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VT 105 Comparative Anatomy and Physiology Nervous System Functions of the Nervous System sensory function – senses stimuli (changes in internal or external environment) integrative function – processes sensory inputs and decides on appropriate responses motor function – sends signals to effector cells, which respond to the stimuli DIVISIONS OF THE NERVOUS SYSTEM Central Nervous System (CNS) – brain and spinal cord main integrative center contains the cell bodies of most neurons Peripheral Nervous System (PNS) – all nervous tissue outside the CNS nerves – bundles of axons following specific paths outside the CNS cranial nerves (12 pairs) – arise from the brain spinal nerves (many pairs) – arise from the spinal cord ganglia – small clusters of neuron cell bodies outside the CNS sensory receptors – dendrites of neurons or specialized cells Functional Divisions of the PNS somatic nervous system (SNS) – voluntary sends sensory information about the external environment or body position to the upper brain, where the inputs are consciously perceived sends motor impulses to skeletal muscles to cause body movements autonomic nervous system (ANS) – involuntary (self-regulated) sends sensory information about the internal environment to the lower brain (not consciously perceived) sends motor impulses to effectors such as smooth muscle, glands, and cardiac muscle HISTOLOGY OF NERVOUS TISSUE Neurons – 3 basic parts 1) cell body – contains the nucleus and cellular organelles carries out vital functions of the cell 2) dendrites – branched receiving portion of neuron receive stimuli from the environment or other neurons vary in number (more dendrites = more stimuli can be received) 3) axon – single, long sending portion of neuron synaptic end bulb – bulb at end of axon which synapses with effector cell synaptic vesicles – store neurotransmitters 1 sensory (afferent) neuron – axon sends impulses to the CNS motor (efferent) neuron – axon sends impulses away from the CNS synapse – site of communication between a neuron and an effector cell (neuron, muscle fiber, gland) Neuroglia Types in CNS: astrocytes – surround and support neurons, structurally and functionally help form the blood-brain barrier oligodendrocytes – produce myelin sheath in the CNS Types in PNS: Schwann cells – produce myelin sheath in PNS satellite cells – surround and protect cell bodies in ganglia Myelination myelin sheath – layers of cell membrane(lipid) wrapped around the axon electrically insulates the axon increases rate of impulse conduction PNS – Schwann cells wrap segments of axons cytoplasm and nucleus of Schwann cell form the outermost layer nodes of Ranvier – gaps in myelin sheath between Schwann cells CNS – oligodendrocytes have multiple, flat processes which wrap several adjacent axon segments White and Grey Matter white matter – areas of CNS that appear white and shiny contains many myelinated axons – myelin is white (lipid) grey matter – grey areas of CNS composed of neuron cell bodies, neuroglia, unmyelinated axons NEURON PHYSIOLOGY – Production of Electrical Impulses electrical current – flow of charged particles (ions in cells) Ion Channels in the Cell Membrane chemically-gated channels – open or close when a particular molecule binds (eg. taste & smell molecules, neurotransmitters) mechanically-gated channels – open or close in response to mechanical forces (eg. touch & pressure, sound waves) voltage-gated channels – open or close in response to change in membrane potential (charge inside cell) 2 Resting Membrane Potential – at rest, neuron cell membrane is polarized (different charges on inside and outside of membrane) Na+/K+ pumps pump Na+ out of neuron (high Na+ concentration outside neuron) K+ positive Na+ balanced by negative Clpump K+ into neuron (high K+ concentration inside neuron) positive K+ balanced by negative protein molecules Neuron has different permeability to ions K+ permeability is 50-100 times greater than Na+ (many K+ leakage channels, almost no Na+ leakage channels) K+ leaks out of neuron, down its concentration gradient Interior of neuron becomes increasingly negative (negative proteins in neuron too large to diffuse out – impermeable) Negative charge in neuron