Week 8-9, Chapter 5 Nerve Cell Physiology

Nerve Cell Types

**nerve cells are classified according to the number of receptive processes coming out of their bodies AND the length of their axons** 1.) Classification by # of receptive processes: MULTIPOLAR -many dendrites, one axon -differ in size and shape -most cells in CNS are multipolar (e.g. spinal interneurons and cerebellar Purkinje cells) BIPOLAR - two processes (dendrite and axon) - one extending from each pole of the body - found in inner ear and retina UNIPOLAR - T-shaped - one process extending from the body and dividing into an axon and dendrite away from the cell body - cells in spinal dorsal root ganglia are unipolar 2.) Classification by length of axons: GOLGI TYPE I -long axon ranging from inches to feet - may form sensory or motor tracts connecting cells across long distances GOLGI TYPE II - short axonal processes (e.g. interneurons that connect with other adjacent cells)

acetylcholine

small molecule neurotransmitter one of the primary neurotransmitters of the PNS that regulates muscular activities as well as parasympathetic function synthesized from choline and acetyl-coenzyme A (accetyl Co-A) by enzyme choline acetyltransferase broken down by acetylcholinesterase disintegration of acetylcholine is necessary to make repetitive nerve impulses effective and to allow for muscle repolarization antibodies that interfere with acetylcholine action on muscles cells affect movement, as seen in Myasthenia gravis deficient cholinergic projections in the hippocampus and orbitofrontal cortex have been implicated in Alzheimer disease SITE OF SECRETION basal forebrain, brainstem, and myoneural junctions FUNCTIONS regulation of forebrain activity inhibition of basal ganglia activity regulation of muscle contractions

Nerve Impulse (continued) DEPOLARIZATION

All stimuli are converted to changes in the membrane potential of the cell membrane potential changes are responses to opening or closing of gated channels changes vary; DEPOLARIZATION is one possible change the neurotransmitter released by the presynaptic cell could cause DEPOLARIZATION ----> Excitatory Postsynaptic Potential (EPSP) ----> EPSP takes membrane potential toward action potential threshold; if EPSP is of sufficient magnitude, the cell will reach threshold for firing an action potential Less negative state triggered by stimuli Cell threshold 55mV depolarization opens specific voltage-gated ion channels, allowing ins to flow in and out of the cell; initially sodium (NA+) flows quickly into the cell, causing large depolarization to potentials increase in positive concentration inside (this means the inside of cell will become positive for brief time) (then the sodium channels close rapidly allowing a return to resting membrane potential)

COMMON TUMOR TYPES (Inside brain) astroctytoma

arises from undifferentiated astroctyes glioblastoma multiform is most malignant and invasive tumor that is most frequent in brains of adults

Nerve Impulse (continued) RESTING MEMBRANE POTENTIAL

RESTING MEMBRANE POTENTIAL -cell is not excited and not conducting an impulse - the difference in charge between the interior and exterior of the cell is called the RESTING MEMBRANE POTENTIAL - resting membrane potential typically between -50 and -70 mV in relation to outside of cell - membrane potential is maintained by an unequal distribution of positively charged sodium (NA+) and postassium (K+) ions and negatively charged choloride (Cl) ions and proteins across the membrane -specific level of membrane potential in which the distribution of positive and negative ions on each side of the membrane is unequal (polarized); therefore there is a difference between electrical charges on the inner and outer sides of cell membrane; NEGATIVE ON INSIDE; POSITIVE ON OUTSIDE - Na+ more concentrated outside; K+ inside -the overall result is that the exterior of the cell has a net positive charge and the interior has a net negative charge - even during resting state, some ions constantly pass through the membrane

COMMON TUMOR TYPES (Inside brain) ependymoma

arises from ependymal cell lining at the ventricle medulloblastoma (the most common form) arises in the posterior fossa from the roof of the fourth ventricle

