Sensory Pathways and the Somatic Nervous System Chapter 15 Neural Integration I: Sensory Pathways and the Somatic Nervous System

fig. 15-1

Sensory Motor General (15) Somatic (15) Special (17) Autonomic (16)

most associated with the skin General senses Special senses smell sight taste hearing special “sense” organs temperature pain touch pressure vibration proprioception most associated with the skin

General senses receptors distributed throughout the body relatively simple

receptors send info to CNS General senses receptors send info to CNS arriving info is called sensation our awareness of it is perception

interface between environment Sensory receptors interface between environment and the body translate stimulus into an AP transduction

Sensory receptors receptors have selective sensivity chemical physical touch light heat transfer receptors may or may not have accessory structures associated with them

size of receptive field Sensory receptors receptive field area monitored by a receptor size of receptive field 70 mm 1 mm specificity fig. 15-2

Sensory receptors stimulus receptor transduction stimulus changes membrane potential receptor potential (+ or -) greater stimulus means larger receptor potential if stimulus is large enough to get to threshold is is called generator potential ( generates an AP) transduction

Sensory receptors stimulus receptor action potential CNS for processing and interpretation (cortical areas)

receptor 2 receptor A receptor B labeled line cortex

a “line” carries the same “type” (modality) of information interpretation is based on which “line” information travels on

receptor 2 receptor A receptor B labeled line optic nerve cortex shut eyes and rub them gently

When CNS receives info… which “line” type of stimulus where “line” ends perception all other attributes (strength, duration, variation) are determined by the frequency and pattern of AP’s

receptor types: tonic: always “on” phasic: only on with stimulus some receptors combine the two greater stimulus higher freq. lesser stimulus lower freq.

adaptation reduction in sensitivity in the presence of a constant stimulus peripheral central change in receptor activity inhibition of nuclei in pathway

peripheral adaptation phasic receptors (aka fast-adapting receptors) example: thermoreceptors you usually don’t notice room temperature unless it changes

central adaptation example: smell you walk into a room and notice a new smell… …but not for long

about 1% of sensory information coming in reaches our awareness adaptation reduces the amount of information reaching the cerebral cortex about 1% of sensory information coming in reaches our awareness

100 Keys (pg 498) “Stimulation of a receptor produces action potentials along the axon of a sensory neuron. The frequency or pattern of action potentials contains information about the strength, duration, and variation of the stimulus. Your perception of the nature of that stimulus depends on the path it takes inside the CNS.”

General senses (from chapter 12) exteroceptors proprioceptors interoceptors outside position inside

based on nature of stimulus General senses classification based on nature of stimulus nociceptors thermoreceptors mechanoreceptors chemoreceptors pain heat flow physical distortion chemical concentration

General senses nociceptors common in: skin joint capsules coverings of bones around blood vessel walls free nerve endings large receptive fields

nociceptors sensitive to: extreme temperature mechanical damage dissolved chemicals (like those release by damaged cells) stimulation causes depolarization

nociceptors two fiber types convey info type A fast pain (cut, etc.,) easy to localize type C slow pain (“burning, aching”) difficult to localize

nociceptors tonic receptors no significant peripheral adaptation as long as the stimulus is present, it will hurt but central adaptation can occur (perception of pain may decrease)

nociceptors sensory neurons bringing in pain info use glutamate and/or substance P as their neurotransmitter these nts can cause facilitation (?) pain may be disproportional (feels worse than it should) pain can be reduced by endorphins and enkephalins (inhibit activity in pathway) [neuromodulators chpt. 12]

nociceptors endorphins pain centers use substance P as nt. endorphins bind to presynaptic membrane and inhibit substance P release, reducing perception of pain

