CH 11 Bio 141 intro to nervous tissue

Functional divisions of the nervous system

Nervous system performs millions of tasks simultaneously every second; fall into three functional categories: sensory, integrative, or motor: Sensory functions -gather information about internal and external environments of body; input is gathered by sensory or afferent division of PNS; further divided into somatic and visceral divisions; Sensory input from both divisions is carried from sensory receptors to spinal cord and/or brain by spinal and cranial nerves Somatic sensory division -consists of neurons that carry signals from skeletal muscles, bones, joints, and skin; also transmits signals from organs of vision, hearing, taste, smell, and balance; sometimes called special sensory division Visceral sensory division -consists of neurons that transmit signals from viscera (organs) such as heart, lungs, stomach, kidneys, and urinary bladder Motor functions - actions performed in response to integration; performed by motor or efferent division of PNS; can be further subdivided into somatic and autonomic divisions, based on organs that neurons contact Motor/efferent division - consists of motor neurons that carry out motor functions; travel from brain and spinal cord via cranial and spinal nerves; organs that carry out effects of nervous system are commonly called effectors. Somatic motor division consists of neurons that transmit signals to skeletal muscle; under voluntary control (aka voluntary motor division) •Autonomic nervous system (ANS) or visceral motor division »Consists of neurons that carry signals to thoracic and abdominal viscera; critical for maintaining homeostasis of body's internal environment »Regulates secretion of certain glands, contraction of smooth muscle, and contraction of cardiac muscle in heart; involuntary (aka involuntary motor division)

Myelination PNS vs CNS

Neurolemma - found on outer surface of a myelinated axon in PNS; composed of Schwann cell nucleus, organelles, and cytoplasm; not present in CNS Number of axons myelinated -oligodendrocytes have multiple processes that can provide myelination for multiple axons in CNS while a Schwann cell only provides myelination for oneaxon in PNS Timingofmyelination - myelination begins early in fetal development in PNS and much later in the CNS; very little myelin present in brain of newborn

OVERVIEW OF NEURONAL SYNAPSES

Neurons must communicate with other cells, including other neurons, in order to carry out their functions—example of Cell-Cell Communication Core Principle •Synapse - where a neuron meets its target cell (in this case another neuron) called a neuronal synapse; can be either electrical or chemical •Neuronal synapses can occur between an axon of one neuron and another part of another neuron

Gliomas and Astrocytomas

Primary brain tumors - originate in brain; most are gliomas (caused by abnormally high rate of division of glial cells) •Predisposing conditions -exposure to ionizing radiation and certain diseases•Most commonly affected cell is astrocyte; resulting tumor is called astrocytoma Range in severity from mild with good prognosis to highly aggressive with a very poor prognosisTreatment - varies with tumor type, age, and health of patient; generally involves surgical removal of mass with chemotherapy and perhaps radiation therapy

three main functional regions of neuron

Receptive region - includes dendrites and cell body Conducting region - includes axon Secretory region - includes axon terminal

regeneration of nervous tissue

Regeneration or replacement of damaged tissue is nearly nonexistent in CNS and is limited in PNS; neural tissue can regenerate only if cell body remains intact

three functional neurons

Sensory or afferent neurons - carry information toward CNS; neuron cell bodies in PNS receive information from sensory receptors and relay information via axons to brain or spinal cord; usually pseudounipolar or bipolar Interneurons or association neurons -relay information within CNS between sensory and motor neurons; make up most of neurons in body; multipolar, communicating with many other neurons Motor or efferent neurons -carry information away from cell body in CNS to muscles and glands; mostly multipolar

multiple sclerosis

certain cells of immune system attack myelin sheaths within CNS; type of autoimmune disorder (patient's own immune system attacks part of body) •Causes progressive loss of myelin sheath; in turn causes loss of current from neurons •Symptoms - result from progressive slowing of action potential propagation; exact symptoms depend on region of CNS affected; most exhibit changes in sensation (e.g., numbness), alterations in behavior and cognitive abilities, and motor dysfunction, including paralysis

biogenic amines

class of five neurotransmitters synthesized from amino acids; used throughout CNS and PNS for many functions such as regulation of homeostasis and cognition; first three form catecholamine subgroup, all of which are made from amino acid tyrosine; mostly excitatory: Norepinephrine (catecholamine, also known as noradrenalin) - found mainly in ANS where it influences heart rate, blood pressure, and digestion; in CNS it regulates sleep/wake cycle, attention, and feeding behaviors Epinephrine (catecholamine, also known as adrenalin) - also used in ANS; has similar functions as norepinephrine; more widely used as a hormone by endocrine system Dopamine (catecholamine) - used extensively by CNS; helps to coordinate movement; involved in emotion and motivation Serotonin - synthesized from amino acid tryptophan; most serotonin-secreting neurons are found in brainstem; axons project into multiple areas of brain; functions include mood regulation, emotions, attention, feeding behaviors, and daily rhythms Histamine - synthesized from amino acid histidine; involved in regulation of arousal and attention

