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Chapter 10

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Chapter 10:  Nervous System I

 

I.  General Functions of the Nervous System

A.  The nervous system is composed predominately of nervous tissue but also includes some blood vessels and connective tissue.

B.  Two cell types of nervous tissue are neurons and neuroglial cells.

C.  Neurons are specialized to react to physical and chemical changes in their surroundings.

D.  Dendrites are small cellular processes that receive input.

E.  Axons are long cellular processes that carry information away from neurons.

F.  Nerve impulses are bioelectric signals produced by neurons.

G.  Bundles of axons are called nerves.

H.  Small spaces between neurons are called synapses.

I.  Neurotransmitters are biological messengers produced by neurons.

J.  The central nervous system contains the brain and spinal cord.

K.  The peripheral nervous system contains cranial and spinal nerves.

L.  Three general functions of the nervous system are sensory, integrative, and motor.

M.  Sensory receptors are located at the ends of peripheral neurons and provide the sensory function of the nervous system.

N.  Receptors gather information.

O.  Receptors convert their information into nerve impulses, which are then transmitted over peripheral nerves to the central nervous system.

P.  In the central nervous system, the signals are integrated.

Q.  Following integration, decisions are made and acted upon by means of motor functions.

R.  The motor functions of the nervous system use neurons to carry impulses from the central nervous system to effectors.

S.  Examples of effectors are muscles and glands.

T.  The two divisions of the motor division are somatic and autonomic.

U.  Somatic nervous system is involved in conscious activities.

V.  The autonomic nervous system is involved in unconscious activities.

 W.  The nervous system can detect changes in the body, make decisions, and stimulate muscles or glands to respond.

X.  The three parts all neurons have are cell body, axon, and dendrites.

Y.  A neurons cell body contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules. It also contains a large nucleus, chromatophilic substance, and cytoplasmic inclusions.

Z.  Neurofibrils are fine threads that extend into axons.

AA.  Chromatophilic substance is membranous sacs that contain rough endoplasmic reticulum.

BB.  Mature neurons generally do not divide but neural stem cells do.

CC.  Dendrites are usually highly branched to provide receptive surfaces to which processes from other neurons communicate.

DD.  Dendritic spines are tiny, thornlike spines on the surface of dendrites.

EE.  A neuron may have many dendrites but will have only one axon.

FF.  An axonal hillock is the initial portion of an axon closest to the cell body.

GG.  An axon is specialized to carry nerve impulses away from the cell body.

HH.  The cytoplasm of an axon includes mitochondria, microtubules, and neruofibrils.

II.  Collaterals are branches of axons.

JJ.  A synaptic knob is a specialized ending of an axon.

KK.  A synaptic cleft is the space between a synaptic knob and the receptive surface of another cell.

LL.  Axonal transport is the process an axon uses to convey biochemicals that are produced in the neuron cell body.

MM.  Schwann cells produce myelin.

NN.  Myelin is a lipid-rich substance.

OO.  A myelin sheath is a coating produced by Schwann cells that is wrapped around an axon.

PP.  A neurilemma is a portion of a Schwann cell outside of the myelin sheath.

QQ.  A node of Ranvier is a narrow gap between myelin sheaths.

RR.  Myelinated axons have myelin sheaths.

SS.  Unmyelinated axons have no myelin sheaths.

TT.  White matter is composed of myelinated axons.

UU.  Gray matter is composed of unmyelinated axons, dendrites, and cell bodies of neurons.

II.  Classification of Neurons and Neuroglia

A.  Classification of Neurons

1.  The three major classifications of neurons based on structural differences are bipolar, multipolar, and unipolar.

2.  Bipolar neurons have two processes; one process is a dendrite and the other an axon.

3.  Bipolar neurons are found within the eyes, ears, and nose.

4.  Unipolar neurons have one process which functions as an axon.

5.  The peripheral process of a unipolar neuron is associated with dendrites near a peripheral body part and the central process enters the brain or spinal cord.

6.  Unipolar neurons are located in ganglia.

7.  Multipolar neurons have multiple dendrites and one axon.

8.  Multipolar neurons are located in the brain and spinal cord.

9.  The three classes of neurons based on functional differences are sensory, motor, and interneurons.

10.  Sensory neurons carry impulses from peripheral body parts to the brain and spinal cord.

11.  Sensory neurons have specialized receptors ends at the tips of their dendrites.

12.  Most sensory neurons are unipolar but some are bipolar.

13.  Interneurons are located in the brain and spinal cord.

14.  Interneurons are multipolar and form links between other neurons.

15.  Motor neurons carry nerve impulses from the brain and spinal cord to effectors.

16.  Two specialized groups of motor neurons are accelerator neurons and inhibitory neurons.

17.  Motor neurons that control skeletal muscle are under conscious control.

18.  Motor neurons that control glands, smooth muscle and cardiac muscle are under involuntary control.

B.  Classification of Neuroglial Cells

1.  In the embryo, neuroglial cells guide neurons to their positions and may stimulate them to grow.

2.  Neuroglial cells also produce growth factors that nourish neurons.

3.  Schwann cells are the neuroglia of peripheral nervous system.

4.  The four neuroglial cells of the central nervous system are astrocytes, oligodendrocytes, microglial cells, and ependymal cells.