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thermoreceptors free nerve endings in the dermis skeletal m. hypothalamus liver warm receptors or cold receptors

thermoreceptors phasic receptors active when temperature is changing, quickly adapting to stable temperature detect transfer of heat heat loss from skin cool heat gain to skin warm

mechanoreceptors contain mechanically regulated ion channels (chapter 12)

c. mechanically regulated channels closed mechanical stimulus- opens remove stimulus- closed fig. 12-10c

mechanoreceptors three classes tactile receptors baroreceptors proprioceptors touch, pressure, vibration pressure changes (gut, genitourinary) position of joints/muscles

mechanoreceptors tactile receptors fine touch/pressure crude touch/pressure small (narrow) receptive field detailed information sensitive wide receptive field poor localization

fig. 15-3

tactile receptors range of complexity free nerve endings root hair plexus tactile discs tactile corpuscles (Meissner’s) lamellated corpuscles (pacinian) Ruffini corpuscles

tactile receptors free nerve endings in epidermis of skin cornea of eye sensitive to touch and pressure tonic receptors small receptive field

tactile receptors root hair plexus around each hair follicle sense movement of hair adapt quickly

tactile receptors tactile discs sensitive, tonic receptors in epidermis fine touch and pressure

tactile receptors tactile corpuscles (Meissner’s) fine touch, pressure , vibration adapt quickly surrounded by Schwann cells in dermis of skin eyelids, fingertips (sensitive areas)

tactile receptors lamellated corpuscles (pacinian) sensitive to deep pressure high-frequency vibrations adapt quickly nerve ending is encapsulated by layers of supporting cells (onion) dermis, pancreas, fingers…

tactile receptors Ruffini corpuscles pressure and skin distortion located deep in the dermis tonic, little if any adaptation

fig. 15-3

sensivitity can be altered infection disease damage to pathway e.g., damage to a spinal nerve would affect an entire dermatome

tickle and itch closely related to touch and pain

baroreceptors free nerve endings in the walls of organs that stretch e.g., blood vessels when pressure changes they expand or contract changes activity of receptors

proprioceptors muscle spindles Golgi tendon organs receptors in joint capsules stretch reflex monitor tendon tension free nerve endings in joints

proprioceptors no adaptation continuously send info to CNS most processed at subconscious level

chemoreceptors respond to chemicals dissolved in the surrounding fluids respiratory centers in brain pH, CO2 levels in blood carotid bodies and aortic bodies pH, CO2, O2 levels in blood

Pathways in the CNS spinothalamic tract corticospinal tract spine to thalamus =sensory corticospinal tract cortex to spine =motor

Pathways in the CNS sensory pathways neurons involved first order neuron second order neuron third order neuron sensory neuron (DRG) in CNS (crosses over) in thalamus

Pathways in the CNS sensory pathways Somatic sensory pathways carry sensory info from skin and muscles of body wall, head, neck, limbs

Pathways in the CNS sensory pathways Somatic sensory pathways posterior column pathway anterolateral column pathway spinocerebellar pathway

fig. 15-4

The Posterior Column Pathway fine touch pressure vibrations proprioception

The Posterior Column Pathway inferior half of body first order neuron in DRG up the fasciculus gracilis to the nucleus gracilis of med. oblong. superior half of body first order neuron in DRG up the fasciculus cuneatus to the nucleus cuneatus of med. oblong.

The Posterior Column Pathway second order neuron in nucleus ? cross to other side and ascend to the ventral nucleus of thalamus third order neuron in thalamus project to the primary sensory cortex

fig. 15-4 fig. 15-5a

The Anterolateral Pathway “crude” touch pressure pain temperature

The Anterolateral Pathway first order neuron in DRG synapses on 2nd order neuron in dorsal horn of spinal cord second order neuron cross to opposite side and ascend

The Anterolateral Pathway second order neuron cross to opposite side and ascend anterior spinothalamic tract lateral spinothalamic tract crude touch and pressure to ventral nucleus of thalamus pain and temperature

The Anterolateral Pathway second order neuron in spinal cord cross to other side and ascend to the ventral nucleus of thalamus third order neuron in ventral thalamus project to the primary sensory cortex

fig. 15-4 fig. 15-5b

The Anterolateral Pathway phantom pain ? activity along pathway, even if “limb” is not there referred pain? viceral pains sensations may stimulate neurons of AL pathway

fig. 15-6

The Spinocerebellar Pathway posterior s.c. tracts axons from same side to cerebellum anterior s.c. tracts axons cross over and ascend to cerebellum information goes to Purkinjie cells in the cerebellum (proprioception)

fig. 15-4 fig. 15-7

100 Keys (pg. 507) Most somatic sensory information is relayed to the thalamus for processing. A small fraction of the arriving information is projected to the cerebral cortex and reaches our awareness.