draws some K+ back into cell At equilibrium, resting membrane potential is about -70mV (70mV more negative inside cell than outside cell) Stimulation of Neuron – small changes in resting membrane potential caused by opening chemically- or mechanically- gated channels on dendrites depolarization – membrane becomes less polarized (less negative inside) Action Potentials (nerve impulses) – large change in resting membrane potential caused by opening voltage-gated channels on axons begins near the cell body and travels down axon to synaptic end bulbs 1) Neuron Stimulated to Threshold – specific level of depolarization that triggers opening of voltage-gated Na+ channels (about -55mV) all-or-none response – if neuron reaches threshold, an action potential occurs (a signal is sent) if threshold isn't reached, no action potential 2) Depolarization Phase – neuron becomes less polarized (less negative inside) voltage-gated Na+ channels open – Na+ rushes into neuron (charge inside neuron becomes positive) 3) Repolarization Phase – neuron becomes polarized again (negative inside) voltage-gated K+ channels open – K+ rushes out of cell (charge inside neuron becomes negative again) (voltage-gated Na+ and K+ channels close again) 4) Na+/K+ pumps restore resting membrane potential 3 Refractory Period – time after action potential begins when cell can’t generate another action potential because voltage-gated channels are not reset Conduction of Action Potentials – traveling of nerve impulse down the axon refractory period results in one-way conduction continuous conduction – step-by-step depolarization of the entire length of an unmyelinated axon relatively slow saltatory conduction – occurs along myelinated axons depolarization leaps from one node of Ranvier to the next the entire axon does not completely depolarize impulse is conducted very rapidly opening ion channels only at nodes means less Na+ and K+ pass through membrane and less ATP (energy) is used to pump them back Synapses Between Neurons presynaptic neuron – sending neuron (axon synaptic end bulb) postsynaptic neuron – receiving neuron (dendrite) synaptic cleft – small space between 2 communicating neurons an action potential in the presynaptic neuron triggers release of neurotransmitter from synaptic vesicles neurotransmitter diffuses across synaptic cleft and binds to receptors (membrane proteins on the postsynaptic neuron that cause change in charge) excitatory neurotransmitter – depolarizes the postsynaptic neuron brings it closer to threshold (may cause an action potential) inhibitory neurotransmitter – hyperpolarizes the postsynaptic neuron postsynaptic neuron becomes more negative (farther from threshold) the postsynaptic neuron can have many synapses summation of all of the excitatory and inhibitory synapses determines whether the postsynaptic neuron reaches threshold and produces an action potential Neurotransmitters – there are many different kinds of neurotransmitters Acetylcholine (ACh) – acts in PNS and CNS excitatory at skeletal muscles – causes contraction inhibitory in the heart – decreases heart rate acetylcholinesterase – enzyme inactivates acetylcholine in synaptic cleft gamma aminobutyric acid (GABA) – common inhibitor in CNS some tranquilizers (valium) enhance action of GABA Catecholamines – excitatory or inhibitory depending on the receptors norepinephrine (NE) – “fight-or-flight” responses epinephrine (E)– hormone from adrenal gland (similar to NE) 4 Removal of Neurotransmitter – effect of neurotransmitter continues until it is removed from the synaptic cleft 3 mechanisms of removal: enzymatic degradation (eg. acetylcholinesterase) uptake by cells – neuron that released it or neuroglial cells diffusion away from synaptic cleft, degraded by other cells How Drugs and Toxins Modify Nervous System Function stimulate or inhibit neurotransmitter synthesis stimulate or inhibit neurotransmitter release block or activate neurotransmitter receptors agonists activate receptors (mimic neurotransmitter) antagonists block receptors (prevent neurotransmitter function) stimulate or inhibit neurotransmitter removal THE BRAIN PROTECTION AND NOURISHMENT OF THE BRAIN Cranium – bones surrounding and protecting the brain Cranial Meninges – 3 connective tissue