5.  Astrocytes are star shaped and are commonly found between neurons and blood vessels.

6.  Astrocytes provide support and hold structures together.

7.  Astrocytes aid metabolism of glucose.

8.  Astrocytes respond to injury of brain tissue and form a special type of scar tissue.

9.  Astrocytes play a role in the blood-brain barrier, which restricts movement of substances between the blood and CNS.

10.  Oligodendrocytes occur in rows along myelinated axons and form myelin in the brain and spinal cord.

11.  Unlike Schwann cells, oligodendrocytes do not form neurilemmal sheaths.

12.  Microglia function to support neurons, and phagocytize bacterial cells and cellular debris.

13.  Ependyma form the inner lining of the central canal of the spinal cord and ventricles of the brain.

14.  Gap junctions join ependymal cells together.

15.  Ependymal cells form a porous layer through which substances diffuse freely between the interstitial fluid of the brain tissues and the fluid within the ventricles.

 16.  Covering the choroids plexus, ependymal cells also regulate the composition of the cerebrospinal fluid.

C.  Regeneration of Nerve Axons

1.  Injury to a neuron cell body usually kills the neuron but damaged peripheral axons usually regenerate.

2.  If a peripheral axon is separated from its cell body, the distal portion of the axon deteriorate but the proximal end of the axon develops sprouts shortly after injury.

3.  Growth of a regenerating axon is slow but eventually the new axon may reestablish the former connection.

4.  Axons within the central nervous system do not regenerate because there is no tube of sheath cells to guide it.

  III.  Cell Membrane Potential

A.  Introduction

1.  Polarized means electrically charged.

2.  When a cell membrane is polarized, the inside is negatively charged with respect to the outside.

3.  The polarization of a cell membrane is due to an unequal distribution of positive and negative ions on either side of the membrane.

B.  Distribution of Ions

1.  Potassium ions are the major intracellular positive ion and sodium ions are the major extracellular cation.

2.  The distribution of potassium and sodium is largely created by the sodium-potassium pump.

3.  The passage of potassium and sodium ions through the cell membrane depends on the presence of channels.

C.  Resting Potential

1.  A resting nerve cell is one that is not being stimulated to send a nerve impulse.

2.  At rest, a cell membrane gets a slight surplus of positive charges outside, and inside reflects a slight negative surplus of impermeable negatively charged ions because the cell membrane is more permeable to potassium ions than sodium ions.   Also the cell may contain anions and proteins that are negatively charged that cannot diffuse out of the cell.

3.  The cell uses ATP to actively transport sodium and potassium ions in opposite directions.

4.  Volts are the electrical differences between two points

5.  A volt is called a potential difference because it represents stored electrical energy that can be used to do work

6.  The membrane potential is the potential difference across the cell membrane and is measured in millivolts.

7.  Resting potential is the membrane potential of a resting neuron and has a value of 70 millivolts.

8.  The negative sign of a resting membrane potential is relative to the inside of the cell and is due to the excess negative charges on the inside of the cell membrane.

D.  Local Potential Changes

1.  Neurons are described as excitable because they can respond to the changes in their surroundings.

2.  Stimuli on neurons usually affect the membrane potential in the region of the membranes exposed to the stimulus.

3.  The stimulus affects the membrane potential of a neuron by opening a gated ion channel.

4.  A membrane is hyperpolarized if the membrane potential becomes more negative than the resting potential.

  5.  A membrane is depolarized if the membrane becomes less negative that the resting potential.

  6.  Local potential changes are graded meaning that the amount of change in the membrane potential is directly proportional to the intensity of the stimulation.

  7.  A threshold potential is sufficient depolarization that triggers an action potential.

  8.  Summation is an additive phenomenon in which two or more depolarizing stimuli are added together.

  9.  At threshold, an axon potential is produced in an axon.

E.  Action Potentials

1.  The trigger zone of an axon is the initial portion of an axon.

2.  The trigger zone contains a great number of voltage-gated sodium channels.

3.  At the resting membrane potential, sodium channels are closed but when threshold is reached, sodium channels open.

4.  As sodium ions rush into the cell, the membrane potential changes and temporarily becomes positive on the inside.

5.  When sodium channels close and potassium channels open, potassium diffuses out across the membrane and the inside of the membrane becomes negatively charged again.

6.  Repolarized means the membrane polar again or returned to its original resting state.

7.  Axons are capable of action potentials but the cell body and dendrites are not.

8.  A nerve impulse is the propagation of action potentials along an axon.

F.  All-or-None Response

1.  A nerve impulse is an all-or-nothing response, meaning if a neuron responds at all to a nerve impulse, it responds completely.

