CNS part 2 & Intro to Sensory Systems

Transcription

1 CNS part 2 & Intro to Sensory Systems Brain Function Important Concepts Functional areas of the cerebral cortex Sensory, Motor, Association Cerebral lateralization each hemisphere has functions not shared by the other The Spinal Cord and Brain Integrate Sensory Information Primary Somatic Sensory Cortex Input from somatic senses The Special Senses localize to other cortical regions Primary Motor Cortex Voluntary movement (Output) Functional Areas of the Cerebral Cortex Brain Function Important Concepts Association Areas - Sensory Information is Processed into Perception ØThe brain s interpretation of the sensory stimuli 1

2 Brain Function Important Concepts The Behavioral State System Controls sensory and cognitive processing Levels of consciousness (awareness of self and the environment) Sleep-wake cycles Diffuse modulatory systems Reticular formation in the brain stem, HT, & limbic system Diffuse Modulatory Systems (4) Regulate brain function by influencing Attention Motivation Wakefulness Memory Motor control Mood Metabolic homeostasis Figure 9.16a Diffuse modulatory systems Figure 9.16b Diffuse modulatory systems Noradrenergic (Norepinephrine) Serotonergic (Serotonin) Functions: Atte ntion, arousa l, slee p-wa k e c y cle s, le a rning, Functions: 1. Lower nuclei: Pain, locomotion memory, anxiety, pain, and mood 2. Upper nuclei: Sleep-wa k e cy c le; m ood and Neurons Originate: Locus coeruleus of the pons To basal nuclei emotional behaviors, such as aggression and depression Thalamus Neurons Terminate: Cerebral cortex, thalamus, hypothalamus, Neurons Originate: Raphe nuclei along brain stem midline olfactory bulb, cerebellum, midbrain, spinal cord Neurons Terminate: 1. Lower nuclei project to spinal cord Hy pothalam us Ce re bellum Ra phe 2. Upper nuclei project to most of brain Locus coeruleus nuclei 2

3 Figure 9.16c Diffuse modulatory systems Figure 9.16d Diffuse modulatory systems Dopaminergic (Dopamine) Cholinergic (Acetylcholine) Functions: 1. Motor control Functions: Sleep-wake cycles, arousal, learning, memory, 2. Reward centers linked to sensory information passing through thalamus Prefrontal cortex To basal nuclei Neurons Originate: addictive behaviors 1. Substantia nigra in midbrain 2. Ventral tegmentum in midbrain Cingulate gyrus Neurons Originate: Base of cerebrum ; pons and midbrain Neurons Terminate: Ce re brum, hippoca m pus, thalam us Ventral Substantia nigra Neurons Terminate: 1. Cortex 2. Cortex and parts of limbic system Fornix tegmental area Pontine nuclei Brain Function: Sleep Four stages with two major phases Slow-wave sleep Adjusts body without conscious commands Rapid eye movement (REM) sleep Brain activity inhibits motor neurons to skeletal muscle, paralyzing them Brain Function: Sleep Sleep disorders Insomnia: inability to sleep Sleep apnea: stopping normal breathing during sleep Somnambulism: sleepwalking Dreaming takes place 3

4 Brain Function: To the N.F.L., 40 Winks Is as Vital as the 40-Yard Dash In the never-ending search for an edge, teams have opened their eyes to sleep as a priority, backed up by science that increasingly points to its importance for physical and mental health. Circadian rhythms Biological cycle of rest and activity Neurons are located in Suprachiasmatic nucleus (SCN) of the hypothalamus SCN has melatonin receptors - linked to circadian rhythms disrupted by shift work and jet lag Brain Function: Motivation Defined as internal signals that shape voluntary behaviors Some states known as drives Figure 9.18 Emotions affect physiology Work with autonomic and endocrine responses Motivated behaviors stop when a person has reached a certain level of satiety Pleasure and addictive behaviors: link to dopamine 4

