Nervous System PDF

Nervous System General Characteristics of the Central Nervous Systems Consists of brain and spinal cord. Brain is largest and most complex portion of nervous system Brain controls sensation, perception, movement, thinking Brain consists of 2 cerebral hemispheres, diencephalon, brainstem, cereb...

Nervous System General Characteristics of the Central Nervous Systems Consists of brain and spinal cord. Brain is largest and most complex portion of nervous system Brain controls sensation, perception, movement, thinking Brain consists of 2 cerebral hemispheres, diencephalon, brainstem, cerebellum Brainstem connects the brain to the spinal cord Both brain and spinal cord connect to the peripheral nervous system (PNS) by way of peripheral nerves Meninges: membranes that protect brain and spinal cord; lie between bone and soft tissues of nervous system Meninges Membranes that protect brain and spinal cord Consist of 3 layers: Dura mater: Outer layer Tough, dense connective tissue Dural sinuses Epidural space Arachnoid mater: Middle layer; web-like Subarachnoid space contains cerebrospinal fluid (CSF) Pia mater: Inner layer; attached to surface of brain, spinal cord Contains blood vessels and nerves Nourishes CNS Ventricle and Cerebrospinal Fluid (CSF) CSF is produced in 4 ventricles, interconnected cavities within cerebral hemispheres, and brainstem Ventricles are continuous with the central canal of the spinal cord The 4 ventricles: 2 Lateral ventricles (called the first and second ventricles) Third ventricle Fourth ventricle Interventricular foramina connect third to lateral ventricles Cerebral aqueduct connects third and fourth ventricles Cerebrospinal Fluid Secreted by the choroid plexuses, special capillaries of pia mater covered by ependymal cells Selective transfer of substances from the blood to form CSF Nutritive and protective of CNS neurons Helps maintain stable ionic concentrations in the CNS Circulates in ventricles, central canal of spinal cord, and subarachnoid space After exchanging substances, CSF is absorbed by the arachnoid granulations Volume is about 140 mL at any time Brain The brain contains centers for/performs the following functions: Neural centers for sensory function Sensations and perceptions Motor commands to skeletal muscles Higher mental functions, such as memory, reasoning Neural centers for coordinating muscular movement Neural centers for regulating visceral activities Personality Brain Development Neural tube gives rise to CNS Brain forms from 3 vesicles (cavities): Forebrain (prosencephalon) Midbrain (mesencephalon) Hindbrain (rhombencephalon) Forebrain divides into the telencephalon and diencephalon Mesencephalon persists Hindbrain divides into metencephalon and myelencephalon These 5 cavities persist in mature brain as ventricles and tubes that connect them: Forebrain becomes cerebrum, basal nuclei, and diencephalon Mesencephalon remains as midbrain Hindbrain becomes cerebellum, pons, and medulla oblongata  Structural Development of the Brain Embryonic Vesicle Spaces Produced Regions of the Brain Produced Forebrain (prosencephalon) Anterior portion (telencephalon) Lateral ventricles Cerebrum Basal nuclei Posterior portion (diencephalon) Third ventricle Thalamus Hypothalamus Posterior pituitary gland Pineal gland Midbrain (mesencephalon) Cerebral aqueduct Midbrain Hindbrain (rhombencephalon) Anterior portion (metencephalon) Fourth ventricle Cerebellum, pons Posterior portion (myelencephalon) Fourth ventricle Medulla oblongata Major Portions of the Brain Major portions of the adult brain: Cerebrum Diencephalon Cerebellum Brainstem Structure of Cerebrum Cerebrum is largest part of brain: Cerebral hemispheres: 2 halves, separated by falx cerebri Corpus callosum: Connects cerebral hemispheres Gyri: Ridges or convolutions Sulci: Shallow grooves in surface; example: Central sulcus Fissures: Deep grooves in surface Longitudinal: separates the cerebral hemispheres Transverse: separates cerebrum from cerebellum Structure of the Cerebrum 5 lobes of the cerebral hemispheres: Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula (Island of Reil): Deep within lateral sulcus 4 of the lobes are named for the bones that they underlie Structure of the Cerebrum Cerebral Cortex: Thin layer of gray matter, which makes up outermost layer of all outer lobes of the cerebrum Contains almost 75% of neuron cell bodies in nervous system White Matter of Cerebrum: Lies under cerebral cortex Makes up most of cerebrum Contains bundles of myelinated axons that connect neuron cell bodies in cerebral cortex to other portions of nervous system Functions of the Cerebral Cortex Cerebral cortex is responsible for higher mental functions: Interpreting impulses from sensory organs Initiating voluntary movements Storing information as memory Retrieving stored information Reasoning Seat of intelligence and personality The cerebral cortex can be divided into sensory, association and motor areas; some overlap exists Each area contains a group of neurons working together to perform a particular function Sensory Areas of the Cortex Cutaneous sensory area: Parietal lobe Interprets sensations on skin Sensory speech area (Wernicke’s area): Temporal/parietal lobe, usually left hemisphere Understanding and formulating language Visual area: Occipital lobe Interprets vision Auditory area: Temporal lobe Interprets hearing Sensory area for taste: Near base of the central sulcus Includes part of insula Sensory area for smell: Arises from centers deep within temporal lobes Association Areas of the Cortex Association Areas: Regions that are not primarily motor or sensory Connect to each other and to other structures in the brain Widespread throughout the cerebral cortex Analyze and interpret sensory experiences Provide memory, reasoning, verbalization, judgment, emotions Association Areas of the Cortex Frontal lobe association areas: Concentrating, planning, complex problem solving Emotional behavior, judging consequences of behavior Parietal lobe association areas: Understanding speech Choosing words to express thoughts and feelings Temporal lobe association areas: Interpret complex sensory experiences (understanding speech, reading) Store memories of visual scenes, music, and complex patterns Occipital lobe association areas: Analyze and combine visual images with other sensory experiences Insula: Translating sensory information into proper emotional responses Motor Areas of the Cortex Primary motor areas (motor cortex): Frontal lobes Control voluntary muscles Most nerve fibers cross over in brainstem Broca’s area: Anterior to primary motor cortex Usually in left hemisphere Controls muscles needed for speech Frontal eye field: Above Broca’s area Controls voluntary movements of eyes and eyelids  Function of Cerebral Lobes Lobe Functions Frontal lobes Association areas carry on higher intellectual processes for concentrating, planning, complex problem solving, and judging the consequences of behavior. Motor areas control movements of voluntary skeletal muscles. Parietal lobes Sensory areas provide sensations of temperature, touch, pressure, and pain involving the skin. Association areas function in understanding speech and in using words to express thoughts and feelings. Temporal lobes Sensory areas are responsible for hearing. Association areas interpret sensory experiences and remember visual scenes, music, and other complex sensory patterns. Occipital lobes Sensory areas are responsible for vision. Association areas combine visual images with other sensory experiences. Hemisphere Dominance The left hemisphere is dominant in most people Dominant hemisphere controls: Language skills of speech, writing, reading Verbal, analytical, and computational skills Nondominant hemisphere controls: Nonverbal tasks Motor tasks involving orientation in space Understanding and interpreting musical and visual patterns Provides emotional and intuitive thought processes Memory Memory is the consequence of learning, and involves persistence of learning 2 types of memory: Short-term (working) memory: Neurons connected in a circuit Circuit is stimulated over and over When impulse flow ceases, memory does