COMMON TUMOR TYPES (outside brain) meningioma

arises from meningeal (arachnoidal cells) membrane mostly benign

COMMON TUMOR TYPES (Inside brain) Oligodendroglioma

arises from oligodendroglia cells most often in the frontal cortex

Multiple Sclerosis (MS)

autoimmune central demyelinating disease linked to viral infections and abnormalities in immune system causing antibodies to attack the body's own myelin at random locations in the CNS early symptoms include vision loss, double vision, vertigo, loss of balance, weakness, dysarthria, and/or numbness in limbs later stage symptoms include nystagmus, scanning speech, and intention tremor other symptoms include delayed or nonexistent evoked responses to visual, auditory, somatic stimuli, cognitive impairments pathogenesis has implicated cytokines, the proteins that regulate the intensity and duration of an immune response within the plaque TREATMENT symptomatic e.g. drug injections and plasma paresis

Regarding synapse development, abnormalities in the development of connectivity, anoxia, and less than optimal aptosis in embryonic development may have biological implications for __________________________________.

many developmental disorders in children who are cognitively-socially-linguistically challenged with a spectrum of behavioral, social ,and intellectual problems

monoamines

subgroup of small molecule neurotransmitters derived from amino acids all monoamine-producing cell clusters lie in the brainstem e.g. norepinephrine, epinephrine, dopamine, and serotonin

medical treatment of tumors

surgical excision, gamma knife (a non-invasive stereotactic radiosurgery method), and chemotherapy

Nerve Impulse (continued) HYPERPOLARIZATION

Because the repolarization of the cell is aided by the opening of voltage-gated potassium channels that allow more potassium to flow down the concentration gradient, it can "undershoot_ and the resulting membrane potential can becoming slightly more HYPERPOLARIZED than the original resting membrane potential e.g. the neurotransmitter released by the presynaptic cell could cause HYPERPOLARIZATION in postsynaptic cell ----> Inhibitory Postsynaptic Potential (IPSP) ----> IPSP takes membrane potential further away from action potential threshold increase in negative concentration inside As cell becomes hyperpolarized, postassium channels also close so the cell can reestablish its membrane potential

Name 2 types of synapses mentioned in the text

CHEMICAL SYNAPSE -action potential triggers the presynaptic neuron to release neurotransmitters ELECTRICAL SYNAPSE -ions flow directly between cells -present where extremely fast communication must occur between cells (such as vision and hearing) or where synchronization of the excitation of cells in necessary (e.g. in the heart) - both surfaces are in close proximity containing large channels embedded in the membranes - small molecules such as ions can flow directly from one cell to the next without the use of extracellular chemical signaling molecules

CNS vs. PNS

CNS: brain and spinal cord -consists of two primary types of cells: 1.) nerve cells (neurons) and neuroglia (glia) cells -In CNS, each oligodendrocyte contributes to myelination of a group of adjacent axons - myelin sheath is produced by glial cells called oligodendrocytes in the CNS PNS: cranial and spinal nerves -cranial nerves: sensory and motor nerves connected to brain stem -spinal nerves: sensory and motor nerves connected to the spinal cord - myelin sheath is produced by Schwann cells in PNS that myelinate fibers in jelly roll manner -In PNS, each Schwann cell is associated with only one axon - unlike CNS, contains ENDONEURIUM, a fibrous connective tissue covering for axon; nerve fibers in PNS are held together by connective tissues, such as collagen fibers secreted by fibroblasts and other cells that form the endonuerial membrane

Synapse development and neuroplasticity

Embryogenesis refers to cellular configuration in the brain Embryogenesis produces overabundance of neurons; most of these cells are active throughout life, but some do not survive postembryonic and postfetal neuronal growth (neuroblast and glioblast) involves cellular reduction, neuronal migration, and rapid synaptic growth, which contributes to a 350% increase in brain size within the first 2 years of life self-programmed cellular death (apoptosis) involves approximately 50% of the original cells and applies to the neurons that develop incorrect, incompatible, or transient synaptic connections to target areas, as well as to neurons unable to develop these connections once a neuron has migrated and is established in its final destination after being functionally differentiated, it extends an axon with growth cones (path finders) coded with sensory or functional inclination these growth cones navigate the directional axonal paths by finding ways through the enormous cellular density and other biological surroundings to connect with target neuron; this process is known as SYNAPTOGENESIS additional factors that contribute to connectivity growth of axons include chemical affinity, guiding cell adhesive molecules, and neuritic inhibitory factors nerve inhibitory factors are some glycoproteins produced by myelin-forming oligodendroglia cells that serve as train tracks to prevent uncontrolled or deviated axonal growth in all directions once connected to target neurons, the presynaptic terminals of the axon and its postsynaptic (receptive) region undergo morphologic and chemical changes the only synaptic connections that survive are those that meet the criterion of biocompatibility on both sides of the synapse connections that are weak, underused, or incorrectly linked, get pruned repeated activation influences brain's organization; the reinforced activation of the neuronal network may underlie the process for allocating functions to different brain regions

