Chapter 16Sensory, Motor & Integrative Systems • Levels and components of sensation • Pathways for sensations from body to brain • Pathways for motor signals from brain to body • Integration Process • wakefulness and sleep • learning and memory
Is Sensation Different from Perception? • Sensation is any conscious or unconscious awareness of external and/or internal stimuli • What are we not aware of? • X-rays, ultra high frequency sound waves, UV light • We have no sensory receptors for those stimuli • Perception is the conscious awareness & interpretation of a sensation. • precisely localization & identification • memories of our perceptions are stored in cortex
Sensory Modalities • Different types of sensations • touch, pain, temperature, vibration, hearing, vision • Each type of sensory neuron can respond to only one type of stimuli • Two classes of sensory modalities • general senses • somatic are sensations from body walls • visceral are sensations from internal organs • special senses • smell, taste, hearing, vision, and balance
Process of Sensation • Sensory receptors demonstrate selectivity • respond to only one type of stimuli • Events occurring within a sensation • stimulation of the receptor • transduction (conversion) of stimulus into a graded potential • vary in amplitude and diminish with distance • generation of impulses when graded potential reaches threshold • integration of sensory input by the CNS
Sensory Receptors • General Sensory Receptors (Somatic and Visceral Receptors) • no structural specializations in free nerve endings that provide us with pain, tickle, itch, temperature sensation • some structural specializations in receptors for touch, pressure & vibration • Special Sensory Receptors (Special Sense Receptors) • very complex structures - vision, hearing, equilibrium, taste, & smell
Classification of Sensory Receptors • Structural classification • Type of response to a stimulus • Location of receptors & origin of stimuli • Type of stimuli they detect
Structural Classification of Receptors • Free nerve endings • bare dendrites • pain, temperature, tickle, itch & light touch • Encapsulated nerve endings • dendrites enclosed in connective tissue capsule • pressure, vibration & deep touch • Separate sensory cells • specialized cells that respond to stimuli • vision, taste, hearing, balance
Structural Classification • Compare free nerve ending, encapsulated nerve ending and sensory receptor cell
Classification by Response to Stimuli • Generator potential • free nerve endings, encapsulated nerve endings & olfactory receptors produce generator potentials • when large enough, it generates a nerve impulse in a first-order neuron • Receptor potential • vision, hearing, equilibrium and taste receptors produce receptor potentials • receptor cells release neurotransmitter molecules on first-order neurons producing postsynaptic potentials • PSP may trigger a nerve impulse • Amplitude of potentials vary with stimulus intensity
Classification by Location • Exteroceptors • near surface of body • receive external stimuli • hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature • Interoceptors • monitors internal environment (BV or viscera) • not conscious except for pain or pressure • Proprioceptors • muscle, tendon, joint & inner ear • senses body position & movement
Classification by Stimuli Detected • Mechanoreceptors • detect pressure or stretch • touch, pressure, vibration, hearing, proprioception, equilibrium & blood pressure • Thermoreceptors detect temperature • Nociceptors detect damage to tissues • Photoreceptors detect light • Chemoreceptors detect molecules • taste, smell & changes in body fluid chemistry • Osmoreceptors
Adaptation of Sensory Receptors • Change in sensitivity to long-lasting stimuli • decrease in responsiveness of a receptor • bad smells disappear • very hot water starts to feel only warm • generator potential amplitudes decrease during a maintained, constant stimulus • Receptors vary in their ability to adapt • Rapidly adapting receptors (smell, pressure, touch) • adapt quickly; specialized for signaling stimulus changes • Slowly adapting receptors (pain, body position) • continuation of nerve impulses as long as stimulus persists
Receptor adaptation ‘Phasic receptors’ ‘Tonic receptors’
Somatic Tactile Sensations • Touch • crude touch is ability to perceive something has touched the skin • discriminative touch provides location and texture of source • Pressure is sustained sensation over a large area • Vibration is rapidly repetitive sensory signals • Itching is chemical stimulation of free nerve endings • Tickle is stimulation of free nerve endings only by someone else
Localization acuity is due to Overlapping Receptor fields 2 neurons used to detect stimuli from 3 locations
Activity in second- order neurons Second- order neurons Frequency of Action potentials X2 Y2 Z2 Inhibitory interneurons Inhibitory interneurons X2 Y2 Z2 Second-order neurons Activity in afferent neurons Afferent neurons (ﬁrst- order neurons) Frequency of Action potentials X1 Y1 Z1 X1 Y1 Z1 Afferent neurons Location of stimulus Precision of location of stimulus (acuity) via lateral inhibition
Meissner’s Corpuscle • Dendrites enclosed in CT (encapsulated) in dermal papillae of hairless skin • Discriminative touch & fine vibration used for recognizing texture • Rapidly adapting (phasic receptors) • Generate impulses mainly at onset of a touch
Merkel’s Disc • Flattened dendrites touching cells (Merkel cells) of stratum basale • Free nerve ending • Tonic (slow adapting) • Used in discriminative touch – shapes, edges, texture • (25% of receptors in hands) “fine surface edge” e.g. sandpaper • Krause end bulb – similar to Merkel’s disc but found in mucous membranes, rather than skin.