also, unless it enters long-term memory via memory consolidation Long-term memory: Holds more memory than short-term, lasts a lifetime Changes structure or function of neurons, makes new synaptic connections by increases branching of processes Long-term potentiation: Increase in neurotransmitter release and effectiveness of synaptic transmission upon repeated stimulation Basal Nuclei Sometimes called basal ganglia, but technically a ganglion is a cluster of neuron cell bodies in the peripheral nervous system Masses of gray matter deep within cerebral hemispheres Consist of caudate nucleus, putamen, and globus pallidus Produce dopamine Help control voluntary movement Diencephalon Between cerebral hemispheres and above the brainstem Surrounds the third ventricle Composed of gray matter Portions of the diencephalon: Thalamus Hypothalamus Optic tracts Optic chiasma Infundibulum Posterior pituitary Mammillary bodies Pineal gland Diencephalon Thalamus: Gateway for sensory impulses ascending to cerebral cortex Receives all sensory impulses (except for sense of smell) Channels impulses to appropriate part of cerebral cortex for interpretation Hypothalamus: Maintains homeostasis by regulating visceral activities, such as heart rate, blood pressure, body temperature, water and electrolyte balance, hunger, body weight, movement and glandular secretion in digestive tract, sleep and wakefulness, pituitary gland function Links nervous and endocrine systems Limbic System: Consists of several structures in various parts of brain, including diencephalon Controls emotional responses, feelings, behavior oriented toward survival Reacts to potentially life-threatening upsets (physical or psychological) Brainstem Connects brain to the spinal cord. Consists of: Midbrain Pons Medulla oblongata Contains nerve fiber tracts and gray matter masses Midbrain Short section of brainstem Lies between diencephalon and pons Contains bundles of fibers that join lower parts of brainstem and spinal cord with higher part of brain Cerebral aqueduct: Connects 3rd to 4th ventricle Cerebral peduncles: Main motor pathways that connect cerebrum to lower portions of nervous system Corpora quadrigemina: Centers for visual and auditory reflexes Red nucleus: Role in postural reflexes Pons Rounded bulge on underside of brainstem Between midbrain and medulla oblongata Relays nerve impulses between medulla oblongata and cerebrum Relays impulses from cerebrum to cerebellum Helps regulate rhythm of breathing Medulla Oblongata Enlarged continuation of spinal cord Conducts ascending and descending impulses between brain and spinal cord Contains cardiac, vasomotor, and respiratory control centers Contains various nonvital reflex control centers (coughing, sneezing, swallowing, and vomiting) Injuries are often fatal Reticular Formation Also called Reticular Activating System Complex network of nerve fibers scattered throughout brainstem Extends into the diencephalon Connects to centers of hypothalamus, basal nuclei, cerebellum, and cerebrum with ascending and descending tracts Filters incoming sensory information, passing some to cerebral cortex, and discarding unimportant information Arouses cerebral cortex into state of wakefulness Decreased activity causes sleep Cerebellum Inferior to occipital lobes Dorsal to pons and medulla oblongata Two hemispheres separated by falx cerebelli Vermis connects hemispheres Cerebellar cortex (gray matter) Arbor vitae (white matter) Cerebellar peduncles Dentate nucleus (largest nucleus) Integrates sensory information concerning position of body parts Coordinates skeletal muscle activity Maintains posture Recent studies suggest other possible roles for the cerebellum. These include interaction with other parts of cerebral cortex, such as the limbic system and the auditory areas. Spinal Cord Slender column of nervous tissue continuous with brain and brainstem Extends downward through vertebral canal Begins at the foramen magnum and terminates at the first and second lumbar vertebrae (L1 to L2) space Consists of 31 segments; each gives rise to a pair of spinal nerves Spinal nerves are grouped according to the level of the vertebra with which they are associated Within each group, the pairs of nerves are numbered in sequence from superior to inferior Structure of the Spinal Cord Portions of the spinal cord (longitudinal section): Cervical enlargement: Supplies nerves to upper limbs Lumbar enlargement: Supplies nerves to lower limbs Conus medullaris: Tapering region below lumbar enlargement Filum terminale: Cord of connective tissue that anchors spinal cord to coccyx Cauda equina: Group of lumbar and sacral nerves extending downward from conus medullaris in vertebral canal Structures of the spinal cord (cross section): Anterior median fissure and posterior median sulcus: Grooves that extend whole length of spinal cord White matter surrounds core of gray matter Gray matter arranged in horns White matter arranged in funiculi Posterior roots contain sensory neurons; cell bodies are outside spinal cord, in posterior root ganglia Anterior roots contain motor neurons Gray commissure surrounds central canal Functions of Spinal Cord 2 main functions of the spinal cord: Center for spinal reflexes Conduit (pathway) for impulses to and from the brain Reflex: Automatic, subconscious response to a stimulus within or outside the body Maintain homeostasis by controlling involuntary processes, such as heart rate, blood pressure, swallowing, coughing Reflex arc: Neural pathway, consisting of a sensory receptor, 2 or more neurons, and an effector Simple reflex arc contains only sensory and motor neurons Most common reflex arc contains sensory neuron, interneurons, motor neurons Reflex Behavior Monosynaptic (stretch) reflex: Contains 2 neurons, sensory and motor Has only 1 synapse in spinal cord Example: Patellar or knee-jerk reflex Helps maintain an upright posture Withdrawal Reflex: Occurs when person touches or steps on something painful (stove, tack) Prevents or limits tissue damage, by removing limb from painful stimulus Polysynaptic: contains sensory neuron, interneuron, motor neuron Reciprocal innervation: flexors contract, extensors are inhibited Crossed Extensor Reflex During withdrawal reflex, flexors on affected (ipsilateral) side contract, and extensors are inhibited At same time, extensors on opposite (contralateral) side contract, flexors are inhibited Also shifts body weight, so person remains upright Peripheral Nervous System Consists of nerves that connect CNS to other body parts; PNS includes: Cranial nerves arising from the brain Spinal nerves arising from the spinal cord Subdivisions of the PNS : Somatic nervous system: cranial and spinal nerves that connect CNS to the skin and skeletal muscles (conscious activities) Autonomic nervous system: cranial and spinal nerves that connect CNS to viscera (subconscious activities) Structure of Peripheral Nerves Nerves are bundles of axons Nerve “fibers” are axons Connective tissue coverings: Endoneurium: Loose connective tissue that surrounds individual axons Perineurium: Loose connective tissue that surrounds fascicles Epineurium: Dense connective tissue that surrounds a group of fascicles Nerve Fiber Classification Sensory nerves: Conduct impulses into brain or spinal cord Motor nerves: Conduct impulses to muscles or glands Mixed nerves: Contain both sensory and motor nerve fibers Most nerves are mixed nerves All spinal nerves are mixed nerves (except the first pair) Classification by origination: Cranial nerves: Originate from brain Spinal nerves: Originate from spinal cord Cranial Nerves 12 pairs on underside of brain Most are mixed nerves Some are sensory, associated with special senses Some are primarily motor, innervate muscles or glands Most are attached to the brainstem, with 2 exceptions First pair has fibers that start in the nasal cavity Second pair originates in eyes, fibers synapse in thalamus Cranial nerves are numbered with Roman numerals (I to XII), from anterior to posterior (for the two pairs associated with the cerebrum), and from superior to inferior (for the remaining pairs) Cranial Nerves Cranial Nerves I, II, and III Olfactory nerve (I): Sensory