Compare IPSPs, EPSPs, and Action Potentials

IPSP (Inhibitory Postsynaptic Potential) graded EPSP ( Excitatory Postsynaptic Potential) graded ACTION POTENTIAL all or nothing

Human Nervous System

Made up of 2 systems, the CNS (brain and spinal cord) and PNS (cranial and spinal nerves) Neuron/nerve cell is basic building block of the human nervous system

cytoskeleton

a dynamic scaffold inside all eukaryotic cells that is responsible for cell shape and motility as well as the transport and organization of intracellular component composed of three classes of protein polymers called microtubules, microfilaments and neurofilaments (long strands of proteins) these 3 components provide a scaffolding that give neurons their characteristic shape and act as roadways for the transport of proteins and small organelles throughout the cell microtubules is largest microfilaments is smallest small staining techniques are used to identify cellular structures in the cell body, like microtubules, microfilaments and neurofilaments abnormalities of cytoskeleton in the form of tangles and subsequently reduced transfer of protein have been associated with Alzheimer disease

cytoplasm

major component of cell body (along with the nucleus) consists of protein molecules and an aqueous (watery) substance enclosed within the cell membrane cytoplasmic material of the cell contains many microscopic subcellular units called ORGANELLES which include mitochondria, ribosomes, lysosomes, rough and smooth endoplasmic reticulum (ER), and the Golgi apparatus primary function of organelles and associated structures within cytoplasmic material is to metabolize protein essential for the maintenance and growth of the cell body (along with its processes) as well as supporting impulse generation and intercellular communication

Guillain-Barre syndrome

autoimmune central demyelinating disease that affects myelin in PNS triggered by an infection related to the bacteria Campylobacter jejuni, which is a food-borne pathogen affects nerve conduction by slowing its speed recoverable condition symptoms include weakness and tingling in the toes, fingers, and legs; weakness spreads to upper limbs and face, along with loss of deep tendon relfexes although recoverable, it can be life threatening in the case of respiratory or bulsar palsy

Myesthania Gravis

autoimmune condition; neuromuscular (myoneural) junction disorder characterized by progressive fatigue and muscle weakness that worsens with physical exercise and improves with rest gradual onset more prevalent in females first 30 years of life, and thereafter more prevalent in men impaired impulse transmission is caused by underactivity of acetylcholine secondary to the loss of acetylcholine receptors at the neuromuscular junction symptoms include ptosis (droopy upper eye lid), diplopia (double vision), altered facial expression, regurgitation, and hypernasality TREATMENT drugs that inhibit acetylcholinesterase plasmapheresis steroids immunoglobulin infusions

(Neuronal Responses to Brain Injuries) Axonal Regeneration in the PNS

sectioned nerve endings proximal to the cell body begin regenerating within 3-4 days regeneration cannot occur unless the proximal and distal ends of the severed nerve are cleaned and attached Schwann cells and fibroblasts contribute significantly to axonal regeneration in PNS Schwann cells fill the interval between opposing ends; sheath of Schwann, or neurilemma, forms a tube from proximal fiber leading to distal end; the neurilemma tube guides the growth of the peripheral axon; as proximal end of axon regenerates, many spouts form low probability that the regenerated axon will reach its previously attached fiber; instead, it might attache to a different sensory or motor fiber improper connection can be problematic; for example, a pain-mediating fiber connected to a touch receptor could result in the sensation of pain from touch nerve fibers connected incorrectly usually atrophy injury closer to cell body is more damaging than a distant nerve lesion because there is less potential for connecting with a functionally incorrect structure

nucleus

component of cell body (along with cytoplasm) control center of nerve cell contains deoxyribonucleic acid (DNA), which are macromolecules with genetic information that determines the characteristics of specific cell types; replication of DNA through cell division provides the mechanism for genetic inheritance visible within the nucleus is the nucleolus, which contains RNA and is the site of ribosome assembly ribosomes are transported out of the nucleolus into the cytoplasm, where they play a key role in protein synthesis; proteins destined to be free in the cytoplasm are synthesized by free ribosomes, whereas all other proteins are synthesized by ribosomes attached to the rough endoplasmic reticulum (ER)

