Chapter 11- Fundamentals of the Nervous System and Nervous Tissue Flashcards

1. Sensory Input

2. Integration

3. Motor Input

information gathered by sensory receptors about internal and external changes

processing and interpretation of sensory input

activation of effector organs (muscle and glands) produces a response

Central nervous system (CNS) and Peripheral nervous system (PNS)

integration and command center. Consists of the brain and spinal cord

Paired spinal and cranial nerves. Carry messages to and from the CNS

Sensory and Motor

afferent, "carrying towards". Visceral afferent fibers- convey impulses from visceral organs.

convey impulses from the body (skin, skeletal muscles, and joints).

convey impulses from visceral organs.

efferent, "carrying away"

Transmits impulses from the CNS to effector organs

Somatic nervous system and autonomic nervous system

voluntary, conscious control of skeletal muscle

involuntary, visceral motor nervous fibers. Regulates smooth muscle and glands

Sympathetic and Parasympathetic

excitable cells that transmit electrical signals

supporting cells.

most abundant versatile, and highly branched glial cells

Cling to neurons, synaptic endings, and capillaries.

Support and brace neurons. Help determine capillary permeability.

Guide migration of young neurons.

Control the chemical environment

Participate in information processing in the brain

Small, ovoid cells with thorny processes that migrate toward injured neurons and phagocytize microorganisms and neuronal debris.

range in shape from squamous to columnar. May be ciliated.

Line the central cavities of the brain and spinal column,

Seperate the CNS interstitial fluid from the cerebrospinal fluids in the cavities.

branched cells. Processes wrap CNS nerve fibers, forming insulating myelin sheathe.

surround neuron cell bodies in the PNS

also known as neurolemmocytes.

Surround peripheral nerve fibers and form myelin sheaths

Vital to regeneration of damaged peripheral nerve fibers

Long-lived (up to 100 years or more)

Amitotic- w/few exceptions

High metabolism rate- depends on continuos supply of glucose and oxygen

Plasma membrane functions in electrical signaling and cell to cell interactions during development

aka Perikaryon or Soma

Biosynthetic center of a neuron

Spherical nucleus with nucleolus

Well- developed Golgi appartua

Rough ER is called Nissl bodies

Network of neurofibrils (neurofilmaents)

Axon hillock

color loviong

cone-shaped area from wich axons arises

nucliei

ganglia

short, tapering and diffusely branched

receptive (input) region of a neuron

convey electrical signals toward the cell body as graded potentials

one axon per cell arising from the axon hillock

long axons (nerve fibers)

occasional branches (axon collaterals)

numerous terminal branches (telodenria)

numerous terminal branches of axons

conducting region of a neuron

generates and transmits nerve impulses (action potentials) away from the cell body

Anterograde and Retrograde

molecules and organelles are moved along axons by motor molecules toward axonal terminal

molecules and organelles are moved along axons by motor molecules towards the cell body

Schwann cells wraps many times around the axon and is the concentric layers of Schwann cell membrane

Neurilemma- peripheral bulge of Schwann cell cytoplasm

Nodes of Ranvier- myelin sheath gaps btw adjacent Schwann cells, sites where axon collaterals can emerge

thin nerve fibers

One Schwann cell may incompletely enclose 15 or more unmyelinated axons

formed by processes of oligodentrocytes, not the whole cells

nodes of Ranvier are present

no neurilemma

thinnest fibers are unmyelinated

mulitpolar- 1 axon and several dendrites. Most abundant. Motor neurons and interneurons

biplolar- 1 axon and 1 dendrite

unipolar (pseudounipolar)- single, short process that has 2 branches: peripheral process: more distal branch, often associated with a sensory receptor. Central process: branch entering the CNS

