Central Nervous System PDF

Central Nervous System 1  The Nervous System The Nervous System is the master controlling and communication system of the body, specialized to quickly detect and respond to stimuli. It is composed of: Central Nervous System (brain, spinal cord) – Integrati...

Central Nervous System 1  The Nervous System The Nervous System is the master controlling and communication system of the body, specialized to quickly detect and respond to stimuli. It is composed of: Central Nervous System (brain, spinal cord) – Integration Peripheral Nervous System (peripheral nerves and receptors). – Afferent Division (Sensory information IN) – Efferent Division (Motor responses OUT) 2  Central nervous system (CNS) Brain and Input Output spinal cord to CNS from from CNS to periphery periphery Peripheral nervous system (PNS) Afferent Efferent division division Stimuli in Sensory Visceral Somatic Autonomic digestive stimuli stimuli nervous system nervous system tract Motor Sympathetic Parasympathetic Enteric nervous neurons nervous system nervous system system KEY Central nervous system Smooth muscle Skeletal Digestive Peripheral nervous system Cardiac muscle muscles organs only Afferent division of Exocrine glands Some endocrine PNS* glands Efferent division Somatic ofsystem nervous PNS Autonomic nervous system Enteric nervous system* Effector organs 3 (made up of muscle and gland tissue) Fig. 5-1, p. 136  Central Peripheral nervous system nervous system (spinal cord) Cell Axon Afferent neuron body terminals Central Peripheral Sensory axon axon receptor (afferent fiber) Interneuron Efferent neuron* Effector organ (muscle or gland) Axon (efferent fiber) Axon Cell terminals body * Efferent autonomic nerve pathways consist of a two-neuron chain between 4 the CNS and the effector organ. Fig. 5-2, p. 137  Protection of the CNS Central Nervous System tissue is easily damaged, so it must be well protected from trauma and harmful substances 3 main ways it is protected: 1.Skull & Meninges 2.Cerebrospinal Fluid (CSF) 3.Blood Brain Barrier 5  Bone & Meninges Bones of the skull – enclose the brain Vertebral Bones- enclose the spinal cord Meninges – three connective tissue membranes wrap the brain and spinal cord: dura, arachnoid, pia mater. 6  Cerebrospinal Fluid Cerebrospinal Fluid (CSF) surrounds and cushions the spinal cord and brain – formed by choroid plexuses in ventricles – about 125-150mL replaced 3 times per day – Absorbs shock if sudden jarring movements occur – exchange of materials and fluids between cells, neuroglia and interstitial fluid low K+, high Na+, very few proteins (vs. blood) – limited exchange between CSF and blood due to blood brain barrier 7  Blood Brain Barrier The Blood Brain Barrier (BBB) is a highly selective network of specialized capillaries that prevent many substances from entering the brain from the blood layer of capillaries that have tight junctions, surrounded by astrocytes and ependymal cells protects the brain from blood-borne pathogens, certain hormones, toxins – lipid soluble, O2, CO2, alcohol, and water can cross – glucose, amino acids, ions transported in by highly selective membrane carriers 8 Metabolic Requirements of the CNS Neurons rely on a constant supply of oxygen and glucose to produce ATP for active transport of ions and neurotransmitters. Oxygen diffuses across the BBB Under normal circumstances glucose is the only energy source for neurons – Glucose is transported from the plasma into the interstitial fluid by insulin independent membrane transporters – Hypoglycemia leads to confusion, unconsciousness and death 9  Functions Of The Brain Homeostasis: regulation of internal environment Emotion Movement Control Sensory Perception Memory Cognition (higher thought, awareness, judgement) 10  CNS Circuits No single area of the CNS is functional on its own. The function of each area is dependant upon the connections that form its input and output. We will look at each area individually, but its important to remember that it is only one part of the many CONNECTIONS that makeup the entire circuit of information flow throughout the CNS. 11 Gray Matter & White Matter Gray Matter: cell bodies, synapses, dendrites, neuroglia – nucleus: CNS gray matter – ganglion: PNS gray matter White Matter: myelinated axons connecting different regions – nerves: PNS – white matter tracts: CNS Gray matter White matter 12  Functional Brain Regions Functional Brain Regions are organized based