurs between two neurons or between a neuron and an effector cell • The term presynaptic neuron refers to the cell that carries a nerve impulse towards a synapse • These cells send signals • The term postsynaptic neuron refers to the cell that receives the signal • These can be postsynaptic neurons that carry a nerve impulse away from the synapse • May also be effector cells that respond to the nerve impulse • Most synapses between neurons are axodendritic, occurring from axon to dendrite • Others are axosomatic, occurring from axon to cell body • Yet others are axoaxonic, occurring from axon to axon

Acetylcholine

• Acetylcholine is the best studied of the neurotransmitters • Is used by neurons in both the PNS and CNS • Can be both excitatory and inhibitory • Excitation occurs from binding of ionotropic receptors • Inhibition occurs from binding of metabotropic receptors • Is inactivated by the enzyme acetylcholinesterase • Cleaves it into acetyl and choline fragments

Action potentials

• Action potentials move much more rapidly along myelinated neurons as compared to unmyelinated neurons • This is due to saltatory conduction of the action potential • In myelinated nerves, there are few ion channels in the myelinated portions, but lots at the nodes of Ranvier where the nerve is unmyelinated • An action potential causes a flow of current through the extracellular fluid and cytoplasm from one node to the next • This current opens voltage gated ion channels in the nodes to open, reach threshold and stimulate another action potential • This action potential will cause another flow of current, and repeat the process at the next node on the neuron

Biogenic amines

• Biogenic amines are amino acids that have been modified and decarboxylated to function as neurotransmitters • Norepinephrine, epinephrine and dopamine are neurotransmitters known as catecholamines • They are derived from the amino acid tyrosine • Catecholamines are either excitatory or inhibitory • Depends on which metabotropic receptor is present in the synapse • Found in the CNS and PNS • Inactivation is achieved by reuptake of the neurotransmitter by the secreting cell • They are either recycled or broken down inside the cell by the enzymes catechol-O-methyltransferase and monoamine oxidase

Bipolar neurons

• Bipolar neurons have one main dendrite and one main axon attached to the cell body • The ends of both the dendrite and axon are usually branched • They are found in the retina of the eye, the inner ear and olfactory area of the brain

Continuous conduction

• Continuous conduction involves step-by-step depolarization and repolarization of the membrane • The influx of sodium stimulates the opening of voltage gated ion channels in adjacent parts of the cell to reach threshold and generate a new action potential • This occurs in unmyelinated neurons and muscle fibers • Overall, the conduction is relatively slow

Tell the difference between efferent and afferent neurons

• Functionally, neurons are classified according to the direction the nerve impulse is conveyed with respect to the CNS • Sensory or afferent neurons contain sensory receptors at their distal ends (dendrites) or are adjacent to sensory receptor cells • In response to stimuli, they will generate nerve impulses that are conveyed towards the CNS • Motor or efferent neurons carry nerve impulses away from the CNS • Interneurons or association neurons are located within the CNS between sensory and motor neurons • Interneurons integrate or process sensory information from sensory neurons, and elicit a motor response by activating the appropriate motor neurons • Allows access • E-Allow EXIT from cns

histologist

• In addition to structural classification, some neurons are named after the histologist who first described them • Like the Purkinje cell in the cerebellum • Others are named for their appearance or shape • Like the Pyramidal cell found in the cerebral cortex of the brain

Multiple Sclerosis

• Multiple sclerosis (MS) is a disease that causes progressive destruction of the myelin sheaths surrounding the neurons in the CNS • Afflicts 350,000 in the US and 2 million world wide • Affects women twice as much as men • Is more common among people of European decent compared to African or Asian decent • MS is an autoimmune disease • The immune system will attack the myelin sheaths of nerves in multiple regions • The sheaths will deteriorate into scleroses or hardened plaques or scars • MRI scans will detect the presence of the plaques in the white matter of the CNS • The destruction of the myelin sheaths will slow then short circuit the propagation of nerve impulses • MS symptoms typically appear between the ages of 20-40 • The most common form is relapsing-remitting MS • Initial symptoms may include feelings of heaviness or weakness in the muscles, abnormal sensations or double vision • The initial "attack" is followed by a period of remission where symptoms disappear • Then in a year or two, the symptoms return in another attack, then abate again • With each attack, there is a progressive loss of function • The exact cause of MS is unclear • Both genetics and exposure to environmental factors, like herpes virus, are believed to be involved • Treatments are limited, but some patients do respond well to beta interferon • Lengthens the time between attacks, decreases severity of attacks and slows the formation of new lesions • Unfortunately, not all patients can tolerate beta interferon treatments and they become less effective with time

Multipolar neurons

• Multipolar neurons usually have multiple dendrites and one axon that extend from the cell body • Most neurons in the brain and spinal cord are of this type, as well as all motor neurons

Describe the function and location of each type of human neuroglia cell. (pgs. 406-408)

• Neuroglia make up about half of the volume of the CNS • Were originally thought to hold the nervous tissue together • We now know they do a lot more than just hold nervous tissue together • Are smaller than neurons and 5-25 times more numerous • Do not generate action potentials and can divide • In injured nervous tissue, neuroglia will actively divide to fill in the space formerly occupied by neurons • Brain tumors arising from neuroglia, called gliomas, are highly malignant and grow rapidly

Describe and recognize the different types of neurons found in the human body, and list the function and location of each.

