The Nervous Tissue PDF

The Nervous System: Nervous Tissue Learning Outcomes ▪ List the structures of the nervous system. ▪ Describe the organization of the nervous system. ▪ Describe the three basic functions of the nervous system. ▪ Contrast the histological characteristics and the functions of neurons and neuroglia....

The Nervous System: Nervous Tissue Learning Outcomes ▪ List the structures of the nervous system. ▪ Describe the organization of the nervous system. ▪ Describe the three basic functions of the nervous system. ▪ Contrast the histological characteristics and the functions of neurons and neuroglia. ▪ Distinguish between gray matter and white matter. ▪ Describe how a nerve impulse is generated and conducted. ▪ Explain the events of synaptic transmission and the types of neurotransmitters used. Structures of the Nervous System ▪ Brain: neurons enclosed within skull ▪ Spinal cord: connects to brain and enclosed within spinal cavity ▪ Nerves: bundles of many axons of neurons – Cranial nerves (12 pairs) emerge from brain – Spinal nerves (31 pairs) emerge from spinal cord ▪ Ganglia: groups of neuron cell bodies located outside of brain and spinal cord ▪ Enteric plexuses: networks in digestive tract ▪ Sensory receptors: monitor changes in internal or external environments Organization of the Nervous System ▪ Central nervous system (CNS) structures: – Brain – Spinal cord ▪ Peripheral nervous system (PNS) structures: – Cranial nerves and branches – Spinal nerves and branches – Ganglia – Sensory receptors Structures of the Nervous System Organization of the Nervous System ▪ Peripheral nervous system (PNS) divisions – Somatic (SNS) ▪ Sensory neurons from head, body wall, limbs, special sense organs ▪ Motor neurons to skeletal muscle: voluntary – Autonomic (ANS) nervous systems ▪ Sensory neurons from viscera ▪ Motor neurons to viscera (cardiac muscle, smooth muscle, glands): involuntary – Sympathetic: “fight-or-flight” – Parasympathetic: “rest-and-digest” – Enteric nervous system (ENS): “brain of the gut” Organization of the Nervous System ▪ Peripheral nervous system (PNS), – Enteric nervous system (ENS): “brain of the gut” ▪ Sensory neurons monitor chemical changes and stretching of GI wall ▪ Motor neurons regulate contractions, secretions and endocrine secretions (involuntary) Organization of the Nervous System Functions of the Nervous System ▪ Sensory receptors and sensory nerves –Monitor changes in internal & external environments –Carry information into brain and spinal cord ▪ Integration: information processing –Perception = awareness of sensory input –Analyzing and storing information to help lead to appropriate responses ▪ Motor activity: efferent nerves –Signals to muscles and glands (effectors) Histology of the Nervous System ▪ Neurons –Can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons ▪ Neuroglia cells: support, nourish and protect neurons –Neuroglia critical for homeostasis of interstitial fluid around neurons Neuronal Structure ▪ Cell body: nucleus, cytoplasm with typical organelles ▪ Dendrites: highly branched structures that carry impulses to the cell body ▪ Axon: conducts away from cell body toward another neuron, muscle or gland –Emerges at cone-shaped axon hillock ▪ Axon terminals: contain synaptic vesicles that can release neurotransmitters Neuronal Structure Structural Classes of Neurons ▪ Multipolar –Have several or many dendrites and one axon –Most common type in brain and spinal cord ▪ Bipolar –Have one dendrite and one axon –Example: in retina of eye and inner ear ▪ Unipolar –Have fused dendrite and axon –Sensory neurons of spinal nerves Functional Classes of Neurons ▪ Sensory (afferent) –Convey impulses into CAN (brain or spinal cord) ▪ Motor (efferent) –Convey impulses from brain or spinal cord out through the PNS to effectors (muscles or glands) ▪ Interneurons (association neurons) –Most are within the CNS –Transmit impulses between neurons, such as between