BIO 210 LabInstructor: Dr. Rebecca Clarke

Chapter 12: Neural Tissue

Nervous System

Includes all neural tissue in the bodyOrgans

Brain and spinal cordSensory receptorsNerves

Anatomical Divisions of the Nervous System • Central nervous system (CNS)

• Brain and spinal cord• Peripheral nervous system (PNS)

• All other neural tissue outside CNS, i.e., receptors, nerves, ganglia

Functions of the CNSActs as control center for nervous

systemIntegrates, processes, coordinates:

Sensory data:Convey info about conditions inside/outside body

Motor commands:Control/adjust activities of peripheral organs, e.g.,

skeletal musclesHigher functions of brain:

Intelligence, memory, learning, emotionProvides short-term control over

activities of other systems

Functions of the PNS Links CNS with other systems and with

sense organs Delivers sensory information from

receptors to CNS Carries motor commands from CNS to

peripheral tissues and systems

Neural Tissue Consists of 2 kinds of cells:

Neurons: Send and receive signalsPerform all communication, information

processing, and control functions of NSNeuroglia (glial cells):

Support and protect neuronsPreserve physical and biochemical structure of

neural tissueEssential to survival and function of neurons

Structure of a NeuronNeuron: basic functional unit of nervous

system

Figure 12–1

Multipolar Neuron Common in the CNSLarge cell body (soma):

= perikaryon + nucleusSeveral short, branched dendritesSingle, long axon

Major Organelles of the Cell Body Large nucleus with nucleolus Cytoplasm (perikaryon)Mitochondria (produce energy)RER and ribosomes (produce

neurotransmitters)Cytoskeleton (supports dendrites and

axon)LACK centrioles

Can’t divide

Nissl BodiesDense areas of RER and ribosomesMake neural tissue appear grayNeuron cell body = gray matter

Dendrites Highly branched processesExtend from cell body Dendritic spines:

Many fine processesReceive information from synaptic end

bulbs/axonal terminals of axons of other neurons

80–90% of neuron surface area

Axon Long process that extends from cell bodyCarries electrical signal (action potential)

away from cell bodyAxon structure is critical to function

Structures of the Axon (1 of 3)Axoplasm:

Cytoplasm of axonAxolemma:

Specialized cell membraneCovers axoplasm

Structures of the Axon (2 of 3)Axon hillock:

Cone-shaped, broad region of cell bodyAttaches to…

Initial segment:Base of axon that attaches to axon hillock of

cell body

Structures of the Axon (3 of 3)Collaterals:

Branches of a single axonEnable axon to communicate with other cells

Telodendria:Fine extensions of distal axon

Synaptic (axonal) terminals/end bulbs/knobs:Tips of telodendria

Myelin/Myelin SheathGlossy white, multilayer, lipid wrap

around axonInsulates axon from contact with

extracellular fluid (like insulation on electrical wires)

Increases speed of action potential along axon

Regions of CNS with many myelinated neurons white matter of CNS

Myelin/Myelin SheathFormed by:

Schwann cells (PNS neuroglia)Oligodendrocytes (CNS neuroglia)

Schwann Cells Wrap around axons of

PNS Myelin sheath

(like oligodendrocytes in CNS)

Last layer of wrap (outer surface) = neurilemmaLoose sheathContains nucleus and cell

organellesInternodes = wrapped

areas of axonNodes (of Ranvier) =

gaps between internodes

Figure 12–5a

Schwann Cells1 Schwann cell sheaths 1 segment of axonMany Schwann cells sheath entire axonAxons branch at nodes collaterals

OligodendrocytesWrap around CNS

axonsCan myelinate

portions of several adjacent axons

Process different from Schwann cellsSchwann cells can

myelinate only one segment of a single axon

Figure 12–4

EndoneuriumConnective tissue layerSurrounds neurilemma of nerve fiber

4 Structural Classifications of NeuronsMultipolar neuronUnipolar neuronBipolar neuronAnaxonic neuron

Multipolar Neuron

2 or more dendrites1 Long axonMost common type in

CNSIncludes all skeletal

muscle motor neuronsIn somatic nervous

system (SNS)