• Neurons are classified using both structural and functional features • Structurally, neurons are classified according to the number of processes extending from the cell body

Neuropeptides

• Neuropeptides are neurotransmitters consisting of 3-40 amino acids linked together by peptide bonds • These are numerous in both the CNS and PNS • They bind to metabotropic receptors, and can have either inhibitory or excitatory effects depending on the particular receptor present • Neuropeptides are produced at the cell body, packaged in synaptic vesicles and moved to the axon terminals • The first neuropeptides were found when studying the opiate receptor in the brain • Wanted to figure out what was naturally binding there • Opioid peptides including enkephalins, endorphins and dynorphins were discovered, and are believed to be the bodies natural pain killers • Substance P is another neuropeptide neurotransmitter • Is released by neurons that transmit pain-related input from peripheral pain receptors • Opioid peptides function by blocking or suppressing the release of substance P

List the various ways that NT effects can be modified given in the "Clinical Connection"

• Neurotransmitter effects can be modified several different ways by naturally occurring substances as well as drugs and toxins • Neurotransmitter synthesis can be stimulated or inhibited • For example, administration of L-dopa, a precursor of dopamine, can stimulate production of dopamine • This is used in treatment of Parkinson's disease • Neurotransmitter release can be enhanced or blocked • For example, amphetamines promote dopamine and norepinephrine release, while botulinum toxin blocks acetylcholine release resulting in flaccid paralysis • Neurotransmitter receptors can be activated or blocked • A compound that enhances or activates receptors is called an agonist • Isoproternol is an agonist of epinephrine and norepinephrine, used to dilate airways during an asthma attack • A compound that blocks neurotransmitter receptors is called an antagonist • Zyprexa is a drug prescribed for schizophrenia which functions as an antagonist of serotonin and dopamine • Neurotransmitter removal can be enhanced or inhibited • For example, cocaine blocks the reuptake of dopamine • This results in euphoria or intensely pleasurable feelings

Nitric oxide

• Nitric oxide is a simple gas (NO) that is a potent neurotransmitter and has widespread effects throughout the body • Note nitric oxide is not nitrous oxide (N2O or laughing gas) • It is found in both the CNS and PNS • Unlike all previously known neurotransmitters, NO is not synthesized and stored in synaptic vesicles; it is produced as needed • This is likely due in part to the fact NO is a highly reactive free radical that will exist for less than 10 seconds in the body before it combines with water and oxygen to form nitrates and nitrites • NO is an excitatory neurotransmitter • The effects of NO are not limited to the synapse, as the molecule is lipid soluble and will diffuse to neighboring cells • In neighboring cells it can activate the second messenger cyclic GMP • Clearance of NO is accomplished by the rapid breakdown of NO into nitrates and nitrites

Sensory or afferent

• Sensory or afferent neurons contain sensory receptors at their distal ends (dendrites) or are adjacent to sensory receptor cells • In response to stimuli, they will generate nerve impulses that are conveyed towards the CNS

amino acids

• Several amino acids are neurotransmitters • Glutamate is an excitatory neurotransmitter • Binds to ionotropic receptors generating an EPSP • Found in the CNS • Glutamate is inactivated by uptake of the glutamate into the secretory cell and neighboring neuroglia • Gamma-aminobutyric acid (GABA) and glycine are inhibitory neurotransmitters • Bind to ionotropic receptors, opening Cl- ion channels • Found in the CNS • Inactivation of GABA is through reuptake by the secreting cell and the surrounding neuroglia cells

