Neural TissueNeural Tissue

Chapter 12Chapter 12

I. Overview of the Nervous SystemI. Overview of the Nervous System

A.A. Provides swift, but brief responses Provides swift, but brief responses to stimuli to stimuli

As opposed to what??As opposed to what??

B.B. Consists of:Consists of:1.1. Neural tissue:Neural tissue:

a.a. Neurons-Neurons- functional units functional units

b.b. NeurogliaNeuroglia (aka (aka Glial CellsGlial Cells))i.i. Separate & protect neuronsSeparate & protect neuronsii.ii. Provide a supportive framework for neural tissueProvide a supportive framework for neural tissueiii.iii. Act as phagocytesAct as phagocytesiv.iv. Help regulate interstitial fluidHelp regulate interstitial fluidv.v. Far outnumber neurons (account for ~ ½ the Far outnumber neurons (account for ~ ½ the

volume of the nervous system).volume of the nervous system).

2.2. Blood vesselsBlood vessels

3.3. Connective tissuesConnective tissues

C.C. Organs of the nervous systemOrgans of the nervous system1.1. BrainBrain2.2. Spinal cordSpinal cord3.3. Sensory receptors for eyes, ears,…Sensory receptors for eyes, ears,…4.4. NervesNerves

D.D. Major anatomical divisions of the nervous Major anatomical divisions of the nervous system:system:

1.1. Central Nervous System (CNS)Central Nervous System (CNS)

a.a. Consists of:Consists of:i.i. BrainBrainii.ii. Spinal cordSpinal cord

b.b. Responsible for:Responsible for:i.i. Integrating, processing, & coordinating sensory data and Integrating, processing, & coordinating sensory data and

motor commands motor commands ii.ii. IntelligenceIntelligenceiii.iii. MemoryMemoryiv.iv. LearningLearningv.v. EmotionEmotion

2.2. Peripheral Nervous System (PNS)Peripheral Nervous System (PNS)

a.a. Consists of neural tissue outside the CNS Consists of neural tissue outside the CNS ((nerve fibersnerve fibers ( (axonsaxons))))

i.i. Cranial nerves-Cranial nerves- connected to the brain connected to the brainii.ii. Spinal nerves-Spinal nerves- connected to the spinal cord connected to the spinal cordiii.iii. Peripheral nerves-Peripheral nerves- aka ‘nerves’ aka ‘nerves’

b.b. Responsible for:Responsible for:i.i. Delivering sensory information to the CNSDelivering sensory information to the CNSii.ii. Carrying motor commands to peripheral tissues & Carrying motor commands to peripheral tissues &

systemssystems

c.c. Functional divisions of the PNSFunctional divisions of the PNSi.i. Afferent division-Afferent division- brings sensory information brings sensory information toto the the

CNS from CNS from receptorsreceptors in peripheral tissues & organs in peripheral tissues & organs

ii.ii. Efferent division-Efferent division- carries motor commands carries motor commands fromfrom the the CNS to CNS to effectorseffectors (muscles & glands) (muscles & glands)

Somatic nervous system (SNS)-Somatic nervous system (SNS)- controls skeletal controls skeletal musclemuscle

** Conscious-Conscious- voluntary voluntary

** Subconscious-Subconscious- involuntary (involuntary (reflexreflex))

Autonomic nervous system (ANS or visceral motor Autonomic nervous system (ANS or visceral motor system)-system)- provides automatic, involuntary provides automatic, involuntary regulation of smooth muscle, cardiac muscle & regulation of smooth muscle, cardiac muscle & glandsglands

** Sympathetic division- Sympathetic division- fight fight or flight activitiesor flight activities

* * Parasympathetic division- Parasympathetic division- activities that activities that conserve energyconserve energy

II. NeuronsII. NeuronsA.A. Representative Neuron Structure Representative Neuron Structure

(Multipolar, most common type in CNS)(Multipolar, most common type in CNS)

1.1. Soma-Soma- cell body cell bodya.a. Large nucleus & noticeable nucleolusLarge nucleus & noticeable nucleolusb.b. Many mitochondriaMany mitochondriac.c. Clusters of ribosomes & RER (Nissl bodies) Clusters of ribosomes & RER (Nissl bodies)

give the cell a grayish appearance, hence the give the cell a grayish appearance, hence the name “name “gray mattergray matter””

d.d. Lack centrioles (can’t divide)Lack centrioles (can’t divide)

