The Peripheral Nervous System: Afferent Division Chapter 6A The 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.

Basal nuclei Thalamus Hypothalamus 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 Ruffini ending Dermis Free nerve ending Meissner’s corpuscle Pacinian corpuscle Hair receptor

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

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 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

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

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

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

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

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

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

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 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

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

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