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Download Ch. 15 – Sensory Pathways and the Somatic Nervous System
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Ch. 15 – Sensory Pathways and the Somatic Nervous System • Pathways = nerves, nuclei, and tracts that link the processing centers of the CNS with the rest of the body • Discussed in Ch. 15: – 1. Sensory pathways • Both somatic sensory pathways and visceral sensory pathways – 2. Motor pathways • Only somatic motor pathways of the somatic nervous system (SNS) are discussed in Ch. 15 – (Visceral motor pathways of the autonomic nervous system (ANS) are discussed in Ch. 16) Another way to look at Ch. 15 Fig. 12-1, p. 387 1 An overview of events occurring along sensory and motor pathways Fig. 15-1, p. 510 Sensation and receptors • Sensory receptors = specialized cells or cell processes that detect stimuli about conditions inside or outside of the body, and send that info to the CNS – The job of a receptor is transduction = the translation of a stimulus into action potentials (the “language” of the NS) • Sensation = the info arrives in the CNS – Vs. perception = you are consciously aware of the sensation (only ~ 1% of sensations are perceived!) – Receptor specificity = (modality specificity) = receptors are specialized to detect different types of stimuli (modalities), e.g. light, sound, chemicals, physical distortion (pressure), etc. • Senses: – General senses: touch, pressure, vibration, temperature, pain, and proprioception (body position) • Receptors for the general senses are distributed throughout the body and are relatively simple in structure – Special senses (Ch. 17): olfaction (smell), vision (sight), gustation (taste), equilibrium (balance), and hearing • Receptors for the special senses are found in special sense organs and are more complex in structure 2 Receptive fields • Receptive field = the area monitored by a single receptor – Smaller receptive fields → more precise localization of the stimulus (e.g. touch by the fingertips) – Larger receptive fields → less precise localization of the stimulus (e.g. touch on the back) Fig. 15-2, p. 511 The interpretation of sensory info • Labeled lines and cortical mapping – Sensory info is sent from a specific receptor in a specific part of the body to a specific area of the cerebral cortex • Labeled line = the neuronal pathway from the receptor to the specific part of the cortex • The sensory cortex can then be mapped (see #1 and #2 below) • The sensory areas of the cerebral cortex interpret the: – 1. Type of stimulus (modality) based upon the cortical destination of the labeled line (e.g. touch → primary sensory cortex, light → visual cortex, sound → auditory cortex, etc.) – 2. Location of stimulus based upon exactly where in particular sensory cortex the labeled line arrives (e.g. consider the sensory homunculus) – 3. Intensity, duration, and other characteristics of the stimulus based upon the action potential frequency and pattern 3 Sensory adaptation • Adaptation = ↓ sensitivity to a constant, painless stimulus – 1. Peripheral adaptation: ↓ receptor sensitivity, ↓ the amount of info that reaches the CNS • Only phasic (fast-adapting) receptors can peripherally adapt (see figure below) – 2. Central adaptation: occurs within the CNS, along sensory pathways, ↓ the amount of info that reaches the cerebral cortex • It may be subconscious or conscious inhibition – E.g. getting used to a smell – subconscious inhibition of olfactory centers in the brain – E.g. “tuning out” background noise on a busy street – subconscious or conscious inhibition of auditory pathways • The CNS may also sometimes subconsciously or consciously ↑ sensitivity (facilitation) to certain stimuli – E.g. “listening carefully” – conscious facilitation of auditory pathways E.g. pain receptors and proprioceptors E.g. temperature receptors and deep pressure receptors Fig. 15-3, p. 512 The general senses – receptor types • General sense receptors can be classified by: – 1. What/where they’re monitoring for changes: • Interoceptors – internal organ systems (e.g. sensing heart rate, blood pressure, deep pressure/pain, etc.) • Exteroceptors – the external environment (e.g. sensing ambient temperature, light, touch, sound, etc.) • Proprioceptors – the position and movement of muscles and joints (i.e., body position) – 2. The nature of the stimulus that excites them: • Nociceptors – pain (see the next slide) • Thermoreceptors – temperature (see the next slide) • Mechanoreceptors – physical distortion (see the two slides after that) • Chemoreceptors – the concentration of dissolved chemicals (e.g. H+, CO2, O2) in certain body fluids – This information is NOT perceived by the cerebral cortex; it is sent to lower brain centers for subconscious homeostatic adjustments 4 Nociceptors and thermoreceptors • Nociceptors (pain receptors) = relatively nonspecific receptors that are sensitive to extreme temperature, mechanical damage, or chemicals released by damaged cells – They’re tonic, so they exhibit very little peripheral adaptation… • However, central adaptation may decrease the perception of pain via the release of endorphins and enkephalins (which are structurally similar to morphine) – There are 2 general types of pain: • 1. Fast pain (prickling pain) – carried by Type A fibers; it is more pinpoint/localizable pain (e.g. deep cut, pin prick) • 2. Slow pain (burning and aching pain) – carried by Type C fibers; it is more general/diffuse pain (e.g. stomach ache, uterine cramps) • Thermoreceptors (temperature receptors) – include both cold receptors and warm receptors – They’re both exteroceptors (e.g. in the skin) and interoceptors (e.g. in the hypothalamus) – They’re phasic (fast-adapting), so they are very active only when the temperature is changing Mechanoreceptors • Are sensitive to stimuli that physically distort their cell