(3 parts of neuron/nerve cell: soma, dendrites, and axon) dendrites

cytoplasmic extension that extends from cell body and mediates nerve impulses receptive structures that receive information from other cells via synaptic sites and transmit information to the cell body tend to be short and have many branches branching dendrites sometimes small spines that add to their arborization (subbranching), which increases the surface available for synapses with other nerve cells

(3 parts of neuron/nerve cell: soma, dendrites, and axon) axons

cytoplasmic extension that extends from cell body and mediates nerve impulses structures that transmit information away rom the cell body to other neurons or target organs do NOT produce their own protein, with the exception of during development periods of injury; because they don't typically produce their own protein, axons depend on the cytoplasmic substance of the cell body for survival originate from a cone-shaped region of the cell (known as the axon hillock, or initial segment) and extend longer distances than dendrites terminate by branching into multiple smaller fibers that include synaptic terminals at their ends (within these synaptic terminal are synaptic vesicles that contain a variety of neurotransmitters that are released upon stimulation of the cell) on their way to a terminal destination, axons give off collaterals that communicate with many intervening nerve cells along the way axon + myelin sheath is commonly referred to as a nerve fiber

hypertrophy

increase in cell size

hyperplasia

increase in number of cells

synaptogenesis

involves the neuronal density that underlies the axonal-dendritic branching patterns and synaptic connections that reinforce the neuronal networks and also regulate the pruning of poorly connected or weak synapses once a neuron has migrated and is established in its final destination after being functionally differentiated, it extends an axon with growth cones (path finders) coded with sensory or functional inclination and these growth cones navigate the directional axonal paths by finding ways through the enormous cellular density and other biological surroundings to connect with target neuron; this process is known as SYNAPTOGENESIS

Peptides (endorphines, enkephalins, and substance P)

large molecule neurotransmitters relevant for pain management SITE OF SECRETION CNS FUNCTIONS regulation of pain perception

COMMON TUMOR TYPES (outside brain) Acoustic Neuroma and Vestibular Schwannoma

arises from the sheath of the nerve at the cerebellopontine angle mostly benign

Common clinical symptoms of brain tumor

progressive weakness, speech or visual loss, anomia, headache, impaired concentration, forgetfulness ,and altered personality common complications are seizures and increased intracranial pressure

Axons USUALLY synapse with dendrites. T or F?

T, this is called an axodendritic synapse **But they may synapse with other axons (axoaxonic synapse) or directly on cell bodies (axosomatic synapse)

glia (neuroglila) cells

protect and support nerve cells actively participate in tissue repair in response to brain injury and disease

Nerve Impulse (continued) EXCITABILITY

-cell's response to various stimuli and the conversion of this response into a nerve impulse or action potential -Excitability: nerve impulse or action potential in response to a stimulus -Basically, all stimuli are converted to changes in the membrane potential of the cell; membrane potential changes are responses to opening or closing of gated channels; changes vary e.g. the neurotransmitter released by the presynaptic cell could cause HYPERPOLARIZATION in postsynaptic cell ----> Inhibitory Postsynaptic Potential (IPSP) ----> IPSP takes membrane potential further away from action potential threshold or e.g. the neurotransmitter released by the presynaptic cell could cause DEPOLARIZATION ----> Excitatory Postsynaptic Potential (EPSP) ----> EPSP takes membrane potential toward action potential threshold; if EPSP is of sufficient magnitude, the cell will reach threshold for firing an action potential

(Glial cells of PNS are Schwann cells an satellite cells. Glial cells in CNS come in 4 TYPES: astrocytes, oligodendrocytes, ependymal, and microglia) Define the 4 types of glial cells in CNS

