the nervous system - colorado.edu spinal cord, and the peripheral nervous system consisting of ......

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Anatomy and Physiology for EngineersSlide 5-1

Reading: Chapters 4,5.Coordinates (with endocrine system) organ system activities in responseto varying external conditions. Composed of the central nervous system (CNS) consisting of the brainand spinal cord, and the peripheral nervous system consisting ofcommunication pathways from CNS to rest of the body.Communication occurs via electrical impulses (high speed pathway).Functions of the nervous system.

Monitors the internal and external environments.Integrates sensory information.Coordinates voluntary and involuntary activities of other organ systems.

These functions are performed by neurons, which are supported byneuroglia (external support cells).

The Nervous System


Organization of the Nervous SystemWithin peripheral nervoussystem, there are the afferentand efferent divisions.Afferent brings sensoryinformation to CNS.Efferent carries motorcommands from CNS tomuscles, glands, etc.Within efferent division, thereare 2 sub-divisions:

Somatic nervous system (SNS)provides voluntary control overskeletal muscles.Autonomic nervous system (ANS)provides involuntary control ofcardiac and smooth muscles, andglands.

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Cellular Organization of Neural Tissue

Neural system consists of all neural tissue in thebody.Neural tissue consists of two types of cells: neuronsand neuroglia (also called glial cells).Neurons are the basic units of the nervous system,and all neural functions involve the communicationof one neuron with another, and with other cells.Neuroglia regulate environment around the neurons,provide structural support for neurons, and act torecycle debris (phagocytes).Neuroglia cells outnumber neurons.


NeuronsClassified into 3 functional groups:

Sensory neurons (Number approximately 106).Make up the afferent division of the peripheral nervous system.Responsible for providing information regarding the externalenvironment (touch, pressure, temperature, smell, hearing), the positionof the skeletal muscles and joints, and the activities of the other organsystems (digestive, cardiovascular, etc).

Motor neurons (Number approximately 500,000).Part of the efferent system (carries info. from CNS to tissues, organs andorgan systems).Somatic motor nuerons target voluntary tissues (skeletal muscles) whilevisceral (autonomic) motor neurons target involuntary tissues (cardiacand smooth muscles, glands).

Interneurons (Number approximately 20 X 109).Located entirely within brain and spinal cord (CNS).Function to interconnect other neurons.Responsible for the analysis of sensory inputs.

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General Structure of Neurons


General Structure of Neurons

Cell body (soma).

Soma of the neuron contains large round nucleus withprominent nucleolus.No centrioles ‡ neurons cannot be replaced once theydifferentiate.Numerous other cellular organelles (mitochondria, free andfixed ribosomes, rough endoplasmic reticulum, etc).Rough ER and ribosomes also found in clusters called Nisslbodies (gray color ‡ gray matter).

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General Structure of NeuronsCell body (soma).Dendrites to receive information.Axon to carry information.Synaptic knobs to transmitinformation to other cell.Projecting from soma are variable number of dendrites and a single longaxon.Stimulation of dendrite or cell body (mechanical, electrical, chemical)produces action potential that travels along axon.Base of axon connected to soma at axon hillock, where action potentialbegins.Axon may branch along its length producing collaterals.Each collateral ends at a synaptic terminal (synaptic knob).Synaptic terminal is part of the synapse ‡ where communication withanother cell takes place.


Structural Classification of Neurons

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Neuroglia (glial cells)Found in both the CNS and PNS, but CNS has greatest diversity.4 types of glial cells found in CNS:

AstrocytesLargest and most numerous neuroglia.Responsible for maintaining the blood-brain barrier, structural stability of CNS,and repair of damaged neural tissue.

OligodendrocytesResponsible for coating axons with cytoplasmic extensions(myelin sheath).

MicrogliaSmallest and rarest of neuroglia in CNS.Phagocytic cells responsible for immune protection.

Ependymal cellsLine central canal of spinal cord and ventricles of the brain.In certain locations, ependymal cells produce cerebrospinal fluid.


Neuroglia in the CNS

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Neuroglia in the Peripheral Nervous SystemSchwann cells aremost important glialcells in peripheralnervous system.Cover every axonoutside CNS (whethermyelinated orunmyelinated).This myelin sheathincreases impulseconduction rate.


NeurophysiologyCell membrane (for generic cell) has excess positivecharges (ions) on outside versus inside.This membrane potential for neurons is around –70mV.Resting membrane potentialis due to relativeconcentrations of ions.

Larger concentration of Na+,Cl- in extracellular fluid.Higher concentrations of K+

and negatively chargedproteins within cell.

Since cell membrane is more permeable to K+ ions than Na+ ionsin resting state, rate of K+ efflux is greater than Na+ influx ‡ cellmembrane is slightly negative.

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Changes in Membrane Potential

Changes in membrane potential can occur due to:Alteration of permeability of cell membrane.Alteration in the activity of an exchange pump.

This can occur due to exposure to chemical,electrical, temperature, ionic, or mechanicalstimulations.Changes can occur almost instantaneously (ex:muscle cell contraction).Usually stimulation opens closed channels withinmembrane, thereby changing permeability of cellmembrane to specific ions.