membranes – e.g. stretching, compression, twisting, etc. – The membranes of mechanoreceptors contain mechanically gated ion channels • There are 3 classes of mechanoreceptors: – 1. Tactile receptors – sense touch, pressure, and vibration – 2. Baroreceptors – sense pressure changes in the walls of hollow organs – 3. Proprioceptors – sense the position of joints and muscles Fig. 12-11c, p. 403 5 • Sense touch, pressure, and vibration • There are 2 main types of tactile receptors: Tactile receptors – 1. Fine touch and pressure receptors – more superficial • They are precisely localized due to smaller receptive fields • E.g. free nerve endings, tactile discs (which include Merkel cells), tactile (Meissner’s) corpuscles – 2. Crude touch and pressure receptors – deeper • They are poorly localized due to larger receptive fields • E.g. lamellated (pacinian) corpuscles and Ruffini corpuscles Fig. 15-4, p. 515 Baroreceptors and proprioceptors • Baroreceptors – monitor pressure in the walls of hollow organs (i.e., they monitor the stretch of the walls due to the pressure of the fluid or gas within the organ) – They’re phasic (fast-adapting) receptors that are found in the walls of the digestive tract, urinary bladder, carotid arteries, aorta, and lungs • Proprioceptors – monitor joint position, tension in tendons and ligaments, and the general state of muscular contraction – They’re tonic – they continuously send info, and do not adapt – Most proprioceptive info is processed subconsciously (e.g. by the cerebellum) – There are 3 main types: • 1. Muscle spindles (shown here) – detect the length or degree of stretch of a muscle • 2. Golgi tendon organs – detect the tension on a muscle and its tendon • 3. Joint capsule receptors – detect pressure, tension, and movement at a joint Fig. 13-16, p. 453 6 Somatic sensory pathways and ascending tracts in the spinal cord • • Carry sensory info from the skin and skeletal musculature There are 3 major somatic sensory pathways: – 1. Posterior column pathway • Tracts: fasciculus cuneatus and fasciculus gracilis – 2. Spinothalamic pathway • Tracts: anterior spinothalamic tract and lateral spinothalamic tract – 3. Spinocerebellar pathway • Tracts: anterior spinocerebellar tract and posterior spinocerebellar tract See Table 15-1 for much more detail than what we’ll cover in lecture Fig. 15-4, p. 503 The organization of somatic sensory pathways • Somatic sensory pathways usually consist of 3 neurons: – 1. First-order neuron = a sensory neuron that sends info to the CNS – 2. Second-order neuron = an interneuron • At some point, it crosses over to the opposite side – 3. Third-order neuron = an interneuron • It travels from the thalamus to the postcentral gyrus (primary sensory cortex) • It must send an AP if the sensation is to reach conscious awareness Remember the idea of a “labeled line”! Fig. 15-6, p. 520 7 The spinothalamic pathway Fig. 15-6, p. 520 Fig. 15-6, p. 521 The posterior column pathway 8 Fig. 15-6, p. 521 The spinocerebellar pathway • Note that there is no thirdorder neuron; processing of this proprioceptive info always occurs subconsciously (in the cerebellum) Visceral sensory pathways • • • • Sensory info from abdominopelvic interoceptors enter the dorsal horn of the spinal cord (SC) via the dorsal root (shown here) Sensory info from interoceptors in the mouth, pharynx, larynx, and thoracic viscera enter the brain stem via cranial nerves (not shown here) Either way, visceral sensory info is sent to the solitary nucleus of the medulla oblongata, which relays it to the appropriate lower (subconscious) brain destinations (e.g. cardiovascular centers, respiratory centers, etc.) Extremely strong visceral pain sensations can cross-stimulate somatic pain interneurons located at the same level of the SC or brain stem, so you feel pain in the corresponding part of the body surface = referred pain (e.g. heart attack, appendicitis, etc.) Fig. 15-7 p. 522 Fig. 13-8 p. 440 9 Somatic motor pathways and descending tracts in the spinal cord • There are 3 major somatic motor pathways: – 1. Corticospinal pathway The names give it away! • Tracts: corticobulbar tract (which terminates at the brain stem), lateral corticospinal tract, and anterior corticospinal tract – 2. Medial pathway • Tracts: vestibulospinal tract, tectospinal tract, and reticulospinal tract – 3. Lateral pathway • Tracts: rubrospinal tract • See Table 15-2 for much more detail than what we’ll cover in lecture Remember: – The somatic nervous system (SNS) sends (mostly) voluntary motor commands to skeletal muscles • Vs. the autonomic nervous system (ANS), or visceral motor system, which sends involuntary motor commands to cardiac muscle, smooth muscle, glands, and adipocytes – Ch. 16 – Somatic motor pathways are monitored and adjusted by the basal nuclei and cerebellum Fig. 15-8, p. 524 Fig. 15-9, p. 525 The organization of somatic motor pathways • Somatic motor pathways consist of at least 2 neurons: – 1. Upper motor neuron (UMN) – it’s actually an interneuron • It facilitates or inhibits a lower motor neuron – 2. Lower motor neuron (LMN) – it’s a true motor neuron • It stimulates the muscle fibers of a motor unit 10 Fig. 15-9, p. 525 The corticospinal pathway • Carries out conscious control of skeletal muscles • The vast majority of corticospinal pathway UMNs decussate at the pyramids of the medulla oblongata Fig. 15-8, p. 524 The medial and lateral pathways • Carry regulatory subconscious somatic motor commands from lower brain levels (e.g., the basal nuclei, diencephalon, or brain stem) • Modify or direct skeletal muscle contractions by stimulating, facilitating, or inhibiting LMNs – Medial pathway: subconsciously helps control muscle tone and gross movements of neck, trunk, and proximal limb muscles – Lateral pathway: subconsciously helps control muscle tone and more precise movements of distal limb muscles 11