Chapter 12: Fundamentals of the Nervous System and Nervous Tissue

what are the basic divisions of the nervous system?

humans have 1 nervous system but for convenience think of it as 2: 1) CNS: brain + spinal cord --- occupy cranium + vertebral canal 2) PNS: part of the nervous system out of CNS, consists of nerves that extend from the brain and spinal cord 3) ganglia: areas where the cell bodies of neurons are clustered

neuroglia

nonexcitable supporting cells that surround and wrap the neurons

what is the sturcture of the soma

nucleus is spherical and clear and contains a dark nucelolus near its center - in all but smallest neuron

neurons

excitable nerve cells that transmit electrical signals

what is a nerve?

- - cable-like organ located in the PNS - - gross anatomy of the neurons - almost all nerves contain both myelinated and unmyelinated sensory and motor fibers - within a nerve, each axon is surrounded by schwann cells - covering schwann cell: delicate layer of loose connective tissue called endoneurium - groups of axons are bound into bundles called nerve fascicles by a wrapping of connective tissue called the perineurium - the whole nerve is surrounded by a tough fibrous sheath -- the epineurium - the 3 layers of connective tissue in nerves corresponds exactly to those in skeletal muscle: endomysium, perimyisum and epimysium - connective tissue in a nerve also contains the blood vessle that nourishes the axons and schwann cells

what is the axon in a neuron?

- 1 axon - arises from a cone shaped regioon of the cell body called the axon hillock - axons are thin processes of uniform doameter throughout their length - by definition, axons are impulse generators and conductors that transmit nerve impulses away from their cell body , - note: the axon receives its proteins from the cell body bc chromatophilic apparatus, Golgi apparatus, ribosomes, and all organelles for protein synthesis are not present in axon - axonal transport: axonal cytoplasm is continuously being renewed so cytoskeletal elements also aid in the transport of subsutances to and from the cell body - axonal length: variable ---- nerve fiber: any long axon - axon diameter: variable --- thicker= faster impulse - axon branching- less frequent than in dendrites but its also possible --- axon collaterals ---usually branches at terminal end in undivided or collateral cells ---end in knobs called axon terminals --- a nerve impulse is typically generated where axon extends from the axon hillock and is conducted along the axon terminals where it causes the release of chemicals called neurotransmitters into the extracellular space

ependymal cells

- CNS retains a central cavity throughout life - ependymal cells form a simple epithelium that lines the central cavoty of the spinal cord and brain - provide a permeable layer bw cerebral spinal fluid that fills the cavity and tissue fluid that bathes cells of CNS - bear cilia that help circulate CSF

neuronal integration

- PNS and CNS are structurally and functionally linked - PNS is composed of axons of sensory (afferent) and motor (efferent) neurons (nerve fibers) bundled together as nerves ---- info pathways to and from the periphery of the body - afferent fibers respond to sesnory stimuli and carry that info to the CNS - efferent: fibers respond to sensory stimuli and carry that info the CNS - efferent: fibers transmit motor stimuli from CNS to muscles and glands, causing them to function (contract or secrete) - interneurons - info enters gray matter and is rerouted to other regions of the CNS - neurons of the PNS synpase with neurons in grey matter creating a network of ramps that link the roadways of PNS (the nerves) with superhighways of the CNS (white matter) - in gray matter, pathways converge and diverge as info is processed and routed to other regions of the CNS - white matter pathways in CNS formed by long myelinated axons that extend out from gray matter -

how is an impulse related to an axon?

- a nerve impulse is typically generated where an axon extends from the axon hillock and is conducted along the axon to the axon terminals where it causes the release of chemicals called neurotransmitters into the extracellular space -

how are the sensory and motor outputs divided according to the body regions they serve?

- a) somatic body region: consists of the structures external to the ventral body cavity (structures of the outer tube- skin, skeletal musculature, bones) - b) visceral body regions: mostly contains the viscera within the central body cavity- structures of the bodys inner tube, digestive tube, lungs, heart, bladder, etc

somatic sensory

- aka afferent - sensory innervation of outer tube- skin, body wall, and limbs - senses whose receptors are spread widely throughtout the outer tube of the body - these include the many senses experienced on the kin, aand in the body wall such as touch, pain, pressure, lubrication and temperature - proprioception: sense that detects amout of stretch in muscles, tendons, and joint capsules ---- informs you of the position and movement of your body in space - "special" somatic senses: somatic senses whose receptors are continued to small areas rather than spread widely throughout the body (special= localized) --- head, hearing, balance, equilibrium and vision