1.) astrocytes - located predominantly in white matter - function as scaffolding and provide skeletal support for brain cells and their processes - in gray matter, astrocytes protect brain by forming limiting membranes; by contacting capillary surfaces with their end feet and through the formation of tight junctions, astrocytes contribute to the blood-brain barrier - in conjunction with the ependymal cells, astrocytes form the internal limiting membrane - regulate extracellular concentration of ion, and, in some instances, can degrade released neurotransmitters - migrates to lesion site after injury (along with microglia) - in case of a large lesion, astrocytes seal cavity created by microglia (that phagocytosed/digested cellular debris) and form a cyst - in case of a limited-size lesion, astrocytes fill the space (created by microglia that phagocytosed/digested cellular debris) with a glial scar 2.) oligodendrocytes - form and maintain the myelin sheath in CNS - each process radiating from oligodendrocyte contributes to forming myelin - oligodendrocytes may supply myelin for 25 or more axons 3.) ependymal - line the inner surface of ventricles - in conjunction with astrocytes, the ependymal cells form the internal limiting membrane - choroid plexus, which secretes CSF consists of vascular membrane surround by epithelial layer of ependymal cells 4.) microglia - migrates to lesion site after injury (along with astrocytes) - phagocytoses (engulfs and digests) cellular debris, leaving a cavity - do not perform day-to-day functions in nervous system, but are called upon during injury - scavengers of the CNS - once they migrate to the injury cite, they transform into MACROPHAGES that phagocytose (engulf) dead tissue debris and patholgens

(3 parts of neuron/nerve cell: soma, dendrites, and axon) cell body (soma)

2 major components: nucleus and cyctoplasm (cytoplasmic material within the cell body contains many microscopic subcellular units called ORGANELLES which include mitochondria, ribosomes, lysosomes, rough and smooth endoplasmic reticulum (ER), and the Golgi apparatus) mitochondria scattered throughout the cell body contain enzymes involved with cellular metabolic energy; lysosomes contain the enzymes that participate in intracellular digestion transport of proteins through the microtubules can occur both from the cell body to axons and dendrites (anterograde) and from axonal processes back to the cell body (retrodgrade)

Disease (associated with nerve cells)

Multiple Sclerosis (MS) Guillain-Barre sndrome Myasthenia gravis

Brain tumors

NEOPLASM (tumor): -uncontrolled growth of body tissue including glia -underlying causes include improper expression of oncogenes (coding proteins involved with cellular growth) and a decreased expression of tumor suppressor genes - brain tumors typically arise from nonnueronal cells in nervous system tissue involved with tumor may retain some original functions PRIMARY vs. METASTATIC 1.) primary: arise from glia or meninges in CNS 2.) metastatic/secondary: arise elsewhere in the body and spread to the brain from an area outside the brain most spreading tumors of the brain come from breast, lung, colon, or lymphatic system; they can also come from melanoma MALIGNANT vs BENIGN 1.) malignant: grow fast, invade surrounding tissue, and are fatal; often multifocal and microscopically undifferentiated from surrounding healthy tissue which makes complete removal difficult 2.) benign: not harmful; differentiated from surround cells COMMON TUMOR TYPES (Inside brain) astroctytoma ependymoma oligodendroglioma (outside brain) meningioma acoustic neuroma and vestibular Schwannoma pituitary adenoma

Neuronal Responses to Brain Injuries

Nerve cells are less capable of further cell division than other cell types; limited cellular regeneration restricts recovery in case of injury a neuron may degenerate if either the presynaptic or postsynaptic terminal degenerates in the case of injured neurons, neurons that innervate the injured neurons can also be affected; the severity depends on the extent to which the other neurons interact with the surrounding noninjured neurons 2 TYPES OF DEGENERATIVE CHANGES (that follow axonal sectioning): 1.) Axonal (retrograde) reaction - Changes in cell body in response to axonal injury 2.) Wallerian (anterograde) degeneration - changes in axonal region detached from cell body - degenerative changes occur in the axon region detached from the cell body - the axonal segment segment still attached is the proximal segment, and the axonal segment that is detached is the distal segment - distal portion of axon begins to degenerate within 12-20 hours; axons degenerate before the myelin sheath - within 7 days, the axon and its myelin are broken into small pieces, which gradually disintegrate and set the stage for the macrophagic action of microglia cells - phagocytosis begins in 7 days and is completed in 3-6 months