nerve only Associated with sense of smell Bipolar neurons, the olfactory receptor cells, pass through cribriform plate of ethmoid bone, and enter olfactory bulbs Optic nerve (II): Sensory nerve only Associated with sense of vision Neuron cell bodies form ganglion layers of retina, and pass through optic foramina of the orbits Oculomotor nerve (III): Primarily motor nerve Motor impulses to several voluntary muscles that raise eyelids, move the eyes Motor impulses to involuntary muscles that focus lens, adjust light entering eye (part of autonomic nervous system) Small sensory component (proprioceptive fibers) Cranial Nerves IV, V, VI, VII Trochlear nerve (IV): Abducens nerve (VI): Primarily motor nerve Smallest pair of cranial nerves Primarily motor nerve Motor impulses to one pair of muscles that move the Motor impulses to one pair of muscles eyes that move the eyes Small sensory component (proprioceptive fibers) Trigeminal nerve (V): Some sensory (proprioceptive fibers) Mixed nerve Largest pair of cranial nerves Facial nerve (VII): 3 large sensory branches: Ophthalmic division: Mixed nerve Sensory from surface of eyes, tear glands, scalp, forehead, and upper eyelids Maxillary division: Sensory from taste receptors Motor impulses to muscles of facial Sensory from upper teeth, upper gum, upper lip, palate, and skin of face Mandibular division: Sensory from scalp, skin of jaw, lower teeth, lower gum, and lower lip expression, tear glands, and salivary Motor impulses to muscles of mastication glands Cranial Nerves VIII, IX, and X Vestibulocochlear nerve (VIII): Also called acoustic or auditory nerve Sensory nerve only 2 branches: Vestibular branch: Sensory from equilibrium receptors of ear Cochlear branch: Sensory from hearing receptors Glossopharyngeal nerve (IX): Mixed nerve Sensory from pharynx, tonsils, part of tongue (the region posterior to the nasal cavity, oral cavity, and larynx) Motor impulses to salivary glands and muscles of pharynx (for swallowing) Vagus nerve (X): Mixed nerve Somatic motor impulses to muscles of pharynx and larynx for speech and swallowing Autonomic motor impulses to heart, other viscera of thorax and abdomen Sensory from pharynx, larynx, esophagus, and viscera of thorax and abdomen Cranial XI and XII Accessory nerve (XI): Primarily motor nerve Formerly called “Spinal Accessory” Contain cranial and spinal branches: Cranial branch: Joins Vagus N.; motor impulses to muscles of soft palate, pharynx, and larynx Spinal branch: Motor to muscles of neck and back; small sensory component (proprioceptive fibers) Hypoglossal nerve (XII): Primarily motor nerve Motor impulses to muscles of the tongue for speaking, chewing, swallowing Small sensory component (proprioceptive fibers) Spinal Nerves All are mixed nerves, except first pair (which is entirely motor) Originate from spinal cord 31 pairs of spinal nerves: 8 cervical nerves, (C1 to C7) 12 thoracic nerves (T1 to T12) 5 lumbar nerves (L1 to L5) 5 sacral nerves (S1 to S5) 1 coccygeal nerve (Co) Cauda equina: Formed by descending roots of lumbar, sacral, and coccygeal nerves Dermatome: An area of skin innervated by the sensory nerve fibers of a particular spinal nerve (in all spinal nerves below C1) Spinal Nerves Each spinal nerve splits into an anterior and posterior root inside the vertebral column: Anterior (ventral or motor) root: Contains axons of motor neurons whose cell bodies are in the spinal cord Posterior (dorsal or sensory) root: Contains axons of sensory neurons Posterior root ganglion: Contains cell bodies of sensory neurons that conduct impulses from periphery into the spinal cord Spinal nerves: Formed by union of anterior root and posterior roots Mixed nerves Branches of spinal nerves outside the spinal cord: Meningeal branch Posterior branch/ramus Anterior branch/ramus Visceral branch (only in thoracic and lumbar) Spinal Nerve Plexuses Nerve plexus: Complex network formed by anterior rami (branches) of spinal nerves Not in T2 through T12; instead the anterior rami become intercostal nerves The fibers of various spinal nerves are sorted and recombined, so all fibers heading to same peripheral body part reach it in the same nerve There are 3 nerve plexuses: Cervical, Brachial, Lumbosacral Cervical plexus: Formed by anterior rami (branches) of C1 to C4 spinal nerves Lies deep in the neck Supply muscles and skin of the neck C3-C4-C5 nerve roots contribute to phrenic nerves, which transmit motor impulses to the diaphragm Brachial Plexus Formed by anterior branches C5 to T1 Lies deep within shoulders 5 major branches of brachial plexus: Musculocutaneous nerve: Supply muscles of anterior arms and skin of forearms Ulnar and Median nerves: Supply muscles of forearms and hands, skin of hands Radial nerve: Supply posterior muscles of arms and skin of forearms and hands Axillary nerve: Supply muscles and skin of anterior, lateral, and posterior arms Lumbosacral Plexus Formed by the anterior branches of L1 to S4 roots Lumbar portions are in lumbar regions of the abdomen, and the sacral portions are in pelvic cavity Major branches of lumbosacral plexus: Obturator nerve: Supply motor impulses to adductors of thighs Femoral nerve: Supply motor impulses to muscles of anterior thigh and sensory impulses from skin of thighs and legs Sciatic nerve: Supply muscles and skin of thighs, legs, and feet; largest and longest nerve in the body Autonomic Nervous System An efferent (motor) part of the peripheral nervous system (PNS) Functions without conscious effort Controls visceral activities Regulates smooth muscle, cardiac muscle, and glands Helps maintain homeostasis Helps body respond to stress Prepares body for exercise, intense physical activity General Characteristics of the ANS Hypothalamus exerts main control over the ANS Reflexes in which sensory (afferent) nerve fibers transmit signals from the viscera and skin to neural centers in CNS regulate some ANS activity In response, motor (efferent) impulses travel through cranial and spinal nerves to ganglia outside CNS Impulses continue to muscles or glands, which respond to nerve impulses by contracting, secreting, or being inhibited 2 divisions of the autonomic nervous system: Sympathetic division: Prepares body for “fight or flight” situations Most active under energy-requiring, stressful, emergency situations Parasympathetic division: Prepares body for “rest and digest” activities Most active under resting, non-stressful conditions Most organs receive input from both divisions Autonomic Nerve Fiber All of the neurons of the ANS are motor (efferent) Somatic motor pathways link the CNS and a skeletal muscle fiber via 1 neuron Autonomic motor pathways contain a chain of 2 neurons: Preganglionic fibers: Axons of preganglionic neurons Neuron cell bodies are in CNS Axons leave CNS and synapse in autonomic ganglion Postganglionic fibers: Axons of postganglionic neurons Neuron cell bodies are in ganglia Axons extend to visceral effector Somatic and Autonomic Motor Pathways Sympathetic Division Thoracolumbar division (T1 to L2) Preganglionic fibers originate in spinal cord, leave via ventral roots, leave spinal nerves through white rami and enter sympathetic chain (paravertebral) ganglia Sympathetic chain ganglia + fibers that connect them make up the sympathetic trunks Sympathetic chain ganglia lie some distance from viscera they regulate Other sympathetic ganglia, such as collateral ganglia, are close to viscera Postganglionic fibers (axons) run from sympathetic ganglia to peripheral effectors Sympathetic Chain Sympathetic Division Preganglionic fibers may do any of the following: Synapse with a postganglionic neuron in a paravertebral ganglion Continue through a paravertebral ganglion, and synapse at another sympathetic ganglion Pass through to collateral ganglia to synapse there Postganglionic fibers: Extend from sympathetic ganglia to visceral effector organs Postganglionic fibers that originate at paravertebral ganglia usually pass a gray rami and return to a spinal nerve before proceeding to an effector Exception: Preganglionic fibers pass through sympathetic ganglia and extend to adrenal medulla; these terminate on hormone-secreting cells that release epinephrine and