Interneurons

- aka association neurons - lie bw motor and sensory neurons - these multipolar neurons are confined entirely in the CNS - link together into chains that form complex neuronal pathways - the fact that the interneurons make up 99.98% of neurons of body reflects the vast amount of info processed in the human CNS - great diversity in size and branching patterns of their processes

what is the efferent signals

- aka motor signals - carried away from the CNS by nerve fibers of the PNS to innervate the muscles and glands, causing these organs to contract or secrete

ganglia

- areas where the cell bodies of neurons are clustered

how do forming neurons meet and form synapses with one another

- as the growing synapses elongate at growth cones, they are attracted by chemical signals such as neurotrophins released from other neurons and astrocytes - at the same time, receiving dendrites send out thin wiggling extensions to reach approaching axons and form synapses

what are the types of neuroglia in the CNS?

- astrocytes, microglia, ependymal cells, oligodendrocytes

what are the functional classifications of neurons: based on dendrites?

- based on direction the nerve impulse travels relative to the CNS 1) sensory neurons (aka afferent neurons) 2) motor neurons (efferent neurons) 3) Interneurons (association neurons)

is neuronal regeneration possible?

- bc of lack of effective replacement of dead or injured neurons after fetal period, neural injuries cause permanent dysfunction in children and adults - but if axons alone are destroyed the cell bodies often survive and the axons may regenerate (in the PNS only) - if a nerve is servered in the PNS, macrophages invade and destroy the axon distal to injury -- axon filaments can grow peripherally from the injured site at a rate of 1.5mm a day within regerneation tubes formed by surviving schwann cells that surrounded the original axon --- thus, reinnervation of target organ with partial recovery of function is sometimes possible in the PNS - in CNS however, neuroglia never form bonds to guide regrowing axons and even hinder them by secreting growth-inhibiting chemicals - therefore, there is no effective regeneration after injury to the spinal cord or brain

CNS

- brain + spinal cord --- occupy cranium + vertebral canal - integrating and command center of nervous system - receives incoming sensory signals, interprets these signals, and dictates motor responses based on past experiences, reflexes, and current conditions

what are ganglia?

- cell bodies that lie along nerve fibers located in the PNS - most cell bodies are located in the CNS but ganglia are the exception

in which areas of the brain does the arrangement of the white and gray matter change? how does it change?

- cerebrum and cerebellum - there is an additional layer of grey matter located superficially- the cortex

what is the treatment for MS?

- directed toward relieving symptoms - anti-inflammatory steriod drugs lessen severity and duration of relapses but they don't reduce frequency of attacks - other drugs such as interferons and glatramer acetate act to control the immune system to decrease frequency of relapses as well as the appearance of new lesions - Mitoxantione a cancer drug that suppresses lymphocyte activity, has been successful in treating more advanced stages of the disease

where does the neuroepithelium of the nervous tissue develop from?

- dorsal ectoderm which invaginates to form the neural tube and neural crest - the neural tube walls begin as a layer of pseudostratified neuroepithelial cells and becomes the CNS --- these cells divide, migrate externally and become neuroblasts (future neurons) which never again divide and neuroglial cells - earliest of the glial cells extend out from neuroepithelium and provide pathways along young neurons to migrate to reach their final destinations - happen in month 2

motor neurons

- efferent neurons - motor division of the PNS - carry impulses away from the CNS to effector organs (muscle and glands) - motor neurons are multipolar and their cell bodies are located in the CNS (except for some neurons of the ANS) - motor neurons form junctions with effector cells, stimulating muscles to contract or glands to secrete

oligodendrocytes

- fewer branches than astrocytes- name means few branch cells - line up in small groups and wrap their cell processes around thicker axon in the CNS, producing insulating coverings called myelin sheaths

myelin sheaths in the PNS

- formed by schwann cells - develop during the fetal period and 1st year of postnatal life 1) schwann cells indent to receive the axon and wrap themselves around the axon repeated in a jelly roll fashion --- note: its plasma membrane is loosely wrapped around its successive layers - 2) cytoplasm of schwann cell is gradually squeezed out from bw the membrane layers 3) when wrapping process is finished, manu concnetric layers of schwann cell plasma membrane ensheathe the axon in a tightly packed coil of membrane that is true myelin sheath 4) nucleus and most of the cytoplasm of the schwann cells end up just external to the myelin layers - external material called "neuroilemma" - bc adjacent schwan cells dont touch each other, there are gaps in the myeline sheaths called: nodes of ranvier - only thick axons are sheathed with myelin-- rapidly conducting - thin axons lack a myelin sheath-- unmyelinted sheath - slow conducting

