G.Response - Stimulus/Response Respond to Stimuli Respond...

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Unformatted text preview: Stimulus/Response Respond to Stimuli Respond to Stimuli Integration & Cognition A key characteristic of Life sensor effector stimulus response Respond to Stimuli: Respond to Stimuli: Nervous System Endocrine System Central Nervous S ystem Sensory Nerves Nervous System stimulus response Motor Nerves effector Endocrine Gland sensor effector = specific muscle response or g land vi sensor stimulus effector response Cellular Communication — Chemical Messengers & Receptors One cell releases a molecule 1. Synapse: the messenger (neurotransmitter) diffuses (messenger) that initiates a across a small gap between a neuron and its target cell change in another cell by binding to a protein receptor (effector). 2. Paracrine: the messenger on that target cell. effector a bl d oo st re am response effector = various target response tissues throughout the body Neurons conduct impulses and transmit electrochemical messages to other cells (paracrine factor, growth factor, cytokine) diffuses to nearby target cells. 3. Endocrine: the messenger (hormone) diffuses into the bloodstream to travel to target cells all over the body. 4. Exocrine: the messenger (pheromone) diffuses outside of the organism ’s body to travel to another organism. Heyer 1 Stimulus/Response Parts of a Neuron • Cell body: location of the cell ’s nucleus and most other major organelles. • Dendrites: highly branched extensions of the cell body membrane to increase surface area for receiving excitatory and inhibitory stimuli. May be several hundred per cell. Electrochemical Signaling • • • • Electrical potential Excitability Conduction Transmission Membranes are electrically charged • Voltage (= electrical potential): Parts of a Neuron • Axon: long extension that can initiate and conduct nerve impulses. – Only one per cell — but may be branched. – Impulse is initiated at the hillock adjacent to the cell body; then conducted outward to the axon terminus. – The axon may be wrapped by glial cells in a segmented myelin coat that increases the conduction rate of the axon. Membranes of neurons are electrically charged • Chemical gradients of ions produces electrical gradients • Electrical gradient produces a membrane potential Significant ions – The difference in charge between two points – Potential energy created by attractive forces between opposite charges separated by distance. • Membrane potential: – difference in charge between inside of cell membrane and outside. – Always measured as the charge inside the cell relative to outside. – Units: millivolts (mV). Heyer Fig. 6.23 2 Stimulus/Response Resting Membrane Potential • Depends upon 2 factors: – Ratio of the concentrations of each ion on the 2 sides of the plasma membrane. – Specific permeability of membrane to each different ion. • Resting membrane potential of most cells ranges from - 65 to – 85 mV. Electrical Activity of Excitable Cells • • The ability to undergo rapid changes in membrane potential in response to stimuli. – Oocytes: rapid block to polyspermy. – Neurons: conduct nerve impulses – Muscle cells: initiate contraction Voltage-regulated Gates (VG) Increase in membrane permeability for specific ion can be measured by placing 2 electrodes (1 inside and 1 outside the cell). • Cell Excitability (= Irritability) Depolarization: – Potential difference reduced (become more positive). • Repolarization : – Return to resting membrane potential (become more negative). • Hyperpolarization : – More negative than RMP. Action Potentials (APs) Action Potentials (APs) (continued) • Stimulus causes depolarization to threshold. • VG Na+ channels open. – Electrochemical gradient inward. • positive feedback loop. – Rapid reversal in membrane potential from –70 to + 30 mV. – VG Na + channels become inactivated. • VG K+ channels open. – Electrochemical gradient outward. • negative feedback loop. – Restore original RMP. Heyer 3 Stimulus/Response Conduction of electrochemical signals in neurons Conduction along an Axon • Spread of depolarization with influx of Na + depolarizes the adjacent region membrane, propagating the AP. • Conduction rate is slow. • Nerves are NOT wires! • Nerve impulses are NOT electricity! • Nerve impulse are a series of action potentials propagated in sequence down the neuron. – AP must be produced at every fraction of micrometer. • Occurs in 1 direction; previous region is in its refractory period. • Only the axons of neurons conduct the nerve impulse. • Initiated at the hillock, • Propagate toward the axon terminus The Domino Effect: propagation of a series of action potentials along the axon The Nervous System(s) Central Nervous System (CNS) • Brain & spinal cord Peripheral Nervous System (PNS) • Sensory Nervous System (SNS) – Conducts nerve impulse to the CNS. – Afferent • Motor Nervous System (MNS) – Conducts nerve impulse from the CNS. – Efferent Heyer Transmission of the signals: the Synapse • Synapse: functional connection between a neuron