ch44 raven biology

7/29/2019 CH44 Raven Biology

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Chapter 44

The Nervous System

Bonus topics: CNS Subdivisions (Table 44.3) sleep and arousal lateralization of function


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Nervous System Organization

Sensory receptors = detect stimulusMotor effectors = respond to itThe nervous system links the two

consists of neurons and supporting cellsVertebrates havethree types of neurons: sensory neurons (afferent neurons)

motor neurons (efferent neurons) interneurons (association neurons)


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Central nervous system (CNS) brain and spinal cord

Peripheral nervous system (PNS) sensory and motor neurons somatic NS stimulates skeletal muscles autonomic NS stimulates smooth & cardiac muscles, glands

sympathetic and parasympathetic NS

counterbalance each other


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Neurons: basic structure cell body dendrites axon

Neurons supported by cells called neuroglia Schwann cells and oligodendrocytes

produce myelin sheaths surroundingaxons

CNS: oligodendrocytes

myelinated axons form white matter dendrites/cell bodies form gray matter

PNS: Schwann cells

axons are bundled to form nerves


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Nerve Impulse TransmissionA potential difference exists across every cells PM negative pole positive pole resting potential

ranges from - 40 to - 90 millivolts (mV)

(average about - 70 mV in neurons)


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The inside of the cell is more negatively charged than theoutside because of

1. sodium-potassium pump (see Fig. 44.5)2. ion leakage channels

buildup of positive charge outside and negative charge insidethe membrane

electrical potential is anattractive force to bring K+ions back into the cell

balance between diffusional

and electrical forces leads tothe equilibrium potential

resting membrane potentialcan be viewed using a

voltmeter and two electrodes


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Two types of potentials:graded potentials and action potentials

graded potentials = small transient changes in membranepotential due to activation of gated ion channels

chemically-gated(ligand-gated channels) ligands are hormones or

neurotransmitters induce opening and cause

changes in cell membrane

permeability


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Depolarization: membrane potential becomes more positive

Hyperpolarization: makes it more negative

summation is the ability of graded potentials to combine


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Action potentials threshold potential

caused by voltage-gated ion channels

two different channels are used:

voltage-gated Na+ channels

voltage-gated K+ channels

when the threshold voltage is reached, Na+ channels open rapidly transient influx of Na+ causes the membrane to depolarize in contrast, potassium channel opens slowly efflux of K+ repolarizes the membrane

the action potential has three phases: rising, falling and undershoot

action potentials are always separate, all-or-none events with thesame amplitude

do not add up or interfere with each other intensity of a stimulus is coded by the frequency, not amplitude,of action potentials


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Movement of the AP along a membrane:

each action potential, in its rising phase,reflects a reversal in membrane polarity

positive charges due to influx of Na+ candepolarize the adjacent region to threshold

the next region produces its own action

potential the previous region repolarizes back to the

resting membrane potential


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Conduction velocity:1. axon diameter

influences resistance to current flow large diameter axons found primarily in invertebrates

2. myelination action potential is only produced at the nodes of Ranvier impulse jumps from node to node (saltatory conduction)


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Synapses

intercellular junctions presynaptic cell postsynaptic cell

two basic types:

electrical synapses chemical synapses synaptic cleft synaptic vesicles neurotransmitters


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action potential triggers influx of Ca2+

synaptic vesicles fuse with cell membrane

neurotransmitter is released by exocytosis diffuses to other side of cleft and binds to chemical- (or ligand)-

gated receptor proteins neurotransmitter action is terminated by enzymatic cleavage or

cellular uptake


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Acetylcholine (ACh) neuromuscular junction

excitatory postsynaptic potential (EPSP) acetylcholinesterase (AChE)

Amino acid neurotransmitters glutamate glycine and GABA (-aminobutyric acid)

produce a hyperpolarization called an inhibitory postsynapticpotential (IPSP)


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Biogenic amines epinephrine (adrenaline) and norepinephrine dopamine serotonin

Neuropeptides

substance P intensity of pain perception depends on enkephalins

and endorphins

Nitric oxide (NO) a gas ; produced as needed from arginine causes smooth muscle relaxation


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Synaptic integration

integration of EPSPs (depolarization) and IPSPs(hyperpolarization) occurs on the neuronal cell body small EPSPs add together to bring the membrane

potential closer to the threshold IPSPs subtract from the depolarizing effect of EPSPs

therefore deter the membrane potential from reachingthreshold

two ways that themembrane can reachthe threshold voltage spatial summation temporal summation


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Memory

involves many regions and connections essential regions in the temporal lobe

amygdala hippocampus

short-term memory is stored in the form of transientneural excitations long-term memory appears to involve structural

changes in neural connections

memory consolidation facilitation structural changes


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Spinal Cord

cable of neurons extending from the brain downthrough the backbone

bodys information highway relays messages between the body and the brain

enclosed and protected bythe vertebral column andthe meninges

functions in reflexes the knee-jerk reflex is monosynaptic however, most reflexes in vertebrates involve an interneuron


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Sensory neurons: axons enter the dorsal surface of the spinal cord and form dorsal root

of spinal nerve cell bodies are grouped outside the spinal cord in dorsal root gangliaMotor neurons: axons leave from the ventral surface and form ventral root of spinal

nerve cell bodies are located in the spinal cord


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functions in reflexes the knee-jerk reflex is monosynaptic

however, most reflexes in vertebrates involve an interneuron


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functions in reflexes the knee-jerk reflex is monosynaptic

however, most reflexes in vertebrates involve an interneuron


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Monosynaptic reflex arc: involves only 1 synapse

example: knee jerk reflex

Polysynaptic reflex arc: involves interneurons most reflexes in vertebrates examples: withdrawal (also

called flexor reflex) andcrossed extension reflex

note: for all of thesereflexes, other interneuronsascend through the s.c. tothe brain (you are aware ofthe various stimuli and howyour body is responding)


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The Peripheral Nervous System

consists of nerves and ganglia

The Somatic Nervous System

skeletal muscle contraction

conscious command or reflex actions antagonist muscle inhibition

The Autonomic Nervous System composed of the sympathetic and

parasympathetic divisions, plus themedulla oblongata


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The Autonomic Nervous System composed of the sympathetic and parasympathetic divisions

in both, efferent motorpathway has 2 neurons

preganglionic neuron:exits the CNS and

synapses at an autonomicganglion postganglionic neuron:

exits the ganglion andregulates visceral

effectors (smooth orcardiac muscle or glands)

* how can Ach have excitatory effect on skeletal muscle but havean inhibitory effect on heart muscle?

see Table 44.5 AutonomicInnervation of Target tissues


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ANSSympathetic division preganglionic neurons

originate in the thoracic andlumbar regions of spinal cord

most axons synapse in twoparallel chains of gangliaright outside the spinal cord

Parasympathetic division preganglionic neurons

originate in the brain andsacral regions of spinal cord

axons terminate in ganglianear or even within internalorgans


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autonomic effects are mediated by the action ofG protein-coupled receptors

receptor is activated by binding to its ligand the G protein is activated; activates the effector protein review G-protein coupled signaling (chapter 9)

ANS

ch44 raven biology

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