Sensory (afferent), motor (efferent) and interneurons (association neurons)

transmit impulses from sensory receptpors toward the CNS

carry impulses from the CNS to effectors

shuttle signals through CNS pathways; mostly are entirely w/in the CNS

highly irritable, respond to adequate stimulus by generating an action potential (nerve impulses), impulse is always the same regardless of stimlus

opposite charges attract each other

energy is required to separate opposite charges across a membrane

energy is liberated when the charges move toward one another

if opposite charges are separated, the system has potential energy

measure of potential energy generated by separated charge

voltage measure between two points

the flow of electrical charge(ions) btw 2 points

hindrance to charge flow (provide by the plasma membrane)

substance w/high electrical resistance

substance w/low electrical resistance

proteins serve as membrane ion channels

2 main types of channels- leakage (nongated) channels and channels (3 types)

chemical gated(ligand-gated) channels- open w/binding of a specific neurotransmitter

voltage-gated channels- open and close in repsonse to changes in membrane potential

Mechanically gated channels- open and close in response to physical deformation of receptors

ions diffuse quickly across the membrane along their electochemical gradients. Along chemical concentration gradients from higher concentration to lower concentration. Along the electrical gradients toward opposite electrical charge/

ion flow creates an electrical current and voltage changes across the membrane

potential difference across the membrane of a resting cell

approximately -70 mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside)

negative interior of the cell is due to much greater diffusion of K⁺ out of the cell than Na+ diffusion into the cell

Sodium-potassium pump stabilizes the Vr by maintaining the concentration gradients for Na⁺ and K⁺

differences in ionic makeup of ICF than ECF. Differential permeability of the plasma membrane

ICF has lower concentration of Na⁺ and Cl^- than ECF

ICF has higher concentration of K⁺ and negatively charged proteins (A^-) then ECF

Impermeable of A^-(protein)

slightly permeable to Na⁺ (through leakage channels)

75 times more permeable to K⁺ (more leakage channels)

freely permeable to Cl^-

concentrations of ions across the membrane change and permeabilty of membrane to ions change

signals used to receive, integrate and send information

graded- incoming short-distance

action- long-distance of axons

Depolarization

Hyperpolarization

• A reduction in membrane potential (toward zero)

• Inside of the membrane becomes less negative than the resting potential

• Increases the probability of producing a nerve impulse

• An increase in membrane potential (away from zero)

• Inside of the membrane becomes more negative than the resting potential

• Reduces the probability of producing a nerve impulse

• Short-lived, localized changes in membrane potential

• Depolarizations or hyperpolarizations

• Graded potential spreads as local currents change the membrane potential of adjacent regions

• Occur when a stimulus causes gated ion channels to open

• E.g., receptor potentials, generator potentials, postsynaptic potentials

• Magnitude varies directly (graded) with stimulus strength

• Decrease in magnitude with distance as ions flow and diffuse through leakage channels

• Short-distance signals

• Brief reversal of membrane potential with a total amplitude of ~100 mV

• Occurs in muscle cells and axons of neurons

• Does not decrease in magnitude over distance

• Principal means of long-distance neural communication

• Resting state

• Only leakage channels for Na + and K + are open

• All gated Na + and K + channels are closed

Each Na + channel has two voltage-sensitive gates

• Activation gates

• Inactivation gates

• Closed at rest; open with depolarization

• Open at rest; block channel once it is open

• Depolarizing local currents open voltage-gated Na + channels

• Na + influx causes more depolarization

• At threshold (–55 to –50 mV) positive feedback leads to opening of all Na + channels, and a

reversal of membrane polarity to +30mV (spike of action potential)

• Na + channel slow inactivation gates close

• Membrane permeability to Na + declines to resting levels

• Slow voltage-sensitive K + gates open

• K + exits the cell and internal negativity is restored

• Some K + channels remain open, allowing excessive K + efflux

• This causes after-hyperpolarization of the membrane (undershoot)

Repolarization

• Restores the resting electrical conditions of the neuron

• Does not restore the resting ionic conditions

• Ionic redistribution back to resting conditions is restored by the thousands of sodium-potassium

pumps

• Na + influx causes a patch of the axonal membrane to depolarize

• Local currents occur

• Na + channels toward the point of origin are inactivated and not affected by the local currents

• Local currents affect adjacent areas in the forward direction

• Depolarization opens voltage-gated channels and triggers an AP

• Repolarization wave follows the depolarization wave

• (Fig. 11.12 shows the propagation process in unmyelinated axons.)