and another neuron or an effector cell (muscle or gland). • Synaptic cleft: a slight gap between the presynaptic cell (axon terminus) and the postsynaptic cell. • Series of action potentials conducted to axon terminus fi causes exocytosis of vesicles containing a chemical messenger (neurotransmitter) into the synaptic cleft. . • Neurotransmitter binds to a receptor protein on the surface of the effector cell fi turns on the receptor . • The intracellular portion of the activated receptor causes a response in the postsynaptic cell. Structures of the Nervous System I. Bundles of axons — in the PNS = “nerve ” (bundle wrapped in connective tissue) • “mixed nerve ” contains the axons of both afferent and efferent neurons — in the CNS = “tract” • In the spinal cord, afferent= “ascending”; efferent=“descending” • Myelinated axons are white in color. Regions of the CNS primarily of axon tracts = “white matter.” II. Clusters of cell bodies — in the PNS = “ganglion ” (cell bodies supported by satellite glia) — in the CNS = “center” (sometimes “ganglion ” is also used) • Since synapses form on cell bodies and their dendrites, these clusters of cell bodies are the areas of complex neural integration. • Since cell bodies are not myelinated (i.e., not white), these centers in the CNS are called “gray matter. ” 4 Stimulus/Response SENSES Sensory Receptors (Sensors) • Transducer: a structure that converts a physical stimulus into a nerve impulse. • Physiological transducers are derived from specialized dendrites, often associated with modified epithelia. • Exteroceptors: sense external environmental stimuli • Interoceptors: sense internal body condition Types of Sensory Receptors: Mechanisms of Transduction Sensations • Somatesthetic senses (“body feeling”) – Cutaneous senses – Proprioceptors • Special senses — via cranial nerves – Gustatory (taste) – Olfactory (smell) – Auditory (hearing) – Equilibrium – Vision Types of Sensory Receptors: Mechanisms of Transduction 1. Chemoreceptors: chemical stimulus binds to specific binding protein on cell surface ‡ open ion gates ‡ depolarization. Interoceptors • – – • 1. Olfaction Taste Chemoreceptors Mechanoreceptors Thermoreceptors Photoreceptors Types of Sensory Receptors: Mechanisms of Transduction 2. Mechanoreceptors: physical distortion of cell membrane ‡ open ion gates ‡ depolarization. Interoceptors • – – – Blood glucose, fatty acids, pH Nocioceptors — chemicals released by damaged cells (pain) Exteroceptors (special senses) – – 1. 2. 3. 4. • Exteroceptors (cutaneous senses) – • Touch & pressure Exteroceptors (special senses) – – Heyer Propioceptors — muscles, tendons, joints Visceral stretch receptors Blood pressure, osmotic pressure Auditory hair cells Equillibrium hair cells 5 Stimulus/Response Types of Sensory Receptors: Mechanisms of Transduction 1. 2. 3. Thermoreceptors: temperature-induced change of membrane protein ‡ open ion gates ‡ depolarization. Interoceptors • – • Body temperature [B T] — hypothalamus Exteroceptors (cutaneous senses) – Types of Sensory Receptors: Mechanisms of Transduction 1. 2. 3. 4. Photoreceptors: light absorbed by pigmentprotein ‡ close ion gates ‡ hyperpolarization. • Exteroceptors (special senses) – Vision — retina rods & cones Hot & cold sensations (“Cold ” is a perception, not a real physical entity.) Divisions of the Nervous Systems Motor Systems Nervous System Central NS Peripheral NS Motor nervous system: transmits efferent nerve stimuli from CNS to peripheral effectors • Somatic Motor System (“voluntary”) Motor PNS Somatic Motor PNS Sensory PNS Autonomic Motor PNS Sympathetic AutonomicMotor PNS Parasympathetic AutonomicMotor PNS Divisions of the Autonomic Motor System ÿ Effectors = skeletal muscles • Autonomic Motor System (“involuntary”) ÿ Effectors = smooth muscles, cardiac muscle, & glands Organs With Dual Innervation Most visceral organs receive dual innervation (innervation by both sympathetic and parasympathetic fibers). • Antagonistic effects: – Actions counteract each other. • Sympathetic and parasympathetic fibers innervate the same cells. – Heart rate. • Sympathetic and parasympathetic fibers innervate different cells. – Iris of the eye. • Cooperative: – Sympathetic and parasympathetic stimulation produce different effects that work together to produce desired effect. • Urination; penile erection. Heyer 6 Stimulus/Response MUSCLES Characteristics of muscle • Muscle Types • Muscle Structure • Muscle Function • • • • Contraction: shorten via internal force Extension: muscle can be lengthened only by an external force Contractility: shorten actively Extensibility: stretch passively Elasticity: recoil to resting length Excitability: respond to stimulation Three types of muscle tissue • Skeletal muscle – Attached to bone (usually) – Striated: contractile proteins stacked in visible columns; contraction is linear – Voluntary: contract only in response to somatic motorneuron stimulation • Cardiac muscle – Found only in heart – Striated – Involuntary: contract in response to intrinsic pacemaker; modifiable by autonomic