Chapter 6A The Peripheral Nervous System: Afferent Division.

Chapter 6AThe Peripheral Nervous System:

Afferent Division

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.

• Cortex– Higher processing

• (Alzheimers)http://scienceblogs.com/neurophilosophy/2007/11/alois_alzheimers_first_case.php

• Basal nuclei– Control of movement, inhibitory, negative

• http://www.vin.com/proceedings/Proceedings.plx?CID=TUFTSBG2007&PID=18599&Category=3036&O=Generic

• 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– http://neuro.psychiatryonline.org/cgi/content/full/16/3/367

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

centers, reticular integration, sleep control

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

– Cerebral cortex further manipulates the data

– Sensation vs. perception

What Do You Perceive?

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

Epidermis

Dermis

Paciniancorpuscle

Hairreceptor

Ruffiniending

Freenerveending

Meissner’scorpuscle

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

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.

Conversion of Receptor and Generator Potentials into Action Potentials

Receptor Potential Generator Potential

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

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, p. 186

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

• 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

Characteristics of Pain

Fast Pain Slow Pain

Occurs on stimulation of mechanical and thermal nociceptors

Occurs on stimulation of polymodal nociceptors

Carried by small, myelinated A-delta fibers

Carried by small, unmyelinated C fibers

Produces sharp, prickling sensation

Produces dull, aching, burning sensation

Easily localized Poorly localized

Occurs first Occurs second, persists for longer time, more unpleasant

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

Somatosensorycortex

Thalamus

Hypothalamuslimbic system

Reticularformation

Noxiousstimulus

Afferent pain fiber

Substance P

Spinalcord

Brainstem

Higherbrain

(Location of pain)

(Perception of pain)

( Alertness)

(Behavioral andemotional responsesto pain)

Nociceptor

Fig. 6-8a, p. 189

Periagueductalgray matter

Opiatereceptor Noxious

stimulus

Afferent pain fiberSubstance P

No perception of painTo thalamus

Transmissionof painimpulses tobrain blocked

Nociceptor

Reticularformation

Endogenous opiate

Fig. 6-8b, p. 189