2.2. Dendrites-Dendrites- sensitive processes sensitive processes

a.a. Extend from somaExtend from soma

b.b. Highly branched w/ dendritic spinesHighly branched w/ dendritic spines

c.c. Neurons receive info from other neurons via Neurons receive info from other neurons via synaptic connections at the dendritic spinessynaptic connections at the dendritic spines

3.3. Axon-Axon- long, cytoplasmic process long, cytoplasmic process

a.a. Connects to the soma at a axon hillock Connects to the soma at a axon hillock (thickened region)(thickened region)

b.b. May have side branches (May have side branches (collateralscollaterals) to ) to communicate w/ multiple cellscommunicate w/ multiple cells

c.c. End in a series of fine extensions (End in a series of fine extensions (telodendriatelodendria))* Telodendria end at synaptic terminals (synaptic bulbs)* Telodendria end at synaptic terminals (synaptic bulbs)

* Synaptic terminals are a part of a * Synaptic terminals are a part of a synapsesynapse

4.4. Synapse-Synapse- site of intercellular site of intercellular communicationcommunication

a.a. There are 2 cells involved:There are 2 cells involved:

i.i. Presynaptic cellPresynaptic cell Has the synaptic terminalHas the synaptic terminal Sends a messageSends a message

ii.ii. Postsynaptic cellPostsynaptic cell Receives the messageReceives the message

b.b. Involves the release of chemicals Involves the release of chemicals ((neurotransmittersneurotransmitters) by the synaptic terminal) by the synaptic terminal

c.c. May occur on a dendrite, the soma, or along the May occur on a dendrite, the soma, or along the axonaxon

B.B. Neuron ClassificationNeuron Classification1.1. Structural Classification Structural Classification

a.a. Anaxonic NeuronsAnaxonic Neuronsi.i. SmallSmall

ii.ii. Dendrites & axons look alikeDendrites & axons look alike

iii.iii. Located in the brain and special sense organsLocated in the brain and special sense organs

iv.iv. Function is poorly understood!Function is poorly understood!

b.b. Bipolar NeuronsBipolar Neurons

i.i. Highly branched dendrites at one end, one Highly branched dendrites at one end, one axon at the other end with the soma in the axon at the other end with the soma in the middlemiddle

ii.ii. Short (30 mm end-to-end)Short (30 mm end-to-end)

iii.iii. Rare- found in special sense organsRare- found in special sense organs

iv.iv. Relay info about sight, smell, hearingRelay info about sight, smell, hearing

c.c. Unipolar Neurons (Pseudounipolar Unipolar Neurons (Pseudounipolar Neuron)Neuron)

i.i. Dendrites and axon are continuous, with Dendrites and axon are continuous, with the soma off to one side.the soma off to one side.

ii.ii. Most sensory neurons of the PNS are Most sensory neurons of the PNS are unipolarunipolar

iii.iii. Axons may extend a meter or more!Axons may extend a meter or more!

d.d. Multipolar NeuronsMultipolar Neurons

i.i. Several dendrites and a single axon w/ one or more Several dendrites and a single axon w/ one or more branchesbranches

ii.ii. Most common type in the CNSMost common type in the CNS

iii.iii. One function- controls skeletal muscleOne function- controls skeletal muscle

iv.iv. Can be as long as unipolar neuronsCan be as long as unipolar neurons

2.2. Functional ClassificationFunctional Classificationa.a. Sensory NeuronsSensory Neurons

i.i. Form the afferent division of the PNS (deliver info from Form the afferent division of the PNS (deliver info from sensory receptors to the CNS)sensory receptors to the CNS) Somatic sensory neurons- Somatic sensory neurons- monitor exterior monitor exterior

conditionsconditions Visceral sensory neurons- Visceral sensory neurons- monitor internal monitor internal

conditionsconditions

ii.ii. Unipolar Unipolar

iii.iii. Axons are known as Axons are known as afferent fibersafferent fibers

iv.iv. ~10 million in the body~10 million in the body

b.b. Motor Neurons (efferent neurons)Motor Neurons (efferent neurons)

i.i. Form the efferent division of the PNS (carry Form the efferent division of the PNS (carry instructions from the CNS to peripheral effectors)instructions from the CNS to peripheral effectors)