motorneurons • Smooth muscle – Found in lining of visceral organs, blood vessels, skin, & elsewhere – Unstriated: contractile proteins aligned in 3-dimensional arrays; contraction may be multi-dimensional – Involuntary: contract either in response to intrinsic reflexes, or from extrinsic autonomic motorneuron stimulation Biceps contracts, elbow bends Triceps exerts force to lengthen biceps, straighten arm Three types of muscle tissue Skeletal muscle • Skeletal muscle – Attached to bone; Striated; Voluntary • Cardiac muscle – Heart muscle; Striated; Involuntary • Smooth muscle – Viscera, vessels, etc.; Unstriated ; Involuntary Heyer • Skeletal muscle cells are long, multi-nuclear fibers. • Most of the cell’s volume is taken up by stacks of protein filaments. • Nuclei and mitochondria are displaced to the periphery. 7 Stimulus/Response Muscle terms origin Muscle fiber organization insertion Mus -, mys - = “mouse” mus cle, myo fiber Sarco - = “meat”; “flesh” Especially with respect to modified cell components of the mulinuclear myofiber : • cytoplasm · sarco plasm • plasmalemma (cell membrane) · sarco lemma • [smooth] endoplasmic reticulum · sarco plasmic reticulum Sarcomere: contractile unit of a myofibril (Z-line to Z-line) Myofilaments — protein engines within the myofibril Two types: thin filaments and thick filaments I-band A-band I-band Sliding Filament model of muscle contraction Actual muscle (TEM) • Thin filaments are pulled over the thick filaments. • Filaments do not change length — but increase degree of overlapping. • A-band does not change size; but Z-lines get closer together (sarcomeres get shorter). • Tiny changes in length of each of the thousands of sarcomeres add to a big change in length for the myofibril/ myofiber /muscle. Heyer 8 Stimulus/Response Cardiac Muscle Smooth Muscle • Striated: Contain actin and myosin arranged in sarcomeres. • Contract via slidingfilament mechanism. • Branched, mononuclear cells. • Adjacent myocardial cells joined by gap junctions. • Does not contain sarcomeres. • Contains > content of actin than myosin (ratio of 16:1). • Myosin filaments attached at ends of the cell to dense bodies. • May contain gap junctions. – APs spread through cardiac muscle through gap junctions. – All cells contribute to contraction. – Single-unit muscle : entire muscle contracts as a single unit Glands: organs specialized for secretion Formation o f E pithelial G lands Endocrine Glands and Hormones • Endocrine glands may be of epithelial or neural origin. • Secrete biologically active molecules into the blood. – Lack ducts. •Exo crine glands: secrete via ducts “out” of body •Endo crine glands: secrete into bloodstream Note: not all hormones are secreted by specialized glands • Bloodstream carries hormones to target cells that contain specific receptor proteins for that hormone. • Target cells can respond in a specific fashion. Pituitary Gland — the “Master Gland” • Pituitary gland is located below the forebrain. • Structurally and functionally divided into: – Posterior lobe — an extension of the hypothalamus neural tissue. – Anterior lobe — epithelial gland derived from the roof of the mouth. Heyer 9 Stimulus/Response Anterior Pituitary Trophic Hormones Trophic: “feeding” • Stimulate activity of other glands. “Master Gland ” • Stimulate growth of target tissues • High blood [hormone] causes target organ to hypertrophy. • Low blood [hormone] causes target organ to atrophy. Feedback Control of the Anterior Pituitary E.g., sex hormones: 1. Hypothalamus secretes releasing homone (GnRH ) to stimulate Ant. Pituitary. 2. Ant. Pituitary secretes gonad otropin s (LH & FSH) to stimulate gonads to: 1. Grow & mature 2. Secrete sex steroids 3. Sex steroids from gonads feedback to inhibit pituitary from secreting more gonadotropin . Regulating the Master Gland 1. Releasing (stimulating) and inhibitory hormones from the hypothalamus. 2. Negative feedback by hormones secreted by the target glands. Feedback Control of the Anterior Pituitary So, … 1. If a body builder takes anabolic steroids (synthetic androgens) … 2. The steroids act like other androgens and inhibit the Ant. Pit. from secreting gonadotropins . … 3. With supressed gonadotropin secretion, the gonads fail to grow. “Their grapes turn to raisins. ” Exogenous sex steroids Feedback Control of the Anterior Pituitary Feedback Control of the Anterior Pituitary Another example: thyroid function: 1. Hypothalamus secretes releasing homone (TRH) to stimulate Ant. Pituitary. 2. Ant. Pituitary secretes thyr otropin (TSH) to stimulate thyroid to: 1. Grow 2. Secrete thyroid hormone (thyroxine ) 3. Thyroxine from thyroid feedback to inhibit pituitary from secreting more TSH. Heyer But, iodine is needed to synthesize thyroxine . 1. If the diet is deficient in iodine, thyroid cannot make thyroxine . 2. If no thyroxine , no negative feedback on Ant. Pituitary. 3. Ant. Pituitary continues to secrete TSH 4. TSH continues to stimulate thyroid to overg row. 5. Overgrown thyroid forms a goiter. 10 ...
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