membranes around brain pia mater – inner membrane which adheres to surface of brain contains blood vessels which supply the brain arachnoid mater – delicate middle membrane has web-like collagen and elastic fibers that extend to the pia mater subarachnoid space – space between arachnoid and pia that contains cerebrospinal fluid (CSF) dura mater – tough, protective outer membrane fuses with periosteum of cranium folds between cerebral hemispheres and between cerebrum and cerebellum help secure brain's position contains large, open veins that collect excess CSF Blood-Brain Barrier – protects brain by preventing passage of many substances from the blood to brain tissue brain capillaries have tight junctions between cells astrocyte processes surround capillaries – selectively pass some substances to neurons but block others glucose crosses by active transport – main energy source for neurons 5 Cerebrospinal Fluid (CSF) – clear fluid which circulates through cavities in brain, spinal cord, and in subarachnoid space Functions of CSF: chemical content helps regulate autonomic functions cushions delicate neurons of brain and spinal cord Formation and Circulation of CSF ventricles – 4 cavities in brain filled with CSF capillary networks in each ventricle filter blood to form CSF neuroglial cells lining ventricles regulate content of CSF CSF circulates from ventricles to central canal of spinal cord and the subarachnoid space CSF returns to blood in veins within dura mater hydrocephalus – excess accumulation of CSF resulting in increased pressure on the brain 4 DIVISIONS OF THE BRAIN - brainstem, diencephalon, cerebellum, cerebrum 1) Brainstem – connects to the spinal cord controls vital autonomic functions and autonomic reflexes such as; swallowing, coughing, sneezing, vomiting Medulla oblongata – caudal brainstem vital centers – control vital autonomic functions cardiovascular center – regulates heart and blood vessels respiratory center – controls respiratory muscles Pons – middle region of brainstem regulates respiratory rhythm Midbrain (Mesencephalon) – cranial brainstem contains reflex centers for vision and hearing 2) Diencephalon – (between brain) between brainstem and cerebrum thalamus – 80% of diencephalon relay station for sensory impulses traveling to the cerebrum hypothalamus – ventral to (below) thalamus has no blood-brain barrier – senses changes in blood and CSF regulates the ANS – involuntary organ functions regulates eating and drinking – thirst center, feeding center regulates body temperature via ANS link between the nervous and endocrine systems produces hormones that regulate anterior pituitary gland produces oxytocin and antidiuretic hormone participates in emotional behavior (eg. fight-or-flight responses) 6 3) Cerebellum – attached to dorsal brainstem coordinates skeletal muscle movements receives voluntary motor impulses from cerebrum receives sensory impulses related to body position and balance the cerebellum compares intended movements with actual movements sends feedback to cerebrum for corrections disorders result in hypermetria – voluntary movements are jerky and exaggerated 4) Cerebrum – largest, most dorsal portion of brain origin of voluntary actions, site of conscious perceptions, center of intellect longitudinal fissure – deep groove that divides cerebrum into 2 hemispheres sulci – shallower grooves that divide hemispheres into lobes Cerebral Cortex – outer gray matter contains neuron cell bodies controlling conscious functions Functional Areas of the Cerebral Cortex Sensory areas – caudal cerebrum primary sensory cortex – receives sensations of pain, touch, temperature from the opposite side of the body parietal lobe visual cortex – receives visual sensations occipital lobe auditory cortex – receives sensations of hearing temporal lobe Motor areas – cranial cerebrum primary motor cortex – controls voluntary contractions of skeletal muscles frontal lobe the cortex sends motor impulses to the opposite side of the body Association areas – located within or near motor and sensory areas allow recognition of sensations control complex, learned motor skills performs abstract functions – prediction, reasoning Cerebral White Matter – deep to cortex contains axon tracts running to and from spinal