0-1 chapter 12 Nervous Tissue Flashcards
Overview of Nervous System
endocrine and nervous system maintain internal coordination
endocrine system
communicates by means of chemical messengers (hormones) secreted into to the blood
nervous system
employs electrical and chemical means to send messages from cell to cell
nervous system carries out its task in three basic steps
-receive information and transmits coded messages
• processes this information
• issue commands
-receive information and transmits coded messages
•sense organs receive information about changes in the body and the external environment, and transmits coded messages to the spinal cord and the brain
• processes this information
•brain and spinal cord processes this information, relates it to past experiences, and determine what response is appropriate to the circumstances
• issue commands
brain and spinal cord issue commands to muscles and gland cells to carry out such a response
Two Major Anatomical Subdivisions of Nervous System
central nervous system (CNS)
peripheral nervous system (PNS)
central nervous system (CNS)
central nervous system (CNS)
–brain and spinal cord enclosed in bony coverings
•enclosed by cranium and vertebral column
peripheral nervous system (PNS)
–all the nervous system except the brain and spinal cord
–composed of nerves and ganglia
nerve
a bundle of nerve fibers (axons) wrapped in fibrous connective tissue
ganglion
a knot-like swelling in a nerve where neuron cell bodies are concentrated
Sensory Divisions of PNS
sensory (afferent) division
motor (efferent) division
sensory (afferent) division
carries sensory signals from various receptors to the CNS
–informs the CNS of stimuli within or around the body
–somatic sensory division –
–visceral sensory division
somatic sensory division
carries signals from receptors in the skin, muscles, bones, and joints
visceral sensory division
carries signals from the viscera of the thoracic and abdominal cavities
•heart, lungs, stomach, and urinary bladder
motor (efferent) division
carries signals from the CNS to gland and muscle cells that carry out the body‟s response
- somatic motor division
- visceral motor division (autonomic nervous system)
somatic motor division
carries signals to skeletal muscles
•output produces muscular contraction as well as somatic reflexes –involuntary muscle contractions
visceral motor division (autonomic nervous system)
carries signals to glands, cardiac muscle, and smooth muscle
- sympathetic division
- parasympathetic division
sympathetic division
–tends to arouse body for action
–accelerating heart beat and respiration, while inhibiting digestive and urinary systems
parasympathetic division
–tends to have calming effect
–slows heart rate and breathing
–stimulates digestive and urinary systems
effectors
cells and organs that respond to commands from the CNS
Universal Properties of Neurons
excitability(irritability)
conductivity
secretion
secretion
when electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell
Functional Types of Neurons
3
sensory (afferent) neurons
interneurons(association) neurons
motor (efferent)neuron
sensory (afferent) neurons
–specialized to detect stimuli
–transmit information about them to the CNS
•begin in almost every organ in the body and end in CNS
•afferent–conducting signals toward CNS
afferent
conducting signals toward CNS
interneurons(association) neurons
–lie entirely within the CNS
–receive signals from many neurons and carry out the integrative function
•process, store, and retrieve information and „make decisions‟ that determine how the body will respond to stimuli
–90% of all neurons are interneurons
–lie between, and interconnect the incoming sensory pathways, and the outgoing motor pathways of the CNS
motor (efferent)neuron
–send signals out to muscles and gland cells (the effectors)
•motor because most of them lead to muscles
•efferent neurons conduct signals away from the CNS
efferent
neurons conduct signals away from the CNS
motor
because most of them lead to muscles
soma
the control center of the neuron
–also called neurosoma, cell body, or perikaryon
–has a single, centrally located nucleus with large nucleolus
cytoplasm
cytoplasm contains mitochondria, lysosomes, a Golgi complex, numerous inclusions, and extensive rough endoplasmic reticulum and cytoskeleton
cytoskeleton
consists of dense mesh of microtubules and neurofibrils (bundles of actin filaments)
•compartmentalizes rough ER into dark staining Nissl bodies
no centrioles
no further cell division
Inclusions
glycogen granules, lipid droplets, melanin, and lipofuscin
lipofuscin
lipofuscin(golden brown pigment produced when lysosomes digest worn-out organelles)
•lipofuscin accumulates with age
•wear-and-tear granules
•most abundant in old neurons