norepinephrine Parasympathetic Division Parasympathetic division: Craniosacral division of the ANS Preganglionic neurons originate in brainstem and S2 to S4 spinal levels Preganglionic fibers extend out on cranial or sacral nerves to terminal ganglia (near or in visceral organs) Short postganglionic fibers continue to specific muscles or glands Preganglionic fibers of the head are included in oculomotor (III), facial (VII), and glossopharyngeal (IX) nerves Preganglionic fibers of thorax and abdomen are parts of vagus (X) nerve, which contains ~75% of all parasympathetic fibers Preganglionic fibers of sacral (S2 to S4) region of spinal cord carry impulses to pelvic viscera Autonomic Neurotransmitters Cholinergic neurons: Release acetylcholine All preganglionic sympathetic and parasympathetic fibers Postganglionic parasympathetic fibers Adrenergic neurons: Release norepinephrine (noradrenaline) Most postganglionic sympathetic fibers Autonomic Neurotransmitters Cholinergic neurons: Release acetylcholine All preganglionic sympathetic and parasympathetic fibers Postganglionic parasympathetic fibers Adrenergic neurons: Release norepinephrine (noradrenaline) Most postganglionic sympathetic fibers Actions of Autonomic Neurotransmitters Most organs are innervated by sympathetic and parasympathetic divisions, with opposite actions Actions result from binding to protein receptors in the membrane of effector cells in synapses or neuromuscular junctions: Cholinergic receptors: Bind to acetylcholine; 2 types: Muscarinic receptors: Excitatory, slow, also activated by fungal toxin, muscarine. Nicotinic receptors: Excitatory, rapid, also activated by tobacco toxin, nicotine. Adrenergic receptors: Bind to epinephrine and norepinephrine; 2 types: Alpha and beta receptors Elicit different responses on various effectors Terminating Autonomic Neurotransmitters Actions After acting at a synapse or neuromuscular junction, neurotransmitters must be removed/inactivated to prevent continued stimulation of the postsynaptic cell: The enzyme acetylcholinesterase (AChE) decomposes acetylcholine released by cholinergic fibers Norepinephrine from adrenergic fibers is removed from synapse by active transport, and inactivated by the enzyme monoamine oxidase (MAO) Control of Autonomic Activity Controlled largely by the hypothalamus Control of the autonomic nervous system (ANS) is involuntary Medulla oblongata regulates cardiac, vasomotor, and respiratory activities Hypothalamus regulates visceral functions, such as body temperature, hunger, thirst, and water and electrolyte balance Autonomic reflex centers occur in medulla oblongata and spinal cord Reflex centers in medulla oblongata control cardiac, vasomotor, respiratory activities Limbic system and cerebral cortex control emotional responses Lifespan Changes Brain cells begin to die before birth, due to apoptosis, a form of normal programmed cell death Over average lifetime, brain shrinks 10% More gray matter than white matter is lost with aging Many cells die in temporal lobes, but few in brainstem By age 90, frontal cortex has lost half its neurons Number of dendritic branches in cerebral cortex decreases Decreased levels of neurotransmitters Action potentials propagation rate declines by 5 to 10% Fading memory Slowed responses and reflexes Increased risk of fainting, falling Changes in sleep patterns that result in fewer sleeping hours General Characteristics of Nervous System Overview of Nervous System Function: Master regulatory system Sends and receives information Sensory input (detects changes) Integration and processing (making decisions) Motor output (stimulates muscles and glands to respond) Maintains homeostasis Acts as center for thought, learning, and memory Main Cell Types of Nervous System: Neurons (nerve cells): Respond quickly to changes/stimuli Conduct electrical impulses via neurotransmitters Neuroglia: Protect, support, insulate, and nourish neurons Do not conduct electrical impulses like neurons Nervous System Structure Central Nervous System (CN S): Brain Spinal cord Peripheral Nervous System (P NS): Connects CNS to other body parts Consists of cranial nerves and spinal nerves Two subdivisions: Afferent (sensory) Efferent (motor) Sensory (Afferent) and Motor Efferent Division Sensory Division: Sensory receptors perform sensory function (detect changes) Receptors convert information into impulses Impulses conducted along peripheral nerves to CNS for integration Motor Division: Neurons that transmit impulses from CNS to effectors perform motor function Effectors are muscles or glands outside nervous system Two subdivisions: Somatic: Transmits voluntary commands to skeletal muscles Autonomic: Transmits involuntary commands to viscera Clinical Application- Migraine Affect 12% of US population Signs: pounding head, nausea, aura (shimmering images in visual field), light or sound sensitivity Environmental triggers: bright light, certain foods, lack of sleep, stress, high altitude, stormy weather, excess caffeine or alcohol Hormonal triggers in women; migraine occurs just prior to menstruation Lasts 4 to 72 hours Period of excitation followed by unresponsiveness in particular neurons stimulates production of pain sensations in areas at base of brain Good first approach is to identify triggers; avoiding triggers can reduce frequency of attacks Clinical Application-Migraine Treatment Drugs called triptans may stop migraine attack; some triptans constrict more blood vessels than others, which is risky to some patients Transcranial magnetic stimulation also works for both acute and chronic migraine Treatments to lower frequency of chronic migraine attacks: injections of botulinum toxin (“botox”), and a drug that binds a neurotransmitter that causes dilation and inflammation of blood vessels associated with migraine Some drugs have been “repurposed” to decrease the number of migraines, such as some antidepressants, anticonvulsants, and drugs used to treat high blood pressure Nervous Tissue Cells : Neuron and Neuroglia Neurons: Vary in size and shape May differ in length, number, and size of axons and dendrites Share certain structural features: Cell body (soma or perikaryon): Contains nucleus, cytoplasm, organelles, neurofilaments, chromatophilic substance (Nissl bodies) Dendrites: Branched receptive surfaces; a neuron may have many Axon: Transmits impulses and releases neurotransmitters to another neuron or effector (another neuron, a muscle cell, or a gland cell); a neuron may have only 1 axon Axons Structural features of axons: Axon hillock: Cone-shaped area of cell body from which axon arises Collaterals: Branches from axon Axon terminal: Specialized endings of extensions from axon Synaptic knob: Rounded ending of a synaptic terminal Schwann cells: Neuroglia of the PNS that wrap around some axons in layers Myelin: Mixture of fats and proteins that fill layers made by Schwann cell membranes Myelin sheath: A wrapped coating around some PNS axons, composed of layers of Schwann cell membranes and myelin; acts as electrical insulator Nodes of Ranvier: Gaps in myelin sheath between Schwann cells Common Neuron Myelinated and Unmyelinated Axons Not all axons are myelinated Myelinated axons: Are coated by a myelin sheath Produced by a series of Schwann cells lined up along axon in PNS Produced by Oligodendrocytes in CNS Groups of myelinated axons in CNS comprise White Matter Increase conduction speed for electrical impulses Unmyelinated axons: Encased by Schwann cell cytoplasm in PNS, but there is no wrapped coating of myelin surrounding the axons Groups of unmyelinated axons in CNS comprise Gray Matter Myelination of Axons Clinical Application- Multiple Sclerosis Destruction of myelin sheaths in CNS by an immune response Myelin is attacked by a person’s own antibodies Scars (scleroses) are left behind, which stop neurons from conducting impulses Without input from motor neurons, muscles stop contracting and atrophy Other symptoms: fatigue, mood problems, blurred vision, and weak, numb limbs Treatments involve drugs that suppress immune activity Classification of Neurons Classification of neurons by structure: Multipolar neurons: Many processes extend from cell