on their adult function. From the top down view, these regions are: – Cerebral Cortex – Basal Nuclei – Thalamus – Hypothalamus – Cerebellum – Brainstem Midbrain Pons Medulla 13  Cerebral Cortex (Cerebrum) The Cerebral Cortex (cerebrum) is the largest, outermost region of the brain. Divided into 4 Lobes: – Frontal – Temporal – Parietal – Occipital Each Lobe can be further divided by functional area 14 Frontal Central sulcus Parietal lobe lobe Occipital lobe Temporal lobe 15 Fig. 5-9, p. 147  Primary Cortex Areas The lobes of the cerebrum contain many functional regions for integration “Primary” regions for motor and sensory integration are responsible for simple, direct and conscious processing of a single type of sensory stimulus or motor command example: the primary visual cortex processing of lights ON and OFF and simple patterns of light. – Primary Motor Cortex: voluntary skeletal movement – Primary Visual Cortex: response to light stimuli – Primary Auditory Cortex: response to sound stimuli – Primary Olfactory Cortex: response to smell – Primary Somatosensory Cortex: response to touch stimuli 16  Complex Cortical Association Areas Association Areas: regions next to or near the primary cortictal areas that integrate multiple sensory stimuli, motor stimuli, and/or memory and emotional stimuli. example: Visual Association Areas- processing of images, faces, – Visual Association Areas – Auditory Association Areas – Olfactory Association Areas – Somatosensory Association Areas – Pre-Motor Cortex: planning and decision making for skeletal movement – Pre-Frontal Cortex: emotional and social processing – Language Areas: facial and motor movements to speak, auditory and visual stimuli to read and express speech. 17  The Cerebral Hemispheres Central sulcus Frontal Lobe (retracted Parietal Lobe to show insula) Primary somatosensory Primary motor cortex cortex (precentral gyrus) (postcentral gyrus) Somatic motor association area Somatosensory (premotor cortex) association area Retractor Occipital Lobe Visual association area Prefrontal cortex Visual cortex Insula Temporal Lobe (retracted Lateral sulcus to show olfactory cortex) Auditory association area Major anatomical landmarks on the surface of Auditory cortex a the left cerebral hemisphere. To expose the Olfactory cortex insula, the lateral sulcus has been pulled open. 18  Occipital Lobe: Primary Visual cortex Primary visual cortex – light, vision (“light”, shading) – receives sensory input from the retina (light receptors in eye) – Function: perception and processsing of light Visual association area – complex processing of visual information Visual Association Areas Primary Visual Cortex 19  Temporal Lobe: Primary Auditory Cortex Primary auditory – sound, hearing – receives sensory input from the ear – Function: perception and processing of sound Auditory association Area – interprets sound into context Primary Auditory Cortex Auditory Association Area 20  Frontal Lobe: Primary Motor Primary motor cortex Cortex – Function: voluntary control of skeletal muscles – contralateral control (neurons cross over before heading down spinal cord) Supplementary Motor Area – movement sequences Pre-motor cortex – learned, planned movement Supplementary Motor Area Primary Motor Cortex Pre-motor Cortex 21  Parietal Lobe: Primary Somatosensory Cortex Primary somatosensory cortex – body sensations – Receives impulses involved in touch, pain, pressure, stretch from contralateral side of the body (axons cross in spinal cord before traveling up) – Function: processing and perception of body sensations, proprioceptive input from skin, joints, muscles Somatosensory association: complex processing of body sensations stimuli – perception of complex patterns such as texture and shape of something you are holding Primary Somatosensory Cortex Somatosensory Association Area 22  Frontal Lobe: Prefrontal Cortex Prefrontal cortex – social and emotional planning and integration involved with intellect, reasoning, judgment, concern for others, personality traits, and management of emotions – Develops later in life and is impacted by social environment – Linked to emotions, via The Limbic System Prefrontal Cortex 23  Language Areas Language areas – speech production and understanding surrounds lateral sulcus in the LEFT hemisphere only – Broca’s area (Left frontal lobe) – motor and pre-motor association, controls muscles involved in speech production – Wernicke’s area (Left temporal lobe) – auditory