cord, between the cerebral hemispheres, and within the same hemispheres corpus callosum – main tracts connecting the 2 cerebral hemispheres 7 CRANIAL NERVES – 12 pairs arising mainly from brainstem sensory nerves – only sensory axons motor nerves – only motor axons mixed nerves – sensory and motor axons Cranial nerve I – olfactory nerve sensory – olfaction (smell) Cranial nerve II – optic nerve sensory – vision Cranial nerve III – oculomotor nerve motor – somatic – most eyeball movements autonomic – inner eye movements (pupil size, focusing lens) Cranial nerve IV – trochlear nerve motor – eyeball movements Cranial nerve V – trigeminal nerve mixed – sensory from face, jaw, and teeth motor to muscles of mastication (chewing) has 3 branches; ophthalmic nerve – sensory maxillary nerve – sensory mandibular nerve – mixed Cranial nerve VI – abducens nerve motor – eyeball movements Cranial nerve VII – facial nerve mixed – sensory from taste buds motor somatic – facial expressions, lip movement autonomic – secretion of tears, saliva, nasal secretions Cranial nerve VIII – vestibulocochlear nerve (auditory, acoustic) sensory – 2 branches vestibular nerve – balance cochlear nerve – hearing Cranial nerve IX – glossopharyngeal nerve mixed – sensory from taste buds, throat motor somatic – swallowing and tongue movement autonomic – secretion of saliva Cranial nerve X – vagus nerve mixed – sensory from larynx, visceral organs, carotid artery motor somatic – swallowing autonomic parasympathetic to most viscera Cranial nerve XI – accessory nerve motor – head and shoulder movements Cranial nerve XII – hypoglossal nerve motor – tongue movements 8 SPINAL CORD AND SPINAL NERVES EXTERNAL ANATOMY OF SPINAL CORD extends from brainstem to lumbar vertebrae in adult cauda equina – bundle of nerve roots in caudal vertebral canal after spinal cord ends spinal nerves – emerge in pairs through the intervertebral foramina most emerge caudal to corresponding vertebra (except cervical nerves) cervical nerves – 1 pair/cervical vertebra + 1 pair between skull and atlas vertebra thoracic nerves – 1 pair/ thoracic vertebra lumbar nerves – 1 pair/lumbar vertebra sacral nerves – 1 pair/sacral vertebra coccygeal nerves – variable numbers SPINAL MENINGES – 3 connective tissue membranes surrounding the spinal cord similar to, and continuous with cranial meminges pia mater – thin, inner membrane on surface of spinal cord with many blood vessels supplying the spinal cord arachnoid mater – thin, middle membrane with a spider’s web of collagen and elastic fibers extending to the pia mater subarachnoid space – space beneath the arachnoid mater containing cerebrospinal fluid (CSF) site for spinal tap to collect CSF dura mater – outer, dense connective tissue sheath surrounding spinal cord and cauda equina suspends spinal cord within vertebral canal epidural space – space above dura mater contains adipose tissue and blood vessels cushions spinal cord site for epidural anesthetic injections SPINAL NERVES – mixed nerves (contain sensory and motor axons) spinal nerves arise at specific spinal cord segments dorsal root – contains sensory axons dorsal root ganglion – swelling on dorsal root containing cell bodies of sensory neurons ventral root – contains motor axons 9 Distribution of Spinal Nerves – spinal nerves branch immediately after passing through intervertebral foramina (gaps between vertebrae) phrenic nerve (C5-C6) – innervates diaphragm thoracic nerves (intercostal nerves) innervate muscles and skin of thorax and abdominal skin nerve plexuses – complex networks of nerves on either side of body brachial plexus (C6-C8 and T1 in cat) innervates shoulder and forelimb musculocutaneous, radial, ulnar, median nerves lumbosacral plexus (L4-L7 and S1-S3 in cat) innervates abdominal muscles, perineum (genitals, anal and urethral sphincters) and lower limb sciatic, femoral, and obturator nerves spinal cord damage causes loss of sensation and voluntary muscle control caudal to site of injury INTERNAL ANATOMY OF SPINAL CORD Gray Matter – butterfly or H shape located centrally contains nuclei – clusters of cell bodies in CNS ventral gray horns – somatic motor nuclei dorsal gray horns – sensory nuclei central canal – contains CSF, lined by ependymal cells White