body (many dendrites, 1 axon) 99% of neurons Most neurons of CNS, some in autonomic NS Bipolar neurons: Two processes extend from cell body (1 dendrite, 1 axon) Not that common Eyes, ears, nose Unipolar (Pseudounipolar) neurons: One process extends from cell body Two branches that function as 1 axon (peripheral and central processes) Cell bodies are mainly found in ganglia of PNS Classification of Neurons by Function Classification of neurons by function: Sensory (Afferent) Neurons: Carry impulses from periphery to CNS (brain or spinal cord) At distal ends, contain sensory receptors to detect changes Most are unipolar, some are bipolar Interneurons (Association or Internuncial Neurons): Link neurons in the CNS Relay information from one part of CNS to another Multipolar Some cell bodies cluster to form nuclei in CNS Motor (Efferent) Neurons: Carry impulses from CNS to effectors (muscles or glands) Multipolar In somatic NS, control voluntary skeletal muscles In autonomic NS, control involuntary smooth and cardiac muscle, glands Classification of Neuroglia General Functions of Neuroglia: Provide structural support for neurons In embryo, guide neurons into position, may stimulate specialization Produce growth factors to nourish neurons and remove excess ions and neurotransmitters Aid in formation of synapses Neuroglia of the CNS Astrocytes: Connect neurons to blood vessels, exchanging nutrients and growth factors Form scar tissue Aid metabolism of certain substances Regulate ion concentrations, K+ Part of Blood Brain Barrier Oligodendrocytes: Myelinate CNS axons; also provide structural support Microglia: Phagocytic cells; also provide structural support Ependyma or ependymal cells: Line central canal of spinal cord & ventricles of brain, cover choroid plexuses Help regulate composition of cerebrospinal fluid Ciliated cuboidal or columnar cells Neuroglia of the PNS Schwann Cells: Produce myelin sheath found on some peripheral axons Speed up speed of nerve impulse transmission Satellite Cells: Support clusters of neuron cell bodies (ganglia) Nourish and balance ionic concentrations Type Characteristics Functions CNS Astrocytes Star-shaped cells between neurons Provide structural support, formation of scar tissue, transport of and blood vessels substances between blood vessels and neurons, communicate with one another and with neurons, mop up excess ions and neurotransmitters, induce synapse formation Oligodendrocytes Shaped like astrocytes, but with Form myelin sheaths in the brain and spinal cord, produce nerve fewer cellular processes, in rows growth factors along axons Microglia Small cells with few processes and Provide structural support and phagocytosis (immune protection) found throughout the CNS Ependyma Cuboidal and columnar cells in the Form a porous layer through which substances diffuse between the lining of the ventricles of the brain interstitial fluid of the brain and spinal cord and the cerebrospinal and the central canal of the spinal fluid cord PNS Schwann cells Cells with abundant, lipid-rich Form myelin sheaths, mop up excess ions and neurotransmitters, membranes that wrap tightly around support neuronal regeneration in PNS the axons of peripheral neurons Satellite cells Small, cuboidal cells that surround Support ganglia, mop up excess ions and neurotransmitters cell bodies of neurons in ganglia Neuroglia and Axonal Regeneration Mature neurons do not divide If cell body is injured, the neuron usually dies Neuron Regeneration in the PNS: If a peripheral axon is injured, it may regenerate Axon separated from cell body and its myelin sheath will degenerate Schwann cells and neurilemma remain Remaining Schwann cells provide guiding sheath for growing axon If growing axon establishes former connection, function will return; if not, function may be lost Neuron Regeneration in the CNS: C N S axons lack neurilemma to act as guiding sheath Oligodendrocytes do not proliferate after injury Regeneration is unlikely Cell Membrane Potential A cell membrane is usually electrically charged, or polarized Having charge difference in local area is called polarity Inside of membrane is negatively charged with respect to the outside Results from unequal distribution of ions on the inside and outside of membrane Important in conduction of impulses in neurons and muscle fibers These excitable cells can rapidly change internal charge Ability to change charge allows for communication between cells Membrane Potential and Distribution of Ions Membrane Potential: Charge inside a cell “Potential” to transport charges across membrane Resting Membrane Potential: Charge inside cell when it is inactive About −70 mV Potentials result from unequal ion distribution across membrane: Potassium (K+) ions are in higher concentration inside than outside cell Sodium (Na+) ions are in higher concentration outside than inside cell Negativity inside cell due mainly to large numbers of negatively charged impermeant proteins and ions (phosphate and sulfate) inside cell Gradients of Na+ and K+ ions allow for excitability Gated channels allow for movement of these ions at certain times Many chemical and electrical factors affect opening and closing of channels Membrane Potential and Distribution of Ions Sodium and potassium ions follow rules of diffusion, moving from higher to lower concentration across membrane Resting membranes are more permeable to K + than Na+ The Na+/K+ Pump maintains balance in ion movement across membrane: When resting potential is disturbed, it pumps 3 Na+ ions out of cell and 2 K+ ions into cell Uses energy of ATP to actively transport these ions in opposite directions across membrane Unequal charge distribution and the Na +/K+ Pump form basis for neurons to conduct electrical impulses for communication Action Potential: Sequence of electrical events in an excitable cell, involving changes in membrane potential, first positive and then negative, to return to resting potential; action potentials are used for communication between cells  (b) The membrane potential, negative on the inside of (a) In a hypothetical neuron before the the membrane, aids sodium diffusion into the cell, and membrane potential is established, opposes potassium diffusion out of the cell. As a result, potassium ions diffuse out of the cell slightly more sodium ions enter the cell than potassium faster than sodium ions diffuse in. A net ions leave. However, the sodium/potassium pump loss of positive charge from the cell balances these movements, maintaining the concentrations of these ions and the resting membrane results. potential. Recording of Action Potential Stimulation, Local Potential Changes, and Action Potential Neurons are excitable cells; can respond to stimuli Stimulus: anything that can change resting potential of −70 mV in either direction Neurons detect stimuli, and respond by changing their resting potential Excitatory stimulus opens chemically gated Na+ channels, and Na+ ions enter cell Local potential change: Change in membrane potential that occurs only in the area of stimulation Local potential changes are graded—the greater the stimulus intensity, the greater the potential change Excitatory stimulus opens chemically gated Na+ channels; this makes inside of neuron less negative Subthreshold stimulus does not result in action potential Graded stimuli (such as neurotransmitters) can add together, to produce a threshold stimulus  Local and Action Potential (b) If sodium or potassium channels open, more of that particular (c) If sufficient sodium ions enter the cell and the membrane ion will cross the cell membrane, altering the resting membrane potential depolarizes to threshold (here −55 millivolts), another potential. The illustration depicts the effect of sodium channels type of sodium channel opens. These channel are found along opening in response to a neurotransmitter. As sodium ions enter the axon, especially near the origin in an area called the “trigger the cell, the membrane potential becomes more positive (or less zone.” Opening of these channels triggers the action potential. negative), changing from −70 millivolts to −62 millivolts in this example. This change in a positive direction is called depolarization. Here the depolarization is subthreshold, and does not generate an action potential. Stimulation, Local Potential Changes, and Action Potential Threshold stimulus: Excitatory stimulus that causes enough Na + ions to flow into cell that it reaches Threshold Potential of −55 mV Now many voltage-gated Na+ channels open, at Trigger Zone, and charge changes to about +30 mV; this is the Action Potential Depolarization: Change from negative to positive charge inside cell, making both sides of membrane positive All-or-None Law: Reaching an action potential; either achieved or not If action potential is reached, it sends signal all the way down the axon Repolarization: Return to resting potential after action potential; occurs as K + channels open and K+ ions rush out of cell; polarity returns Hyperpolarization: Slight overshoot at end of repolarization, in which potential drops below −70 mV for a moment before returning to −70 mV Na+/K+ pump now returns ions to original locations and concentrations Ion Movements During Action Potentials Events Leading to Impulse Conduction 1. Nerve cell membrane maintains resting potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients. 