C H A P T E R 47 SOMATIC SENSATIONS: GENERAL ORGANIZATION, THE TACTILE AND POSITION SENSES

CLASSIFICATION OF SENSORY RECEPTORS General senses: somatic and visceral. Somatic: tactile and proprioceptive sensations,thermal, pain. Visceral: provide information about conditions within internal organs. Special senses- smell, taste, vision, hearing and equilibrium or balance.

CLASSIFICATION OF SENSORY RECEPTORS BASED ON THE LOCATION Exteroceptors Interoceptors Proprioceptors

TACTILE SENSATIONS Include touch, pressure, vibration Tactile receptors in the skin are Meissner corpuscles, hair end organ, Merkel discs, Ruffini corpuscles, pacinian corpuscles, and free nerve endings.

MEISSNER’S CORPUSCLE Dendrites enclosed, locate in hairless skin Discriminative touch & vibration-- rapidly adapting Generate impulses mainly at onset of a touch 5

Rapidly adapting touch receptors found in the hairy skin. Free nerve endings wrapped around hair follicles. Detect movements on the skin surface that disturb hairs.

MERKEL’S DISC Slowly adapting touch receptors. Saucer-shaped, flattened free nerve endings. Found in the fingertips, hands, lips, and external genitalia. 7

Iggo dome receptor. Note the multiple numbers of Merkel’s discs connecting to a single large myelinated fiber and abutting tightly the undersurface of the epithelium

RUFFINI CORPUSCLE Found deep in dermis of skin Detect heavy touch, continuous touch, & pressure 9

PACINIAN CORPUSCLE Onion-like connective tissue capsule enclosing a dendrite Found in subcutaneous tissues & certain viscera Sensations of pressure or high-frequency vibration Fast adapting receptors 10

THERMAL SENSATIONS Thermoreceptors are free nerve endings. Two distinct thermal sensations: cold receptors warm receptors

TICKLE AND ITCH The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite, and the elicited signals then activate the scratch reflex or other maneuvers that rid the host of the irritant. Itch can be relieved by scratching if this removes the irritant or if the scratch is strong enough to elicit pain. The pain signals are believed to suppress the itch signals in the cord by lateral inhibition

SOMATIC SENSORY PATHWAYS Tracts (pathways) in the spinal cord carries information Three major pathways carry sensory information  Posterior column pathway  Anterolateral pathway  Spinocerebellar pathway Sensations that originate in different areas of the body can be distinguished because sensory neurons from each body region synapse in a specific brain region.

THE ORGANIZATION OF SENSORY PATHWAYS First order neurons  Sensory neurons that deliver sensory information to the CNS Second order neurons  First order neurons synapse on these in the brain or spinal cord Third order neurons  Found in the thalamus  Second order neurons synapse on these First, second, and third order neurons

POSTERIOR COLUMN PATHWAY Posterior column pathway carries sensation of highly localized touch Pressure Vibration psition

Cross section of the spinal cord Cross section of the spinal cord, showing the anatomy of the cord gray matter and of ascending sensory tracts in the white columns of the spinal cord.

DORSAL COLUMN / MEDIAL LEMNISCUS SYSTEM Signals ascend spinal cord via the dorsal (white) columns.  large myelinated fibers (types I and II) fine touch and proprioception  rapid transmission  high degree of spatial orientation Fig. 13.5

POSTERIOR COLUMN / MEDIAL LEMNISCUS SYSTEM 1 st order sensory fibers ascend ipsilaterally in dorsal (posterior) columns to the medulla, and synapse in the medulla. 2 nd order fibers cross over in the medulla and travel through the brain stem via the medial lemniscus to the thalamus. 3 rd order fibers project to the somatosensory areas of the cerebral cortex.  post central gyrus of the parietal lobe

The dorsal column– medial lemniscal pathway for transmitting critical types of tactile signals.

Projection of the dorsal column– medial lemniscal system through the thalamus to the somatosensory cortex.

Structurally distinct areas, called Brodmann’s areas, of the human cerebral cortex. Note specifically areas 1, 2, and 3, which constitute primary somatosensory area I, and areas 5 and 7, which constitute the somatosensory association area.

