Chapter 6A The Peripheral Nervous System: Afferent Division
Dec 26, 2015
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
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
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
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
Acuity
• Refers to discriminative ability
• Influenced by receptive field size and 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