Chapter 6AThe Peripheral Nervous System:

Afferent

Divisionhttp://www.brainline.org/multimedia/interactive_brain/the_human_brain.html?gclid=CJroxvfmjaACFVth2godUkI6eA

• Describe the components (afferent and efferent) of the peripheral nervous system. This will be measured by lecture and laboratory exams.

Outline

• Pathways, perceptions, sensations• Receptor Physiology

– Receptors have differential sensitivities to various stimuli.– A stimulus alters the receptor’s permeability, leading to a graded

receptor potential.– Receptor potentials may initiate action potentials in the afferent

neuron.– Receptors may adapt slowly or rapidly to sustained stimulation.– Each somatosensory pathway is “labeled” according to modality

and location.– Acuity is influenced by receptive field size and lateral inhibition.– PAIN– Stimulation of nociceptors elicits the perception of pain plus

motivational and emotional responses.– The brain has a built-in analgesic system.

Peripheral Nervous System

• Consists of nerve fibers that carry information between the CNS and other parts of the body

• Afferent division– Sends information from internal and external

environment to CNS• Visceral afferent

– Incoming pathway for information from internal viscera (organs in body cavities)

• Sensory afferent– Somatic (body sense) sensation

» Sensation arising from body surface and proprioception

– Special senses» Vision, hearing, taste, smell

Perception

• Conscious interpretation of external world derived from sensory input

• Why sensory input does not give true reality perception

– Some information is not transduced

– Some information is filtered out

– Cerebral cortex further manipulates the data

– Sensation vs. perception

What Do You Perceive?

Proof !

Receptors

• Structures at peripheral endings of afferent neurons

• Detect stimuli (change detectable by the body)

• Convert forms of energy into electrical signals (action potentials)

– Process is called transduction

Types of Receptors• Photoreceptors

– Responsive to visible wavelengths of light• Mechanoreceptors

– Sensitive to mechanical energy• Thermoreceptors

– Sensitive to heat and cold• Osmoreceptors

– Detect changes in concentration of solutes in body fluids and resultant changes in osmotic activity

• Chemoreceptors– Sensitive to specific chemicals– Include receptors for smell and taste and receptors that detect

O2 and CO2 concentrations in blood and chemical content of digestive tract

• Nociceptors – Pain receptors that are sensitive to tissue damage or distortion

of tissue

Subcutaneoustissue

Dermis

Epidermis

Myelinatedneuron

Shaft of hair inside follicle Skin surface

Merkel’sdisc: light,sustainedtouch

Meissner’scorpuscle:light,flutteringtouch

Ruffini endings:deep pressure

Pacinian corpuscle:vibrations and deeppressure

Hairreceptor:hairmovementand verygentle touch

Figure 6-5 p190

Muscle Receptors

• Two types of muscle receptors.

• Both are activated by muscle stretch, but monitor different types of information.

• Muscle spindles monitors muscle length.

• Golgi tendon organs detect changes in tension.

Spinalcord

Type IIsensory neuron

Type lAsensory neuron

Alpha motor neuron

Gamma motor neuron

Golgi tendon organ

Intrafusal musclefibers

Nuclear bagfiber

Nuclear chainfiber

Nuclei of musclefibers

Motor end plate

Extrafusal musclefibers

Muscle spindle (proprioceptor)regulates rate of change of length,And length

Like pg. 289

Capsule

Alpha motorneuron axon

Gamma motorneuron axon

Afferent neuronaxons

Extrafusal (“ordinary”)muscle fibers

Noncontractilecentral portionof intrafusal fiber

Contractile end portions of intrafusal fiber

Intrafusal (spindle)muscle fibers

Fig. 8-25a, p. 289

Uses For Perceived Information

• Afferent input is essential for control of efferent output

• Processing of sensory input by reticular activating system in brain stem is critical for cortical arousal and consciousness

• Central processing of sensory information gives rise to our perceptions of the world around us

• Selected information delivered to CNS may be stored for further reference

• Sensory stimuli can have profound impact on our emotions

Receptors

• May be– Specialized ending of an afferent neuron– Separate cell closely associated with peripheral ending of

a neuron• Stimulus alters receptor’s permeability which leads to graded

receptor potential• Usually causes nonselective opening of all small ion channels• This change in membrane permeability can lead to the influx

of sodium ions. This produces receptor (generator) potentials.

• The magnitude of the receptor potential represents the intensity of the stimulus.

• A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS.