sensory and motor neurons Neuroglia ▪ Cells smaller but much more numerous than neurons ▪ Can multiply and divide and fill in brain areas ▪ Gliomas: brain tumors derived from neuroglia ▪ Functions – Do not conduct nerve impulses – Do support, nourish and protect neurons Neuroglia ▪ Astrocytes: help form blood brain barrier ▪ Oligodendrocytes: produce myelin in CNS ▪ Microglia: protect CNS cells from disease ▪ Ependymal cells: form CSF in ventricles ▪ Schwann cells: produce myelin around PNS neurons; help to regenerate PNS axons ▪ Satellite cells: support neurons in PNS ganglia Myelination ▪ Axons covered with a myelin sheath – Many layers of lipid and protein: insulates neurons – Increases speed of nerve conduction – Appears white (in white matter) ▪ Nodes of Ranvier: gaps in the myelin – Nodes are important for rapid signal conduction ▪ Some diseases destroy myelin: – Multiple sclerosis – Tay-Sachs Neuron Regeneration ▪ Regeneration of PNS neurons – Axons and dendrite in the PNS can be repaired if cell body is intact and Schwann cells functional. These form a regeneration tube and grow axons or dendrites if scar tissue does not fill the tube ▪ Regeneration of CNS neurons – Very limited even if cell body is intact – Inhibited by neuroglia and by lack of fetal growth- stimulators Collections of Nervous Tissue ▪ Clusters of neuron cell bodies – Ganglion: cluster of cell bodies in PNS – Nucleus: cluster of cell bodies in CNS ▪ Bundles of axons – Nerve: bundle of axons in PNS – Tract: bundle to axons in CNS Gray and White Matter ▪ White matter: primarily myelinated axons ▪ Gray matter: cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia ▪ Locations of gray and white matter – Spinal cord: white matter (tracts) surround centrally located gray matter “H” of “butterfly” – Brain: gray matter in thin cortex surrounds white matter (tracts) Action Potentials ▪ Action potentials = nerve impulses = electrical signal ▪ Require –A membrane potential: a charge difference across cell membrane (polarization) –Ion channels: allow ions to move by diffusion from high to low concentration ▪ Leakage channels: allow ions to leak through membrane; there are more for K+ than for Na+ ▪ Gated channels – Open and close on command – Respond to changes in membrane so can generate and conduct action potentials Resting Membrane Potential ▪ Typically –70 mV – Inside of membrane more negative than outside ▪ Caused by presence of ions: – Inside (more negative) because cytosol has: ▪ Many negative ions (too large to leak out): amino acids (in cellular proteins) and phosphates (as in ATP) ▪ K+ that easily leaks out through many K+ channels – Outside (more positive) because interstitial fluid has: ▪ Few negative ions ▪ Na+ that does not leak out of cell: few Na+ channels ▪ Membrane “pumps” that quickly pump out Na+ that does leak (diffuse) into cell Resting Membrane Potential Action Potential Generation ▪ Series of events that activate cell membrane in neuron or muscle fiber ▪ An initial event (stimulus) is required – Triggers resting membrane to become more permeable to Na+ – Causes enough Na+ to enter cell so that cell membrane reaches threshold (~ –55 mv) – If so, the following events occur: action potential which spreads along neuron or muscle fiber Action Potential Action Potential Generation ▪ Depolarizing phase –Na+ channels open → as more Na+ enters cell, membrane potential rises and becomes positive (–70 → 0 → + 30 mv) ▪ Repolarizing phase –K+ channels open → as more K+ leave cell, membrane potential is returned to resting value (+ 30 → 0 → –70 mv) –May overshoot: hyperpolarizing phase Typically depolarization and repolarization take place in about 1 millisecond (1/1000 sec) Action Potential Action Potential Action Potential Rule ▪ Recovery – Levels of ions back to normal by action of Na+/K+ pump – Refractory period (brief): even with adequate stimulus, cell cannot be activated ▪ All-or-none principle – If a stimulus