COMMON TUMOR TYPES (outside brain) pituitary adenoma

arises from the pituitary gland most common cause of hormonal and visual deficits mostly benign

What methods is modern neuroscience employing to stop or slow the rate of cellular death in the brains of stroke patients?

administering nerve growth proteins and incorporating the brain's own trophic factors

Nerve Impulse (continued) REDEPOLARIZATION

aided by opening of voltage-gated potassium channels that allow more potassium to flow down the concentration gradient K+ flows out of cell

action potential

all or nothing not all stimuli are strong enough to cause a cell to reach the threshold for firing an action potential passively conducted a short distance in the axon by sodium entering the cell membrane interior of the axon becomes more positive than the adjacent neighboring area; this gradually changes the membrane potential in the neighboring area and the action potential continues to allow positively charged ions to enter the cell membrane as it moves along the axon action potential conduction in a myelinated axon is the same as in an unmyelinated axons, except the action potential conduction is faster in the myelinated axon (as the action potential jumps from one node of Ranvier to the other) conduction velocities depend on the diameter of the axon and myelination axons with larger diameters have greatest conduction velocity; larger axons also are typically myelinated axons with smaller diameters have lower conduction velocity for a period following an action potential, the cell is incapable of producing a second action potential (this is called the ABSOLUTE REFRACTIVE PERIOD)

Neuron (nerve cell)

basic building block of nervous system 3 parts: soma, dendrites, and axon make up part gray matter (gross appearance of nerve cells, glia cells, and unmyelinated fibers) make up part white matter (axonal bundles in CNS) responsible for generating, receiving, transmitting, and synthesizing electrical impulses as well as influencing other neurons or effector tissue uses mechanisms to synthesize protein, thereby using and transforming energy through excitatory and inhibitory impulses, nerve cells in CNS drive the power of the mind by regulating higher mental functions (attention, problem solving, memory, thinking, reasoning, calculation, and language) and sensorimotor activities protected and supported by glia cells receive impulses primarily through the dendrites, and secondarily through the soma and the axons conduct nerve impulses through axonal fibers, which travel various distances and synapse on the receptive ends of other nerve cells and target organs; whether the effect of a nerve impulse is excitatory or inhibitory on the target cell depends on the identity of the neurotransmitter released by the the neuron have a high metabolic activity that depends on the availability of glucose in addition to conducting nerve impulses, nerve cell synapses transmit nutritive (trophic) substances between neurons compared to other cell types, neurons contain a large amount of rough endoplasmic reticulum (ER), as well as free ribosomes for protein production, which are viewed as Nissl bodies when stained with basic dyes (such as methylene blue) for histology; further processing of proteins occurs in the smooth ER and golgi apparatus less capable of further cell division than other cell types; limited cellular regeneration restricts recovery of skills after injuries

neuroplasticity

brain's intrinsic ability to functionally reorganize and continually optimize its functioning developing brain displays greater organizational flexibility and is not as vulnerable to an injury as is the brain in adults developing brains show notable improvement after the injury than do the adult brains

(Neuronal Responses to Brain Injuries) Axonal Reaction

microscopic structures (organelles) of the soma undergo structural-degenerative alterations that are evident within 24-48 hours after injury first signs of changes in cell body are swelling of the organelles and dissolution of course clumps of Nissl bodies into fine granules cellular edema, caused by an altered blood-brain barrier, obliterates structural details of the gray and white matter and triggers nuclear shrinking; edema is maximal within 90-100 hours chromatolysis: structural changes that accompany a cell body disintegration in response to an injury; associated changes are marked by swelling, liquefaction, dissolution of cellular organelles (specifically Nissl bodies), and shifting of the nucleus from its central position to the periphery chromatolytic process may continue for 10-18 days (see next flashcard for more details) by the end of the week, swelling may go down, but necrotic tissues are invaded by a considerable number of new capillaries (hyperplasia) and proliferation of macrophages (atrocytes and microglia); period after 1st week is marked by liquefaction of necrotic tissue and phagocytosis, in which lipid-laden macrophagic microglia cells engulf and remove dead tissue phagocytosis may take 3 or more months in case of large lesion, tissue removal is likely to leave a cavity filles with some macrophagic cells and CSF, this cavity area is outlined by a sheet of astocytes which create a cystic cavity