dendrites
vast number of branches coming from a few thick branches from the soma
–primary site for receiving signals from other neurons
axon
axon(nerve fiber) –originates from a mound on one side of the soma called the axon hillock
–cylindrical, relatively unbranched for most of its length
axoplasm
cytoplasm of axon
axolemma
plasma membrane of axon
–only one axon per neuron
synaptic knob
(terminal button) –little swelling that forms a junction (synapse) with the next cell
•contains synaptic vesicles full of neurotransmitter
axon collaterals
branches of axon
terminal arborization
distal end, axon has terminal arborization –extensive complex of fine branches
multipolar neuron
–one axon and multiple dendrites
–most common
–most neurons in the brain and spinal cord
bipolar neuron
–one axon and one dendrite
–olfactory cells, retina, inner ear
unipolar neuron
–single process leading away from the soma
–sensory from skin and organs to spinal cord
anaxonic neuron
–many dendrites but no axon
–help in visual processes
axonal transport
two-way passage of proteins, organelles, and other material along an axon
anterograde transport
movement down the axon away from soma
retrograde transport
movement up the axon toward the soma
microtubules
microtubules guide materials along axon
–motor proteins (kinesin and dynein) carry materials “on their backs” while they “crawl” along microtubules
kinesin
motor proteins in anterograde transport
dynein
motor proteins in retrograde transport
fast axonal transport
occurs at a rate of 20 –400 mm/day
fast anterograde transport
(up to 400 mm/day)
•organelles, enzymes, synaptic vesicles and small molecules
fast retrograde transport
•for recycled materials and pathogens -rabies, herpes simplex, tetanus, polio viruses
–delay between infection and symptoms is time needed for transport up the axon
slow axonal transport or axoplasmic flow
-0.5 to 10 mm/day
–always anterograde
–moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons
–damaged nerve fibers regenerate at a speed governed by slow axonal transport
Neuroglial Cells
- about a trillion (10-12) neuronsin the nervous system
* neurogliaoutnumber the neurons by as much as 50 to 1
neuroglia or glial cells
–support and protect the neurons
–bind neurons together and form framework for nervous tissue
six Types of Neuroglial Cells
•four types occur only in CNS
oligodendrocytes
ependymal cells
microglia
astrocytes
oligodendrocytes
- form myelin sheaths in CNS
* each arm-like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction
ependymal cells
•lines internal cavities of the brain
•cuboidal epithelium with cilia on apical surface
•secretes and circulates cerebrospinal fluid (CSF)
–clear liquid that bathes the CNS
microglia
- small, wandering macrophages formed white blood cell called monocytes
- thought to perform a complete checkup on the brain tissue several times a day
- wander in search of cellular debris to phagocytize
astrocytes
- most abundant glial cell in CNS
* cover entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS
astrocytes
diverse functions
–form a supportive framework of nervous tissue
–have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood-brain barrier
–convert blood glucose to lactate and supply this to the neurons for nourishment
nerve growth factors
secreted by astrocytes promote neuron growth and synapse formation
astrocytosis or sclerosis
when neuron is damaged, astrocytes form hardened scar tissue and fill space formerly occupied by the neuron
SCAR TISSUE
Six Types of Neuroglial Cells
•two types occur only in PNS
Schwann cells
satellite cells
Schwann cells
- envelop nerve fibers in PNS
- wind repeatedly around a nerve fiber
- produces a myelin sheath similar to the ones produced by oligodendrocytes in CNS
- assist in the regeneration of damaged fibers
Neurilemmocytes
Schwann cells
satellite cells
- surround the neurosomas in ganglia of the PNS
- provide electrical insulation around the soma
- regulate the chemical environment of the neurons
tumors
masses of rapidly dividing cells
–mature neurons have little or no capacity for mitosis and seldom form tumors
brain tumors arise from:
–meninges (protective membranes of CNS)
–by metastasis from non-neuronal tumors in other organs
–most come from glial cells that are mitotically active throughout life
gliomas
gliomas grow rapidly and are highly malignant
–blood-brain barrier decreases effectiveness of chemotherapy
–treatment consists of radiation or surgery
myelin sheath
an insulating layer around a nerve fiber
–formed by oligodendrocytes in CNS and Schwann cells in PNS
–consists of the plasma membrane of glial cells
•20% protein and 80 % lipid
myelination
production of the myelin sheath
–begins the 14thweek of fetal development