Nodes of Ranvier

- gaps in bw the myelin sheaths of the PNS bc Schwann cells dont touch each other - occur at regular intervals - in myelinated axons, nerve impulses do not travel along myelin-covered regions of axonal membrane but instead jump from membrane of one node of ranvier to next in a way that speeds impulse conduction -

visceral sensory

- general visceral senses include stretch, pain, and temperature which can be felt widely in the digestive and sensory tracts, reproductive organs and other viscera - sensations such as hunger, and nausea are also great visceral sensations - taste and smell are considered special visceral sensations ---- these senses, referred to as the chemical senses have their sensory receptors localized to the tongue and nasal cavity

gray matter of the CNS

- gray colored zone that surrounds hollow central cavity of the CNS - in spinal cord: butterfly- shaped region in which dorsal 1/2 contains cell bodies of motor neurons - gray matter is the site where neuron cell bodies are clustered contains neuron cell bodies, dendrites, short unmyelinated neurons, and neuroglia

what is the structural link bw the PNS and the CNS?

- gray matter of CNS -

neuroglia in the CNS: structure and neurons vs. neuroglia in CNS

- have branching processes + cell body - but can be distinguished from neurons by: --- small size ---darker skin ---unlike neurons, neuroglia can divide - glial cells outnumber neurons by 10:1 , make 1/2 mass of brain

what ways is info processed in the nervous system?

- in 2 ways: 1) serial processing in a series-- neurons linked this way pass their signal along a single pathway from one neuron to the next like links in a chain - ex: reflex arc and a long chain if interneurons carrying a sensory signal to the brain is another 2) parallel- neurons joined in parallel - info from a single neuron is sent along 2 or more parallel pathways--> parallel processing - parallel processing occurs when a single sensory stimulus results in multiple perceptions - happens instantaneously-- allows brain to evaluate stimuli with incredible speed and enables info to be integrated along numerous pathways --- ex: as you watch a friend towards you, multiple paths process visural stimuli from your retina in parallel evaluating the color, shape, spatial location and movement

what is the role of the plasma membrane of a cell body of most neurons?

- in most neurons, plasma membrane of cell body acts as a receptive surface that receives signals from other neurons

can neurons be replaced?

- in the postnatal period, there is no obvious replacement of dead neurons by any neural stem cells --- for this reason stem cells were thought to not exist ---- BUT new research shows that the "no-new-neurons" doctrine was recently overturned with the discovery of neural stem cells in adults ---- some dividing cells in the subependymal zone have been shown to form new neurons in 2 regions of adult brain: hippocampus and olfactory bulb --- perhaps such stem cells can one day be induced to form new neurons and new neural networks in people with brain injuries or degenerative brain diseases

what is the role of a cell body during embryonic development?

- it is the focal point of outgrowth of a neuron and process during embryonic development

how to diagnose MS?

- lumbar puncture to take some CSF from space around spinal cord and identify antibodies in CSF that are MS-specific-- but invasive procedure - less invasive: tests for delay in time it takes visual signals to travel from eyes through the brain as measured by monitoring brain waves detected on outside of the skull - MRI can reveal the disease characteristic lesions

convergence circuit

- many neurons synapse on a single postsynaptic neuron - ex: when a single motor neuron receives excitatroy and inhibitory impulses from many other neurons - these impulses are integrated by the target motor neuron and influence whether or not it will initiate a nerve impulse - located throughout CNS as neurons integrate info

polysynaptic reflexes

- more common - 1 or more interneurons are part of relflex oathway bw sensory and motor neurons - presence of even one interneuron means that there have to be at least 2 synapses un this type of arc - polysynaptic - ost simple reflex arcs have 1 interneuron and a total of 3 neurons - withdrawal reflexes by which we pull away from danger are 3-neuron polysynaptic reflxes - ex: pricking a finger with a tack --- initiates an impulse in the sensory neuron which activates the interneuron in the CNS ----> the interneuron then signals the motor neuron to contract the muscle that withdrws the hand

multipolar neurons

- more than 2 processes - numerous dendrites and a single axon - but some small multipolar neurons have no axon and rely strictly on their dendrites for conducting signals - more than 99% of the neurons in the body are multipolar

when do these events of nervous tissue development happen?