• Membrane is depolarized by 15 to 20 mV

• Na + permeability increases

• Na influx exceeds K + efflux

• The positive feedback cycle begins

• Subthreshold stimulus—weak local depolarization that does not reach threshold

• Threshold stimulus—strong enough to push the membrane potential toward and beyond

threshold

• AP is an all-or- none phenomenon—action potentials either happen completely, or not at all

• All action potentials are alike and are independent of stimulus intensity

• How does the CNS tell the difference between a weak stimulus and a strong one?

• Strong stimuli can generate action potentials more often than weaker stimuli

• The CNS determines stimulus intensity by the frequency of impulses

• Absolute refractory period

• Relative refractory period

• Time from the opening of the Na + channels until the resetting of the channels

• Ensures that each AP is an all-or- none event

• Enforces one-way transmission of nerve impulses

• Follows the absolute refractory period

• Most Na + channels have returned to their resting state

• Some K + channels are still open

• Repolarization is occurring

• Threshold for AP generation is elevated

• Exceptionally strong stimulus may generate an AP

Larger diameter fibers have less resistance to local current flow and have faster impulse conduction

• Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons

• Myelin sheaths insulate and prevent leakage of charge

• Saltatory conduction in myelinated axons is about 30 times faster

• Voltage-gated Na + channels are located at the nodes

• APs appear to jump rapidly from node to node

• An autoimmune disease that mainly affects young adults

• Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and

urinary incontinence

• Myelin sheaths in the CNS become nonfunctional scleroses

• Shunting and short-circuiting of nerve impulses occurs

• Impulse conduction slows and eventually ceases

Some immune system–modifying drugs, including interferons and Copazone:

• Hold symptoms at bay

• Reduce complications

• Reduce disability

• Group A fibers

• Large diameter, myelinated somatic sensory and motor fibers

• Group B fibers

• Intermediate diameter, lightly myelinated ANS fibers

• Group C fibers

• Smallest diameter, unmyelinated ANS fibers

Diameter, Degree of myelination, and Speed of conduction

• A junction that mediates information transfer from one neuron:

• Postsynaptic neuron—transmits impulses away from the synapse

• Axodendritic—between the axon of one neuron and the dendrite of another

• Axosomatic—between the axon of one neuron and the soma of another

• Less common types:

• Axoaxonic (axon to axon)

• Dendrodendritic (dendrite to dendrite)

• Dendrosomatic (dendrite to soma)

chemical and electrical synpases

• Less common than chemical synapses

• Neurons are electrically coupled (joined by gap junctions)

• Communication is very rapid, and may be unidirectional or bidirectional

• Are important in:

• Embryonic nervous tissue

• Some brain regions

• Specialized for the release and reception of neurotransmitters

• Typically composed of two parts

• Axon terminal of the presynaptic neuron, which contains synaptic vesicles

• Receptor region on the postsynaptic neuron

• Fluid-filled space separating the presynaptic and postsynaptic neurons

• Prevents nerve impulses from directly passing from one neuron to the next

• Transmission across the synaptic cleft:

• Is a chemical event (as opposed to an electrical one)

• Involves release, diffusion, and binding of neurotransmitters

• Ensures unidirectional communication between neurons

• AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca 2+ channels

• Synaptotagmin protein binds Ca 2+ and promotes fusion of synaptic vesicles with axon membrane

• Exocytosis of neurotransmitter occurs

• Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the

postsynaptic neuron

• Ion channels are opened, causing an excitatory or inhibitory event (graded potential)

• Within a few milliseconds, the neurotransmitter effect is terminated

1. Degradation by enzymes

2. Reuptake by astrocytes or axon terminal

3. Diffusion away from the synaptic cleft

• Neurotransmitter must be released, diffuse across the synapse, and bind to receptors

• Synaptic delay—time needed to do this (0.3–5.0 ms)

• Synaptic delay is the rate-limiting step of neural transmission

• Graded potentials

• Strength determined by:

• Amount of neurotransmitter released

• Time the neurotransmitter is in the area

• EPSP—excitatory postsynaptic potentials

• IPSP—inhibitory postsynaptic potentials

• Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of

Na + and K + in opposite directions

• Na + influx is greater that K + efflux, causing a net depolarization

• EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated

channels

• Neurotransmitter binds to and opens channels for K + or Cl –

• Causes a hyperpolarization (the inner surface of membrane becomes more negative)

• Reduces the postsynaptic neuron’s ability to produce an action potential

• Repeated use increases the efficiency of neurotransmission

• Ca 2+ concentration increases in presynaptic terminal and ostsynaptic neuron

• Brief high-frequency stimulation partially depolarizes the postsynaptic neuron

• Chemically gated channels (NMDA receptors) allow Ca 2+ entry

• Ca 2+ activates kinase enzymes that promote more effective responses to subsequent

stimuli

• Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of

another neuron via an axoaxonic synapse

• Less neurotransmitter is released and smaller EPSPs are formed

• Most neurons make two or more neurotransmitters, which are released at different stimulation

frequencies

• 50 or more neurotransmitters have been identified

• Classified by chemical structure and by function

• Broadly distributed in the brain

• Play roles in emotional behaviors and the biological clock

Acetylcholine (Ach)

Biogenic amines- Catecholamines and Indolamines

Amino acids

Peptides (neuropeptides)

Purines such as ATP

Gases and lipids

• Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)

• Determined by the receptor type of the postsynaptic neuron

• GABA and glycine are usually inhibitory

• Glutamate is usually excitatory

• Acetylcholine

• Excitatory at neuromuscular junctions in skeletal muscle

• Inhibitory in cardiac muscle

Neurotransmitter Action:

Direct Action and Indirect action

• Neurotransmitter binds to channel-linked receptor and opens ion channels

• Promotes rapid responses

• Examples: ACh and amino acids

• Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular

second messenger

• Promotes long-lasting effects

• Examples: biogenic amines, neuropeptides, and dissolved gases

1. Channel-linked receptors

2. G protein-linked receptors

• Ligand-gated ion channels

• Action is immediate and brief

• Excitatory receptors are channels for small cations

• Na + influx contributes most to depolarization

• Inhibitory receptors allow Cl – influx or K + efflux that causes hyperpolarization

• Transmembrane protein complexes

• Responses are indirect, slow, complex, and often prolonged and widespread

• Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides

• Neurotransmitter binds to G protein–linked receptor

• G protein is activated

• Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, diacylglycerol or Ca 2+

• Second messengers

• Open or close ion channels

• Activate kinase enzymes

• Phosphorylate channel proteins

• Activate genes and induce protein synthesis

functional groups and simple

• Integrate incoming information

• Forward the processed information to other destinations

• Single presynaptic fiber branches and synapses with several neurons in the pool

• Discharge zone—neurons most closely associated with the incoming fiber

• Facilitated zone—neurons farther away from incoming fiber

Diverging circuit

Converging circuit

Reverberating circuit

Parallel after-discharge circuit

Serial processing and parallel processing

• Input travels along one pathway to a specific destination

• Works in an all-or- none manner to produce a specific response

• Example: reflexes—rapid, automatic responses to stimuli that always cause the same

response

• Reflex arcs (pathways) have five essential components: receptor, sensory neuron, CNS

integration center, motor neuron, and effector

• Input travels along several pathways

• One stimulus promotes numerous responses

• Important for higher-level mental functioning

• Example: a smell may remind one of the odor and associated experiences

• The nervous system originates from the neural tube and neural crest formed from ectoderm

• The neural tube becomes the CNS

• Neuroepithelial cells of the neural tube undergo differentiation to form cells needed for

• Cells (neuroblasts) become amitotic and migrate

• Neuroblasts sprout axons to connect with targets and become neurons

development

• Growth cone at tip of axon interacts with its environment via:

• Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules or

• Neurotropins that attract or repel the growth cone

• Nerve growth factor (NGF), which keeps the neuroblast alive

• Astrocytes provide physical support and cholesterol essential for construction of synapses

N-CAMs)

• About 2/3 of neurons die before birth

• Death results in cells that fail to make functional synaptic contacts

• Many cells also die due to apoptosis (programmed cell death) during development