Somatic motor neurons- Somatic motor neurons- (in SNS) innervate (in SNS) innervate skeletal muscleskeletal muscle

Visceral motor neurons- Visceral motor neurons- (in ANS) innervate all (in ANS) innervate all other peripheral effectorsother peripheral effectors

ii.ii. ~ ½ million in the body~ ½ million in the body

iii.iii. Axons are known as Axons are known as efferent fibersefferent fibers

c.c. Interneurons (association neurons)Interneurons (association neurons)

i.i. Responsible for distribution of sensory info & Responsible for distribution of sensory info & coordination of motor activitycoordination of motor activity

ii.ii. May be situated between sensory & motor neuronsMay be situated between sensory & motor neurons

iii.iii. Located entirely w/in brain & spinal cordLocated entirely w/in brain & spinal cord

iv.iv. ~20 billion in the body~20 billion in the body

III. Neuroglia III. Neuroglia

A.A. Neuroglia of the CNS:Neuroglia of the CNS:

1.1. Ependymal CellsEpendymal Cells

a.a. Epithelial cells (cuboidal to columnar in shape)Epithelial cells (cuboidal to columnar in shape)

b.b. Line the ventricles of the brain and central canal Line the ventricles of the brain and central canal of the spinal cord (both are filled with of the spinal cord (both are filled with cerebrospinal fluid cerebrospinal fluid (CSF))(CSF))

2.2. AstrocytesAstrocytesa.a. Largest, most numerous glial cellsLargest, most numerous glial cellsb.b. Functions:Functions:

i.i. Maintaining Maintaining blood-brain barrier blood-brain barrier - isolates the CNS from - isolates the CNS from the general circulationthe general circulation

ii.ii. Creating a framework for the CNS (microfilaments)Creating a framework for the CNS (microfilaments)

iii.iii. Repairing damaged neural tissue-stabilize tissue & Repairing damaged neural tissue-stabilize tissue & prevent further damageprevent further damage

iv.iv. Guiding neuron development (in embryos)Guiding neuron development (in embryos)

v.v. Controlling the interstitial environmentControlling the interstitial environment

3.3. OligodendrocytesOligodendrocytes

a.a. Myelinate CNS axons Myelinate CNS axons ((myelinmyelin= multilayered = multilayered membraneous sheath)membraneous sheath)

a.a. Increases speed of communicationIncreases speed of communication

b.b. Myelinated areas appear white, hence “white Myelinated areas appear white, hence “white matter”matter”

c.c. Tie clusters of axons togetherTie clusters of axons together

4.4. MicrogliaMicroglia

a.a. Remove cell debris, wastes, and pathogens by Remove cell debris, wastes, and pathogens by phagocytosisphagocytosis

B.B. Neuroglia of the PNS:Neuroglia of the PNS:1.1. Soma are clustered together in masses Soma are clustered together in masses

called called gangliaganglia

2.2. Soma & axons are completely insulated by Soma & axons are completely insulated by processes of glial cells:processes of glial cells:

a.a. Satellite cells (amphicytes)- Satellite cells (amphicytes)- surround soma in surround soma in peripheral gangliaperipheral ganglia

b.b. Schwann cells (neurilemmocytes)- Schwann cells (neurilemmocytes)- form a form a sheath around every peripheral axon (even sheath around every peripheral axon (even unmyelinated)unmyelinated)

3.3. DemyelinationDemyelinationa.a. Progressive destruction of myelin sheaths in the Progressive destruction of myelin sheaths in the

CNS & PNSCNS & PNS

b.b. Results in a gradual loss of sensation & motor Results in a gradual loss of sensation & motor control that leaves affected areas numb & control that leaves affected areas numb & paralyzedparalyzed

i.i. Heavy metal poisoningHeavy metal poisoningii.ii. DiphtheriaDiphtheriaiii.iii. Multiple sclerosis (MS)Multiple sclerosis (MS)

Neural Responses to InjuryNeural Responses to Injury

Read page 407 in your text to learn how a Read page 407 in your text to learn how a damaged axon is able to (sometimes) damaged axon is able to (sometimes) repair itself.repair itself.