2.  A greater intensity of stimulation on the neuron produces more impulses per second but not a stronger impulse.

G.  Refractory Period

1.  The refractory period is the period in which a threshold stimulus will not trigger another impulse on an axon.

2.  An absolute refractory period is the period when an axons membrane cannot be stimulated and is the first part of the refractory period.

3.  A relative refractory period is the period in which a stronger stimulus can trigger an impulse.

4.  The refractory period limits how many action potentials may be generated in a neuron in a given amount of time.

  H.  Impulse Conduction

1.  Myelin serves as an insulator.

2.  Saltatory conduction is the type of nerve impulse conduction that occurs only at nodes. 

3.  Myelinated axons exhibit salutatory conduction.

4.  Myelinated axons send nerve impulses faster than unmyelinated axons.

5.  The diameter of an axon also affects the speed of a nerve impulse.

IV.  The Synapse

A.  Introduction

1.  Synapses are the places where impulses are passed from one neuron to another.

2.  A presynaptic neuron is the neuron that brings the impulse to the synapse.

3.  A postsynaptic neuron is the neuron that is stimulated by the presynaptic neuron.

4.  Synaptic transmission is the process by which the impulse in the presynaptic neuron signals the postsynaptic neuron.

B.  Synaptic Transmission

1.  A nerve impulse travels along an axon to the axon terminals.

2.  The synaptic knobs of axons contain sacs called synaptic vesicles.

3.  Synaptic vesicles contain neurotransmitters.

4.  When a nerve impulse reaches a synaptic knob, calcium diffuses inward from the extracellular fluid.

5.  The calcium inside the synaptic knob initiates a series of events that causes the synaptic vesicles to fuse with the cell membrane, releasing the neurotransmitter by exocytosis.

6.  Released neurotransmitters diffuse across the synaptic cleft and react with specific receptors on the postsynaptic neuron.

7.  Some neurotransmitters cause ion channels to open, some cause ion channels to close.

8.  Synaptic potentials are local potentials created by changes in chemically gated ion channels.

C.  Synaptic Potentials

1.  Synaptic potentials are graded and can depolarize or hyperpolarize the receiving cell membrane.

2.  An excitatory postsynaptic potential is a type of membrane change in which the receiving cell membrane is depolarized.

  3.  An inhibitory postsynaptic potential is a type of membrane change in which the receiving cell membrane is hyperpolarized.

4.  Within the brain and spinal cord, each neuron may receive the synaptic knobs of a thousand or more axons on its cell body and dendrites.

5.  The integrated sum of EPSPs and IPSPs  determines whether an action potential results.

D.  Neurotransmitters

1.  The nervous system produces at least thirty different kinds of neurotransmitters.

2.  Acetylcholine stimulates skeletal muscle contractions.

3.  Examples of monoamines are epinephrine, norepinephrine, dopamine, and serotonin.

4.  Examples of unmodified amino acids that act as neurotransmitters are glycine, glutamic acid, aspartic acid, and GABA.

 5.  Examples of peptides are enkephalins and substance P.

6.  Most neurotransmitters are synthesized in the cytoplasm of synaptic knobs and stored in synaptic vesicles.

7.  The more calcium that enters the synaptic knob, the more neurotransmitters that are released.

8.  After a vesicle releases its neurotransmitter, it becomes part of the cell membrane.

9.  The enzyme acetlycholinesterase functions to break down acetylcholine.

10.  The process of reuptake is when neurotransmitters are transported back into the synaptic knobs of the presynaptic neurons.

11.  Monoamine oxidase functions to inactivate epinephrine and norepinephrine after reuptake.

E.  Neuropeptides

1.  Neuropeptides are substances that alter a neurons response to a neurotransmitter or block the release of a neurotransmitter.

2.  Three examples of neuropeptides are enkephalins, beta endorphin, and substance P.

3.  Enkephalins function to relieve pain sensations.

4.  Endorphins function to relieve pain.

5.  The function of substance P is to transmit pain impulses into the spinal cord and on to the brain.

V.  Impulse Processing

A.  Neuronal Pools

1.  Neuronal pools are groups of neurons that make synaptic connections with each other and work together to perform a common function.

2.  Neuronal pools may have excitatory or inhibitory effects on other pools or on peripheral effectors.

3.  Facilitation is a condition created in which a neuron is brought closer to threshold.

B. Convergence

1.  Axons originating from different parts of the nervous system leading to the same neuron exhibit convergence.

2.  Convergence allows the nervous system to collect, process, and respond to information.

  C.  Divergence

1.  Axons may branch many times.

2.  Impulses leaving a neuron of a neuronal pool may exhibit divergence by reaching several other neurons.

3.  Diverging axons can amplify and impulse.

 

 



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