myelin sheath

multilayered lipid material that insulates and protects the nerve fiber produced by glial cells called oligodendrocytes in the CNS produced by Schwann cells in PNS an important function of this insulation provided by myelin sheath, is to prevent the escape of electrical energy during action potential transmission, which affects the speed of nerve impulses speed of nerve conduction is determined by the diameter of the nerve and its myelin sheath myelin sheath is formed in small segments that are interrupted by intervals called the nodes of Ranvier; segment of myelin between two nodes is the internode myelin sheath + nodes of Ranvier = sausage-link appearance action potentials jump from one node of Ranvier to the next (saltatory conduction), which facilitates rapid impulse conduction of up to 120 m/s (typical rate = 10 m/s) myelin formation process begins during fetal period, continues to cortical maturity, and moves beyond into adulthood growth rate and time span for myelin information have implications for development of sensorimotor functions, learning, and speech-language-cognitive skills damaged myelin in CNS impairs nerve impulse conduction, a deficit found inMS PNS vs. CNS -In PNS, each Schwann cell is associated with only one axon -In CNS, each oligodendrocyte contributes to myelination of a group of adjacent axons **This difference has important implication for differential recovery following injuries in the CNS and PNS***

Nerve Impulse

nerve cells communicate with one another through impulses that represent ALL neuronal activity nerve impulses have a chemical component that underlies the electric potential of the cells excitability of nerve cells depends on ion channels in neuronal membrane action potential results from charged particles (ions) moving through the cell membranes nerve impulses activate the release of a neurotransmitter in a presynaptic neuron neurotransmitter often causes adjacent postsynaptic receptors to open select ion channels by selectively opening or closing ion channels, the released neurotransmitter controls the excitability as well as the inhibition of the interconnecting neuron (see more details on next slide)

(Neuronal Responses to Brain Injuries) Neuroglial Responses

neuroglial cells react to cellular injuries and brain tissue necrosis by multiplying in number (hyperplasia) and by increasing their size (hypertrophy) the infection-fighting neutrophils (white blood cells) arrive at the lesion site within a few days of injury this is followed by the migration of microglia and the proliferation of the astrocytes and other macrophages in the region of the dying cells breakdown of blood-brain barrier allows monocytes (phagocytic white blood cells) to invade brain tissues in case of small lesions, the astrocytes form a glial scar, called replacement gliosis in case of large lesions, the astrocytes outline the fluid-filled space and form a cystic cavity

Neurotransmitter

one of a variety of molecules within axon terminals released into the synaptic cleft in response to a nerve impulse, which affects the membrane potential of the postsynaptic neuron released at a synapse, and transmits signals across neurons also called a transmitter substance most have more than 1 receptor type more than 1 neurotransmitter may be secreted into a single synapse 2 TYPES OF NEUROTRANSMITTERS (in nervous system): 1.) Small molecule e.g. acetylcholine, dopamine, norepinephrine, serotonin, glutamate, and GABA (y-aminobutyric acid) - short lasting effects on postsynaptic nerve cell 2.) Large molecule (peptides) - long lasting effects on postsynaptic nerve cell

(Neuronal Responses to Brain Injuries) Axonal Regeneration in the CNS

prognosis for recovery of axotomized neurons in CNS is poor Axons severed in CNS undergo regrowth and sprouting similar to the PNS, but unknown factors prevent damaged neurons from reconnecting to the distal axonal segment one factor for the failure of damaged neurons to reconnect, like in the PNS, is the lack of the growth protein in the CNS; there are no Schwann cells or endoneural tissue to help guide growth

Synapse

site of communication between 2 neurons or between a neuron and a target cell 3 PARTS OF SYNAPSE 1.) presynaptic terminal - may be a clearly defined bouton, or occur merely where there is close opposition of the membrane of 2 cells - contain vesicles filled with neurotransmitters that mediate communication between cells 2.)synaptic cleft 3.)postsynaptic cell - membrane of this cell contains receptor proteins, the sites for binding the neurotransmitter molecules PROCESS OF NERVE IMPULSES (more in later flashcard) communication at the synapse occurs through a neurotransmitter released from the presynaptic terminals presynaptic cell is stimulated to release its neurotransmitter by an action potential that travels down the axon action potential initiates depolarization (cell interior becomes less negative) and causes the vesicles at the axon terminals to release stored neurotransmitters into the synaptic cleft area activation of the postsynaptic receptors can have various outcomes (e.g. depolarizing the postsynaptic membrane, hyperpolarizing the postsynaptic membrane, and activating messenger pathways) if depolarization of postsynaptic membrane is large enough to cause the membrane potential to reach threshold, the cell will fire an action potential that will move up the dedrite to the cell body - nerve impulse!