and visual association area involved in speech processing, language comprehension 24  Wernicke’s area Angular gyrus of parietal-temporal-occipital Language Processing (plans content of association cortex spoken words) (integrates sensory input) Primary motor cortex (commands facial and tongue muscles to speak words) 4 2 3 Broca’s area 1 (programs sound b pattern of speech) 1 a Primary visual cortex (perceives sight) Primary auditory cortex (perceives sound) Hear words See words 25 Fig. 5-12, p. 152  Basal Nuclei Basal Nuclei - integration and fine tuning of motor, sensory and emotional input/output gray matter deep in the cerebrum – Adjust stopping, starting and intensity of movements after receiving input from cerebral motor cortex – Sensory and motor processing – Emotional processing in the Amygdala Affected in Parkinson’s Disease 26  Cerebellum Cerebellum: balance, movement planning and movement execution highly folded, large region beneath the occipital lobe receives visual, somatic, cortical input Function: subconscious control of motor coordination 27  Cerebellum Motor cortex Sends intended muscle Movement to cerebellum Adjustments made by Cerebellum sent back to Cerebellum Motor cortex Coordinate motor intent with sensory input Sensory input from proprioceptors, visual and equilibrium pathways 28  Thalamus Thalamus – sensory relay station Function: filter, process, relay sensory information to cortex regions, i.e. screens sensory impulses and decides if it should be passed onto the cortex and where it should be sent Thalamus 29  Hypothalamus Hypothalamus – homeostasis Function: links the endocrine system, autonomic systems to directly regulate internal body environment Hypothalamus 30  Hypothalamus Functions Autonomic control center – controls ANS centers in the brain stem and spinal cord Emotions –basic primitive drives such as fear, anger, pleasure Regulates body temperature – thermostat, initiates cooling or heating mechanisms Sleep-wake cycles Hunger – responds to changes in levels of nutrients and hormones Water balance and thirst- detects concentrations of body fluids, triggers thirst centers Secretes hormones – controls the release of hormones from the pituitary 31  Limbic System Limbic areas – emotional response and processing A circuit of regions in Limbic Association Cortex, Basal Nuclei (Amygdala), Thalamus, Hypothalamus Motivation, basic emotion, social, sexual behavioral patterns, basic survival instinctual behaviors – example: stimulate Amygdala- fear sensations 32  Brainstem Brainstem: 3 regions that link spinal cord to higher brain regions – Midbrain – Pons – Medulla Midbrain Pons Medulla oblongata 33  Reticular Formation Reticular Activating System (RAS): interconnect regions of the brainstem that receive and integrate sensory input Function: filter sensory input, attention, arousal of cerebral cortex, some control of sleep/wake states Reticular Formation 34  Reticular activating system Cerebral cortex Cerebellum Visual impulses Reticular Brain Auditory impulses formation stem Spinal cord Ascending Descending motor 35 sensory tracts tracts Fig. 5-21, p. 171  Midbrain Midbrain – superior portion of the brain stem that contains: – Corpora quadrigemina Superior colliculi - visual reflexes Inferior colliculi - auditory reflexes Midbrain Pons Medulla oblongata 36  Pons Pons – bulging region between midbrain and medulla, anterior to cerebellum – Pneumotaxic respiratory center – works with medulla to maintain rhythmic breathing Midbrain Pons Medulla oblongata 37  Medulla Oblongata Medulla Oblongata – base of brain stem, blends inferiorly with the spinal cord – Pyramids – contains motor tracts that cross over (decussation) before they continue down the spinal cord – Olives – relay information to the cerebrum & cerebellum Autonomic Nuclei – Cardiovascular center – adjusts heart rate and blood pressure – Respiratory center – controls rate and depth of breathing, works with pons for rhythm – Vomiting, swallowing, coughing, sneezing, hiccups Midbrain Pons Medulla oblongata 38  Cerebral cortex Cerebral Cerebral cortex cortex Basal nuclei (lateral to Basal nuclei thalamus) Thalamus Thalamus (medial) Hypothalamus Cerebellum Hypothalamus Cerebellum Midbrain Brain stem Brain stem Pons Spinal cord Medulla 39 Table 5-2a, p. 144  Cerebral cortex Basal nuclei Thalamus Hypothalamus Cerebellum Brain stem 40 Table 5-2b, p. 145  Electroencephalogram (EEG) An electroencephalogram is a record of postsynaptic activity in cortical neurons. Extracellular current flow arising from electrical activity within the cerebral cortex can be detected by placing recording electrodes on the scalp to produce a graphic record known as an electroencephalogram, or EEG. These “brain waves” for the most part are not due to action potentials but instead represent the momentary collective postsynaptic potential activity in the cell bodies and dendrites located in the cortical layers 41 Electroencephalogram (EEG)…(Cont’d)  Electrical activity can always be recorded from the living brain, even during sleep and unconscious states.  