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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

BIOL 2401 : Anatomy/ Physiology

PNS

Collin County Community College


Peripheral Nervous System (PNS)

PNS – all neural structures outside the brain andspinal cord

Includes sensory receptors, peripheral nerves,associated ganglia, and motor endings

Provides links to and from the externalenvironment

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PNS in the Nervous System

Figure 13.1


Sensory Receptors

Structures specialized to respond to stimuli

Activation of sensory receptors results indepolarizations that trigger impulses to the CNS

The realization of these stimuli, sensation andperception, occur in the brain

Sensory Receptors can be specialized cells closely associated with peripheral

endings of sensory neurons

or specialized regions of sensory neurons.

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Receptor Classification by Stimulus Type

Mechanoreceptors – respond to touch, pressure,vibration, stretch, and itch

Thermoreceptors – sensitive to changes intemperature

Photoreceptors – respond to light energy (e.g.,retina)

Chemoreceptors – respond to chemicals (e.g.,smell, taste, changes in blood chemistry)

Nociceptors – sensitive to pain-causing stimuli


Receptor Class by Location: Exteroceptors Respond to stimuli arising outside the body

Found near the body surface

Sensitive to touch, pressure, pain, and temperature

Include the special sense organs

Receptor Class by Location: Interoceptors Respond to stimuli arising within the body

Found in internal viscera and blood vessels

Sensitive to chemical changes, stretch, and temperature changes

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Receptor Class by Location: Proprioceptors

Respond to degree of stretch of the organs theyoccupy

Found in skeletal muscles, tendons, joints,ligaments, and connective tissue coverings ofbones and muscles

Constantly “advise” the brain of one’s movements


Receptors are structurally classified as eithersimple or complex

Most receptors are simple and includeencapsulated and unencapsulated varieties

Complex receptors are special sense organs

Receptor Classification by StructuralComplexity

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Simple Receptors: Unencapsulated Free dendritic nerve endings

Respond chiefly to temperature and pain

Merkel (tactile) discs

Hair follicle receptors

Simple Receptors: Encapsulated Meissner’s corpuscles (tactile corpuscles)

Pacinian corpuscles (lamellated corpuscles)

Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles

Joint kinesthetic receptors


Receptor Density• Receptors vary in terms of

abundance relative to each other.• For example, there are far more

pain receptors than temperaturereceptors in the body.

• Receptors also vary in terms ofthe concentration of theirdistribution over the surface ofthe body

• The fingertips having far moretouch receptors than the skin ofthe back of the hand. The figureshows the distribution oftemperature receptors in the skinby area.

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Simple Receptors: Unencapsulated

Table 13.1.1


Simple Receptors: Encapsulated

Table 13.1.2

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From Sensation to Perception Survival depends upon sensation and perception

Sensation is the awareness of changes in the internal andexternal environment

Perception is the conscious interpretation of those stimuliand of the external world from a pattern of differentsensory nerve impulses via the sensory receptors.

Some perceptions are indeed integrated compoundsensations such as for example “wetness” ( touch, pressureand thermal input…. there is no such thing as a “wet-receptor”)


Organization of theSomatosensory System

Input comes from exteroceptors,proprioceptors, and interoceptors

The three main levels of neuralintegration in the somatosensorysystem are:

Receptor level – the sensorreceptors

Circuit level – ascendingpathways

Perceptual level – neuronalcircuits in the cerebral cortex

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Processing at the Receptor Lever The receptor must have specificity for the stimulus energy

( temperature, touch, pressure, light,…)

The receptor’s receptive field must be stimulated

Stimulus energy must be converted into a gradedpotential

If the receptive field is in the same neuron that generatesthe action potential, we call it a generator potential.

If the receptive field is in a separate cell, it is called areceptor potential. If summed up to reach threshold, hhiswill then release neurotransmitters in order to excite theassociated sensory neuron.


Processing at the Receptor Lever The steps in formation of a generator potential are not known

for every receptor, but where it has been studied the start of thegenerator potential usually results from an increase in thepermeability of the membrane of the receptor to all small ions

Usually, the ion furthest from its electrochemical equilibriumand in greatest concentration, namely sodium, contributes thegreatest current. ( and thus results in EPSP’s)

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Processing at the Receptor LeverStimulus (1)

GeneratorPotential (2)

ActionPotential (3)

Stimulus (1)

ReceptorPotential (2)

ActionPotential (3)

N.T. release (3)


Example : Muscle spindle• Muscle spindles are

composed of 3-10 intrafusalmuscle fibers that lackmyofilaments in their centralregions, are noncontractile,and serve as receptivesurfaces

• They inform the body of themuscle tone and length of amuscle. They becomeactivated when stretched andsend sensory impulses to theCNS.

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Example : Muscle spindle• The figure shows the graded

responses of the musclespindles when the muscle isstretched.

• The amplitude of thegenerator potentials increasewith increasing stimulusstrength. Different amounts ofmuscle stretch ( as shown bythe heights in the lower trace)resulted in the graded seriesof generator potentials shownin the upper trace.


Example : Pacinian Corpuscle Pacinian corpuscles are

present in the skin, somemucous membranes etc. Theyare mechanoceptors,responding to pressure, or anykind of mechanical stimuluscausing a deformation of thecorpuscle.

The Pacinian corpuscle has asingle afferent nerve fiber. Itsend is covered by a sensitivereceptor membrane whosesodium channels will openwhen the membrane isdeformed in any way.