- most events take part in month 2 but neurons continue to form rapidly until month 6

where are most neuronal cell bodies located?

- most neuron cell bodies are located within the CNS

what are possible disorders of the nervous tissue?

- multiple sclerosis (MS) - neuronal regeneration

what is the nervous tissue?

- nervous tissue is made up of 2 main types of cells: 1) neurons 2) neuroglia

what part of the nervous tissue develops external to the neuroepithelium?

- neuroblasts cluster into an alar plate and a basal plate -- the future gray mater - dorsally, the neuroblasts of the alar plate become interneurons which remain in the CNS - ventrally, the neuroblasts of basal plate become motor neurons and sprout axons that grow out to the effector organs --- some interneurons also form from the basal plate and remain ventrally among the motor neurons - axons that sprout from the young unterneurons form the white matter by growing out along the length of the CNS --- these events occur in the spinal cord and brain - happen in month 2

what happens after month 6 of nervous tissue development?

- neuron formation slows rapidly although it may continue at a slow rate until childhood - just before neuron formation slows, the neuroepithelium differentiates into astrocytes and oligodendrocytes - as the division of its cells slows, the neuroepithilium differentiates into the layers of ependymal cells

neuron vs. nerve fiber vs. nerve

- neuron= nerve cell - nerve fiber = a long axon - nerve= collection of nerve fibers in the PNS

what is a reflex arc?

- simplest chain of neurons that cause our simplest reflexive behvaiors and reflect the basic structural plan of nervous system - reflex accouonts for reflexes which are defined as rapid, automatic motor responses to stimuli ---- involuntary stimuli - reflexes are either somatic reflexes or visceral reflexes

which neurons are terminated?

- neurons that make "bad" connections are signaled to die via apoptosis (programmed cell death) --- of the many neurons formed during embryonic period, about 1/2 are before birth - initial overproduction of neurons ensures that all necessary neural connections will be made and that mistaken connection will be eliminated - fully differentiated neurons do not divide

what is the basic structural unit of the nervous system?

- neurons- highly specialized cells that conduct electrical signals from one part of the body to another

do unipolar neurons have dendrites? why?

- no bc both the peripheral and central process have features similar to an axon - although the peripheral process travels towards the cell body, it is still considered an axon bc its fine structure is identical to that of a true axon

myelin sheaths in the CNS

- oligodendrocytes form myelin sheaths in brain and spinal cord - in contrast to schwann cells, each oligodendrocyteshas multiple processses that coil around several different axons - nodes of Ranvier are present but more widely spread than those in the PNS - as in the PNS, the thinnest axon in the CNS are myelinated - unmyelinated axons are covered by many long processes of glial cells such as astrocytes that are so abundant in the CNS

reverberating circuit

- one neuron in the circuit receives feedback from another neuron in same circuit - a branch off axon of one neuron circles back and synapses with a previous neuron in a circuit - signal continues to be sent until fatigue or inhibition by another signal interrupts the circuit - ex: involved in the control of many rhythmic activities such as breathing and swinging arms when walking

diverging circuit

- one presynaptic neuron branches with several other neurons - ex: stretch of a muscle stimulates sensory neurons - each sensory neuron synapses directly with 100-150 neurons in the gray matter of the spinal cord - some are motor neurons that directly innervate the stretched muscle and stimulate contraction - others are interneurons that act to inhibit activity of the antagonistc muscle group and still others are interneurons that project sensory info to higher regions of the CNS - as a result of divergence, info is distributed through multiple neuronal pathways

PNS

- part of nervous system out of CNS, consists of nerves that extend from brain and spinal cord - cranial nerves- carry signals to and from the brain - spinal nerves- carry signals to and from the spinal cord - these peripheral nerves serve as communication lines that link all regions of the body to the CNS - consists of afferent (sensory) and efferent nerves (motor)

white matter

- part of the CNS - external to gray matter - no neuron cell bodies but many axons - white color comes from myelin sheaths around axons - most ascend from spinal cord to brain or descent from brain to spinal cord - allowing 2 regions of the CNS to communicate with each other - thus, white matter consists of axons running bw different parts of the CNS - with the white matter, axons traveling to similar destinations from axon bundles called tracts

what kind of neuron do most neurons in the CNS functions as?

