Control Systems and Homeostasis

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• General properties of sensory systems • Somatic senses • Chemoreception: smell and taste • The ear: hearing • The ear: equilibrium • The eye and vision

• Stimulus as physical energy  sensory receptor – Receptor acts as a transducer • Intracellular signal  usually change in membrane potential • Stimulus  threshold  action potential to CNS • Integration in CNS  cerebral cortex or acted on subconsciously

© 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Receptors to Particular Forms of Energy • Naked (“free”) nerves • Complex neural receptors encased in connective tissue capsules • Smell receptors are neurons • Non-neural receptors for four special senses

© 2016 Pearson Education, Inc. Receptors to Particular Forms of Energy • Chemoreceptors respond to chemical ligands: taste, smell • Mechanoreceptors respond to mechanical energy pressure and sound: hearing • Thermoreceptors respond to temperature • Photoreceptors for vision respond to light

Simple receptors are neurons Complex neural receptors have nerve Most special senses receptors are cells with free nerve endings. They endings enclosed in connective tissue capsules. that release neurotransmitter onto sensory may have myelinated or This illustration shows a Pacinian corpuscle, neurons, initiating an action potential. The unmyelinated axons. which senses touch. cell illustrated is a hair cell, found in the ear.

Stimulus Stimulus Stimulus

Enclosed nerve Free nerve endings Specialized receptor ending cell (hair cell) Layers of connective tissue Synaptic vesicles Synapse

Unmyelinated axon

Myelinated axon Myelinated axon

Cell body Cell body

Cell body of sensory neuron

• Stimulus energy converted into information processed by CNS – Ion channels or second messengers initiate membrane potential change • Adequate stimulus: form of energy to which a receptor is most responsive • Threshold: minimum stimulus • Receptor potential: change in sensory receptor membrane potential

• Sensory information – Spinal cord to brain by ascending pathways – Directly to brain stem via cranial nerves • Visceral reflexes integrated in brain stem or spinal cord usually do not reach conscious perception.

• Perceptual threshold: level of stimulus necessary to be aware of particular sensation • Habituation: decreased perception through inhibitory modulation – Falls below perceptual threshold • Each major division of the brain processes one or more types of sensory information

Primary somatic sensory cortex Gustatory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum

Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium

Equilibrium pathways project primarily to the cerebellum. Tongue

FIGURE QUESTION Which sensory pathways shown do not synapse in the thalamus? Somatic senses

Primary somatic sensory cortex Gustatory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum

Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium

Equilibrium pathways project primarily to the cerebellum. Tongue

Somatic senses

Primary somatic sensory cortex Gustatory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum

Nose Thalamus Sound

Brain stem Equilibrium

Tongue

Somatic senses

Primary somatic sensory cortex Gustatory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum

Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium

Tongue

Somatic senses

Primary somatic sensory cortex Gustatory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum

Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium

Equilibrium pathways project primarily to the cerebellum. Tongue

Somatic senses

• Four properties of a stimulus – Modality – Location – Intensity – Duration

• Modality indicated by – Which sensory neurons are activated – Where neurons terminate in brain • Each receptor type is most sensitive to a particular modality of stimulus • Labeled line coding – 1:1 association of receptor with sensation

• According to which receptive fields are activated • Auditory information is an exception – Ear neurons sensitive to different frequencies – Brain uses timing to locate

• Lateral inhibition – Increases contrast between activated receptive fields and inactive neighbors • Population coding – Multiple receptors functioning together

Sound takes longer to reach left ear.

Signals coming Left from the right reach Right the brain first.

Stimulus Stimulus Pin

Skin

A B C

Tonic level

Primary neuron A B C response is proportional to stimulus strength.

Primary Frequency of potentials action sensory neurons

Pathway closest to Secondary the stimulus inhibits neurons neighbors.

A B C

Inhibition of lateral Tonic level Tertiary neurons enhances neurons perception of stimulus.

A B C Frequency of actionpotentials

© 2016 Pearson Education, Inc. Properties of Stimulus: Intensity and Duration • Intensity – Coded by number of receptors activated and frequency of action potentials called frequency coding • Duration – Coded by duration of action potentials – Some receptors can adapt, or cease to respond • Tonic receptors versus phasic receptors

Cell body Transduction site Trigger zone Myelinated axon Axon terminal

Stimulus

Moderate Stimulus 20 0 Amplitude 20

40 Threshold 60 80 0 5 10 0 5 10 0 5 10 Duration Membrane potential (mV) Time (sec)

Longer and 20 Stronger Stimulus 0 20 40 60 80

0 5 10 0 5 10 0 5 10 Membrane Membrane potential (mV)

Frequency of action Receptor potential Receptor potential Neurotransmitter potentials is proportional strength and is integrated at the release varies to stimulus intensity. duration vary with trigger zone. with the pattern Duration of a series of the stimulus. of action potentials action potentials is arriving at the axon proportional to stimulus terminal. duration.

Stimulus Stimulus

Receptor

Receptor potential

Axon of sensory neuron Action potentials in sensory neuron Time

Stimulus Stimulus

Receptor

Receptor potential

Axon of sensory neuron Action potentials in sensory neuron Time

• Touch • Proprioception • Temperature • Nociception – Pain – Itch

• Free nerve endings • Terminate in subcutaneous layers • Cold receptors – Lower than body temperature • Warm receptors – Above body temperature to about 45°C – Pain receptors activated above 45°C • Thermoreceptors use cation channels called transient receptor potential (TRP) channels

• Respond to strong noxious stimulus that may damage tissue • Free nerve endings • Primary sensory fibers • A fibers • C fibers

• Pain – Subjective perception – Fast pain • Sharp and localized—by A fibers – Slow pain • Duller and more diffuse—by C fibers • Itch – Histamine activates C fibers, causing itch – From skin nociceptors

Nociceptors Pathways

• Reflexive protective response – Integrated in spinal cord – Withdrawal reflex • Ascending pathway to cerebral cortex – Becomes conscious sensation (pain or itch)

• Modulated by local chemicals – Substance P is secreted by primary sensory neurons – Mediate inflammatory response – Inflammatory pain • Ischemia is lack of adequate blood flow • Referred pain • Chronic pain is a pathological (neuropathic) pain

Pain in internal organs is often sensed on the surface of the body, a sensation known as referred pain.

Heart

Liver and gallbladder

Stomach Small intestine Ureters Appendix Colon

One theory of referred pain says that nociceptors from several locations converge on a single ascending tract in the spinal cord. Pain signals from the skin are more common than pain from internal organs, and the brain associates activation of the pathway with pain in the skin. Based on H.L. Fields, Pain (McGraw Hill, 1987).

Skin (usual stimulus)

Primary sensory neurons

Kidney (uncommon stimulus) Secondary Ascending sensory sensory neuron path to somatosensory cortex of brain

• Gate control theory: Aβ fibers synapse on inhibitory interneurons and increase inhibition – Integrated response from Aβ and C fibers decreases the perception of pain.

• Analgesic drugs – Aspirin • Inhibits prostaglandins, decreases inflammation, and slows transmission of pain to site of injury – Opioids • Block pain perception by decreasing primary sensory neuron neurotransmitter release and by postsynaptic inhibition of secondary sensory neurons • Endorphins, enkephalins, dynorphins