Mammalian Physiology. Sensory Nervous System UNLV. PHYSIOLOGY, Chapter 7 Berne, Levy, Koeppen, Stanton UNIVERSITY OF NEVADA LAS VEGAS

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1 Mammalian Physiology Sensory Nervous System UNLV 1 UNIVERSITY OF NEVADA LAS VEGAS PHYSIOLOGY, Chapter 7 Berne, Levy, Koeppen, Stanton

2 Objectives Describe the classifications of sensory receptors Describe sensory nerve pathways Describe the operation of a sensory neuron Describe the function and operation of the muscle spindle Describe the function and operation of the Golgi tendon organ 2

3 Basic Functions of the Nervous System Sensory Input - provides the central nervous system with information about the internal and external environment Integration - CNS takes all the incoming information, processes it, then selects an appropriate action Motor Output effects the physical responses dictated by the central nervous system 3

4 Sensory Receptors 1. Mechanoreceptors Skin tactile sensibilities free nerve endings, Deep tissue sensibilities Pacinian corpuscles, muscle spindles, Golgi tendon organ Hearing sound receptors of cochlea Equilibrium vestibular receptors Arterial pressure baroreceptors Mechanoreceptors respond to mechanical stimuli and are either fast or slow adapting 4

5 2. Thermoreceptors Cold Warm Sensory Receptors Both types are slow adapting discharge spontaneously and are active over a broad temperature range Receptors are reciprocal Cold receptors become inactive at skin temps above 37º C Warm receptors are inactive above 45º C cannot sense heat pain 5

6 Sensory Receptors 3. Nociceptors Pain free nerve endings Mechanical Polymodal Sensitization 6 Sensitization damage to cell release of proteolytic enzymes leads to production of bradykinen which binds to receptor and increases receptor s sensitivity to subsequent stimuli

7 Sensory Receptors 3. Nociceptors Pain free nerve endings Inflammation Rubor Calor Rumor Dolor 7 Activation of the receptor releases peptides, substance P, CGRP into the skin - P and CGRP cause vasodilation and increased capillary permeability, augmenting other factors released from damaged cells histamines, etc

8 Sensory Receptors 4. Electromagnetic receptors Vision rods and cones 5. Chemoreceptors Taste taste buds Smell olfactory epithelium Arterial oxygen aortic and carotid bodies Osmolality anterior hypothalamus Blood carbon dioxide CNS (medulla) 8

9 Sensors in the Skin For all sensory receptors, excitation by a stimulus results in a change in the receptor s membrane electrical potential the receptor potential 9

10 Mechanisms of Receptor Potentials Mechanical deformation of the receptor Stretches the receptor membrane & opens ion channels Application of a chemical to the membrane Opens ion channels Change in temperature of the membrane Altars permeability of the membrane Effects of electromagnetic radiation Directly or indirectly changes membrane characteristics allowing ions to flow through membrane channels 10

11 Receptor Potential Action Potential Action potentials appear when receptor potential reaches threshold potential Frequency of action potentials increases, the more receptor potential rises above threshold 11

12 Pacinian Corpuscle Mechanical deformation of receptor initiates an action potential Corpuscle contains gel-like substance which redistributes after initial response (pressure applied equally on all sides of central nerve) decreasing sensor output (adaptation) 12

13 Pacinian Corpuscle Receptor output in response to mechanical stress in a pacinian corpuscle 13

14 Adaptation Mechanoreceptors Rapid adapting receptor -discharges at the onset of the stimulus, ceases if stimulus persists at a steady rate, may also discharge at the end of stimulus 14

15 Adaptation Mechanoreceptors Slow adapting receptor -discharge continues as long as stimulus persists 15

16 Sensor Adaptation When a continuous sensory stimulus is applied, the receptor responds initially with a high impulse rate, which declines until the rate of action potentials is few or none (accommodation). 16

17 Spatial Summation Receptive fields define degree of spatial discrimination sensor responds to any stimulus in the receptive field Strength of sensory signal is a function of the number of fibers activated 17