- presynaptic (info sending) and postsynaptic ( receving info)

what are dendrites in a neuron?

- processes that branch from the cell body - all organelles that occur in the cell body also occur in dendrites and chromatophillic bodies (rough ER) extend into basal part of each dendrite - dendrites function as receptive sites providing an enlarged surface area for receiving signals from other neurons - by definition dendrites conduct electrical signals toward the cell body

myelin sheaths

- produced by oligodendrocytes in the CNS and schwann cells in the PNS - segmented structures composed of lipoprotein myelin and surround the thicker axons of the body - each segment of myelin consists of the plasma membrane of a supporting cell rolled in concentric layers around the axon - myelin sheath form an insulating layer that: 1) prevents leakage of electrical current from axon 2) increases the speed of impulse conduction along the axon 3) makes impulse propagation more energy-efficient

bipolar neurons

- rare - 2 processes that extend from opposite sides of the cell body - occur in special sensory orgrans (inner ear, olfactory epithelium of nose, retina of the eye) - where they mostly serve as sensory neurons

visceral motor

- regulates the contraction of smooth and cardiac muscle and secretion by the body's many glands - general visceral motor neurons make up the ANS which control the function of the visceral organs - bc we have no voluntary control over such activities as pumping of the heart and movement of food through digestive tract, the ANS is also called involuntary nervous system - ANS divides into: sympathetic and parasympathetic division --- a) sympathetic division: gets the body ready for activity (fight or flight) ---b) parasympathetic division: conserves energy and promotes digestion

proprioception

- sense that detects amount of stretch in muscles, tendons, and joint capsules - informs you of the position and movement of your body in space - a part of your somatic sensory

what is the afferent signal

- sensory signal - picked up by sensory receptors located throughout the body and carried by nerve fibers of the PNS into the CNS

unipolar neurons

- short, single process that emerges from the cell body and divides like am innervated T into 2 long branches - most unipolar neurons start out as bipolar neorons whse 2 processes fuse together near the cell body during development---> more properly called "pseudounipolar neurons" - location: sensory ganglia in PNS where the function as sensory neurons - the short, single process near the neuron cell body divides into 2 longer branches: 1) runs centrally into the CNS: central process 2) branch extends peripherally to the receptors: peripheral pricess - central process is clearly an axon bc it meets the 2 criteria of an axon: 1) carries nerve impulse 2) carries a nerve impulse away from the cell body - peripheral process is ambiguously an axon bc: 1) carried nerve impulse 2) but travels toward the cell body, a fundamental feature of dendrites - but still considered an axon bc its fine structure is identical to that of a true axon - unipolar neurons have no dendrites-- receptive endings at terminal end of the peripheral axon receives sensory stimuli and transmit signals through the peripheral axon toward the cell body

monosynaptic reflex

- simplest of all reflex arcs - no interneuron bw the sensory neuron and the motor neurons - only one synapse present - ex: knee-jerk reflex- impact of a hammer stretches the quadriceps, this stretch initiates an impulse in a sensory neuron that directly activates a motor neuron in the spinal cord, which then signals the quadriceps muscle to contract - this contraction counteracts the original stretching caused by a hammer - many muscles participate- helps maintain equilibrium and upright posture - bc they contain just one synapse, stretch reflexes are fastest of all reflexes- speed is essential to keep one from falling

what is a synapse?

- site at which neurons communicate - most synapses in nervous system transmit info through chemical messengers - but some neuons in certain areas of the CNS transmit signals electrically through gap junctions - bc signals pass across most synpases in one direction only synpases determine the direction of info through the nervous system

microglia

- smallest and least abundant neuroglia of the CNS - elongated cell bodies and cell processes with many pointed projections (like thorns) - they are phagocytes, the macrophages of the CNS - they migrate to and engulf invading microorganisms and injured or dead neurons - derived from blood cells called monocytes--- unlike typical neuroglia and like other macrophages ---- monocytes that become microglia migrate to CNS during embryonic and fetal periods

astrocytes

- star-shaped - most abundant glial cells of CNS - bulbous ends in radiating processes - bulbs bind to neurons of capillaries - functions are not fully understood - known functions: 1) regulating neurotransmitter levels by increasing the rate of neurotransmitter uptake in regions of high neuronal activity 2) signaling increased blood flow through capillaries in active regions of the brain 3) controlling the ionic environment around neurons - active role in neuronal activity - can help: synapses form in developing neuronal tissue, produce molecules necessary for neuronal growth (brain-derived trophic factor-BOTF) and propagate Ca2+ signals that may be involved with memory