Polarization and Potential

Depolarization: making the membrane potential morepositive.

Ex: opening of selective Na+ channels, allowing extracellular Na+ todiffuse faster into the cell, causing membrane potential to rise toward0 mV.

Hyperpolarization: making the membrane potential morenegative:

Ex: opening of selective K+ channels, causing intracellular K+ todiffuse out of cell.

Graded Potential: Variation in cell membrane permeabilityand polarization at a local site.Action Potential: Variation in polarization across entire cellmembrane.Graded potential acts as the stimulus that produces the actionpotential

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Generation of the Action Potential


Conduction of an Action PotentialContinuous (unmyelinated) and saltatory (myelinated) conduction.

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Synaptic CommunicationAction potential carries information from one point to another via thenervous system.Information transfer at the end of an axon occurs via the release ofchemicals known as neurotransmitters.The location at which this information transfer takes place is the synapse.

Synaptic cleft: space betweenthe two cell membranes.Presynaptic neuron ‡ transmitsinformation to postsynapticneuron, muscle or gland cell.Presynaptic neuron containsspecific neurotransmittersenclosed in vesicles.


Events in the Synaptic Connection

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The Reflex Arc


Anatomy of the Nervous SystemIntroduction to terminology.In peripheral nervous system:

Ganglia: groups of neuron cell bodies.Nerves: bundles of axons (spinal nerves connect to spinal cord; cranial nervesconnect to brain).

In central nervous system:Centers: collection of neuron cell bodies that share a particular function.Nucleus: center that has a distinct anatomical boundary.Neural Cortex: gray matter on the surface of the brain.Tracts: bundles of axons within CNS that share common origin, destinationand function.Columns: groups of tracts.Pathways: avenues that link brain centers to rest of the body (sensorypathways ‡ bring information to brain; motor pathways ‡ transmitinformation from brain to muscles).

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Anatomy of the Nervous System


Anatomy of the Nervous SystemProtection of central nervous system is provided by glialcells and by the meninges and the blood-brain barrier.Meninges:

Series of three specialized membranes.Dura Mater

Tough fibrous outermost covering of the CNS.Epidural space: between dura mater of spinal cord and walls ofvertebral column.

ArachnoidIntervening space containing a layer of squamous cells.Subarachnoid space: deeper layer within meninges that containsthe cerebrospinal fluid.

Pia MaterInnermost layer, in direct contact with underlying neural tissue.Highly vascular ‡ blood to supply CNS function.

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Anatomy of the Nervous System


Blood-Brain Barrier

Neural tissue is isolated from rest ofcirculation by this barrier.Maintained by astrocytes, which causecapillary membranes in the CNS to beimpermeable to various compounds.

Lipid soluble substances can get through.Water soluble substances get through only viafacilitated diffusion.

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The Spinal CordCentral canal filled withcerebrospinal fluid (CSF).Consists of 31 segments, eachidentified by letter (vertebralsection) and number (individualnerve).Internal diameter decreases as itextends inferiorly (with twoexceptions).

Cervical enlargement ‡ nerves topectoral girdle and upper limbs.Lumbar enlargement ‡ nerves topelvis and lower limbs.


The Spinal CordEach spinal segmentcontains a pair of dorsalroot ganglia (group ofneuron cell bodies).Each ganglion contains adorsal root (axons throughwhich sensory informationarrives) and a ventral root(axons through whichinformation is sent out).

Dorsal and ventral roots unite into a single spinal nerve.Spinal cord extends only to L1 or L2. After this, nervesextend inferiorly without spinal cord protection.

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The BrainAdult brain consists of 6 major regions:

Cerebrum2 large paired hemispheres (left and right).Home of conscious thought processes, sensations, intellectual functions, memory storageand retrieval, and complex motor functions.

Diencephalon (thalmus and hypothalmus)Hollow internal structure connected to the cerebrum.Relaying and processing sensory information (thalmus), and processing emotions,autonomic function and hormone production (hypothalmus).

Midbrain (also called the mesencephalon)Part of the brain stem which connects brain to spinal cord.Processes auditory and visual information; generates involuntary motor responses.

Pons (Also part of the brain stem)Involved with somatic and visceral motor control.

Medulla Oblongata (Direct attachment to spinal cord)Relays sensory information to various brain centers; also contains centers for autonomicregulation of heart rate, digestion, blood pressure, respiration, etc.

CerebellumFound posterior and inferior (covers the brain stem).Adjustment of voluntary and involuntary activities based on input and stored memories.


The BrainCerebral Hemispheres and the Cerebellum

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The BrainBrain Stem and the Cerebellum


The BrainDiencephalon, Brain Stem, and Cerebellum

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Ventricles and Cerebrospinal FluidBrain contains 4 ventricles (hollow chambers) filled with cerebrospinal fluid.Two larger lateral ventricles located in each cerebral hemisphere.Third ventricle located in diencephalon.Fourth ventricle located within brain stem and is continuous with spinal canal.Cerebrospinal fluid important for shock absorption, buoyancy, and transport ofnutrients and waste products.CSF constantly produced by ependymal cells and circulated within the choroidplexus (vascular network that extends into each ventricle).CSF is in direct contact with interstitial fluid and free exchange takes placebetween these fluids.