2. Neurons receive stimulation, causing local potentials, which may sum to reach threshold. 3. Sodium channels in the trigger zone of the axon open. 4. Sodium ions diffuse inward, depolarizing the membrane. 5. Potassium channels in the membrane open. 6. Potassium ions diffuse outward, repolarizing the membrane. 7. The resulting action potential causes an electric current that stimulates adjacent portions of the membrane. 8. The action potential propagates along the length of the axon. Refractory Period During an impulse, the portion of the axon actively conducting the action potential is not able to respond to another threshold stimulus; this is the Refractory Period, and has 2 parts: Absolute Refractory Period: Time when threshold stimulus cannot generate another action potential + Voltage-gated Na channels are briefly unresponsive Relative Refractory Period: Time when only high-intensity stimulus can generate another action potential Repolarization is not complete, and membrane is re-establishing resting Refractory period limits number of action potentials generated per second Impulse Conduction Speed of impulse conduction varies with myelination Unmyelinated axons conduct impulses over entire length Myelin is rich in lipids, and prevents water and water-soluble substances (such as ions) from crossing membrane; acts as electrical insulator Ions can cross membrane only through gaps in myelin sheath, the Nodes of Ranvier Myelinated axons transmit impulses through saltatory conduction, in which action potentials “jump” from node to node down the axon Saltatory conduction is much faster than impulse conduction in unmyelinated axons. Axon diameter also affects conduction speed; thick axons transmit faster than thin axons: Thick, myelinated axons: 120 m/sec. Thin, unmyelinated axons: 0.5 m/sec. Action Potential Propagation Saltatory Conduction Clinical Application- Factors Affecting Impulse Conduction Changes in permeability of axons to certain ions affect impulse conduction: Increase in concentration of K+ in extracellular fluid + Reduces gradient for K to leave cell; threshold potential reached with stimulus of lower intensity; leads to excitable neurons, perhaps convulsions. Decrease in concentration of K+ in extracellular fluid Neurons can become hyperpolarized; action potentials are not generated; lack of impulses leads to muscle paralysis Decrease in permeability to K+ ions: Can be caused by some anesthetic drugs; stops impulses from passing through tissue fluid around axon; impulses do not reach brain, and there is no perception of touch and pain Synapse Neurons communicate with each other at a synapse: A site at which a neuron transmits a nerve impulse to another neuron Presynaptic neuron sends impulse Postsynaptic neuron receives impulse Synaptic cleft separates the 2 neurons Synaptic Transmission: Process by which presynaptic neuron sends impulse to postsynaptic neuron One-way process, using neurotransmitters to transfer the message Synaptic Transmission Transmission of a nerve impulse from one neuron to another Released neurotransmitters cross the synaptic cleft and react with specific receptors in the membrane of postsynaptic neuron Effects of neurotransmitters vary; some open ion channels and others close ion channels Chemically gated ion channels respond to neurotransmitters. Local potentials resulting from changes in chemically gated ion channels are called synaptic potentials. + Excitatory neurotransmitters increase permeability to Na ions, bring membrane closer to threshold; increase likelihood of generating impulses. Inhibitory neurotransmitters move membrane farther from threshold, decrease likelihood of generating impulses. Synaptic Potentials Excitatory postsynaptic potential (EPSP): Membrane change in which neurotransmitter opens Na + Channels. Depolarizes membrane of postsynaptic neuron, as Na + enters axon. Action potential in postsynaptic neuron becomes more likely Inhibitory postsynaptic potential (I PSP): Membrane change in which neurotransmitter opens K + channels (or Cl− channels). Hyperpolarizes membrane of postsynaptic neuron, as K + leaves axon Action potential of postsynaptic neuron becomes less likely EPSPs and IPSPs are added together in a process called summation Net excitatory effect leads to great probability of an action potential Net inhibitory effect does not generate action potentials Summation of all inputs usually occurs at the trigger zone Excitatory and Inhibitory Stimuli Neurotransmitters There are at least 100 neurotransmitters Acetylcholine stimulates skeletal muscle contraction Neurotransmitters may be monoamines, amino acids, peptides Neurotransmitters are produced in the rough ER or cytoplasm When impulse reaches synaptic knob of an axon, neurotransmitters are released by exocytosis Vesicle trafficking: Process of membrane recycling Synaptic vesicle becomes part of cell membrane as it releases neurotransmitter Endocytosis returns membrane to cytoplasm; forms new vesicles  Neurotransmitters Neurotransmitter Location Major Actions Acetylcholine CNS Controls skeletal muscle actions PNS Stimulates skeletal muscle contraction at neuromuscular junctions; may excite or inhibit at autonomic nervous system synapses Biogenic amines Norepinephrine CNS Creates a sense of well-being; low levels may lead to depression PNS May excite or inhibit autonomic nervous system actions, depending on receptors Dopamine CNS Creates a sense of well-being; deficiency in some brain areas associated with Parkinson disease PNS Limited actions in autonomic nervous system; may excite or inhibit, depending on receptors Serotonin CNS Primarily inhibitory; leads to sleepiness; action is blocked by L S D, enhanced by selective serotonin reuptake inhibitor antidepressant drugs Histamine CNS Release in hypothalamus promotes alertness Neurotransmitters Amino acids GABA and glycine CNS Generally inhibitory Glutamate CNS Most abundant excitatory neurotransmitter in the CNS Neuropeptides Enkephalins, endorphins CNS Generally inhibitory; reduce pain by inhibiting substance P release Substance P PNS Excitatory; pain perception Gases Nitric oxide CNS May play a role in memory PNS Vasodilation Events Leading to Neurotransmitters Release 1. Action potential passes along an axon and over the surface of its synaptic knob. 2. Synaptic knob membrane becomes more permeable to calcium ions, and they diffuse inward. 3. In the presence of calcium ions, synaptic vesicles fuse to synaptic knob membrane. 4. Synaptic vesicles release their neurotransmitter by exocytosis into the synaptic cleft. 5. Synaptic vesicle membrane becomes part of the cell membrane. 