is strong enough to cause depolarization to threshold level, the impulse will travel the entire length of the neuron at a constant and maximum strength. Conduction of Nerve Impulses ▪ Nerve impulse conduction (propagation) – Each section triggers the next locally as even more Na+ channels are opened (like row of dominos) ▪ Types of conduction – Continuous conduction ▪ In unmyelinated fibers; slower form of conduction ▪ Speed 1 millisecond – Saltatory conduction ▪ In myelinated fibers; faster as impulses “leap” between nodes of Ranvier ▪ Speed 100 millisecond ▪ Factors that increase rate of conduction – Myelin, large diameter and warm nerve fibers Conduction of Nerve Impulses Conduction of Nerve Impulses Conduction of Nerve Impulses Synaptic Transmission ▪ Synapse is the site of signal transmission from one neuron to the next – Neuronal junction (neuron-neuron) – Neuromuscular junction (neuron-muscle fiber) – Neuroglandular junction (neuron-gland) ▪ Triggered by action potential (nerve impulse) ▪ Components of synapse: – Presynaptic neuron – Synaptic cleft – space between neurons – Postsynaptic cell Synaptic Transmission Synaptic Transmission Synapse Chemical Electrical ▪ Neurotransmitters ▪ Gap junction – Cytoplasm of adjacent cells are ▪ One way directional connected by clusters of ion transmission channels ▪ Discrete and highly focused ▪ Bidirectional transmission signals ▪ Large group of interconnected ▪ CNS, PNS neurons simultaneously synchronous ▪ Cardiac muscle, smooth muscle of GI, blood vessels Chemical Synaptic Transmission ▪ Action potential arrives at presynaptic neuron’s end bulb ▪ Opens voltage gated Ca2+ channels → Ca2+ flows into presynaptic cytosol ▪ Increased Ca2+ concentration → exocytosis of synaptic vesicles ▪ Neurotransmitter (NT) released into cleft ▪ NT diffuses across cleft and binds to receptors in postsynaptic cell membrane Chemical Synaptic Transmission ▪ NT serves as chemical trigger (stimulus) of ion channels ▪ Postsynaptic cell membrane may be depolarized or hyperpolarized –Depends on type of NT and type of postsynaptic cell –1000+ neurons converge on synapse; the sum of all of their NTs determines effect ▪ If threshold reached, then postsynaptic cell action potential results Chemical Synaptic Transmission ▪ One-way transmission only because – Only presynaptic cells release NT – Only postsynaptic cells have receptors for NT binding ▪ Finally, NT must be removed from the cleft. Three possible mechanisms – Diffusion out of cleft – Destruction by enzymes (such as ACh-ase) in cleft – Transport back (recycling) into presynaptic cell Signal Transmissio n at the Chemical Synapse 50 Neurotransmitters ▪ Acetylcholine (ACh): common in PNS – Stimulatory (on skeletal muscles) – Inhibitory (on cardiac muscle) ▪ Amino acids – Glutamate, aspartate, gamma aminobutyric acid (GABA), glycine ▪ Modified amino acids – Norepinephrine (NE), dopamine (DA), serotonin ▪ Neuropeptides such as endorphins ▪ Nitric oxide (NO) THE NERVOUS SYSTEM : CENTRAL NERVOUS SYSTEM Part 1 – The Brain Learning Outcomes ▪ Discuss how the brain is protected and supplied with blood. ▪ Name the major parts of the brain and explain the function of each part. ▪ Describe how the spinal cord is protected. ▪ Describe the structure of the spinal cord. ▪ Describe three somatic sensory and somatic motor pathways. ▪ Describe the components of a reflex arc. Learning Outcomes ▪ Describe the composition, coverings, and distribution of spinal nerves. ▪ Identify the 12 pairs of cranial nerves by name and number and give the functions of each. ▪ Describe the location and function of the receptors for tactile, thermal, and pain sensations. ▪ Identify the receptors for proprioception and describe their functions. Nervous System Brain: Major Parts ▪ Cerebrum: largest part and most superior ▪ Diencephalon: superior to brain stem ▪ Thalamus and hypothalamus ▪ Brain stem: continuous with spinal cord ▪ Medulla oblongata, pons, midbrain ▪ Cerebellum: posterior and inferior ▪ Means “little brain” ▪ Cranial meninges: dura mater, arachnoid mater, and pia mater Brain: Major Parts Brain: Major Parts Copyright 2010, John Wiley & Sons, Inc. Cerebrum: Structure ▪ Cerebral cortex is gray mater ▪ Internal white mater ▪ Deep gray nuclei ▪ Surface folds of cerebral cortex: gyri ▪ Grooves between gyri: sulci ▪ Longitudinal fissure: divides cerebrum into left and right hemispheres ▪ Hemispheres connected by corpus collosum Cerebrum: Structure ▪ Each hemisphere has 4 lobes ▪ Frontal, parietal, temporal, occipital ▪ Central sulcus separates frontal, parietal ▪ Precentral gyrus anterior to sulcus: primary motor area ▪ Postcentral gyrus: primary somatosensory area ▪ Deep gray nuclei: basal ganglia ▪ Globus pallidus, putamen, caudate nucleus Cerebrum Cerebrum Cerebrum Functional Areas of Cerebral Cortex ▪ Specialized areas in specific regions of cerebral cortex ▪ Sensory areas receive input → perception ▪ Motor areas → initiate movements ▪ Associative areas → complex integration: memory, emotion, reasoning, judgment Cerebrum: Functional Areas Sensory Areas ▪ Primary somatosensory area: postcentral gyrus ▪ Input includes: touch, proprioception, pain, itching, tickle, temperature ▪ Primary visual area: occipital lobe ▪ Primary auditory area: temporal lobe ▪ Primary gustatory (taste) area: base of postcentral gyrus ▪ Primary olfactory (smell) area: medial aspect of temporal lobe Motor Areas ▪ Located anterior to central sulcus ▪ Primary motor area: precentral gyrus ▪ Broca’s speech area ▪ Interacts with premotor area and primary motor area to regulate breathing and speech muscles ▪ Is in left hemisphere in 97% of persons Association Areas ▪ Adjacent to sensory and motor areas and connected via association tracts ▪ Integrate and interpret information ▪ Examples ▪ Somatosensory association area ▪ Posterior to primary somatosensory area ▪ Integrates sensation: exact shape and texture of object compared with stored memories ▪ Wernicke's area: left temporal, parietal lobes ▪ Interprets meaning of speech: words → thoughts ▪ Right hemisphere adds emotional content Cerebrospinal Fluid (CSF) ▪ Formed in the 4 ventricles of brain ▪ Lateral (1st and 2nd) → 3rd → 4th ventricle ▪ Formed in choroid plexuses ▪ By filtration and secretion of blood plasma ▪ In specialized capillary networks (covered by ependymal cells) in walls of ventricles ▪ Pathway ▪ Through 4 ventricles → central canal of spinal cord and within subarachnoid space → ▪ Reabsorbed through arachnoid villi into blood in superior sagittal sinus ▪ Cushions brain and provides nutrients Cerebrospinal Fluid (CSF) Cerebrospinal Fluid (CSF) Cerebrospinal Fluid (CSF) Cerebrospinal Fluid (CSF) Cerebrospinal Fluid (CSF) Cerebrospinal Fluid (CSF) Brain Blood Supply and Blood-Brain Barrier ▪ Requires 20% of the body’s O2 supply ▪ 4 min lack → permanent damage ▪ Requires continuous glucose supply ▪ Protected by blood-brain barrier ▪ Allows passage of lipid soluble materials: O2, CO2, alcohol, anesthetic agents ▪ But controls entry of most harmful materials ▪ Created by tight capillaries and astrocytes Diencephalon ▪ Thalamus: major sensory relay center ▪ Also motor, autonomic, and consciousness functions ▪ Hypothalamus: lies inferior to thalamus ▪ Control of pituitary and hormone production ▪ Works with ANS regulating many viscera ▪ Involved with feelings and behavior patterns ▪ Regulation of eating, drinking, fluid levels ▪ Control of body temperature ▪ Regulation of circadian rhythms, sleep, waking Diencephalon Brain Stem Brain Stem: Midbrain ▪ Connects pons to diencephalon ▪ Large tracts: cerebral peduncles ▪ Nuclei: ▪ Substantia nigra: related to Parkinson disease ▪ Red nuclei: help coordinate movements ▪ Origin of cranial nerves III and IV (control eye movements) ▪ Superior colliculi: nuclei involved in ▪ Scanning eye movements ▪ Responses to visual stimuli ▪ Inferior colliculi: responses to auditory