18 Spatial Discrimination The horizontal bars represent the minimum distance at which two points can be perceived as distinct at various locations over the body Spatial discrimination depends on -Receptive field size -Density of the receptors The higher the density of receptors, the finer the discrimination 18

19 Temporal Summation Strength of signal is a function of the frequency of impulses in any given nerve fiber 19

20 Proprioceptros Muscle Spindle and Golgi Tendon Organ 20

21 Muscle Spindle Apparatus More than one type of motor neuron - alpha motor neuron - gamma motor neuron More than one type of sensory neuron - primary afferent - secondary afferent Many interneurons Two types of muscle fibers - intrafusal muscle fibers - extrafusal muscle fibers 21

22 Muscle Spindle Muscle spindle Changes in muscle length - spatial position Rate of change in muscle length - stretch reflex Primary afferents fast Rate of stretch - Phasic stretch Function: react to oppose stretch Secondary afferents slow Final length of muscle - Tonic stretch Function: maintain muscle tone, posture, positional awareness 22

23 Muscle Spindle Sensory End of System Primary sensory nerve fibers (1a afferent fibers) Found integrated within the intrafusal muscle fiber and respond to distortion (rate of stretch) 23

24 Muscle System Motor End of System Alpha motor (α) neurons innervate the extrafusal muscle fibers. When the alpha nerve fires the motor unit generates tension and/or shortens. 24

25 Stretch Reflex Quick stretch of muscle distorts nuclear bag Afferent signal via primary sensory nerve Monosynapse in spinal cord with α-motorneuron Efferent signal via α-motor neuron Muscle contraction results to oppose stretch Reciprocal innervation Efferent signal to antagonist relaxation Inhibitory interneuron (Renshaw cell) 25

26 26 Schema of Basic Stretch Reflex

27 Coactivation the Gamma Loop Gamma motor (γ) neurons innervate the intrafusal muscle fibers. When the gamma nerve fires the intrafusal muscle fiber generates tension and/or shortens. α-motor neuron and γ-motor neuron are activated in parallel Muscle spindle shortens in proportion to muscle fiber shortening maintains constant rate of signaling. 27

28 Monitors tension developed in muscle Prevents damage during excessive force generation Stimulation results in reflex relaxation of muscle Inhibitory interneuron, not mono-synapse Golgi Tendon Organ 28

29 Golgi Tendon Organ Inhibitory Interneuron 29

30 Sensory Pathways 1 st order neurons Primary afferents Peripheral processes receptors Central process collateral branches 2 nd order neurons Transmit information to thalamus Cross-over to contralateral thalamus 3 rd order neurons Project to somatosensory regions of the cerebral cortex Dorsal column-medial lemniscus tract Spinothalamic tract 30

31 Sensory Pathways White matter Myelinated fibers bundled into large fibers and tracts Commissural connect corresponding gray areas Association connect different parts of same hemisphere 31 Projection connect cortex to lower centers

32 Sensory Pathways Ascending Tracts Specific pathways dorsal column medial lemniscus tract Sensory input from precise locations Discriminatory touch, vibrations Spinocerebellar tracts - information from muscle or tendon stretch to cerebellum 32

33 Somatotopic Arrangement of Neurons Dorsal column medial lemniscus pathway Somatosensory cortex Leg- medial aspect VPL thalamic nucleus Arm-lateral aspect Face-dorsal to lateral aspect VPM nucleus Distortion regions that have more dense innervation, use more cortical circuitry 33 Sensory homunculus

34 Sensory Pathways Ascending Tracts Non-specific pathways anterolateral pathways Lateral spinothalamic tract Sensory for pain, temperature, and course touch Non-discriminating localization 34

35 Motor Pathways Descending Tracts Pyramidal tracts - direct pathways from pyramidal neurons to spinal cord to motor neurons Regulates fast & fine motor skills 35

36 Motor Pathways Descending Tracts Extrapyramidal tract All motor pathways except pyramidal pathways including brain stem motor nuclei Balance, course limb movements, - heavy dependence on reflex activity 36

37 37 Integration of Input and Output