6. The added membrane provides material for endocytotic vesicles. Neuropeptides Many neurons in the brain or spinal cord synthesize neuropeptides Some neuropeptides act as neurotransmitters Other neuropeptides act as neuromodulators: substances which alter a neuron’s response to a neurotransmitter or block the release of a neurotransmitter. Examples: Enkephalins: relieve pain sensations Beta endorphin: relieves pain sensations; potent, long-lasting Substance P: found in neurons that conduct pain impulses; enkaphalins and endorphins inhibit release of Substance P Impulse Processing The way the nervous system processes nerve impulses and acts upon them reflects the organization of neurons and axons in the brain and spinal cord. Neuronal pools and facilitation Neuronal Pools: Groups of interneurons that make synaptic connections with each other, and are located completely within the CNS Cell bodies may be in different parts of the C NS Interneurons work together to perform a common function Each pool receives input from other neurons Each pool generates output to other neurons Pools may affect other pools or peripheral effectors Facilitation: Repeated impulses on an excitatory presynaptic neuron may cause that neuron to release more neurotransmitter in response to a single impulse. Increases likelihood that postsynaptic cell will reach threshold. Convergence and Divergence Convergence: One neuron receives input from several neurons Incoming impulses often represent information from different types of sensory receptors Allows nervous system to collect, process, and respond to information Makes it possible for a neuron to sum impulses from different sources Divergence: One neuron sends impulses to several neurons, via branching of its axon Can amplify an impulse Impulse from a single neuron in CNS may activate several motor units in a skeletal muscle Impulse from a sensory receptor may reach different regions of the C NS for processing Convergence and Divergence Senses General Characteristics of Sensory Function Maintain homeostasis, by providing information about the outside world and the internal environment Two types of senses: General senses: Receptors that are widely distributed throughout the body Skin, various organs, and joints Special senses: Specialized receptors confined to structures in the head Eyes, ears, nose, and mouth Sensory receptors: Collect information from the environment, and relay it to the CNS on sensory neurons Link nervous system to internal and external changes or events Can be specialized cells or multicellular structures General Characteristics of Sensory Function Pathway of sensory information from sensory receptors to Central Nervous System (CNS, brain or spinal cord): A stimulus first stimulates sensory receptors Stimulus is converted into graded receptor potentials; this is called transduction Receptor potentials can trigger action potentials, which are conducted along sensory neurons to CNS; this is called transmission occurs in the brain or spinal cord of the CNS Receptors, Sensation, and Perception Sensory receptors: Respond to specific stimuli Particularly sensitive to a certain type of environmental change, and less sensitive to other stimuli Allow body to interpret sensory events Receptor Types 5 types of sensory receptors in the body: Chemoreceptors: Respond to changes in chemical concentrations Smell, taste, oxygen concentration Pain receptors (nociceptors): Respond to tissue damage Mechanical, electrical, thermal energy Thermoreceptors: Respond to moderate changes in temperature Mechanoreceptors: Respond to mechanical forces that distort receptor Touch, tension, blood pressure, stretch Photoreceptors: Respond to light Eyes Sensory Impulses Sensory receptors can take the form of ends of neurons or cells near extensions of the neurons Stimulation of receptor causes local change in its membrane potential, causing graded potential according to stimulus intensity If receptor is part of a neuron, the membrane potential may generate an action potential If receptor is not part of a neuron, the receptor potential must be transferred to a neuron to trigger an action potential Peripheral nerves transmit impulses to CNS where they are analyzed and interpreted in the brain Sensation and Perception Sensation: Occurs when action potentials make the brain aware of a sensory event Example: Awareness of pain Perception: Occurs when brain interprets sensory impulses Example: Realizing that pain is a result of stepping on a tack Projection: Process in which cerebral cortex interprets sensation as being derived from certain receptors Brain projects the sensation back to the apparent source It allows a person to locate the region of stimulation Information Flow Smell Taste Sight Hearing Sensory receptors Olfactory receptor Taste bud receptor cells Rods and cones in Hair cells in cells retina cochlea ↓ ↓ ↓ ↓ ↓ Impulse in sensory fibers Olfactory nerve Sensory fibers in Optic nerve fibers Auditory nerve fibers various cranial nerves fibers ↓ ↓ ↓ ↓ ↓ Impulse reaches CNS Cerebral cortex Cerebral cortex Midbrain and cerebral Midbrain and cortex cerebral cortex ↓ ↓ ↓ ↓ ↓ Sensation (new experience, A pleasant smell A sweet taste A small, round, red A crunching recalled memory) object sound ↓ ↓ ↓ ↓ ↓ Perception The smell of an The taste of an apple The sight of an apple The sound of apple biting into an apple Sensory Adaptation Ability to ignore unimportant (or continuous) stimuli Involves a decreased response to a particular stimulus from the receptors (peripheral adaptation) or along the CNS pathways leading to the cerebral cortex (central adaptation) When sensory adaptation occurs, sensory impulses become less frequent and may cease Stronger stimulus is then required to trigger impulses Best accomplished by thermoreceptors and olfactory receptors General Senses Senses with small, widespread sensory receptors, associated with skin, muscles, joints, and viscera General Senses are divided into 3 groups: Exteroceptive senses: Senses associated with body surface Examples: Touch, pressure, temperature, and pain Interoceptive (visceroceptive) senses: Senses associated with changes in the viscera Examples: Blood pressure stretching blood vessels Proprioceptive senses: Senses associated with changes in muscles, tendons, and joints, body position Examples: Stimulated when changing position or exercising Touch and Pressure Sense 3 types of mechanoreceptors respond to touch and pressure: Free nerve endings: Simplest receptors Sense itching and other sensations Tactile (Meissner’s) corpuscles: Abundant in hairless portions of skin and lips Detect fine touch and texture Distinguish between 2 points Lamellated (Pacinian) corpuscles: Nerve endings encased in large ellipsoidal structures Common in deeper subcutaneous tissues, tendons, and ligaments Detect heavy pressure and vibrations Temperature Sense Temperature receptors (thermoreceptors): Free nerve endings in skin 2 types of thermoreceptors: Warm receptors: Sensitive to temperatures above 25°C (77°F) Unresponsive to temperature above 45°C (113°F) Cold receptors: Sensitive to temperatures between 10° (50°F) and 20°C (68°F) Pain receptors: Respond to temperatures below 10°C; produce freezing sensation Respond to temperatures above 45°C; produce burning sensation Sense of Pain Pain receptors/nociceptors: Consist of free nerve endings Widely distributed Nervous tissue of brain lacks pain receptors Stimulated by tissue damage, chemicals, mechanical forces, or extremes in temperature, oxygen deficiency Adapt very little, if at all Visceral Pain Pain receptors are the only receptors in viscera whose stimulation produces sensations Pain receptors in viscera respond differently to stimulation than those of surface tissues. Visceral pain may feel as if coming from some other part of the body; this is called referred pain Example of referred pain: Heart pain often feels like it is coming from the left shoulder or medial portion of left arm Referred pain results from common nerve pathways, in which sensory impulses from the visceral organ and a certain area of the skin synapse with the same neuron in the CNS Special Sense Senses that have sensory receptors are within large, complex sensory organs in the head Types of special senses and their organs: Smell: Olfactory organs in nasal cavity Taste: Taste buds in oral cavity Hearing and equilibrium: Inner ears Sight: Eyes Sense of Smell: Olfaction The sense of smell Olfactory receptors: Olfactory receptor cells are chemoreceptors Respond to chemicals