Scwann cells

encircle axons found in PNS to provide them with myelination

neurons

excitable cell type responsible for sending and receiving signals in form of action potentials; most consist of three parts

satellite cells

found surrounding cell bodies of neurons in PNS to provide supportive functions (still not well defined)

astrocytes

large star-shaped cells whose many processes terminate in structures called end-feet; function to: oAnchor neurons and blood vessels in place; help define and maintain three-dimensional structure of brain oFacilitate transport of nutrients and gases between blood vessels and neurons; regulate extracellular environment of brain oAssist in formation of blood-brain barrier; protective structure that surrounds capillary endothelial cells and makes them impenetrable to most polar compounds and proteins oRepair damaged brain tissue by rapid cell division

postsynaptic potential

local potentials found in membranes of postsynaptic neuron Membrane potential of postsynaptic neuron moves closer to threshold; caused by a small local depolarization (sodium or calcium channels open) called an excitatory postsynaptic potential (EPSP) Membrane potential of postsynaptic neuron moves farther away from threshold; caused by a small local hyperpolarization (potassium or chloride ion channels open) called an inhibitory postsynaptic potential (IPSP)

Cell body (soma)

most metabolically active region of neuron; manufactures all proteins needed for whole neuron; the following organelles support this high level of biosynthetic activityBoth free ribosomes and rough endoplasmic reticulum for protein synthesis; Nissl bodies are RER that can be seen with microscopeGolgi apparatus (vesicular transport) and large or multiple nucleoli (ribosomal RNA)Mitochondria supply energy required for high metabolic activity Cytoskeleton - contains microtubules; provide structural support and a means for chemical transportation between cell body and axon Neurofibrils - composed of intermediate filaments of cytoskeleton; provide structural support that extends into neuron processes Processes - cytoplasmic extensions that originate at cell body and include dendrites and axons; allow neurons to communicate with other cells

neuroglial cells

not only provide structural support and protection for neurons but also maintain their environment Able to divide and fill in space left behind when a neuron dies; form of each type of neuroglial cell is specialized for its function, another example of the Structure-Function Core Principle

EPSP

postsynaptic neuron moves closer to threshold; caused by a small local depolarization (sodium or calcium channels open) called an excitatory postsynaptic potential (EPSP)

dendrites

short, branched processes; receive input from other neurons, which they transmit to toward cell body in form of electrical impulses; each neuron may have multiple dendrites

Microglia

small and scarce cells; activated by injury into wandering phagocytic cells within CNS; ingest disease-causing microorganisms, dead neurons, and cellular debris

Acetylcholine

small molecule neurotransmitter widely used by nervous system Cholinergic synapses bind to ACh; found in neuromuscular junction, within brain and spinal cord and within autonomic nervous system Largely excitatory but it does exhibit some inhibitory effects in PNS Synthesized from choline and acetyl-CoA and packed into synaptic vesicles Quickly degraded by acetylcholinesterase (AChE) an enzyme in synaptic cleft; by-products of reaction are taken back into presynaptic neuron for recycling and reuse

major neurotransmitters

•Binding of neurotransmitter to receptor leads to either an EPSP (with excitatory effects) or an IPSP (with inhibitory effects) •Most neurotransmitters can have both effects depending on which postsynaptic neuron receptors they bind; single neurotransmitter may have several receptor types •Major neurotransmitters are classified into four groups based on chemical structure

local anesthetics

•Local anesthetics - (like lidocaine) commonly administered agents for surgical or dental procedures; produce temporary numbness in specific area •Block voltage-gated sodium channels of neurons in treated area; prohibits depolarization and therefore action potentials relaying pain are not transmitted to CNS •Nonselective; also affect sodium channels in muscles of area; causes temporary paralysis; reason for crooked smiles and drooling following dental work

types of neuroglia

4 types reside in CNS: oAstrocytes oOligodendrocytes oMicroglia oEpendymal cells 2 types reside in PNS: oSchwann cells oSatellite cells

electrophysiology of neurons

All neurons are excitable or responsive in presence of various stimuli: chemical signals, local electrical signals, and mechanical deformation•Stimuli generate electrical changes across neuron plasma membrane; rapidly conducted (conductivity) along entire length of membrane•Two forms of electrical changes occur in neurons: Local potentials -travel short distances Action potentials - travel entire length of axon

types of neural synapses

Axodendritic synapse -synapse between axon of one neuron and dendrite of another neuron Axosomatic synapse -synapse between axon of one neuron and cell body of another neuron Axoaxonic synapse -synapse between axon of one neuron and axon of another neuron