Matter – mainly myelinated axons located peripherally sensory (afferent) tracts – axons carrying sensory impulses to the brain motor (efferent) tracts – axons carrying impulses from the brain to skeletal muscles or autonomic effectors SENSORY AND MOTOR PATHWAYS sensations – nerve impulses stimulated by internal or external stimuli perception – conscious awareness and interpretation of sensations (occurs in cerebral cortex) SOMATIC SENSORY PATHWAY Sensory Receptors – specialized cell or dendrites that detect stimuli in the internal or external environment touch receptors – have mechanically-gated channels stimulated by touch pain receptors – have chemically-gated channels stimulated by chemicals released by tissue damage or inflammation 10 First-order neurons – carry nerve impulses from receptors to CNS cranial nerves – from face and mouth to brainstem spinal nerves – from head, neck, thorax, abdomen, and limbs to spinal cord Second-order neurons – carry nerve impulse from brain stem or spinal cord to thalamus cross over in medulla or spinal cord before going to thalamus almost all sensory information from one side of the body goes to the opposite cerebral cortex Third-order neurons – carry nerve impulses from thalamus to cerebral cortex impulses transmitted to the appropriate sensory area of the cortex SOMATIC MOTOR PATHWAYS – from cerebral cortex to skeletal muscles Upper motor neurons (UMNs) – from motor area of cerebral cortex to brainstem or spinal cord initiate voluntary movements Lower motor neurons (LMNs) – from CNS to skeletal muscles cranial nerves – brainstem to face, mouth, and neck spinal nerves – spinal cord to thorax and limbs direct motor pathways – UMNs from cortex synapse directly with LMNs cross over in medulla or spinal cord (motor impulses from cortex control skeletal muscles on opposite side of body) indirect motor pathways – UMNs from cortex form complex pathways that regulate muscle tone, posture and balance, coordinate movements cerebellum – compares motor commands from UMNs with actual movements and provides feedback to UMNs to correct REFLEXES – fast, automatic responses to specific stimuli somatic reflexes – involve skeletal muscle protective reflexes autonomic reflexes – involve smooth muscle, cardiac muscle or glands maintain homeostasis in the body Reflex Arc – pathway for nerve impulses of a reflex 1) sensory receptor – detects a stimulus 2) sensory neuron – generates a sensory impulse and carries it to the CNS 3) integrating center in CNS – interneurons in gray matter of spinal cord or brainstem synapse with sensory neuron and other neurons to determine a response 4) motor neuron – carries impulse from CNS to an effector 5) effector – part of body that responds to the motor impulse (muscle, gland) the effector’s automatic response to the stimulus is called a reflex 11 STRETCH REFLEX – suddenly stretching skeletal muscle causes it to contract protects the muscle from overstretching; helps set muscle tone 1) muscle spindles – sensory receptors in muscle; stimulated by stretching 2) sensory neuron excited – sends an impulse through dorsal root of spinal nerve into spinal cord 3) spinal cord gray matter – sensory neuron makes an excitatory synapse with a motor neuron 4) motor neuron excited – sends an impulse through ventral root of spinal nerve to neuromuscular junctions of the muscle that was stretched 5) muscle excited to contract by release of acetylcholine at NMJ ipsilateral reflex – sensory impulse triggers a motor reflex on the same side of spinal cord reciprocal innervation – an axon collateral of the sensory neuron synapses with inhibitory interneurons that cause simultaneous relaxation of antagonistic muscles WITHDRAWAL REFLEX – a painful stimulus causes withdrawal of a body part 1) pain receptors stimulated (eg. stepping on thorn) 2) sensory neuron excited and sends an impulse to spinal cord 3) spinal gray matter – sensory neuron synapses with multiple excitatory interneurons, which then synapse with multiple motor neurons 4) motor neurons send impulses to NMJs of flexor muscles of limb 5) flexor muscles contract, pulling limb away from stimulus reciprocal innervation also occurs; this reflex is also ipsilateral CROSSED EXTENSOR REFLEX – initiated by same painful stimulus