dissolved in liquids Sense of smell provides 75 to 80% of sense of taste Olfactory organs: Contain olfactory receptor cells (bipolar neurons) and supporting epithelial cells Olfactory neurons have knobs at the distal ends of their dendrites covered with cilia Cover upper parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum Odorants may bind to any of almost 400 types of olfactory membrane receptors, resulting in depolarization and action potentials Olfactory Pathways Once olfactory receptors are stimulated, nerve impulses travel through openings in cribriform plates of ethmoid bone Olfactory nerves → olfactory bulbs → olfactory tracts → limbic system (for emotions) and olfactory cortex (for interpretation) Olfactory bulbs process sensory impulses Limbic system, center for memory and emotion, provides emotional responses to certain odorant molecules Sense of Taste: Gustation Sense of taste Taste buds: Organs of taste Located on papillae of tongue, roof of mouth, linings of cheeks, and walls of pharynx About 10,000 taste buds, each with 50 to 150 taste cells Taste receptors: Chemoreceptors Taste cells: modified epithelial cells that function as receptors Taste hairs: microvilli that protrude from taste cells through pores of taste buds; sensitive parts of taste cells Taste cells are replaced every 3 days Taste Sensation 5 primary taste sensations: Sweet: Stimulated by carbohydrates + Sour: Stimulated by acids (H ) + + Salty: Stimulated by salts (Na or K ) Bitter: Stimulated by many organic compounds, Mg and Ca salts Umami: Stimulated by some amino acids, MSG Each flavor results from 1 primary taste sensation or a combination Spicy foods may stimulate a class of pain receptors Taste receptors undergo rapid adaptation Taste Pathways Sensory impulses from taste receptor cells travel on fibers of 3 different cranial nerves, according to the location of the taste cells: Facial nerve (VII) Glossopharyngeal nerve (IX) Vagus nerve (X) Cranial nerves conduct impulses into medulla oblongata Impulses then proceed to the thalamus Impulses are interpreted in the gustatory cortex in the insula Sense of Hearing Organ of hearing and equilibrium 3 sections of the ear: Outer/external ear Middle ear Inner/internal ear Outer (External) Ear Parts of the Outer Ear: Auricle (Pinna): Funnel-shaped Collects sounds waves External acoustic meatus: S-shaped tube Lined with ceruminous glands Carries sound to tympanic membrane Terminates at tympanic membrane Tympanic membrane (Eardrum): Vibrates in response to sound waves Middle Ear Tympanic cavity: Air-filled space in temporal bone Auditory ossicles: 3 tiny bones Vibrate in response to tympanic membrane vibrations; amplify force Malleus, incus, and stapes (hammer, anvil, and stirrup) Oval window: Opening in wall of tympanic cavity Stapes vibrates against it to move fluids in inner ear Middle Ear: Tympanic Reflex Muscle contractions that occur during loud sounds, to lessen the transfer of sound vibrations to inner ear, and prevent damage to hearing receptors Muscles involved in reflex: Tensor tympani: Pulls malleus away from tympanic membrane Stapedius: Pulls stapes away from oval window Auditory Tube Connects middle ear to throat Helps maintain equal air pressure on both sides of tympanic membrane Usually closed by valve-like flaps in throat Inner (Internal) Ear Inner ear is a complex system of labyrinths: Osseous (bony) labyrinth: Bony canal in temporal bone Filled with fluid called perilymph Membranous labyrinth: Tube of similar shape that lies within osseous labyrinth Filled with fluid called endolymph Three portions of labyrinths: Cochlea functions in hearing Semicircular canals function in dynamic equilibrium Vestibule functions in static equilibrium Cochlea Spiral, snail-shaped tube, widest at its base, becomes narrower toward tip Coiled around bony core, the modiolus Spiral lamina is a bony shelf that coils around cochlea Three compartments of the cochlea: Scala vestibuli: Upper compartment, part of bony labyrinth Leads from oval window to apex of spiral Scala tympani: Lower compartment, part of bony labyrinth Extends from apex of the cochlea to round window Cochlear duct: Middle compartment, portion of membranous labyrinth Contains receptor organ for hearing (spiral organ, organ of Corti) Membranes of Cochlea The cochlea contains these membranes: Vestibular membrane: Separates scala vestibuli from cochlear duct Basilar membrane: Separates cochlear duct from scala tympani Forms floor of cochlear duct Tectorial membrane: Extends partially into cochlear duct Forms roof of hearing receptor organ, the Spiral Organ  Cochlear branch of vestibulocochlear nerve Auditory Pathways Medulla oblongata Midbrain Thalamus Auditory cortex in temporal lobe of cerebrum Sense of Sight: Vision Visual receptors are found in the eye Accessory organs for sense of sight: Upper and lower eyelids (palpebrae, protection) Eyelashes (protection) Lacrimal apparatus (tear production) Extrinsic eye muscles (eye movement) Visual Accessory Organs: Eyelids Composed of 4 layers: Skin: Thinnest in body Muscle: Orbicularis oculi closes eyelid, Levator palpebrae superioris muscle opens it Connective tissue: Contains tarsal glands, which secrete oil on to eyelashes Conjunctiva: Mucous membrane that lines eyelid and covers portion of eyeball Visual Accessory Organs: Lacrimal Apparatus Lacrimal gland: In orbit, lateral to eye Secretes tears Canaliculi: 2 ducts that collect tears Lacrimal sac: Collects tears from canaliculi Lies in groove in lacrimal bone Nasolacrimal duct: Collects from lacrimal sac Empties tears into nasal cavity Lysozyme: Antibacterial component of tears Structure of the Eye Hollow, spherical organ of sight Wall of eye has 3 layers: Outer (fibrous) tunic Middle (vascular) tunic Inner (nervous) tunic Outer (Fibrous) tunic Consists of cornea and sclera Portions of outer tunic: Cornea: Anterior one-sixth Transparent window of eye Helps focus light rays Transmits and refracts light Sclera: Posterior five sixths White, opaque, tough Protects eye, attaches muscles Pierced by optic nerve and blood vessels Anterior Portion of the Eye Cavity between cornea and lens Filled with a watery fluid, aqueous humor Lens: Transparent, biconvex, lies behind iris, elastic Held in place by suspensory ligaments of ciliary body Helps focus light rays Accommodation: A change in the shape of the lens, to view close objects Lens thickens and becomes more convex when focusing on close object Lens thins and becomes flatter when focusing on distant objects The ciliary muscle relaxes the suspensory ligaments during accommodation Iris and Aqueous Humor The Iris: Controls amount of light entering the eye Consists of connective tissue and smooth muscle (colored portion of eye) Pupil is window or opening in center of iris Dim light stimulates radial muscles and pupil dilates Bright light stimulates circular muscles and pupil constricts Amount and distribution of melanin determines eye color Aqueous Humor: Fluid in anterior cavity of eye Fills both anterior and posterior chambers of anterior cavity; circulates through pupil Secreted by epithelium on inner surface of the ciliary body Provides nutrients and maintains shape of anterior portion of eye Leaves cavity through scleral venous sinus Visual Pathways Proceeds from the ganglion cells of the retina to the optic nerve, optic chiasma, optic tracts, the thalamus, optic radiations, and visual cortex in occipital lobe of cerebrum A few fibers branch off before reaching the thalamus, and enter nuclei for visual reflexes Medial fibers from each optic nerve cross over to opposite side in optic chiasma, to help with depth perception Some fibers travel from thalamus to brainstem for visual reflexes, and controlling head and eye movements while tracking objects Life Span Change Age-related hearing loss due to: Damage to hair cells in spiral organ Degeneration of neural pathways to the brain Tinnitus Age-related visual problems include: Dry eyes Floaters (crystals in vitreous humor) Loss of elasticity of lens, decreasing accommodation (presbyopia) Glaucoma Cataracts Macular degeneration Age-related smell and taste problems due to: Loss of olfactory receptors (anosmia)