different types of human synapses

• Synapses can be either electrical or chemical in nature • At an electrical synapse, action potentials are conducted directly between the plasma membranes of adjacent neurons • This will occur through gap junctions • Remember that gap junctions form little tunnels that connect the cytoplasm of adjacent cells • Ions will flow from one cell through the gap junctions to the next cell • This will spread the action potential from one cell to another • Gap junctions are common in visceral smooth muscle cardiac muscle and the developing embryo; they also occur in the brain • There are two main advantages of electrical synapses • They transmit the action potential faster then chemical synapses • Transmission is direct, not slowed by the events that take place in a chemical synapse • Electrical synapses can synchronize a group of neurons or muscle fibers • A large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions • Important in visceral or cardiac muscle, where coordinated contraction will result in movement of food or generation of a heartbeat • The membrane of the pre and post synaptic neurons are close, but they do not touch • This gap is called the synaptic cleft • Is roughly 20-50nm across and is filled with interstitial fluid • Nerve impulses can not cross this space directly, but rely on an indirect form of communication • In response to the arrival of an action potential, the presynaptic neuron will release a neurotransmitter • This chemical will diffuse across the synaptic cleft • The post synaptic neuron will receive the chemical signal and produce a postsynaptic potential • This is a graded potential • The time it takes for the release of the neurotransmitter, the diffusion across the synaptic cleft and generation of the postsynaptic potential is about 0.5msec • This is why electrical synapses are faster than chemical synapses

The ANS consists of

• The ANS consists of: • Sensory neurons that convey information from autonomic sensory receptors in the visceral organs to the CNS • Motor neurons that conduct impulses from the CNS to smooth and cardiac muscle and glands • Because we typically cant control these functions consciously, the action of the ANS is involuntary • The ANS is further subdivided into two branches, the sympathetic division and parasympathetic division • With only a few exceptions, effectors receive signals from both divisions, and the two divisions have opposite effects • For example, sympathetic neurons increase heart rate and parasympathetic neurons slow the heart rate down • In general, sympathetic division support exercise and emergency (fight or flight) responses, whereas the parasympathetic division supports the "rest and digest" activities

The SNS consists of

• The SNS consists of: • Sensory neurons that convey information from somatic receptors in the head, body wall, limbs and special senses to the CNS • Motor neurons that conduct impulses from the CNS to the skeletal muscles • This part of the PNS is under voluntary control (we can consciously control the skeletal muscles)

ATP

• The adenosine portion of ATP contains a purine ring structure attached to a ribose sugar • Purines, including ATP, ADP and AMP are all neurotransmitters • These are excitatory neurotransmitters • Found in both the PNS and CNS • ATP is often released from the same synaptic vesicle at the same time as another neurotransmitter • In sympathetic neurons, ATP is released with norepinephrine • In some parasympathetic neurons, ATP and acetylcholine are in the same vesicles • ATP is inactivated by nucleotidases present in the synapse • These cleave the ATP into it's base components, preventing further binding to the receptor

cell body

• The cell body (also called the perikaryon or soma) contains the nucleus, cytoplasm and typical organelles • Also contain free ribosomes and prominent clusters of rough endoplasmic reticulum called Nissl bodies • Newly synthesized proteins from the Nissl bodies are used to replace cellular components, provide material for neuronal growth and regenerate damaged axons in the PNS. • The cytoskeleton consists of neurofibrals, composed of bundles of intermediate filaments which provide shape to the cell and microtubules which assist movement of materials from the cell body to the axons • Aging neurons also contain lipofuscin • A yellow-brown pigment found in granules in the cytoplasm • Is a product of the lysosomes that accumulates, but does not harm the cell

The central nervous system

• The central nervous system, consisting of the brain and spinal cord • The brain is located within the skull and contains about 85 billion neurons • The spinal cord is connected to the brain through the foramen magnum, is encircled by the bones of the vertebral column and is composed of ~100 million neurons • Functions of the CNS include: • Processing incoming sensory information • Producing thoughts, emotions and memories • Producing most of the signals that stimulate muscle contraction and gland secretion

Describe the manner in which a neuron might repair itself after an injury.

• The inner layer is the myelin sheath, and the outer layer is called the neurolemma o The neurolemma aids regeneration of the PNS by forming a regeneration tube that guides and stimulates regrowth of the axon

axon

• The single axon of a neuron is responsible for propagating nerve impulses to another cells(neuron, muscle or gland) • Are long, thin, cylindrical projections connected to the cell body by a cone shaped elevation called the axon hillock • The part of the axon closest to the axon hillock is called the initial segment • In most neurons, the nerve impulses arise at the junction of the initial segment and the axon hillock, and travel along the axon towards their destination • Axons contain mitochondria, microtubules and neurofibrals, but no endoplasmic reticulum • No ER means no protein synthesis in the axon • The cytoplasm of the axon is called axoplasm, and the plasma membrane is called the axolemma • Along the length of an axon, side branches may occur, typically at a right angle to the axon • These are called axon collaterals • Axons, and their axon collaterals terminate by dividing into many fine processes called axon terminals

Unipolar neurons

• Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body • These are more appropriately called psuedounipolar neurons because they begin in the embryo as bipolar neurons • During development, the axon and dendrites fuse together into a single process • The dendrites of most unipolar neurons function as sensory receptors • These detect stimuli like touch, pressure, pain and thermal stimuli