axodendritic synapse

- synapses that occur bw the axon terminal of one neuron and dendrites of another neuron - on presynaptic side, axon terminals contains sympatic vesicles --- membrane-bound sacs filled with neurotransmitters, the molecules that transmit signals across the synapse - mitochondria are abundant in axon terminal bc secretion of neurotransmitter requires a great deal of energy - at the synapse, plasma membranes of the 2 neurons are separated by a synaptic cleft

why do the sensory neurons lie outside of the CNS?

- the sensory neurons arise not from the neural tube but from the neural crest - like motor and interneurons, the sensory neurons stop growing at fetal period

neuroglia function

- they are the non-nervous supporting cells of the Nervous tissue - covering in all non-synaptic parts of a neuron- insulating them and keeping the electrical activities of adjacent neurons from interfering with each other

how are signals transmitted?

- they are transmited along the plasma membrane in the form of nerve impulses or action potentials - in addition to their ability to conduct electrical signals, neurons have other specialized characteristics: 1) extreme longevity 2) do not

where does the nervous tissue develop from? are there any exceptions? what is the exception?

- they develop from the same embryonic tissue: neural tube and neural crest - the exception is microglia that develop from blood cells called monocytes like other macrophages

unmyelinated sheath in PNS

- thin, slow conducting - lack a myelin sheath - schwann cells surround such axons but dont wrap around them in concentric layers of membrane - in unmyelinated sheath, 1 schwann cell partly encloses more than or equal to 15 unmyelinated axon, each of which occuppies a separate tubular recess in the surface of schwann cells - unmyelinated axons found in portions of ANS and some sensory fibers

what does the cytoplasm of a cell body contain?

- usual organelles - distinctive chromatographic (Nissl) bodies ---- large clusters of rough ER and free ribosomes that stain dark with basic dyes --- continuously renew membranes of the cell and protein part of the cytosol - neurofibrils-- bindles of intermediate filaments (neurofilaments) that run in a network bw the chromotophillic bodies ----- keeps the cell from being pulled apart when it is subjected to tensile strength -

what do neurons have in common?

- vary in structure but all have a cell body from which 1 or more processes project

what is the arrangement of white and gray matter throughout the CNS?

- white matter is external to gray matter which surrounds the hollow central cavity

how is it determined which synapses are made and which persist?

1) amount of neurotrophin initially received by an axon 2) degree to which a synapse is used after being established -

what are the 2 main types of synapses?

1) axodendritic synpase 2) axosomatic synapse

what are the types of neuronal circuits

1) diverging circuits 2) converging circuit 3) reverberating circuit

what are the types of reflex arcs?

1) monosynaptic reflex 2) polysynaptic reflex

what are the classifications of neurons by structure?

1) multipolar neurons 2) Bipolar neurons 3) unipolar neurons

what is the function of the interneurons?

1) process received info 2) direct this info to specific regions of CNS 3) initiate appropriate motor response 4) transport info (sensory and motor) from one region of the CNS to another - 1st 3 accomplished by interneurons in gray matter - 4th one accomplished by axons in the white matter

what are the 5 essential components if a reflex arc?

1) receptor= site where stimulus acts-- receptors are located at terminal end if peripheral process of a sensory neuron 2) sensory neuron transmits afferent impulses to the CNS 3) integration center consists of one or more synapses in the gray matter in the CNS - simplest reflex arcs: integration center is a single synapse bw a sensory neuron and a motor neuron - in more complex reflexes, it can involve multiple synapses that send signals through long chains of interneurons to other portions of the CNS, for ex---> to portions of the brain 4) motor neuron conducts efferent impulses from integration center to an effector 5) effector is muscle or gland that responds to the efferent impulses by contracting or secreting

what are the neuroglia in the PNS?

1) schwann cells 2) satellite cells

what are the 4 main subdivisions of the PNS?

1) somatic sensory (afferent) 2) visceral sensory 3) somatic motor 4) visceral motor

what are the 2 ways neurons are classified?

1) structure 2) function

what are specialized characteristics of neurons?