input Brain Stem: Pons ▪ Serves as a “bridge” ▪ Connects medulla to midbrain and above ▪ Contains ascending and descending tracts ▪ Connects left and right sides of cerebellum ▪ Contains nuclei ▪ Motor relays from cerebrum to cerebellum ▪ Helps control breathing ▪ Cranial nerves V-VIII attached here Brain Stem: Medulla Oblongata ▪ Most inferior part of brainstem ▪ White matter connects spinal cord and other parts of brain ▪ Contains vital nuclei ▪ Cardiovascular center ▪ Regulates heart rate, blood pressure ▪ Medullary rhythmicity area ▪ Adjusts respiratory rhythm ▪ Other sensory and reflex motor areas ▪ Cranial nerves VIII-XII attached here Cerebellum ▪ Location: posterior to medulla and pons, inferior to cerebrum ▪ Attached to brain stem by cerebellar peduncles ▪ Structure: ▪ Two cerebellar hemispheres ▪ Cerebellar cortex: gray matter ▪ Tree-like appearance (seen in sagittal section) of white matter and gray nuclei Cerebellum ▪ Functions ▪ Receives wide range of sensory input from muscles, joints, tendons, eyes, inner ears ▪ Compares actual movements with intended ones ▪ Helps produce smooth, coordinated movements ▪ Helps execute skilled motor activities ▪ Regulates posture and balance Cranial meninges ▪ 3 layers ▪ Dura mater ▪ Arachnoid mater ▪ CSF circulates in subarachnoid space ▪ Pia mater ▪ Continuous with the spinal meninges ▪ Functions ▪ To protect the brain from traumatic injury, as a shock absorber and anchor the brain from moving around in the skull ▪ To support the brain blood vessels, nerve and lymphatics Limbic System ▪ Ring of structures on inner border of cerebrum and floor of diencephalon ▪ Called “emotional brain”: plays primary role in pain, pleasure, anger, affection and in behavior ▪ Involuntary activity related to survival ▪ Important in memory development Limbic System Reticular Formation ▪ Netlike arrangement of gray and white matter ▪ Contains ascending and descending tracts ▪ Ascending part = reticular activating system (RAS) ▪ Carries sensory pathways to cerebral cortex ▪ Helps maintain consciousness ▪ Helps induce sleep Reticular Formation Lateralization ▪ Brain controls opposite side of the body: all sensory and motor pathways cross in CNS ▪ Left side of the brain controls right side of body ▪ Right side of brain controls left side of body ▪ Left hemisphere important for spoken and written language, numerical and scientific skills, and reasoning ▪ Right side more involved with spatial and pattern recognition and emotional content Memory ▪ Process for storing and retrieving information ▪ Involves structural and functional changes ▪ Involves association areas, parts of limbic system, diencephalon and spinal cord ▪ Skill memory also involves cerebellum and basal ganglia Brain Development Brain Development Brain Waves Sleep Disorders THE NERVOUS SYSTEM : CENTRAL AND PERIPHERAL NERVOUS SYSTEM Part 2 – The Spinal Cord, The Spinal Nerves, Somatic Nervous System, Learning Outcomes ▪ Describe how the spinal cord is protected. ▪ Describe the structure of the spinal cord. ▪ Describe the composition, coverings, and distribution of spinal nerves. ▪ Describe the components of a reflex arc. ▪ Describe three somatic sensory and somatic motor pathways. Learning Outcomes ▪ Identify the 12 pairs of cranial nerves by name and number and give the functions of each. ▪ Describe the location and function of the receptors for tactile, thermal, and pain sensations. ▪ Identify the receptors for proprioception and describe their functions. Gross Anatomy of Spinal Cord ▪ Extends from medulla oblongata of brain to L2 vertebra ▪ It is protected by the vertebral column ▪ Cauda equina (horse’s tail) ▪ Extends inferior to end of spinal cord ▪ Consists of roots of lumbar, sacral and coccygeal spinal nerves ▪ Left and right halves partially separated by ▪ Anterior median fissure and posterior median sulcus ▪ Small central canal (filled with CSF) in middle ▪ Enlargements: cervical and lumbar regions ▪ Points of origins of nerves to upper and lower limbs Gross Anatomy of Spinal Cord Gross Anatomy of Spinal Cord Spinal Cord Structure: Protection and Coverings ▪ Spinal cord is covered by spinal meninges ▪ Three layers of connective tissue ▪ Dura mater ▪ Arachnoid mater ▪ Pia mater ▪ Continuous with cranial meninges Spinal Meninges and Spaces ▪ Epidural space: between vertebrae and dura mater ▪ Dura mater- tough ,dense connective tissue ▪ Extends to vertebra S2 (well beyond spinal cord) ▪ Arachnoid mater: resembles spider’s web ▪ Extends into subarachnoid space ▪ Subarachnoid space ▪ CSF circulates in this space ▪ Pia mater: thin, delicate layer ▪ Adheres to surface spinal cord (and brain) ▪ Contains blood vessels Spinal Meninges and Spaces Internal Structure of Spinal Cord ▪ Gray matter forms “H” (or “butterfly”) ▪ Three horns on each side; sites of cell bodies ▪ Posterior gray horns: contain sensory neurons ▪ Anterior gray horns: contain somatic motor neurons ▪ Lateral: contain autonomic motor neurons ▪ White matter (surrounds gray “H”) ▪ Consists of white columns ▪ Posterior, anterior, and lateral columns ▪ Contain tracts (bundles of axons) ▪ Sensory tracts: ascending to brain ▪ Motor tracts: descending from brain Internal Structure of Spinal Cord Spinal Cord Functions ▪ To transmit electrical signals between the brain and the spinal nerves ▪ Pathways for nerve impulse within tracts ▪ Ascending (sensory). Example: spinothalamic ▪ Descending (motor). Example: corticospinal ▪ Reflexes: fast, involuntary sequences of actions in response to stimuli ▪ Can be simple (withdrawal) or complex (learned sequence such as driving car) ▪ Levels ▪ Spinal (reflex arc): simple ▪ Cranial: more complex Spinal Cord function – reflex action Reflex Arc Components 1. Sensory receptor: responds to stimulus 2. Sensory neuron: through dorsal root ganglion and root → posterior horn 3. Interneuron or Integrating center: single synapse between sensory and motor neurons 4. Motor neuron: from anterior horn → ventral root → spinal nerve → 5. Effector: muscle responds Example of Reflex Arc: Patellar Reflex 1. Sensory receptor is stimulated by tap on patellar tendon 2. Sensory neuron: through dorsal root → spinal cord 3. Integrating center: single synapse in spinal cord 4. Motor neuron: through ventral root → spinal nerve → femoral nerve → 5. Effector: quads contract, extend leg Example of Reflex Arc: Patellar Reflex Reflex Arc Reflex Arc Cervical Spinal Cord Injury Gross Anatomy of Spinal Nerves ▪ Part of the peripheral nervous system ▪ 31 pairs ▪ Named according to level of vertebra ▪ C1-C8, T1-T12, L1-L5, S1-S5, 1 coccygeal ▪ Emerge from spinal cord through intervertebral foramina ▪ Nerves attached to spinal cord by 2 roots ▪ Dorsal root: made of axons of sensory neurons ▪ Dorsal root ganglion: swelling containing cell bodies of sensory neurons ▪ Ventral root: composed of axons of motor neurons ▪ Both somatic motor and autonomic motor Spinal Nerve Composition ▪ Formed by 2 spinal nerve roots ▪ Formed from dorsal root (sensory) and ventral root (motor) root ▪ Connective tissue coverings ▪ Individual axons wrapped in endoneurium ▪ Axons grouped in fascicles wrapped in perineurium ▪ Outer covering = epineurium Spinal Nerves Peripheral Nervous System – the spinal nerve Spinal Nerve Composition Distribution of Spinal Nerves ▪ Spinal nerves branch after pass through intervertebral foramina ▪ Some join with branches from neighboring nerves to form plexuses ▪ Nerve names relate to region innervated ▪ Has 4 spinal plexuses ▪ Spinal nerves T2-T12 do not form plexuses ▪ Called intercostal nerves ▪ Supply abdominal muscles, skin of chest and back, and muscles between ribs. Spinal Nerve Plexuses ▪ Cervical plexus ▪ Supplies posterior head, neck, shoulders, and diaphragm ▪ Important nerves: phrenic to diaphragm ▪ Brachial plexus ▪ Supplies upper limbs + some neck and shoulder muscles ▪ Important nerves: radial, ulnar, axial, median to