Ruffini Corpuscle • Found deep in dermis of skin, joint capsules • Encapsulated • Tonic receptor • Detect skin stretch used for joint positioning in fingers, deep touch, continuous touch, stretch & pressure
Pacinian Corpuscle • Onion-like connective tissue capsule enclosing a dendrite = encapsulated • Phasic = rapidly adapting • Found in subcutaneous tissues (deep in dermis, especially on hands, feet, breasts, genitals) & certain viscera (periosteum, joint capsules, pancreas) • Sensations of deep pressure, high-frequency vibration, diffuse vibration e.g. tapping with a pencill, stretch, tickle.
Hair Root Plexus • Free nerve endings found around follicles, detects movement of hair “localized flutter”, phasic = rapidly adapting
ReceptorsSurfaceDeep Receptive fieldSmallLarge Rapidly adaptingMeissnerPacinian encapsulatedencapsulated Hair root free ending Slow adaptingMerkel (& Krause) Ruffini free ending encapsulated
Thermal Sensations • Free nerve endingswith 1mm diameter receptive fields on the skin surface • Cold receptorsin the stratum basale respond to temperatures between 50-105 degrees F • Warm receptorsin the dermis respond to temperatures between 90-118 degrees F • Both adapt rapidly at first, but continue to generate impulses at a low frequency • Pain is produced below 50 and over 118 degrees F.
Pain Sensations • Nociceptors = pain receptors • Free nerve endings found in every tissue of body except the brain • Stimulated by excessive distension, muscle spasm, & inadequate blood flow • Tissue injury releases chemicals such as K+, kinins, seretonin, histamine, or prostaglandins that stimulate nociceptors • Bradykinin the most potent pain stimulus known • No (or little) adaptation occurs
Types of Pain • Fast pain (acute) • occurs rapidly after stimuli (0.1 second) • sharp pain like needle puncture or cut • not felt in deeper tissues • larger A nerve fibers • Slow pain (chronic) • begins more slowly & increases in intensity • diffuse, dull • aching or throbbing pain of toothache • in both superficial and deeper tissues • smaller C nerve fibers
Localization of Pain • Superficial Somatic Pain arises from skin areas • Deep Somatic Pain arises from muscle, joints, tendons & fascia • Visceral Pain arises from receptors in visceral organs • localized damage (cutting) intestines causes no pain • diffuse visceral stimulation can be severe • distension of a bile duct from a gallstone • distension of the ureter from a kidney stone • Phantom Pain (amputated limb sensations)
Referred Pain • Visceral pain that is felt just deep to the skin overlying the stimulated organ or in a surface area far from the organ. • Skin area & organ are served by the same segment of the spinal cord. • Heart attack is felt in skin along left arm since both reach the cord segment T1-T5 and ascend using the same neurons. The brain can not distinguish if the sensation is coming from the heart or chest/arm.
Pain Relief • Aspirin and ibuprofenblock formation of prostaglandins that stimulate nociceptors • Novocaineblocks conduction of nerve impulses along pain fibers (votage gated Na+ channels) • Morphinelessen the perception of pain in the brain. • Endogenous analgesics – enkephalins (200 x more potent than morphine), endorphins, and dynorphins; • There are some people who are born WITHOUT the sense of pain - "congenital insensitivity to pain".
“Spinal” Gate Control Theory Pain can be stopped at the dorsal horn of the spinal cord A famous theory concerning how pain works is called the Gate Control Theory devised by Patrick Wall and Ronald Melzack in 1965. Although the gate control theory has support from some experiments and does explain some observations seen in pain patients during therapy, it does not explain everything.
One mechanism of the gating involves competition between mechanoreceptor information and nociceptor information. The theory states that pain is a function of the balance between the information traveling into the spinal cord through large nerve fibers and information traveling into the spinal cord through small nerve fibers. Large nerve fibers carry non-nociceptive information and small nerve fibers carry nociceptive information. If the relative amount of activity is greater in large nerve fibers, there should be little or no pain. However, if there is more activity in small nerve fibers, then there will be pain.
Pain fiber C fiber relay nociception Stimuli other than pain I = "Inhibitory Interneuron“ tonically active; P = "Projection Neuron“ = ascending pathway (-)= inhibition (blocking); (+) = excitation (activation)
The theory step by step: 1. Without any stimulation, both large and small nerve fibers are quiet and the inhibitory interneuron (I) blocks the signal in the projection neuron (P) that connects to the brain. The "gate is closed" and therefore NO PAIN. 2. With non-painful stimulation, large fibers are activated primarily. This activates the projection neuron (P), BUT it ALSO activates the inhibitory interneuron (I) which then BLOCKS the signal in the projection neuron (P) that connects to the brain. The "gate is closed" and therefore NO PAIN. 3. With pain stimulation, small fibers become active. They activate the projection neurons (P) and BLOCK the inhibitory interneuron (I). Because activity of the inhibitory interneuron is blocked, it CANNOT block the output of the projection neuron that connects with the brain. The "gate is open", therefore, PAIN!