The waveforms vary, depending on the degree of activity in the cerebral cortex.  Often the waveforms appear irregular, but sometimes distinct patterns in the wave’s amplitude and frequency can be observed. 42 Electroencephalogram (EEG)…(Cont’d) Replacement of an alpha rhythm on an EEG with a beta rhythm when the eyes are opened. 43 Electroencephalogram (EEG)…(Cont’d) The EEG has three major uses: 1.The EEG is often used as a clinical tool in the diagnosis of cerebral dysfunction. Diseased or damaged cortical tissue often gives rise to altered EEG patterns. One of the most common neurologic diseases accompanied by a distinctively abnormal EEG is epilepsy. Epileptic seizures occur when a large collection of neurons undergo abnormal, synchronous action potentials that produce involuntary spasms and alterations in behavior. 44 Electroencephalogram (EEG)…(Cont’d) 2. The EEG is also used in the legal determination of brain death. Even though a person may have stopped breathing and the heart may have stopped pumping blood, it is often possible to restore and maintain respiratory and circulatory activity if resuscitative measures are instituted soon enough. The brain is susceptible to O2 deprivation, irreversible brain damage may occur before lung and heart function can be reestablished, resulting in the paradoxical situation of a dead brain in a living body. 45 Electroencephalogram (EEG)…(Cont’d) 3. EEG is also used to distinguish various stages of sleep. EEG pattern during paradoxical(REM ) sleep is similar to that of an alert, awake person, whereas the pattern during slowwave sleep displays distinctly different waves. 46  Spinal Cord Functions: – Pathway between the body and the brain – Contains ascending and descending nerve tracts of the CNS, relaying information to the brain – Initiates basic reflexes independent of the brain 47  Review of Sensory & Motor Pathways Sensory pathway: sensory input into the spinal cord and brain provides information on internal and external changes in stimuli – Afferent & Ascending Motor pathway: motor response based on CNS decision (voluntary or involuntary) to change activity in skeletal muscle, organ or gland – Efferent & Descending Reflex – involuntary, rapid predictable motor response to a sensory stimulus 48  Reflexes Deep Tendon Reflex: – stimulus: brisk tap of muscle tendon activates a stretch receptor – sensory neuron: located in dorsal root ganglion  dorsal (posterior) horn of spinal cord – integration: spinal cord interneuron or synapse – motor output: motor neuron from ventral (anterior) horn – NOTE: can be modulated by cerebral cortex and brainstem nuclei  Stretch Receptor (muscle spindle) Stimulus Spinal cord REFLEX ARC Patellar Reflex KEY Effector Sensory neuron (stimulated) Motor neuron Contraction (stimulated) The patellar reflex is controlled by muscle spindles in the quadriceps muscle group. The stimulus is a reflex hammer striking the muscle Response tendon, stretching the spindle fibers. This results in a sudden increase in the activity of the sensory neurons, which synapse on spinal motor neurons. The response occurs upon the activation of motor units in the quadriceps group, which produces an immediate increase in muscle tone and a reflexive kick. Muscle Physiology  Functions of Skeletal Muscles 1) Voluntary Skeletal Movement 2) Guard body entrances and exits 3) Maintain posture / body position 4) Support soft tissues 5) Maintain body temperature 6) Store nutrients  Muscles are Organs Skeletal Muscles are organs that contain bundles of skeletal muscle cells, blood vessels, connective tissue and nerves Bundle Arrangement Connective Tissue Layers Microscopic Proteins Skeletal Muscle Organization SKELETAL MUSCLE Surrounded by: Epimysium Contains: Muscle fascicles MUSCLE FASCICLE Surrounded by: Perimysium Contains: Muscle fibers MUSCLE FIBER Surrounded