Cells that complete myelin sheath

Axons in both CNS and PNS are generally longer than neuroglial cells so multiple cells must provide a complete myelin sheath Internodes - segments of axon that are covered by neuroglia Node of Ranvier -gap between adjacent neuroglia; where myelin sheath is absent •Small axons in CNS and PNS are usually unmyelinated •White matter -composed of myelinated axons that appear white •Gray matter -composed of neuron cell bodies, unmyelinated dendrites and axons that appear gray

ependymal cells

Ciliated cells that line hollow spaces found within CNS (brain and spinal cord); function to manufacture and circulate cerebrospinal fluid

action potential steps

Depolarization -sodium channels open, allowing positively charged sodium ions to flow into cell; membrane potential becomes morepositive Repolarization -potassium ion channels open; allows positively charged potassium ions to flow out of cell; cell becomes more negative, returning to resting membrane potential Hyperpolarization -cell becomes more negative than its normal resting membrane potential due to loss of potassium ions (cations) plus loss of anions such as chloride Sodium ion channels inactivate and voltage-gated potassium ion channels activate: sodium ions stop flowing into axon and potassium begins exiting axon as repolarization begins 4.Sodium ion channels return to resting state and repolarization continues 5.Axolemma may hyperpolarize before potassium ion channels return to resting state; then axolemma returns to resting membrane potential

IPSP

Membrane potential of postsynaptic neuron moves farther away from threshold; caused by a small local hyperpolarization (potassium or chloride ion channels open) called an inhibitory postsynaptic potential (IPSP)

three types of neurons

Multipolar neurons - with a single axon and multipledendrites, make up over 99% of all neurons Bipolar neurons - with one axon and one dendrite and a cell body between them; found in eye and olfactory epithelium in nasal cavity Pseudounipolar neurons - have only one fused axon that extends from cell body and divides into two processes: one process carries sensory information from sensory receptors to cell body; other process carries sensory information from cell body to spinal cord; sensory neurons that carry information related to pain, touch, and pressure

Anatomical Divisions of the Nervous System

Divided anatomically into central nervous system (CNS) and peripheral nervous system (PNS) CNS - includes brain and spinal cord Brain- made up of billions of nerve cells or neurons; protected by bones of skull Spinal cord begins at foramen magnum and continues through vertebral foramina of first cervical to first or second lumbar vertebra •Made up of millions of neurons; much fewer than brain •Enables brain to communicate with most of body below head and neck PNS -consists of all nerves in body outside protection of skull and vertebral column Nerves consist of axons of neurons bundled together with blood vessels and connective tissue; carry signals to and from CNS; classified based on origin or destination •12 pairs of nerves traveling back to or from brain; called cranial nerves •31 pairs of nerves traveling back to or from spinal cord; called spinal nerves

Axon (nerve fiber)

Each neuron has only one axon or nerve fiber that can generate and conduct action potentials; axon may have following distinct regions Axon hillock - region where axon originates from cell body Axon collaterals - branches that extend from main axon Telodendria - small branches that arise from axon and axon collaterals near where these extensions end Axon terminals or synaptic bulbs -arise from telodendria; components that communicate with a target cell

3 main amino acid neurotransmitters

Glutamate - most important excitatory neurotransmitter in CNS; binds to its ionotropic postsynaptic receptors and opens channels that allow for flow of both sodium and calcium ions; generate EPSPs in postsynaptic neuron Glycine and GABA - both major inhibitory neurotransmitters; induce IPSPs on postsynaptic neurons by opening chloride ion channels; hyperpolarize axolemma

Oligodendrocytes

also found in CNS; have radiating processes with flattened sacs that wrap around axons of nearby neurons to form myelin

myelin sheath

composed of repeating layers of plasma membrane of Schwann cell or oligodendrocyte in PNS and CNS respectively :•Myelination -process that forms myelin sheath from plasma membranes of neuroglial cells; wrap themselves around axon forming multiple layers of membrane (myelin)Electric current - generated by movement of ions in body fluids Lipid content of myelin sheath insulates axon (prevents ion movements) like rubber around copper wire; increases speed of action potential conduction Myelinated axons conduct action potentials about 15-20 times faster than unmyelinated axons

psychiatric disorders and treatments

•Psychiatric disorders affect thought processes; generally treated by modifying synaptic transmission to change how neurons communicate with each other •Psychopharmacology (study of drugs that affect higher brain functions) targets either action potential generation or some aspect of neurotransmitter physiology: Schizophrenia - characterized by repetitive psychotic episodes (periods during which patient is unable to appropriately test beliefs and perceptions against reality); thought to result from excessive release of dopamine; management involves blocking postsynaptic dopamine receptors Depressive disorders -marked by disturbances in mood; thought to result from deficiency in synaptic transmission of serotonin, norepinephrine, and/or dopamine; most widely used antidepressants are selective serotonin reuptake inhibitors (SSRIs); block serotonin transporter (only), preventing reuptake by presynaptic neuron Anxiety disorders - characterized by exaggerated and inappropriate fear responses; believed to stem from abnormalities in norepinephrine, serotonin, and GABA transmission; drugs for treatment include antidepressants, GABA activity enhancers, and others that modulate norepinephrine transmission Bipolar disorders - characterized by episodes of abnormal elevated mood (mania) followed by depression; treatments involve decreasing ease of action potential generation; generally block sodium channels in axolemma