Chapter 1 A Perspective on Human Genetics

1
Chapter 6A The Peripheral Nervous System
Afferent Division
2
Outline

• Pathways, perceptions, sensations
• Receptor Physiology
• Receptors have differential sensitivities to
  various stimuli.
• A stimulus alters the receptors 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.

3

• Cortex
• Higher processing
• (Alzheimers)http//scienceblogs.com/neurophilosoph
  y/2007/11/alois_alzheimers_first_case.php
• Basal nuclei
• Control of movement, inhibitory, negative
• http//www.vin.com/proceedings/Proceedings.plx?CID
  TUFTSBG2007PID18599Category3036OGeneric
• 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

4
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

5
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

6
What Do You Perceive?
7
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

8
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

9
Epidermis
Ruffini ending
Dermis
Free nerve ending
Meissners corpuscle
Pacinian corpuscle
Hair receptor
10
Golgi tendon organ
Type II sensory neuron
Spinal cord
Intrafusal muscle fibers
Nuclear bag fiber
Type lA sensory neuron
Nuclear chain fiber
Nuclei of muscle fibers
Motor end plate
Alpha motor neuron
Extrafusal muscle fibers
Gamma motor neuron
11
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

12
Receptors

• May be
• Specialized ending of an afferent neuron
• Separate cell closely associated with peripheral
  ending of a neuron
• Stimulus alters receptors 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.

13
Conversion of Receptor and Generator Potentials
into Action Potentials
Receptor Potential
Generator Potential
14
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

15
Fig. 6-5, p. 185
16
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

17
(No Transcript)
18
Table 6-1, p. 186
19
Fig. 5-11, p. 145
20
Acuity

• Refers to discriminative ability
• Influenced by receptive field size and lateral
  inhibition

21
Lateral inhibition
Fig. 6-7, p. 187
22
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

23

• 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

24
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

25
Characteristics of Pain
26
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

27
Somatosensory cortex
(Location of pain)
(Perception of pain)
Higher brain
Thalamus
(Behavioral and emotional responses to pain)
Hypothalamus limbic system
Reticular formation
Brain stem
( Alertness)
Noxious stimulus
Spinal cord
Afferent pain fiber
Substance P
Nociceptor
Fig. 6-8a, p. 189
28
No perception of pain To thalamus
Periagueductal gray matter
Opiate receptor
Reticular formation
Noxious stimulus
Endogenous opiate
Transmission of pain impulses to brain blocked
Afferent pain fiber
Substance P
Nociceptor
Fig. 6-8b, p. 189