BRAIN FUNCTION: FUNCTIONAL AREAS OF THE CEREBRAL CORTEX Figure 9-15 Skeletal muscle movement Visual association area OCCIPITAL LOBE Visual cortex Auditory association area Auditory cortex TEMPORAL LOBE FRONTAL LOBE Sensory association area Olfactory cortex Hearing Vision Smell Prefrontal association area Primary somatic sensory cortex Motor association area (premotor cortex) Primary motor cortex Gustatory cortexTaste PARIETAL LOBE

areas I and II.

Homunculus - Motor and Sensory

Astereognosis amorphosynthesis

Layers of the Somatosensory Cortex and Their Function Incoming sensory signal excites neuronal layer send axons to related portions of the cerebral cortex on the opposite side of the brain through the corpus callosum Layers I and II receive diffuse, nonspecific input signals from lower brain centers The neurons in layers V and VI send axons to the deeper parts of the nervous system

Transmission of a pinpoint stimulus signal to the cerebral cortex.

Transmission of signals to the cortex from two adjacent pinpoint stimuli. The blue curve represents the pattern of cortical stimulation without “surround” inhibition, and the two red curves represent the pattern when “surround” inhibition does occur.

Weber-Fechner Principle-Detection of "Ratio" of Stimulus Strength Gradations of stimulus strength are discriminated approximately in proportion to the logarithm of stimulus strength. the greater the background sensory intensity, the greater an additional change must be for the psyche to detect the change.

Graphical demonstration of the “power law” relation between actual stimulus strength and strength that the psyche interprets it to be. Note that the power law does not hold at either very weak or very strong stimulus strengths.

POSITION SENSES (1) static position sense, which means conscious perception of the orientation of the different parts of the body with respect to one another, (2) rate of movement sense, also called kinesthesia or dynamic proprioception. Position Sensory Receptors Processing of Position Sense Information in the Dorsal Column–Medial Lemniscal Pathway

PROPRIOCEPTIVE OR KINESTHETIC SENSE Awareness of body position & movement  walk or type without looking  estimate weight of objects Proprioceptors adapt only slightly Sensory information is sent to cerebellum & cerebral cortex  signals project 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 back to the CNS Spindle sensory fiber monitor changes in muscle length Brain regulates muscle tone by controlling gamma fibers 33

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 & resulting in the muscle’s relaxation 34

Typical responses of five different thalamic neurons in the thalamic ventrobasal complex when the knee joint is moved through its range of motion.

ANTEROLATERAL SYSTEM 1 st order sensory neurons enter spinal cord and synapse in the posterior horn. 2 nd order fibers cross over in the spinal cord, and ascend spinal cord to the thalamus via the anterior (ventral) and lateral spinothalamic tracts. 3 rd order fibers project to the somatosensory areas of the cerebral cortex.  post central gyrus of the parietal lobe

ANTEROLATERAL PATHWAY Anterolatheral pathway provide conscious sensations of poorly localized (crude) touch, pressure, pain and temperature Anterolatheral pathway includes:  Lateral spinothalamic tract – relays information concerning pain and temperature  Anterior spinothalamic tract – carry (crude) touch, pressure sensation.

Anterior and lateral divisions of the anterolateral sensory pathway.

CHARACTERISTICS OF TRANSMISSION IN THE ANTEROLATERAL PATHWAY (1) the velocities of transmission are only one third to one half those in the dorsal column–medial lemniscal system, ranging between 8 and 40 m/sec; (2) the degree of spatial localization of signals is poor; (3) the gradations of intensities are also far less accurate, with most of the sensations being recognized in 10 to 20 gradations of strength, rather than as many as 100 gradations for the dorsal column system; and (4) the ability to transmit rapidly changing or rapidly repetitive signals is poor

Segmental Fields of Sensation- Dermatomes Each spinal nerve innervates a "segmental field" of the skin called a dermatome. Each spinal nerve innervates a “segmental field” of the skin called a dermatome.