1. The Peripheral Nervous System: Afferent Division
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 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.

3. Basal nuclei Thalamus Hypothalamus Cortex Higher processing
(Alzheimers)
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

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
Meissner’s
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 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.

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 Figure 6.5: Tonic and phasic receptors.
(a) Tonic receptor. This receptor type does not adapt at all or adapts slowly to a sustained stimulus and thus provides continuous information about the stimulus. (b) Phasic receptor. This receptor type adapts rapidly to a sustained stimulus and frequently exhibits an off response when the stimulus is removed. Thus, the receptor signals changes in stimulus intensity rather than relaying status quo information.
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

18. Table 6-1, p. 186

19. Figure 5.11: Somatotopic map of the primary motor cortex.
(a) Top view of cerebral hemispheres. (b) Motor homunculus showing the distribution of motor output from the primary motor cortex to different parts of the body. The distorted graphic representation of the body parts indicates the relative proportion of the primary motor cortex devoted to controlling skeletal muscles in each area.
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 Figure 6.7: Lateral inhibition.
(a) The receptor at the site of most intense stimulation is activated to the greatest extent. Surrounding receptors are also stimulated but to a lesser degree. (b) The most intensely activated receptor pathway halts transmission of impulses in the less intensely stimulated pathways through lateral inhibition. This process facilitates localization of the site of stimulation.
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. Basal nuclei Thalamus Hypothalamus Cortex Higher processing
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
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

26. Pain Two best known pain neurotransmitters Substance P Glutamate
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) Higher brain
(Perception of pain)
Thalamus
(Behavioral and
emotional responses
to pain)
Hypothalamus
limbic system
Brain
stem
Reticular
formation
( Alertness)
Noxious
stimulus
Spinal
cord
Afferent pain fiber
Substance P
Nociceptor
Fig. 6-8a, p. 189

28. receptor 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