–proceeds rapidly during infancy
–completed in late adolescence
–dietary fat is important to nervous system development
Schwann cell
in PNS, Schwann cell spirals repeatedly around a single nerve fiber
–lays down as many as a hundred layers of its own membrane
–no cytoplasm between the membranes
neurilemma
thick outermost coil of myelin sheath
•contains nucleus and most of its cytoplasm
endoneurium
external to neurilemma is basal lamina and a thin layer of fibrous connective tissue
oligodendrocytes
in CNS –oligodendrocytes reaches out to myelinate several nerve fibers in its immediate vicinity
–anchored to multiple nerve fibers
–cannot migrate around any one of them like Schwann cells
–must push newer layers of myelin under the older ones
•so myelination spirals inward toward nerve fiber
–nerve fibers in CNS have no neurilemma or endoneurium
myelin sheath
segmented
nodes of Ranvier
internodes
nodes of Ranvier
gap between segments
internodes
myelin covered segments from one gap to the next
initial segment
short section of nerve fiber between the axon hillock and the first glial cell
trigger zone
the axon hillock and the initial segment
•play an important role in initiating a nerve signal
Diseases of Myelin Sheath
multiple sclerosis
Tay-Sachs disease
multiple sclerosis
- oligodendrocytes and myelin sheaths in the CNS deteriorate
- myelin replaced by hardened scar tissue
- nerve conduction disrupted (double vision, tremors, numbness, speech defects)
- onset between 20 and 40 and fatal from 25 to 30 years after diagnosis
- cause may be autoimmune triggered by virus
Tay-Sachs disease
a hereditary disorder of infants of Eastern European Jewish ancestry
•abnormal accumulation of glycolipid called GM2in the myelin sheath
–blindness, loss of coordination, and dementia
•fatal before age 4
mesaxon
neurilemma wrapping of unmyelinated nerve fibers
speed at which a nerve signal travels along a nerve fiber depends on two factors
–diameter of fiber
–presence or absence of myelin
conduction speed
–small, unmyelinated fibers -0.5 -2.0 m/sec
–small, myelinated fibers -3 -15.0 m/sec
–large, myelinated fibers -up to 120 m/sec
–slow signals supply the stomach and dilate pupil where speed is less of an issue
–fast signals supply skeletal muscles and transport sensorysignals for vision and balance
Regeneration of Peripheral Nerves
regeneration of a damaged peripheral nerve fiber can occur if:
–its soma is intact
–at least some neurilemma remains
regeneration tube
formed by Schwann cells, basal lamina, and the neurilemma near the injury
–regeneration tube guides the growing sprout back to the original target cells and reestablishes synaptic contact
denervation atrophy
of muscle due to loss of nerve contact by damaged nerve
electrophysiology
cellular mechanisms for producing electrical potentials and currents
–basis for neural communication and muscle contraction
electrical potential
a difference in the concentration of charged particles between one point and another
electrical current
a flow of charged particles from one point to another
Resting Membrane Potential
RMP exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF)
RMP results from the combined effect of three factors
–ions diffuse down their concentration gradient through the membrane
–plasma membrane is selectively permeable and allows some ions to pass easier than others
–electrical attraction of cations and anions to each other
potassium ions (K+)
have the greatest influence on RMP
–plasma membrane is more permeable to K+ than any other ion
–leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached and net diffusion of K+ stops
–K+ is about 40 times as concentrated in the ICF as in the ECF
cytoplasmic anions
can not escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA)
Na+/K+ pumps out
3 Na+ for every 2 K+ it brings in
cellIonic Basis of Resting Membrane Potential
- Na+ concentrated outside of cell (ECF)
* K+ concentrated inside cell (ICF)
local potentials
disturbances in membrane potential when a neuron is stimulated
depolarization
case in which membrane voltage shifts to a less negative value
differences of local potentials from action potentials
are graded
decremental
reversible
either excitatory or inhibitory
graded
vary in magnitude with stimulus strength
•stronger stimuli open more Na+gates
decremental
get weaker the farther they spread from the point of stimulation
•voltage shift caused by Na+ inflow diminishes rapidly with distance
reversible
when stimulation ceases, K+ diffusion out of cell returns the cell to its normal resting potential
either excitatory or inhibitory
some neurotransmitters (glycine) make the membrane potential more negative –hyperpolarize it –becomes less sensitive and less likely to produce an action potential
action potential
