Chapter 6A The Peripheral Nervous System: Afferent Divisionhttp:// man_brain.html?gclid=CJroxvfmjaACFVth2godUkI6eA

Describe the components (afferent and efferent) of the peripheral nervous system. This will be measured by lecture and laboratory exams.

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.

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 –Some information is not transduced –Some information is filtered out –Cerebral cortex further manipulates the data –Sensation vs. perception

What Do You Perceive? Proof !

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 O 2 and CO 2 concentrations in blood and chemical content of digestive tract Nociceptors –Pain receptors that are sensitive to tissue damage or distortion of tissue

Subcutaneous tissue Dermis Epidermis Myelinated neuron Shaft of hair inside follicleSkin surface Merkel’s disc: light, sustained touch Meissner’s corpuscle: light, fluttering touch Ruffini endings: deep pressure Pacinian corpuscle: vibrations and deep pressure Hair receptor: hair movement and very gentle touch Figure 6-5 p190

Muscle Receptors Two types of muscle receptors. Both are activated by muscle stretch, but monitor different types of information. Muscle spindles monitors muscle length. Golgi tendon organs detect changes in tension.

Spinal cord Type II sensory neuron Type lA sensory neuron Alpha motor neuron Gamma motor neuron Golgi tendon organ Intrafusal muscle fibers Nuclear bag fiber Nuclear chain fiber Nuclei of muscle fibers Motor end plate Extrafusal muscle fibers Muscle spindle (proprioceptor) regulates rate of change of length, And length Like pg. 289

Capsule Alpha motor neuron axon Gamma motor neuron axon Afferent neuron axons Extrafusal (“ordinary”) muscle fibers Noncontractile central portion of intrafusal fiber Contractile end portions of intrafusal fiber Intrafusal (spindle) muscle fibers Fig. 8-25a, p. 289

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.

Stimulus Stimulus strength Time (sec) Off Stimulus strength Magnitude of receptor potential Receptor potential (mV) Frequency of action potentials in afferent fiber Afferent fiber potential (mV) Rate of neurotransmitter release at afferent terminals Sensory receptor Afferent fiber Afferent terminals On Off Rest +30 –70 Figure 6-3 p189

Conversion of Receptor Potentials into Action Potentials

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 Tonic -Takes longer for the membrane Voltage to drop (maintaining the signal i.e position) Phasic- Membrane potential drops More rapidly (intensity i.e pressure)

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 p192

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 –Role of asprin 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

Table 6-2 p194

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

Higher processing of pain Substance P –Different destinations Cortex – localizes the pain Thalamus- perception of pain Reticular formation- increases alertness Hypothalamus/limbic system- emotional and behavioral responses Glutamate –AMPA receptors Ap’s in the dorsal horn –NMDA receptors Ca entry makes dorsal horn neuron more sensitive

(Behavioral and emotional responses to pain) (Perception of pain) (Localization of pain) Higher brain Brain stem Spinal cord Dorsal horn excitatory interneurons Reticular formation Noxious stimulus (a) Substance P pain pathway Thalamus ( Alertness) Substance P Afferent pain fiber Nociceptor Hypothalamus; limbic system Somatosensory cortex Figure 6-9a p195

Dorsal horn excitatory interneurons Noxious stimulus Substance P Afferent pain fiber Nociceptor Reticular formation Medulla Periaqueductal gray matter Opiate receptor Endogenous opiate No perception of pain To thalamus Transmission of pain impulses to brain blocked Inhibitory interneuron in dorsal horn (b) Analgesic pathway Figure 6-9b p195