arm, forearm, hand Spinal Nerve Plexuses ▪ Lumbar plexus ▪ Supplies abdominal wall, external genitalia, and part of lower limbs ▪ Important nerves: femoral (to anterior thigh: quads) ▪ Sacral plexus ▪ Supplies buttocks, perineum, and most of lower limbs ▪ Important nerves: gluteal, sciatic (to posterior thigh and all of leg and foot) Spinal nerve distribution and plexuses Spinal Nerves Brachial Plexus Lumbar Plexus and Pelvic Plexus Dermatomes ▪ Areas of the skin whose sensory distribution is innervated by the afferent nerve fibers from the specific single spinal nerve root. Dermatomes Organization of the Nervous System Somatic Nervous System: Sensory Pathways ▪ Relay sensory information from periphery to cerebral cortex ▪ 3 neurons in each pathway ▪ Cell body #1 in dorsal root ganglion ▪ Cell body #2 in spinal cord or brain stem ▪ Cell body #3 in thalamus; axon extends to cerebral cortex (somatosensory area in postcentral gyrus) ▪ Most sensory input to right side of body reaches left side of brain (and vice versa) Somatic Sensory Pathways ▪ Posterior column - medial lemniscus pathway senses ▪ Fine touch: body location, texture, size ▪ Proprioception: position and motion of body parts ▪ Vibrations: fluctuating touch stimuli ▪ Spinothalamic pathways ▪ Anterior and lateral spinothalamic tracts ▪ Relay impulses for pain, tickle, itch, hot, and cold sensations Somatic Sensory Pathways Somatic Motor Pathways ▪ Signals come from ▪ Upper motor neurons: via corticospinal tracts ▪ Basal ganglia: help with muscle tone ▪ Cerebellum: coordination ▪ Sensory neurons or interneurons via reflexes ▪ Impulses activate lower motor neurons ▪ Cell bodies in anterior gray of spinal cord ▪ Axons → ventral root → spinal nerve → muscle → voluntary movements Somatic Motor Pathways Somatic Sensory and Motor Pathway Sensory Nervous System ▪ Somatic – conscious awareness ▪ Tactile: touch, pressure, vibration ▪ Thermal (warm, cold) ▪ Pain ▪ Proprioception (joint, muscle position sense; movements of limbs, head) ▪ Visceral – unconscious awareness ▪ Autonomic nervous system ▪ Blood pressure, blood pH, blood volume Somatic Senses ▪ The faculty of bodily perception – conscious awareness ▪ Somatic receptors in skin, mucous membranes, muscles, tendons, and joints ▪ Include tactile, thermal, pain, proprioceptive, visual, auditory, olfactory, gustatory ▪ Adaptation: decreased receptor response during prolonged stimulation ▪ Decreased perception ▪ Adaptation speed varies with receptor ▪ Rapid adaptation: pressure, touch, smell ▪ Slow adaptation: pain, body position, chemical levels in blood Definition of Sensation ▪ Conscious or subconscious awareness of change in external or internal environment ▪ Requires 1. Stimulus 2. Sensory receptor 3. Neural pathway 4. Brain region for integration Sensory Receptors: Structural Types ▪ Free nerve endings ▪ Pain, thermal, tickle, itch, some touch receptors ▪ Encapsulated nerve endings ▪ Touch, pressure and vibration ▪ Separate, specialized cells ▪ Hair cells in inner ear – hearing, balance ▪ Photoreceptors in retina of eye ▪ Chemoreceptors in tongue for taste and nasal cavity for smell Sensory Receptors: Functional Types ▪ Mechanoreceptors ▪ Proprioception – joint position ▪ Balance ▪ Muscle stretch ▪ Tactile - Touch, pressure, vibration, itch, tickle ▪ Thermoreceptors: temperature ▪ Nociceptors: pain ▪ Photoreceptors: light ▪ Chemoreceptors: taste, smell, blood pH ▪ Osmoreceptors - Osmotic pressure of body fluid ▪ Baroreceptor – blood pressure Tactile Sensations ▪ Touch, pressure, vibration ▪ Encapsulated nerve endings ▪ Itch and tickle ▪ Free nerve endings Touch ▪ Rapidly adapting receptors for touch ▪ Meissner corpuscles ▪ Hair root plexuses: detect hair movement ▪ Slowly adapting receptors for touch ▪ Type I mechanoreceptors: Merkel discs or tactile discs ▪ Surface receptors: in epidermis ▪ Type II mechanoreceptors: Ruffini corpuscles ▪ Deep in dermis and tendons Pressure and Vibration ▪ Pressure ▪ Pacinian (lamellated) corpuscles: layers like onion ▪ Rapid adapting ▪ Widely distributed: in dermis, subcutaneous, around joints, tendons, muscles, periosteum ▪ Vibration ▪ Response to rapidly repetitive stimuli ▪ Receptors: Meissner and Pacinian Itch and Tickle ▪ Itch: chemical stimulation of free nerve endings ▪ Bradykinin from inflammation response ▪ Tickle: from free nerve endings and Pacinian corpuscles ▪ Tickle requires stimulus from outside of self ▪ Effects of attempts to tickle oneself are blocked by signals to/from cerebellum Structure and Location of Sensory Receptors Thermal Sensations ▪ Two kinds of thermoreceptors ▪ Cold receptors: 10˚–40˚ C (50–105˚ F) ▪ Located in epidermis ▪ Warm receptors: 32˚–48˚ C (90–118˚ F) ▪ Located in dermis ▪ Both adapt rapidly but continue slow signals during prolonged stimulus ▪ Outside these ranges: nociceptors detect pain Pain Sensations ▪ Nociceptors ▪ Free nerve endings in every tissue except brain ▪ Can respond to any excessive stimulus ▪ Minimal adaptation ▪ Types of pain ▪ Fast pain: acute, sharp pain ▪ Well localized ▪ Slow pain: chronic, burning, aching, throbbing ▪ More diffuse (not localized) ▪ Referred pain is visceral pain displaced to skin surface Distribution of Referred Pain Proprioception ▪ Awareness of ▪ Body position, movements, weight of objects ▪ Sites of receptors ▪ Muscles (muscle spindles) ▪ Tendons (tendon organs) ▪ Joint kinesthetic receptors (synovial joints) ▪ Inner ear (hair cells): head position ▪ Tracts to ▪ Somatosensory area of cerebral cortex and ▪ Cerebellum ▪ Slight adaptation Proprioception receptor Functional Areas of the Cerebrum Aging ▪ Rapid brain growth during first few years of life ▪ Due to increase in size of neurons and proliferation of neuroglia ▪ Increase in development of dendritic branches and synaptic contacts ▪ From early adulthood through old age: ▪ Decline in brain mass ▪ Fewer synaptic contacts brain function ▪ Some decrease in brain function THE NERVOUS SYSTEM : PERIPHERAL NERVOUS SYSTEM Part 3 – The Cranial Nerves Learning Outcomes ▪ Discuss how the brain is protected and supplied with blood. ▪ Name the major parts of the brain and explain the function of each part. ▪ Describe how the spinal cord is protected. ▪ Describe the structure of the spinal cord. ▪ Describe three somatic sensory and somatic motor pathways. ▪ Describe the components of a reflex arc. Learning Outcomes ▪ Describe the composition, coverings, and distribution of spinal nerves. ▪ Identify the 12 pairs of cranial nerves by name and number and give the functions of each. ▪ Describe the location and function of the receptors for tactile, thermal, and pain sensations. ▪ Identify the receptors for proprioception and describe their functions. Cranial Nerves Cranial Nerves I. Olfactory: special sensory—smell II. Optic: special sensory—vision III. Oculomotor: motor—control of eye movements IV. Trochlear: motor—control of eye movements V. Trigeminal: mixed ❑General sensory: touch, pain, pressure, hot, cold in face ❑Motor: to muscles used for chewing Trigeminal Nerve Cranial Nerves VI. Abducens: motor—control of eye movements VII. Facial: mixed ❑ Special sensory (taste) from anterior of tongue ❑ Motor to muscles of facial expression, tear glands, and some salivary glands VIII. Vestibulocochlear: special sensory—ear Facial Nerve Cranial Nerves IX. Glossopharyngeal: mixed ❑ Sensory for posterior of tongue, pharynx, and palate ❑ Motor to pharyngeal muscles (swallowing), salivary gland (parotid) X. Vagus: mixed (the major parasympathetic nerve) ❑ Sensory from pharynx, ear, diaphragm, visceral organs in thoracic and abdominal cavities ❑ Motor to palatal and pharyngeal muscles (swallowing and voice); to viscera in thoracic and abdominal cavities Vagus Nerve Cranial Nerves XI. Accessory: motor to voluntary muscles including sternocleidomastoid and trapezius (move head, shoulders) XII. Hypoglossal: motor to tongue (swallowing and speech)