by: Endomysium Contains: Myofibrils  Muscle Cells have Organelles Like any other cell, muscle cells have organelles Muscle Fiber (cell) Organelles are specialized to Myofibril Endomysium support contraction, high levels of Sarcoplasm activity Mitochondrion sarcoplasm: cytoplasm sarcolemma: cell membrane mitochondria Sarcolemma sarcoplasmic reticulum: modified ER Nucleus for calcium storage and release T-tubules: invagination of cell membrane terminal cisternae: ends of T-tubules myofibrils****: specialized contractile organelle A Muscle Cell (Fiber)  Myofibrils are contractile organelles within a single muscle cell/fiber, there are densely packed myofibrils myofibrils are the contractile organelles of the muscle cell surrounded by sarcoplasmic reticulum (stores Ca2+) units of thick and thin contractile proteins; sarcomere arrangement MYOFIBRIL Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line)  Sarcomere is the Contractile Unit When a muscle contracts, myosin pulls the actin inward toward the M- line, causing the sarcomere to shorten A muscle fiber contraction at the microscopic level is “shortening” of the muscle fiber Sarcomere Z line H zone I band A band Z line Relaxed I band A band H zone shorter same shorter width Contracted Thick filament Thin filament Sarcomere shorter Fig. 8-7, p. 264  Sarcomere Sarcomere Pattern: – Z-lines- boundaries of 1 sarcomere – I band (light)– thin filaments only, spans 2 sarcomeres – A band (dark)– contains thick and thin filaments overlapping – H band – center, thick filaments only – M–line – center line, middle proteins that link adjacent myosin I band A band Contains: Thick filaments Thin filaments M line Z line Z line H band Figure 10-6 Levels of Functional Organization in a Skeletal Muscle. Skeletal Muscle Myofibril Surrounded by: Surrounded by: Epimysium Sarcoplasmic Epimysium reticulum Contains: Muscle fascicles Consists of: Sarcomeres (Z line to Z line) Sarcomere I band A band Muscle Fascicle Contains: Thick filaments Surrounded by: Perimysium Thin filaments Perimysium Contains: Muscle fibers Z line M line Titin Z line H band Muscle Fiber Surrounded by: Endomysium Endomysium Contains: Myofibrils  Muscle Proteins NAME LOCATION FUNCTION A band (thick Myosin filament) Contraction; hydrolyzes ATP and develops tension I band (thin Contraction; activates myosin ATPase and interacts with Actin filament) myosin Regulatory protein; in presence of Ca++, promotes actin- Troponin Thin filament myosin activation Regulatory and structural function; links filaments, Tropomyosin Thin filament controls filament length Alpha (α) Regulatory and structural function; links filaments, actin Z band controls filament length Beta (β) Regulatory and structural function; links filaments, actin Z band controls filament length M line (center of Regulatory and structural function; provides enzyme M protein thick filaments) creatine kinase A band (thick C protein filaments) Possible structural role Z line (thick Titin filament) Interconnects thin filaments in Z line Creatine kinase M line Catalyzes the phosphorylation of ADP to form ATP Desmin Z line Interconnects thin filaments in Z line Interconnects thin filaments in Z line; stabilizes Filamin∗ Z line membrane Nebulin∗ Z line Determines filament length  Sarcomere Actinin Z line Titin a The attachment Sarcomere of thin filaments to the Z line H band Troponin Active site Nebulin Tropomyosin G actin molecules F actin strand Myofibril b The detailed structure of a thin filament M line Z line Titin c The structure of Myosin thick filaments M line head Myosin tail Hinge d A single myosin molecule detailing the structure and movement of the myosin head after cross-bridge binding occurs Figure Thin and Thick Filaments  Excitation – Contraction Coupling EXCITATION of the muscle cell by the motor neuron Motor neuron CAUSES contraction ACTION POTENTIAL Contraction of the TRAVELING muscle is due to individual muscle fibers contracting, each from microscopic events at the sarcomere Muscle Fiber  Muscles are controlled by Motor Units Motor Neuron: one motor neuron within a cranial or spinal nerve that connects to a skeletal muscle – a nerve is a bundle of neurons traveling to/from targets Skeletal Muscle Fiber: a single skeletal muscle cell that contracts in response to electrical input – one muscle is made up of many muscle fibers Motor Unit: one motor neuron and all the muscle fibers it connects to – one motor neuron may connect to several muscle