more dramatic change produced by voltage-regulated ion gates in the plasma membrane
–only occur where there is a high enough density of voltage-regulated gates
soma
soma (50 -75 gates per m2 ) -cannot generate an action potential
trigger zone
(350 –500 gates per m2 ) –where action potential is generated
threshold
critical voltage to which local potentials must rise to open the voltage-regulated gates
•-55mV
action potential
process
look at slides
spike
action potential is often called a spike–happens so fast
characteristics of action potential versus a local potential
–follows an all-or-none law
•if threshold is reached, neuron fires at its maximum voltage
•if threshold is not reached it does not fire
–nondecremental-do not get weaker with distance
–irreversible-once started goes to completion and can not be stopped
refractory period
the period of resistance to stimulatio
two phases of the refractory period
–absolute refractory period
•no stimulus of any strength will trigger AP
•as long as Na+gates are open
•from action potential to RMP
–relative refractory period
•only especially strong stimulus will trigger new AP
–K+gates are still open and any effect of incoming Na+ is opposed by the outgoing K+
unmyelinated fiber
has voltage-regulated ion gates along its entire length
saltatory conduction
the nerve signal seems to jump from node to node(
presynaptic neuron
1st neuron in the signal path is the presynaptic neuron
•releases neurotransmitter
postsynaptic neuron
2nd neuron is postsynaptic neuron
•responds to neurotransmitter
presynaptic neuron may synapse with
a dendrite, soma, or axon of postsynaptic neuron to form axodendritic, axosomaticor axoaxonic synapses
Discovery of Neurotransmitters
synaptic cleft
gap between neurons was discovered by Ramón y Cajal through histological observations
Discovery of Neurotransmitters
Otto Loewi, in 1921, demonstrated that neurons communicate by releasing chemicals
later renamed acetylcholine, the first known neurotransmitter
electrical synapses
–some neurons, neuroglia, and cardiac and single-unit smooth muscle
–gap junctions join adjacent cells
•ions diffuse through the gap junctions from one cell to the next
electrical synapses
Advantages and disadvantages
advantage of quick transmission
•no delay for release and binding of neurotransmitter
•cardiac and smooth muscle and some neurons
–disadvantage is they cannot integrate information and make decisions
•ability reserved for chemical synapses in which neurons communicate by releasing neurotransmitters
Structure of a Chemical Synapse
- synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter
- postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates
Neurotransmitters
4 classes
acetylcholine
amino acid neurotransmitters
monoamines
neuropeptides
acetylcholine
in a class by itself •formed from acetic acid and choline
amino acid neurotransmitters
•include glycine, glutamate, aspartate, and -aminobutyric acid (GABA)
monoamines
•synthesized from amino acids by removal of the –COOH group
•retaining the –NH2(amino) group
•major monoamines are:
–epinephrine, norepinephrine, dopamine(catecholamines)
–histamine and serotonin
neuropeptides
chains of 2 to 40 amino acids
–beta-endorphin and substance P
•act at lower concentrations than other neurotransmitters
•longer lasting effects
Function of Neurotransmitters at Synapse
- they are synthesized by the presynaptic neuron
- they are released in response to stimulation
- they bind to specific receptors on the postsynaptic cell
- they alter the physiology of that cell
Effects of Neurotransmitters
•a given neurotransmitter does not have the same effect everywhere in the body
•multiple receptor types exist for a particular neurotransmitter
–14 receptor types for serotonin
•receptor governs the effect the neurotransmitter has on the target cell
neurotransmitters are diverse in their action
–some excitatory
–some inhibitory
–some the effect depends on what kind of receptor the postsynaptic cell has
–some open ligand-regulated ion gates
–some act through second-messenger systems
synaptic delay
time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell
–0.5 msec for all the complex sequence of events to occur
three kinds of synapses with different modes of action
–excitatory cholinergic synapse
–inhibitory GABA-ergic synapse
–excitatory adrenergic synapse
Excitatory Cholinergic Synapse
cholinergic synapse –employs acetylcholine (ACh) as its neurotransmitter
–ACh excites some postsynaptic cells
•skeletal muscle
–inhibits others
Inhibitory GABA-ergic Synapse
- GABA-ergic synapse employs -aminobutyric acid as its neurotransmitter
- nerve signal triggers release of GABA into synaptic cleft
- GABA receptors are chloride channels