Figure 12–3)

Unipolar Neuron

1 Long axonFused dendrite and axonCell body to one sideFound in sensory

neurons of PNS

Figure 12–3

Bipolar Neuron

Small cell1 Dendrite

Branches extensively at distal tip dendritic spines

1 AxonCell body between axon

and dendriteRare

Found in special sense organs, e.g., retina of eye

Figure 12–3

Anaxonic Neuron

SmallAll cell processes look

alikeFound in brain and sense

organs

Figure 12–3

Functional Classifications of Neurons

Sensory Neuron – carries sensory (afferent) information from sensory receptors to the CNSSomatic sensory neurons – monitor outside world and

our position in itVisceral sensory neurons – monitor internal conditions

Interneuron – between the sensory and motor neurons; information processing

Motor Neuron – carries motor (efferent) information from the CNS to the effectorSomatic motor neuron – carries info to skeletal

musclesVisceral motor neuron – carries info to smooth m.,

cardiac m., glands, and adipose tissue

NeurogliaSupporting cellsHalf the volume of the nervous systemMany types of neuroglia in CNS and PNS

Neuroglia of the CNSEpendymal cellsMicrogliaAstrocytesOligodendrocytes

Neuroglia of the CNS: Ependymal Cells

Columnar epithelial cellsForm ependyma

Line fluid (CSF)-filled cavities in:Spinal cord (central canal)Brain (ventricles)

Have slender processes on “nonpassageway” side of cell

Assist in:Producing, circulating

and monitoring composition of CSF Figure 12–4

Cerebrospinal fluid (CSF)Surrounds:

BrainSpinal cord

Protective cushionTransports dissolved gases, nutrients,

wastes, etc.

Neuroglia of the CNS: Microglia

Smallest, rarest neuroglia in CNS

Many fine-branched processes

Phagocytic macrophageMigrates through

neural tissue; wandering police force/janitorial service

Cleans up cellular debris, waste products, pathogens

Figure 12–4

Neuroglia of the CNS: Astrocytes

Star-shapedLargest and most

numerous neuroglia in CNS

Have “feet” on end of many processes

Wide range of functions

Figure 12–4

Functions of AstrocytesMaintain blood-brain barrier

“Feet” extensions wrap around capillariesCreate 3-D framework for CNSRepair damaged neural tissue

Structural repairs that stabilize tissue/prevent further injury

Guide neuron development in embryonic brainControl interstitial environment

Regulate ion, nutrient, dissolved gas concentrations and transport

Control blood flow through capillariesAbsorb and recycle NTs

Neuroglia of the CNS: Oligodendrocytes

Smaller cell bodyFewer processesProcesses

Contact other neuron cell bodies

Wrap around axons to form myelin sheath myelinated axon

Many oligodendrocytes on each axon

Each oligodendrocyte myelinates segments of several axons

Figure 12–4

Neuroglia of the PNSSchwann cellsSatellite cells

Neuroglia of the PNS:Schwann Cells

Wrap around peripheral axons of PNS

Myelin sheath(like oligodendrocytes in CNS)

Figure 12–5a

Neuroglia of the PNS:Satellite CellsSurround ganglia (collections of neuron cell

bodies)Regulate environment around neurons (like

astrocytes in CNS)

White Matter and Gray MatterWhite matter of the CNS

Myelinated axonsGray matter of the CNS

Neuron cell bodies, unmyelinated axons, and neuroglia

Terminology

Collection of: CNS PNS

Neuron Cell Bodies

Nucleus Ganglion

Axons Tract Nerve

Neural Responses to Injuries

Wallerian degeneration

Axon distal to injury degenerates

Schwann cells

Form path for new growth

Wrap new axon in myelin

Nerve Regeneration in CNS

Limited by chemicals released by astrocytes

that

Block growth

Produce scar tissue

Neural Responses to Injuries

Neural Responses to Injuries

Neural Responses to Injuries

Neural Responses to Injuries

Neural Responses to Injuries