© 2012 Pearson Education, Inc. Nervous system All organisms must sense and react to their environment Hydra – simplest nervous system with a nerve net.

© 2012 Pearson Education, Inc.

Nervous system

•All organisms must sense and react to their environment•Hydra – simplest nervous system with a nerve net•Arthropods and annelids – ganglia•We have neurons•Neurons consist of a cell body, dendrites and axon


Types of neurons

1. Sensory – receive impulses from the environment and bring them to the body

2. Motor – transmits the impulse to the muscles or glands in response

3. Interneurons – link sensory and motor neurons – they are found in the brain and spinal cord

© 2012 Pearson Education, Inc. Figure 7.4a

Dendrite Cellbody

Nissl substance

Axonhillock

Neurofibrils Collateralbranch

OneSchwann cell

Node ofRanvier

Schwann cells,forming the myelin sheath on axon

Nucleus

Axonterminal

Mitochondrion

Axon

(a)

© 2012 Pearson Education, Inc. Figure 7.6

Dendrites Peripheralprocess (axon)

Ganglion

Cellbody

Sensoryneuron

Central process (axon)

Spinal cord(central nervous system)

Motor neuron

Interneuron(associationneuron)

Afferenttransmission

Peripheralnervous system

Receptors

To effectors (muscles and glands)

Efferent transmission

Reflex arc


Nerve Impulses

•Resting neuron•The plasma membrane at rest is polarized•Fewer positive ions are inside the cell than outside the cell

© 2012 Pearson Education, Inc. Figure 7.9, step 1

Resting membrane is polarized. In the resting state, theexternal face of the membrane is slightly positive; its internalface is slightly negative. The chief extracellular ion is sodium(Na+), whereas the chief intracellular ion is potassium (K+).The membrane is relatively impermeable to both ions.

[Na+]

++++ + + + +[K+]

– ––– – – – –

++++ + + +– ––– – – –

1


Na+

++++ + + + +– ––– – – – –

++++ + + +– ––– – – –+

2 Stimulus initiates local depolarization. A stimuluschanges the permeability of a local "patch" of the membrane,and sodium ions diffuse rapidly into the cell. This changes thepolarity of the membrane (the inside becomes more positive;the outside becomes more negative) at that site.


Na+

++–– + + + ++ ––+ – – – –

++–– + + ++ ––+ – – –+

3 Depolarization and generation of an action potential.If the stimulus is strong enough, depolarization causesmembrane polarity to be completely reversed and an actionpotential is initiated.


–––– – + + ++ +++ + – – –

–––– + + ++ +++ – – –

Propagation of the action potential. Depolarization ofthe first membrane patch causes permeability changes in theadjacent membrane, and the events described in step arerepeated. Thus, the action potential propagates rapidly alongthe entire length of the membrane.

4

2


+++++ – – – –

– ––– + + + +

–+++ – – –– +–– + + +K+

Repolarization. Potassium ions diffuse out of the cell asthe membrane permeability changes again, restoring thenegative charge on the inside of the membrane and thepositive charge on the outside surface. Repolarization occursin the same direction as depolarization.

5


Diagram on Pg. 193 in Cliffs


Initial ionic conditions restored. The ionic conditionsof the resting state are restored later by the activity of thesodium-potassium pump. Three sodium ions are ejected forevery two potassium ions carried back into the cell.

6

Cellexterior Na+

Na+Na+

K+

Dif

fusi

on

Na+

Dif

fusi

on

Na+ – K+

pump

Plasmamembrane

K+

K+

K+K+

Cellinterior


Parts of the Nervous System

Central Nervous System

•Brain and spinal cord

Peripheral Nervous System

•All other nerves

•Somatic and autonomic

•Somatic – voluntary activities – skeletal muscle contraction

•Autonomic – involuntary activities – digestion/heartbeat

•These systems sometimes work together


Nervous Tissue: Neurons

•Neurons = nerve cells

•Cells specialized to transmit messages

•Major regions of neurons

•Cell body—nucleus and metabolic center of the cell

•Processes—fibers that extend from the cell body



•Cell body

•Nissl bodies

•Specialized rough endoplasmic reticulum

•Neurofibrils

•Intermediate cytoskeleton

•Maintains cell shape

•Nucleus with large nucleolus


Dendrite Cellbody

Nissl substance

Axonhillock

Neurofibrils Collateralbranch

OneSchwann cell

Node ofRanvier

Schwann cells,forming the myelin sheath on axon

Nucleus

Axonterminal

Mitochondrion

Axon

(a)