Ventricles and Cerebrospinal Fluid

Cerebrospinal fluidcirculates within andaround brain and spinalcord.CSF forms at thechoroid plexus,circulates within theventricles, down thespinal canal, out intothe sub-arachnoidspace, and back up andaround the brain.

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The CerebrumLargest structure of the brain.Two hemispheres: left hemisphere responsible for motor andsensory control over right side of body, and vice versa.Two hemispheres linked via region of white matter known asthe corpus callosum.Responsible for conscious thought, intellectual function,limited input and control of sensory information.Includes gray and white matter,Key structures:

Cerebral cortex: thick blanket of gray matter outer covering surface.Gyri (gyrus): ridges on cerebral cortex.Sulci (sulcus): depressions within cerebral cortex.Longitudinal fissure: groove separating the hemispheres.Lobes: well defined regions within each hemisphere.


The Cerebrum

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The Cerebral Nuclei

Masses of gray matter that lie within the 2 larger ventricles(lateral ventricles) and within the white matter of eachcerebral hemisphere.Important in control of learned movement patterns.

The Cerebrum


The Limbic SystemFunctional group that includes the olfactory cortex, various cerebralnuclei, gyri and tracts along border between cerebrum and diencephalon.

The Cerebrum

Functions include:Establishingemotional states andrelated behavioraldrives.Linking consciousand intellectualfunctions ofcerebrum tounconsciousautonomic functionsof brain stem.

Long term memory storage and retrieval.

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The DiencephalonProvides control and relaying of conscious and unconscious sensory and motorinformation.Structures contained within diencephalon:

Third ventricle and pineal gland (endocrine structure that secretes hormone melatonin).Thalmus

Final relay point for ascending sensory information (except smell).Passes only a small portion of this information to active consciousness (primarysensor cortex within cerebrum).Remaining is relayed to cerebral nuclei and other centers within brain stem.Also important for coordination of voluntary and involuntary motor commands.

Hypothalmus:Located deep within diencephalon.Contains centers associated with emotions (rage, pleasure, etc).Adjusts and controls autonomic activities of pons and medulla oblongata.Coordinates neural and endocrine activities.Produces variety of hormones (Anti-diuretic hormone - ADH, oxytocin, etc).Coordinates various voluntary and autonomic functions.Maintains body temperature.


The Diencephalon

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The Midbrain

Contains various nuclei andbundles of ascending anddescending nerve fibers.Home of cranial nervesresponsible for eyemovement.Also home to the reticularformation.

Network of interconnectednuclei responsible forwaking and sleeping states.

Midbrain also responsible forcontrol of muscle tone.


The PonsLinks cerebellumwith midbrain,diencephalon,cerebrum and medullaoblongata.Sensory and motorcontrol for cranialnerves.Also responsible forgenerating depth andpace of respiration.

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The CerebellumTwo important functions:

Maintains posture andbalance by makingrapid adjustments inmuscle tone andposition.Programs and fine-tunes variousinvoluntary andvoluntary movements.

These are performed by adjusting and regulating neural activitybetween cerebrum and brain stem.


The Medulla Oblongata

Provides physical connectionbetween brain and spinal cord.All communication betweenthese two structures must passthrough medulla oblongata.Many nerve pathways end insynapses here, and informationis relayed onward throughconnecting neurons.Very important forcardiovascular and respiratorycontrol.

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On to the Peripheral Nervous Systemand Integrated Functions

PNS is the link between neurons in CNS and rest of the body.Although PNS contains < 2% of all neural tissue, it is vital asa pathway between brain and body.Certain decisions may be made without or before enteringcerebral cortex and conscious awareness.This is done via synaptic communication within brain stemand spinal cord.PNS is dominated by nerves (axons bundled together byconnective tissue) coming to or from the CNS.PNS also contains sensory and motor neurons of theautonomic nervous system (ANS).PNS includes the cranial and spinal nerves.


The Cranial Nerves

Components of the PNS that connect to thebrain rather than the spinal cord.12 pairs of cranial nerves, numberedaccording to their position along long-axis ofbrain.Responsible for olfactory, optic, oculomotor,trochlear, facial muscles, acoustic, taste,swallowing, speech, vocal cords, etc.

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The Cranial Nerves


The Spinal Nerves31 pairs of spinal nerves.Grouped according to locationalong vertebral column.8 cranial pairs (C1-C8).12 thoracic pairs (T1-T12)5 lumbar pairs (L1-L5).5 sacral pairs (S1-S5).1 coccygeal pair (C1).Several nerve axons bundled toform a plexus with commondestination (ex: cervical plexusinnervates neck and diaphragm).

the nervous system - colorado.edu spinal cord, and the peripheral nervous system consisting of ......

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