Stimulus

Stimulus strength Time (sec)Off

Sti

mu

lus

stre

ng

th Magnitude of receptor potential

Rec

epto

r p

ote

nti

al (

mV

)

Frequency of action potentials in afferent fiber

Aff

eren

t fi

ber

po

ten

tial

(m

V)

Rate of neurotransmitter release at afferent terminals

Sensoryreceptor

Afferentfiber

Afferentterminals

On OnOff

Rest

+30

–70

Figure 6-3 p189

Conversion of Receptor Potentials into Action Potentials

Receptors

• May adapt slowly or rapidly to sustained stimulation

• Types of receptors according to their speed of adaptation

– Tonic receptors• Do not adapt at all or adapt slowly

• Muscle stretch receptors, joint proprioceptors

– Phasic receptors • Rapidly adapting receptors

• Tactile receptors in skin

Fig. 6-5, p. 185

Tonic -Takes longer for the membraneVoltage to drop (maintaining the signal i.e position)

Phasic- Membrane potential dropsMore rapidly (intensity i.e pressure)

Somatosensory Pathways

• Pathways conveying conscious somatic sensation

• Consists of chains of neurons, or labeled lines, synaptically interconnected in particular sequence to accomplish processing of sensory information

– First-order sensory neuron• Afferent neuron with its peripheral receptor that first

detects stimulus

– Second-order sensory neuron• Either in spinal cord or medulla

• Synapses with third-order neuron

– Third-order sensory neuron• Located in thalamus

Table 6-1 p192

Fig. 5-11, p. 145

Acuity

• Refers to discriminative ability

• Influenced by receptive field size and lateral inhibition

Fig. 6-7, p. 187

Lateral inhibition

Pain

• Primarily a protective mechanism meant to bring a conscious awareness that tissue damage is occurring or is about to occur

• Storage of painful experiences in memory helps us avoid potentially harmful events in future

• Sensation of pain is accompanied by motivated behavioral responses and emotional reactions

• Subjective perception can be influenced by other past or present experiences

• Cortex– Higher processing

• Basal nuclei– Control of movement, inhibitory, negative

• Thalamus– Relay and processing of sensory information– Awareness, a positive screening center for information

• Hypothalamus– Hormone secretion, regulation of the internal environment

• Cerebellum– Important in balance and in planning and executing voluntary

movement

• Brain Stem– Relay station (posture and equilibrium), cranial nerves,

control centers, reticular integration, sleep control

Pain

• Presence of prostaglandins (lower nociceptors threshold for activation) greatly enhances receptor response to noxious stimuli

– Role of asprin

• Nociceptors do not adapt to sustained or repetitive stimulation

• Three categories of nociceptors

– Mechanical nociceptors• Respond to mechanical damage such as cutting, crushing, or

pinching

– Thermal nociceptors• Respond to temperature extremes

– Polymodal nociceptors• Respond equally to all kinds of damaging stimuli

Table 6-2 p194

Pain

• Two best known pain neurotransmitters

– Substance P• Activates ascending pathways that transmit nociceptive

signals to higher levels for further processing

– Glutamate • Major excitatory neurotransmitter

• Brain has built in analgesic system

– Suppresses transmission in pain pathways as they enter spinal cord

– Depends on presence of opiate receptors• Endogenous opiates – endorphins, enkephalins,

dynorphin

Higher processing of pain

• Substance P– Different destinations

• Cortex – localizes the pain• Thalamus- perception of

pain• Reticular formation-

increases alertness• Hypothalamus/limbic

system- emotional and behavioral responses

• Glutamate– AMPA receptors

• Ap’s in the dorsal horn

– NMDA receptors• Ca entry makes dorsal

horn neuron more sensitive

(Behavioral andemotional responsesto pain)

(Perception of pain)

(Localization of pain)

Higherbrain

Brainstem

Spinalcord

Dorsal hornexcitatoryinterneurons

Reticularformation Noxious

stimulus

(a) Substance P pain pathway

Thalamus

( Alertness)

Substance P

Afferent pain fiber

Nociceptor

Hypothalamus;limbic system

Somatosensorycortex

Figure 6-9a p195

Dorsal hornexcitatoryinterneurons

Noxiousstimulus

Substance P

Afferent pain fiber

Nociceptor

ReticularformationMedulla

Periaqueductalgray matter

Opiatereceptor

Endogenousopiate

No perception of painTo thalamus

Transmissionof painimpulses tobrain blocked

Inhibitoryinterneuronin dorsal horn

(b) Analgesic pathway

Figure 6-9b p195