•Processes outside the cell body

•Dendrites—conduct impulses toward the cell body

•Neurons may have hundreds of dendrites

•Axons—conduct impulses away from the cell body

•Neurons have only one axon arising from the cell body at the axon hillock



•Axons •End in axon terminals•Axon terminals contain vesicles with neurotransmitters

•Axon terminals are separated from the next neuron by a gap•Synaptic cleft—gap between adjacent neurons

•Synapse—junction between nerves



•Myelin sheath—whitish, fatty material covering axons

•Schwann cells—produce myelin sheaths in around axons (PNS)

•Nodes of Ranvier—gaps in myelin sheath along the axon – saltatory conduction

•Oligodendrocytes—produce myelin sheaths around axons of the CNS


Transmission of a Signal at Synapses

•When the action potential reaches the axon terminal, the electrical charge opens calcium channels


Axon oftransmittingneuron

Receivingneuron

Actionpotentialarrives.

Dendrite

Vesicles

Synapticcleft

Axon terminal

1


Vesicle fuses with plasma membrane.

2 Transmitting neuron

Synapticcleft

Ion channels

Neurotransmittermolecules

Receiving neuron



2

3

Transmitting neuron

Neurotrans-mitter isreleased into synaptic cleft.

Synapticcleft

Ion channels


Receiving neuron



4

Transmitting neuron

Neurotrans-mitter isreleased into synaptic cleft.

Neurotrans-mitter bindsto receptoron receivingneuron's membrane.

Synapticcleft

Ion channels


Receiving neuron

2

3



• If enough neurotransmitter is released, graded potential will be generated• Inhibitory or excitatory•Eventually an action potential (nerve impulse) will occur in the neuron beyond the synapse


Receptor

Ion channel opens.

Neurotransmitter

Na+

5


Neurotransmitter isbroken down andreleased.

Ion channel closes.

Na+

6



•The electrical changes prompted by neurotransmitter binding are brief •The neurotransmitter is quickly removed from the synapse


Acetylcholine

•Neurotransmitter that is released from the axon as a result of Ca2+ entering the axon terminal•Stimulates muscle contraction•Active in parasympathetic nervous system•Norepinephrine and GABA are other neurotransmitters


Centralnervous system Peripheral nervous system Effector organs

Somatic nervous system

Sympatheticdivision

Autonomicnervoussystem

Parasympatheticdivision

KEY:

Preganglionic axons(sympathetic)

Postganglionic axons(sympathetic)

Myelination Preganglionic axons(parasympathetic)

Postganglionic axons(parasympathetic)

Acetylcholine

Acetylcholine

Acetylcholine

Epinephrine andnorepinephrine

Bloodvessel

Adrenal medullaAcetylcholine

Ganglion

Ganglion

Norepinephrine

Skeletalmuscle

Smooth muscle(e.g., in stomach)

Glands

Cardiacmuscle

© 2012 Pearson Education, Inc. Figure 6.4a(a)

Spinal cord

Motor unit 1

Motor unit 2

Axon terminals at neuromuscular junctions

Nerve

Axon ofmotorneuron

Motor neuroncell bodies

Muscle Muscle fibers


Action potential reaches axonterminal of motor neuron.

Calcium (Ca2+) channelsopen and Ca2+ enters the axon terminal.

Ca2+ entry causes somesynaptic vesicles to release theircontents (acetylcholine, a neurotransmitter) by exocytosis.

Acetylcholine diffuses acrossthe synaptic cleft and binds toreceptors in the sarcolemma.

Synaptic vesicle containing ACh

Axon terminal of motor neuron

Mitochondrion

Ca2+

Fusing synapticvesicleSarcoplasmof muscle fiber

Folds ofsarcolemma

Ca2+

AChreceptor

ACh

1

2

3

4

SarcolemmaSynapticcleft


ACh binds and channels openthat allow simultaneous passageof Na+ into the muscle fiber and K+ out of the muscle fiber. MoreNa+ ions enter than K+ ions leaveand this produces a local changein the electrical conditions of themembrane (depolarization), whicheventually leads to an actionpotential.