It is surrounded by several concentric capsules of connectivetissue, with a viscous gel between them.

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Example : Pacinian Corpuscle

In the resting state, a cross-section through the corpusclelooks something like this

Now, if the skin over thecorpuscle is touched, it will bedeformed and make a nuisanceof itself:

But the viscous gel between thecapsules will move and allow thenerve ending to resume itsnormal shape:

If the pressure is now released,the corpuscle as a whole willresume its original shape, butthe nerve ending will bedeformed in the process:

The viscous gel will then flowback, and soon we are back atthe beginning.


Example : Pacinian Corpuscle

The result is two generator potentials; one when pressure isapplied and one when pressure is released.

This system is thus very good for picking up vibrations.

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Adaptation of Sensory Receptors To a certain extent, the duration of the generator potential

depends upon the duration of the stimulus.

However, some receptors have generator potentials that lastonly a short time, no matter how long the stimulus ismaintained.

We refer to a decrease in the amplitude of the generatorpotential or the frequency of discharge of the sensory fiber inthe face of a persisting, constant stimulus as adaptation.


Adaptation of Sensory Receptors

Some receptors are fast adapting, some are slow adapting. Those that adapt fast arecalled phasic receptors. Thos that adapt slow or not at all are called tonic receptors.

Receptors responding to pressure, touch, and smell adapt quickly

Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, andinteroceptors that respond to chemical levels in the blood

Pain receptors and proprioceptors do not exhibit adaptation

There is purpose to these differences. Tonic (slowly-adapting) receptors are important in situations where

constant information about a stimulus is important ( they thus sendinformation about ongoing stimulation)

Phasic (rapidly-adapting) receptors send information related to changingstimuli. They stop responding to a maintained stimulus, but when thestimulus is removed, they respond again

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Adaptation of Sensory Receptors


Tonic and Phasic Receptors

Diagram showing the differences and effects of tonic andphasic receptors

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Information Coding

Any stimulus contains within it certain features that are of interest to thebody. Stimuli have

• intensities or strengths• locations or sites of application• frequencies of application• rates of application• modalities

Modality, broadly speaking, is a class of sensations that are referred to asingle type of receptor. Vision, hearing, touch, smell, and taste are allmodalities ( energy forms).

Sensory receptors may be sensitive to different kind of energies. Forexample, putting pressure on the eye cause you to see light flashes,although the function of the eye receptors is to detect light.


Information Coding

• Doctrine of Specific Nerve Energies, as formulated by JohannusMüller, says that, although a sense organ may be sensitive to manyforms of stimulus energy other than its real stimulus (called theadequate stimulus), the sensation evoked is always like that associatedwith the adequate stimulus, no matter what kind of energy was applied.

• For example : electrical stimulation of the optic nerve, does not resultin an electric shock; the sensation evoked is one of seeing light.

• The doctrine of specific nerve energies implies that the modality orsubmodality of a sensation is determined not by the stimulus, but bywhat specific receptor or nerve fiber is stimulated. The doctrine alsoimplies that the subjective qualities of a modality are determined, notin the receptors themselves, but in the central nervous system. (in thiscase for the optic nerve, it is determined by the visual cortex).

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Processing at theCircuit Level

Chains of three (3) neurons conductsensory impulses upward to thebrain

First-order neurons – soma residein dorsal root or cranial ganglia, andconduct impulses from the skin tothe spinal cord or brain stem

Second-order neurons – somareside in the dorsal horn of thespinal cord or medullary nuclei andtransmit impulses to the thalamus orcerebellum

Third-order neurons – located inthe thalamus and conduct impulsesto the somatosensory cortex of thecerebrum


Processing at the Circuit Level• Neuronal signals from skin and deeper structures are segregated in the

spinal cord.

• For pain, temperature and the less discriminative aspects of touch,neurons in the dorsal horn have axons that cross in the spinal cord andascend via the spinothalamic tract

• For discriminative touch and for conscious proprioception, the axons ofprimary sensory neurons ascend ipsilaterally ( do not cross over) in thedorsal funiculus (either gracile or cuneate fasciculus) and end in thegracile or cuneate nucleus. Fibers arising in these nuclei cross in themedulla and ascend in the medial lemniscus, which is near the midline inthe medulla and shifts to a lateral location in the midbrain.

• The differences between the two main ascending somatosensory pathwaysare important functionally and clinically. Specific lesions within the spinalcord can thus results in specific loss of sensations in the body.

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Processing at the Circuit Level

Simple touch,temperature, pain

Discriminative touch, consciousproprioception


Processing at the Perceptual Level Both spinothalamic tract and the medial lemniscus

terminate in the ventral posterior nucleus of the thalamus.

The thalamus projects fibers to:

The somatosensory cortex of postcentral gyrus

Sensory association areas

First one modality is sent, then those considering morethan one

The result is an internal, conscious image of the stimulus

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Main Aspects of Sensory Perception Perceptual detection – detecting that a stimulus has

occurred and requires summation

Magnitude estimation – how much of a stimulus is acting

Spatial discrimination – identifying the site or pattern ofthe stimulus

Feature abstraction – used to identify a substance that hasspecific texture or shape

Quality discrimination – the ability to identifysubmodalities of a sensation (e.g., sweet or sour tastes)

Pattern recognition – ability to recognize patterns instimuli (e.g., melody, familiar face)

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