- Cl-enters cell and makes the inside more negative than the resting membrane potential
- postsynaptic neuron is inhibited, and less likely to fire
Excitatory Adrenergic Synapse
- adrenergic synapse employs the neurotransmitter norepinephrine(NE) also called noradrenaline
- NE and other monoamines, and neuropeptides acts through second messenger systems such as cyclic AMP (cAMP)
- receptor is not an ion gate, but a transmembrane protein associated with a G protein
Cessation of the Signal
stop adding neurotransmitter and get rid of that which is already there
–stop signals in the presynaptic nerve fiber
–getting rid of neurotransmitter
getting rid of neurotransmitter by:
- diffusion
- reuptake
- degradation in the synaptic cleft
neuromodulators
hormones, neuropeptides, and other messengers that modify synaptic transmission
–may stimulate a neuron to install more receptors in the postsynaptic membrane adjusting its sensitivity to the neurotransmitter
–may alter the rate of neurotransmitter synthesis, release, reuptake, or breakdown
neural integration
the ability of your neurons to process information, store and recall it, and make decisions
neural integration is based on
the postsynaptic potentials produced by neurotransmitters
excitatory postsynaptic potentials (EPSP)
excitatory postsynaptic potentials (EPSP)
–any voltage change in the direction of threshold that makes a neuron more likely to fire
glutamate and aspartate
inhibitory postsynaptic potentials (IPSP)
any voltage change away from threshold that makes a neuron less likely to fire
glycine and GABA produce IPSPs and are inhibitory
acetylcholine (ACh) and norepinephrineare
excitatory to some cells and inhibitory to others
summation
the process of adding up postsynaptic potentials and responding to their net effect
–occurs in the trigger zone
temporal summation
occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades
–allows EPSPs to add up over time to a threshold voltage that triggers an action potential
spatial summation
occurs when EPSPs from several different synapses add up to threshold at an axon hillock.
–several synapses admit enough Na+ to reach threshold
–presynaptic neurons cooperate to induce the postsynaptic neuron to fire
facilitation
a process in which one neuron enhances the effect of another one
–combined effort of several neurons facilitates firing of postsynaptic neuron
presynaptic inhibition
process in which one presynaptic neuron suppresses another one
–the opposite of facilitation
neural coding
the way in which the nervous system converts information to a meaningful pattern of action potentials
qualitative information
depends upon which neurons fire
–labeled line code –each nerve fiber to the brain leads from a receptor that specifically recognizes a particular stimulus type
quantitative information
information about the intensity of a stimulus is encoded in two ways:
–one depends on the fact that different neurons have different thresholds of excitation
–other way depends on the fact that the more strongly a neuron is stimulated, the more frequently it fires
Kinds of Neural Circuits
4
diverging circuit
converging circuit
reverberating circuits
parallel after-discharge circuits
diverging circuit
–one nerve fiber branches and synapses with several postsynaptic cells
–one neuron may produce output through hundreds of neurons
converging circuit
–input from many different nerve fibers can be funneled to one neuron or neural pool
–opposite of diverging circuit
reverberating circuits
–neurons stimulate each other in linear sequence but one cell restimulates the first cell to start the process all over
–diaphragm and intercostal muscles
memory trace
physical basis of memory is a pathway through the brain called a memory trace or engram
synaptic plasticity
the ability of synapses to change
synaptic potentiation
the process of making transmission easier
kinds of memory
immediate, short-and long-term memory
immediate memory
the ability to hold something in your thoughts for just a few seconds
–essential for reading ability
short-term memory (STM)
lasts from a few seconds to several hours
–quickly forgotten if distracted
–calling a phone number we just looked up
–reverberating circuits
types of long-term memory
declarative
procedural
declarative
retention of events that you can put into words
procedural
retention of motor skills
Alzheimer Disease
memory loss for recent events, moody, combative, lose ability to talk, walk, and eat
•show deficiencies of acetylcholine (ACh) and nerve growth factor (NGF)
Parkinson Disease
progressive loss of motor function beginning in 50‟s or 60‟s -no recovery
–degeneration of dopamine-releasing neurons in substantia nigra
•dopamine normally prevents excessive activity in motor centers (basal nuclei)
•involuntary muscle contractions