that causes the withdrawal reflex causes extension of opposite limb to support body weight and maintain balance 1) pain receptor stimulated 2) sensory neuron sends nerve impulse to spinal cord 3) spinal gray matter - sensory neuron synapses with multiple excitatory interneurons, which cross to the other side of the spinal cord to synapse with multiple motor neurons 4) motor neurons conduct impulses to NMJs of extensor muscles of limb opposite to the one that stepped on the thorn 5) extensors of opposite limb contract to support extra weight as stimulated limb is lifted contralateral reflex arc – motor reflex on side opposite to sensory stimulus 12 Animals with brain or spinal cord injuries can still have reflexes if the reflex arc is intact. hyperreflexive – exaggerated reflexes due to loss of UMN regulation of muscle tone hyporeflexive – decreased reflexes due to damage to any part of the reflex arc NEURONAL CIRCUITS – pathways for impulses diverging circuits – spread of an impulse from 1 neuron to multiple neurons one stimulus produces multiple responses eg. pain causes the withdrawal reflex, cross extensor reflex, and reciprocal innervation of many muscles converging circuits – multiple neurons synapse with 1 postsynaptic neuron multiple stimuli can produce the same response eg. moving the limbs can be due to voluntary impulses from UMNs or reflex responses to external stimuli AUTONOMIC NERVOUS SYSTEM – regulates activities of smooth muscle, cardiac muscle, and glands operates mainly via reflex arcs Comparison of Somatic and Autonomic Nervous Systems SOMATIC stimuli – conscious sensations higher brain functions integration – cerebral cortex, lower brain & spinal cord voluntary control motor – somatic motor neurons 1-neuron pathways from CNS effectors – skeletal muscles AUTONOMIC stimuli – mainly subconscious sensations integration – hypothalamus brain stem, spinal cord little voluntary control motor – autonomic motor neurons 2-neuron pathways from CNS effectors – smooth muscles, cardiac muscle, glands neurotransmitters – ACh & NE response – excitation or inhibition neurotransmitter – ACh response – excitation AUTONOMIC REFLEX PATHWAYS sensory receptors – mainly receptors in organs, blood vessels, or hypothalamus sensory neurons – carry impulses to spinal cord, brainstem, or hypothalamus integrating centers – highest level of integration is in the hypothalamus little conscious perception or voluntary control autonomic motor neurons – 2-neuron pathways from CNS preganglionic neurons – from brainstem or spinal cord to autonomic ganglia autonomic ganglion – site of synapse between 2 autonomic motor neurons postganglionic neurons – run from autonomic ganglia to effectors autonomic effectors – smooth muscle, cardiac muscle, glands 13 DUAL INNERVATION – many organs receive autonomic motor innervation from 2 divisions separate divisions of the ANS one division is usually excitatory, the other inhibitory Sympathetic division – motor neurons that trigger fight-or-flight responses due to physical or emotional stress pupils dilate increased heart rate and force increased blood flow to skeletal muscles, lungs airways dilate, respiratory rate increased energy released from storage in liver and adipocytes increased metabolism and sweating inhibition of non-essential activities – digestive & urinary function Parasympathetic division – motor neurons that trigger rest-and-digest activities when no stress is occurring function in production and storage of energy pupils constrict decreased heart rate and force airway constriction stimulation of digestive and urinary functions increased nutrient absorption in cells increased energy storage Sympathetic Division (Thoracolumbar Division) preganglionic neurons – arise at thoracic and lumbar spinal cord segments T1-L2 axon releases ACh at synapse in the autonomic ganglion always excitatory to postganglionic neuron autonomic ganglia – near spinal cord sympathetic chains – rows on either side of spine prevertebral ganglia – within abdominal cavity ventral to spine celiac, cranial and caudal mesenteric ganglia postganglionic neuron – long; from ganglia to effectors all over body most release NE, which may be excitatory or inhibitory adrenal medulla – modified sympathetic ganglion