5Ion channel insarcolemma opens;ions pass.

Na+ K+


ACh effects are ended by itsbreakdown in the synaptic cleft bythe enzyme acetylcholinesterase.

6

Ion channel closed;ions cannot pass.

Acetylcholinesterase

Na+

Degraded AChACh

K+


Short term

Spinal cord

Catecholamines(epinephrine and norepinephrine)

Adrenalmedulla

1. Increased heart rate2. Increased blood pressure3. Liver converts glycogen to glucose and releases glucose to blood4. Dilation of bronchioles5. Changes in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity6. Increased metabolic rate

1. Retention of sodium and water by kidneys2. Increased blood volume and blood pressure

1. Proteins and fats converted to glucose or broken down for energy2. Increased blood sugar3. Suppression of immune system

Short-term stress response Long-term stress response

Preganglionicsympatheticfibers

Nerve impulses

Hypothalamus

More prolongedStress

Releasing hormones

Corticotropic cells ofanterior pituitary

ACTH Adrenalcortex

Mineralocorticoids Glucocorticoids


Brain


Brain

•Cerebrum – convoluted folds, two halves

•Corpus callosum – allows the cerebral hemispheres to communicate

•Left side – dominant in math, language, logic

•Right – nonverbal thinking and image recognition

•Hypothalamus and pituitary gland – hormone release, limbic system (thirst, pain, sexual desire)

•Cerebellum – coordination and muscle movement


Skeletal Muscle Functions

•Produce movement

•Maintain posture

•Stabilize joints

•Generate heat


Microscopic Anatomy of Skeletal Muscle

•Sarcolemma—specialized plasma membrane

•Myofibrils—long organelles inside muscle cell

•Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum


Sarcolemma

Myofibril

Dark(A) band

Light(I) band

Nucleus

(a) Segment of a muscle fiber (cell)

© 2012 Pearson Education, Inc. Figure 6.3b

(b) Myofibril or fibril (complex organelle composed of bundles of myofilaments)

Z disc H zone Z disc

I band A band I band M line

Thin (actin) filament

Thick (myosin) filament


Microscopic Anatomy of Skeletal Muscle

•Sarcomere—contractile unit of a muscle fiber

•Organization of the sarcomere

•Myofilaments

•Thick filaments = myosin filaments

•Thin filaments = actin filaments

© 2012 Pearson Education, Inc. Figure 6.3c

Z disc

Sarcomere

M lineZ disc

Thin (actin) filament

Thick (myosin) filament

(c) Sarcomere (segment of a myofibril)


The Nerve Stimulus and Action Potential

•Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract

•Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron

© 2012 Pearson Education, Inc. Figure 6.4a(a)

Spinal cord

Motor unit 1

Motor unit 2

Axon terminals at neuromuscular junctions

Nerve

Axon ofmotorneuron

Motor neuroncell bodies

Muscle Muscle fibers



The Sliding Filament Theory of Muscle Contraction

•Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament

•Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere

•This continued action causes a sliding of the myosin along the actin

•The result is that the muscle is shortened (contracted)

© 2012 Pearson Education, Inc. Figure 6.7a–b

Myosin Actin

Z H

I

Z

A I

(a)

(b)

Z

I A I

Z


Protein complex

Myosinmyofilament

Actinmyofilament(a)

In a relaxed muscle cell, the regulatory proteins formingpart of the actin myofilaments prevent myosin binding(see a). When an action potential (AP) sweeps along itssarcolemma and a muscle cell is excited, calcium ions(Ca2+) are released from intracellular storage areas (thesacs of the sarcoplasmic reticulum).

© 2012 Pearson Education, Inc. Figure 6.8b

Myosin-binding siteCa2+

Upper part of thick filament only

(b)

The flood of calcium acts as the final trigger forcontraction, because as calcium binds to the regulatoryproteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thinfilaments. This action exposes myosin-binding sites onthe actin, to which the myosin heads can attach (see b),and the myosin heads immediately begin seeking out binding sites.


The Cross Bridge cycle

© 2012 Pearson Education, Inc. Nervous system All organisms must sense and react to their environment Hydra – simplest nervous system with a nerve net.

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