dopamine

small molecule neurotransmitter 2 most important projections are the mesostriatal (includes dopaminergic projectons from substantia nigra to striatum) and mesocortical degeneration of substantia nigra reduces projection and transmission of dopamine and is associate with Parkinson disease excessive dopamine activity in forebrain contributes to schizophernia SITE OF SECRETION brainstem and forebrain FUNCTIONS modulation of limbic and prefrontal functions regulation of basal ganglia motor functions participation in reward pathways

serotonin

small molecule neurotransmitter important neurotransmitter of CNS, though 95% of it is found in blood platelets in the gastrointestinal tract firing rate of serotonin fluctuates with sleep and wakefulness and thus might be involved in general activity level of CNS thought to be associated with overall level of arousal and slow-wave sleep low levels of serotonin are associated with depression, suicidal tendency, and mental illness Prozac is an anti-depressant that enhances serotonin concentration by reducing its uptake absorption SITE OF SECRETION brainstem and limbic system FUNCTIONS regulation of arousal, emotions, and pain perception

glutamate

small molecule neurotransmitter major excitatory neurotrasnmitter of CNS mediates fast, synaptic transmission SITE OF SECRETION CNS (diffuse presence) FUNCTIONS facilitation of fast synaptic transmissions in the CNS

GABA (y-aminobutyric acid)

small molecule neurotransmitter major inhibitory neurotrasnmitter of CNS derivative of glutamate mediates fast inhibitory transmission and slow loss of GABA-producing neurons is associated with Huntington-chorea SITE OF SECRETION CNS (diffuse presence) FUNCTIONS regulation of excitability of neurons regulation of pain perception inhibition of basal ganglia movements

norepinephrine

small molecule neurotransmitter one of the primary neurotransmitters of the PNS responsible for fight-or-flight reaction In the CNS, norepinephrine-containing neurons are in the pons and medulla noradrenergic (norepinephrine-containing) cells are thought to be involved in generating paradoxical sleep (deep sleep with brain wave patterns of a wakeful state) and maintaining attention and vigilances SITE OF SECRETION reticular formation and ANS FUNCTIONS regulation of sleep, attention, and mood in conjunction with reticular projections

chromatolysis:

structural changes that accompany a cell body disintegration in response to an injury; associated changes are marked by swelling, liquefaction, dissolution of cellular organelles (specifically Nissl bodies), and shifting of the nucleus from its central position to the periphery begins between the axon hillock and the cell nucleus depending on severity of injury, chromatolytic process may continue for 10-18 days; cellular RNA production, protein synthesis, and formation of the plasma membrane increase during this time to regenerate the severed axon and to prevent the cell body from dying if connection of severed axon is properly restored, the chromatolysis process ends, and the cell may return to its normal appearance with some function restoration can take months cells that are severely injured do not survive; they shrink and assume irregular shapes because of degenerated organelles; they gradually atrophy and leave as debri

organelles

structures that have specific functions; a membrane bound structure found within a cell body every cell in your body contains organelles organelles are found in the cytoplasm Just like organs in the body, each organelle contributes in its own way to helping the cell function well as a whole The nucleus, mitochondria and chloroplasts are all organelles

Neuroglial cells (glial cells)

support and protect nerve cells located in gray and white matter of brain 40-50 times as many glial cells as nerve cells small and don not participate in generation and transmission of nerve impulses glial cells of PNS are Schwann cells an satellite cells glial cells in CNS come in 4 TYPES: 1.) astrocytes 2.) oligodendrocytes 3.) ependymal 4.) microglia

necrosis

tissue death

Threshold for triggering action potential

varies from cell to cell typically 5-10 mV depolarized from resting membrane potential