in center of adrenal gland preganglionic neurons trigger release of NE (noradrenalin) and epinephrine (adrenalin) as hormones released into interstitial space and diffuse into bloodstream circulate throughout body and affect any tissue with receptors 14 Summary of Sympathetic Division: thoracolumbar division preganglionic neuron – short axon; releases ACh arise at spinal cord segments T1-T12 and L1-L2 autonomic ganglia sympathetic chain ganglia – lateral to spinal column prevertebral ganglia – ventral to spinal column postganglionic neuron – long axon; most release NE has rapid, widespread affect innervates most tissues and organs of the body circulation of adrenal hormones through the blood Parasympathetic Division (Craniosacral Division) preganglionic neurons – arise as cranial nerves or sacral nerves cranial nerves III, VII, IX, X spinal cord segments S2-S4 axons release ACh at synapse in autonomic ganglia always excitatory to postsynaptic neuron autonomic ganglia – near or within effector organs terminal ganglia postganglionic neurons – short release ACh, which may be excitatory or inhibitory Summary of Parasympathetic Division craniosacral division preganglionic neuron – long axon; releases ACh arise at brainstem nuclei of c.n.III, VII, IX, X and spinal cord segments S2-S4 autonomic ganglia terminal ganglia – near or within effectors postganglionic neuron – short axon; releases ACh affect is less widespread innervates only specific glands and organs no hormonal component as in sympathetic division 15 NEUROTRANSMITTERS OF THE ANS CHOLINERGIC NEURONS – release acetylcholine (ACh) all preganglionic neurons all parasympathetic postganglionic neurons (also somatic neurons at NMJ) cholinergic receptors – 2 types nicotinic receptors – found on all postganglionic neurons and skeletal muscles always cause excitation (open Na+ channels) muscarinic receptors – on all parasympathetic effectors may cause excitation or inhibition acetylcholinesterase – enzymatically inactivates ACh rapidly ACh has a short duration of effect at cholinergic receptors agonist – mimics neurotransmitter and activates receptor (eg. nicotine, muscarine) antagonist – binds or blocks receptor and prevents activation (eg. atropine – blocks muscarinic receptors = parasympatholytic) ADRENERGIC NEURONS – release norepinephrine (NE) most sympathetic postganglionic neurons adrenergic receptors – most sympathetic effectors activated by NE or epinephrine (from adrenal medulla) may cause excitation or inhibition due to different receptors alpha receptors beta receptors NE and E are inactivated by; reuptake into synaptic end bulbs where it is broken down by monoamine oxidase (MAO) diffusing away to be removed by other cells which break it down using catechol-O-metyl transferase (COMT) effect lasts longer than ACh 16 EFFECTS OF SYMPATHETIC AND PARASYMPATHETIC DIVISION EFFECTOR adrenal medullae lacrimal glands pancreas salivary glands posterior pituitary sweat glands digestive glands liver iris of eye ciliary muscles of eye lungs – bronchiole muscles gall bladder and ducts stomach and intestines spleen urinary bladder arrector pili muscles blood vessels CARDIAC MUSCLE SYMPATHETIC GLANDS secretion of epinephrine and norepinephrine no known effect inhibits secretion of digestive enzymes inhibits secretion secretion of ADH increases secretion decreases secretion release of glucose (various mechanisms) decreases bile secretion SMOOTH MUSCLE dilation of pupil relaxation – distant vision dilates airways relaxation – no bile release decreased motility and tone; sphincter contraction contraction – releases stored blood into circulation relaxation of muscle wall; sphincter contraction erection of hairs alters diameter increased heart rate and force of contraction 17 PARASYMPATHETIC no known effect secretion of tears increases secretion of digestive enzymes increases secretion no known effect no known effect increases secretion glucose storage (as glycogen) increases bile secretion contraction of pupil contraction – close vision constricts airways contraction – releases bile increased motility and tone; sphincter relaxation no known effect contraction of muscle wall; sphincter relaxation no known effect no known effect decreased heart rate and force of contraction