5 Brain Function: Moods Similar to emotions but longer-lasting Mood disorders Fourth leading cause of illness worldwide today Depression Sleep and appetite disturbances Alterations of mood and libido May affect function at school or work or in personal relationships Antidepressant drugs alter synaptic transmission Brain Function: Learning and Memory Learning The acquisition of knowledge Two broad types Associative learning 2 stimuli are associated together Nonassociative learning Repeated exposure to single stimulus Habituation and sensitization Brain Function: Language Speaking a Written Word Left cerebral hemisphere Integration of spoken language involves two regions Damage to Wernicke s area causes receptive aphasia Unable to understand sensory input Damage to Broca s area causes expressive aphasia Unable to understand complex sentences Motor cortex Broca s area Wernicke s area Read words Visual cortex 5

6 Speaking a Heard Word PET Scans of Functional Brain Areas Motor cortex Broca s area Figure 9-17 Hear words Auditory cortex Wernicke s area Brain Function: Personality Combination of experience and inheritance Schizophrenia Both genetic and environmental basis 6

7 Sensory Physiology Important Concepts The Somatic Senses Touch Temperature Pain Itch Proprioception The Special Senses Vision, Hearing, Taste, Smell, Equilibrium Sensory Physiology General properties of sensory systems Somatic senses Chemoreception: smell and taste The ear: hearing The ear: equilibrium The eye and vision Central Nervous System Peripheral Nervous System Afferent (Sensory) 7

8 Sensory Physiology Important Concepts Somatosensory Pathways Chain of neurons from the receptor organ to the cerebral cortex responsible for the perception of the senses Somatic Sensory Pathways to the Cerebral Cortex Sensory Pathways 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 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 8

9 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 with free nerve endings. They may have myelinatedor unmyelinated axons. Complex neural receptors have nerve endings enclosed in connective tissue capsules. This illustration shows a Paciniancorpuscle, whic h senses touch. Mo s t s p e c i a l s e n s e s rec e p to rs are c e l ls that re lea s e neurotra nsm itte r onto se nsory neurons, initiating an action potential. The cell illustrated is a hair cell, found in the ear. Sensory Receptors and Sensory Neurons Stimulus Stimulus Stimulus Free nerve endings Enclosed nerve ending Layers of connective tissue Specialized receptor cell (hair cell) Synaptic vesicles Synapse Unmy e linate d axon Myelinated axon Myelinated axon Cell body Cell body Ce ll body of sensory neuron 9

10 Sensory Transduction Conversion of a physical stimulus into an electrical signal 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 Transduction Receptive Field of a Sensory Neuron 10

11 Two-Point Discrimination Test Sensory Pathways Integration by CNS 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. Integration by CNS 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 11

12 1. Stimulus modality is encoded by specific neural pathways to the cerebral cortex. 2. Representation of stimulus location in the somatosensory cortex. Perception of Sound Location Lateral Inhibition - enhances contrast between adjacent receptive fields - sharpens perception of stimulus location 12

13 3. Stimulus intensity is encoded by the number of receptors activated, and... Stimulus intensity is encoded by the frequency of action potentials in sensory neurons Receptor Adaptation 13

14 Somatic Senses: Modalities Touch Proprioception Temperature Nociception Pain Itch Temperature Receptors 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 Nociceptors Respond to strong noxious stimulus that may damage tissue Free nerve endings Primary sensory fibers Aδ fibers C fibers Nociceptors Pain Subjective perception Fast pain Sharp and localized by Aδ fibers Slow pain Duller and more diffuse by C fibers Itch Histamine activates C fibers, causing itch From skin nociceptors 14

15 NociceptorsPathways Reflexive protective response Integrated in spinal cord Withdrawal reflex Ascending pathway to cerebral cortex Becomes conscious sensation (pain or itch) NociceptorsPathways Pain in internal organs is often sensed on the surface of the body, a sensation known as referred pain. Figure 10.11a Referred pain 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 Liver and gallbladder He a rt Stomach Appendix Colon Small intestine Ure te rs 15

16 Figure 10.11b Referred pain One the ory of re ferre d pa in s a y s tha t 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) Pain Modulation Gate control theory: Aβ fibers synapse on inhibitory interneurons and increase inhibition Integrated response from Aβ and C fibers decreases the perception of pain. Primary sensory neurons Kidney (uncommon stimulus) Secondary sensory neuron Ascending sensory path to somatosensory cortex of brain Pain Modulation 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 16