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Pathways in the CNS sensory pathways Somatic sensory pathways posterior column pathway anterolateral column pathway spinocerebellar pathway

fig. 15-4

Pathways in the CNS sensory pathways Somatic sensory pathways posterior column pathway anterolateral column pathway spinocerebellar pathway

Pathways in the CNS sensory pathways Somatic sensory pathways Visceral sensory pathways info from interoceptors (internal organs)

Pathways in the CNS Somatic sensory pathways Visceral sensory pathways nociceptors, thermoreceptors, tactile receptors, baroreceptors, chemoreceptors

Somatic sensory pathways Visceral sensory pathways Pathways in the CNS Somatic sensory pathways Visceral sensory pathways CN V, VII, IX, X carry info from pharynx, mouth, palate, larynx, trachea and esophagus project to solitary nucleus (medulla oblongata)

Somatic sensory pathways Visceral sensory pathways Pathways in the CNS Somatic sensory pathways Visceral sensory pathways T1 to L2 abdominal organs S2 to S4 pelvic organs first order neurons project to interneurons which travel up the anterolateral pathway to sol. nuc. usually subconscious

Pathways in the CNS sensory pathways motor pathways the somatic nervous system (SNS) autonomic nervous system (ANS) voluntary involuntary

motor pathways in the CNS the somatic nervous system (SNS) always involve at least two neurons upper motor neuron lower motor neuron inside CNS (+ or -) stimulates a motor unit

motor pathways in the CNS motor information follows one of three main pathways: corticospinal pathway medial pathway lateral pathway

motor pathways in the CNS corticospinal pathway (aka., pyramidal system) upper motor neurons are pyramidal cells in primary motor cortex synapse on lower motor neurons (ventral horn of spinal cord) also project to other control centers

motor pathways in the CNS corticospinal pathway three pairs of tracts: corticobulbar tracts to motor nuclei of CN III, IV, V, VI, VII, IX, XI, XII conscious control of eye, jaw and face muscles…

motor pathways in the CNS corticospinal pathway three pairs of tracts: corticobulbar tracts lateral corticospinal tracts anterior corticospinal tracts

fig. 15-9

Pathways in the CNS motor pathways motor information follows one of three main pathways: corticospinal pathway medial pathway lateral pathway

fig. 15-8

corticospinal pathway medial pathway Pathways in the CNS motor pathways corticospinal pathway medial pathway neck trunk proximal limbs muscle tone gross movement

reflexive head position Pathways in the CNS motor pathways medial pathway UMN in: vestibular nuclei (hind) superior colliculus (mid) reticular formation (brain stem) posture & balance reflexive head position various

Pathways in the CNS motor pathways lateral pathway control of muscle tone precise movement of distal limbs UMN in red nucleus (mid) descend down rubrospinal tract

Basal Nuclei background patterns of movement (walking, running, etc.) adjust activities of UMN in cortex normally: inactive active two populations: ACh stimulatory GABA inhibitory inhibited

Cerebellum monitors (sensory): proprioception visual vestibular (balance) spinocerebellar tract superior colliculus vestibular nucleus output continually adjusts UMN activity

Several conditions ALS cerebral palsy anencephaly amyotrophic lateral sclerosis (aka Lou Gerhig’s disease) degeneration of UMN’s and/or LMN’s atrophy of muscle cerebral palsy affect voluntary muscle performance trauma, exposure to drugs etc., genetics cerebrum, cerebellum, basal nuclei, hippocampus, thalamus abnormal motor skills, posture, speech… anencephaly lack of higher brain development

100 Keys (pg. 513) “Neurons of the primary motor cortex (UMN) innervate motor neurons in the brain and spinal cord (LMN) responsible for stimulating skeletal muscles. Higher centers in the brain can suppress or facilitate reflex responses; reflexes can complement or increase the complexity of voluntary movements”