Example of the pathway: What is one of the first things you do after you bump your head or pinch a finger by accident? You probably rub it and it feels better. Rubbing your bumped head or pinched finger would activate non-nociceptive touch signals carried into the spinal cord by large nerve fibers. According to the theory, the activity in the large nerve fibers would activate the inhibitory interneuron that would then block the projection neuron and therefore block the pain.
A second mechanism of pain gating involves descending pathways. A nociceptor (1st order neuron, releases substance P) stimulates 2nd order neuron (in the spinothalamic tract), the second order neuron synapses in the thalamus where it synapses on 3rd order neuron. The 3rd order neuron reaches the cortex (conscious awareness of pain). Pain blocking occurs when descending pathway from the cortex and hypothalamus enter the midbrain (autonomic and conscious influences on pain), and down into the reticular formation in the medulla. The medulla projects serotonin-secreting analgesic fibers (in the reticulospinal tract) which terminate on interneurons in the dorsal horn of the spinal cord at all levels. These interneurons secrete enkephalins and synapse on the 2nd order ascending neurons, thus, inhibiting them.
Proprioceptive or Kinesthetic Sense • Awareness of body position & movement • walk or type without looking • estimate weight of objects • Proprioceptors adapt only slightly (aretonic) • Sensory information is sent to cerebellum & cerebral cortex • from muscle, tendon, joint capsules & hair cells in the vestibular apparatus
Muscle Spindles • Specialized intrafusal muscle fibers enclosed in a CT capsule and innervated by gamma motor neurons • Stretching of the muscle stretches the muscle spindles sending sensory information to the CNS • The response is muscle contraction • Spindle sensory fiber monitor changes in muscle length • Brain regulates muscle tone by controlling gamma fibers
Golgi Tendon Organs • Found at junction of tendon & muscle • Consists of an encapsulated bundle of collagen fibers laced with sensory fibers • When the tendon is overly stretched, sensory signals head for the CNS & result in the muscle’s relaxation
Joint Receptors • Ruffini corpuscles • found in joint capsule • respond to pressure, stretch • Pacinian corpuscles • found in connective tissue around the joint • respond to acceleration & deceleration of joints
Somatic Sensory Pathways • First-order neuron conduct impulses to brainstem or spinal cord • either spinal or cranial nerves • Second-order neurons conducts impulses from spinal cord or brainstem to thalamus, cross over to opposite side before reaching thalamus • Third-order neuron conducts impulses from thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)
All somatic sensory input enters the spinal cord through the dorsal roots of spinal nerves • Sensory signals travel to the brain in 3 pathways: • Dorsal/Posterior column-medial lemniscal system; • Anterolateral/spinothalamic system; • Anterior and posterior spinocerebellar • 1. Dorsal column-medial lemniscal system is used by • - fine touch • - stereognosis = ability to recognize size, shape, texture • - proprioception = awareness of precise body position and weight discrimination • - kinesthesia = awareness of direction of movement • - vibration • 2. Spinothalamic/Anterolateral system • - pain • - thermal sensation (cold and warm) • - crude touch and pressure • - itch and tickle • - sexual sensations • 3. Spinocerebellar (anterior and posterior) - proprioception
1. Posterior (dorsal) Column- Medial Lemniscus Pathway of CNS • Proprioception, vibration, discriminative touch, weight discrimination & stereognosis • Signals travel uninterrupted up the spinal cord in posterior column (1st order neurons) • Fibers synapse on 2nd order neurons and decussate in medulla to become the medial lemniscus pathway ending in thalamus • Thalamic fibers, 3rd order neurons, reach cortex somatic sensory area I (postcentral gyrus) • Spacial orientation of nerve fibers is maintained throughout the tract.
2. Spinothalamic/Anterolateral Pathways • in contrast to dorsal column pathways, spinothalamic pathways transmit signals that do not require precise localization or fine gradation discrimination; • e.g. pain, temperature, crude touch, itch, tickle, sexual sensations • - receptors from neck, trunk and limbs - 1st order neurons synapse immediately after entering the spinal cord (dorsal horn) • 2nd order neuron decussate in the anterior commissure of the cord to the • opposite anterior and lateral white columns, where they turn upward • forming anterior spinothalamic tract and lateral spinothalamic tract which synapse in thalamus • 2 tracts in spinal cord: • - Anterior spinothalamic tract carries input from itch, tickle, pressure, • vibrations and crude touch; • Lateral spinothalamic tract carries signals from pain and thermal receptors • 3rd order neurons transmit impulses from thalamus to somatosensory cortex (on opposite side of stimulus)