1) their ability to conduct electrical signals 2) extreme longevity 3) do not divide - as fetal neurons assume their role as communication links in the nervous system they lose their ability to undergo mitosis - high price: neurons cant replace themselves if destroyed - exceptions: neural stem cells un CNS 4) high metabollic rate - require a continuous and abundant amount of glucose and oxygen - neurons can suvive for more than a few minutes wo O2

what are the 3 functions of the nervous tissue?

1) uses millions of sensory receptors to changes occurring in/out of the body - each change= stimuli - gathered info= sensory input 2) processes and interprets the sensory input and makes decisions about what should be done at each moment- integration - brain- 90% of the nervous body= integration nerve 3) dictates a response by activating effector organs (our muscles or glands) - response is called a motor output

how does a synapse function?

1) when an impulse travels along the axon of the presynaptic neuron, impulse stimulates the synaptic vesicles to fuse with the presynaptic membrane 2) fused area then ruptures causing the vesicles to release their neurotransmitter molecules which diffuse across the synaptic cleft and bind to post-synaptic membrane 3) binding changes the membrane charge on post-synaptic neuron, influencing membranes ability to generate a nerve impulse

what are the pathways that a stimulus elicits?

1) withdrawal reflex--> painful stimulus triggers nerve impulses in a sensory neuron which initiates the polysynaptic withdrawal reflex 2) parallel processing --> simultaneously, the nerve impulses travel on an axon branch that extends into the white matter -- this ascending fiber carried the nerve impulses to the brain 3) integration into the grey matter--> multiple interneurons process teh nerve impulses to localize the stimulus, identify its source amd plan a response 4) voluntary motor response - a nonreflexive motor response is initiated in the gray matter and transmitted down a descending fiber in the white matter to stimulate somatic motor neurons ex: if you burn your finger, immediate response is withdrawal reflex of moving your hand away - however, sensory info is passed through a long chain of interneurons to the area of the brain that interupts sensory signals and you feel the sensation of pain - you dont generally feel the pain until after you have refelxively moved your hand away bc CNS needs time to process the info - in response to the pain, the motor area of your brain may initiate a non reflexive motor response (ex: running finger under cold water)

gray and white matter of the CNS

The CNS has two kinds of tissue: grey matter and white matter, Grey matter, which has a pinkish-grey color in the living brain, contains the cell bodies, dendrites and axon terminals of neurons, so it is where all synapses are. White matter is made of axons connecting different parts of grey matter to each other.

sensory neurons

aka afferent neurons - sensory division of the PNS - they transmit impulses towards the CNS from sensory receptors in the PNS - virtually all sensory neurons are pseudo-unipolar and cell bodies are in ganglia out of CNS - the peripheral process extends from a sensory receptor; central process terminates in the CNS

what are neuron processes? what types are there?

armlike processes that extend from the cell bodies of all neurons - 2 types: dendrites and axons

multiple sclerosis

progressive disease that destroys patches of myelin in the brain and spinal cord disrupting neuronal signals in the CNS and leading to sensory disorders and weakened musculature - characterized by periods of relapse and remission --- remission is possible bc of unexplained halting of immune response which gives axons time to repair themselves and recover to their functions --- but after 10-15 years of periodic relapses, accumulated damage to the axon is too great and 50% of MS sufferers decline rapidly in their 2nd decade of disease - common signs and symptoms: numbness or pain on the skin of some part of the body, diturbances of vision, muscle weakness, or paralysis, difficulty in maintaining balance, slurred speech, bladder incontience, fatigue and depression - cause: --- autoimmune disease in which the immune system attacks the myelin around axons in the CNS, thereby interfering with the conduction of signals along the axons --- immune system cells called lymphocytes break down the myelin, and macrophages conume the rest through phagocytosis --- damage is also accompanied by inflammation which can destroy the axons themselves

what is the simplest example of neuronal integration?

reflex arc

what is another name for the cell body

soma

somatic motor

stimulates contractions of skeletal muscles in the body - bc we have voluntary control over the contraction of our skeletal muscles, the somatic motor system is often called the voluntary nervous system

schwann cells

surround all axons in the PNS and form myelin sheaths around many of the axons

satellite cells

surround neuron cell bodies within the ganglia

axosomatic synapse

synapses that occur bw axons and neuron cell bodies

presynaptic neuron

the neuron that conducts signals toward a synapse

postsynaptic neuron

the neuron that transmits signals away from the synpase

what is the somatic motor also called?

voluntary nervous system