fibers – one muscle will have many motor neurons from a single nerve branch that control it  Neuromuscular Junction A connection between a single neuron and a single muscle fiber is called a Neuromuscular Junction (NMJ) Neuromuscular Junction Neuron Skeletal muscle Neuromuscular Junction Anatomy Neuron: nervous system cell – Axon: branch of a neuron – Axon Terminal: the end of the neuron that contacts the muscle Synaptic cleft: the space between the neuron and the muscle fiber Muscle Fiber: skeletal muscle cell – motor end plate: the region of the muscle fiber that the neuron connects to – sarcolemma: muscle cell membrane  Excitation- Contraction Coupling Summary Axon terminal Excitation Excitation Sarcolemma T tubule Cytosol 1) NEURON Action Potential Sarcoplasmic reticulum 2) Neurotransmitter (Ach) Calcium ion release released Ca2+ Ca2+ ATP 3) Neurotransmitter receptors activated (AchR) Thick-thin 4) MUSCLE membrane is filament interaction Ca2+ Myosin tail depolarized (thick filament) Tropomyosin Cross-bridge formation 5) Ca 2+ release Troponin G-actin Ca2+ 6) Ca enters the sarcomere (thin filament) 2+ Ca2+ Nebulin Active site 7) Ca 2+ binds to troponin 8) Myosin-Actin cross-bridge In a resting sarcomere, the When calcium ions enter the Cross-bridge tropomyosin strands cover sarcomere, they bind to formation then 9) Myosin-Actin powerstroke the active sites on the thin filaments, preventing troponin, which rotates and swings the tropomyosin away occurs, and the contraction cycle cross-bridge formation. from the active sites. begins. 10)myosin detaches  Sliding Filament Theory The Sliding Filament Theory describes the action of myosin and actin: – Calcium is the excitation signal, required to free the myosin binding sites – ATP required for Power Stroke – new ATP also required for crossbridge release, relaxation lack of ATP = no release upon death, no ATP available, rigor mortis (state of constant fixed contraction) sets in  Sliding 1 Energized...or... No Ca2+ Filament 2b Resting Theory present (excitation) 4a Detachment Cross- 2a Binding bridge cycle Fresh ATP available 3 Bending...or... No ATP (after death) 4b Rigor complex Figure Cross-bridge cycle.  Muscle Fiber Types Muscle fibers differ in their methods of metabolism based on: – Pathways they use to produce ATP; aerobic or glycolytic – How quickly their ATPases work; speed of contraction cycles – levels of myoglobin; increased ability to bind to oxygen 3 types: – Slow; Slow Oxidative (Type I) – Intermediate; Fast Oxidative (Type IIa) – Fast; Fast Glycolytic (Type IIx) Muscles will have different fiber composition depending on their function: muscles that have continuous activity (ex: postural muscles) vs. muscles that need to respond quickly (ex: rapid eye movement)  Types of Muscle Fibers Slow – oxidative (SO) muscle fibers (Type I) slow myosin ATPase activity: endurance aerobic metabolism Numerous mitochondria, small in diameter, high myoglobin (oxygen SO storage) content  DARK RED FG SO Intermediate – Fast oxidative (FO) muscle FG FO SO SO fibers (Type IIa) Fast myosin ATPase activity, SO FO FG are fast to contraction but resistant to fatigue FG FG FO – intermediate levels of mitochondria, aerobic FO SO metabolism, low myoglobin  LIGHT RED FO SO Fast glycolytic (FG) (Type IIx) muscle LM 250x fibers Fast myosin ATPase activity: power © G W Willis/Getty Images and speed – few mitochondria, high glycogen reserves, glycolysis (lactic acid build up), reduced myoglobin  nearly WHITE in color  Muscle Twitch A single, quick stimulus produces a single muscle fiber activation or twitch A single muscle fiber twitch has 3 phases: Tension 1) latent period: delay between action potential stimulus arrival and calcium release from SR 2) contraction: calcium Stimulus ions bind to troponin, crossbridges formed Time between myosin and actin (msec) Latent Contraction Relaxation 3) relaxation: calcium phase phase phase levels decrease as calcium pumped back into SR and crossbridges detach  Latent Contraction Relaxation period time time Muscle Muscle Contraction twitch Tension Timing Contractile After excitation, the response duration of the muscle A few msec contraction (15-50 msec) and muscle 30- 100 msec relaxation (additional Membrane potential (mV) 15-50msec) is very long compared to the +30 duration of the action Action 0 potential potential (1-2 msec) –90 1-2 msec 50 100 Stimulation Time (msec)  Muscle Tension A whole muscle is made up of many muscle fibers Sarcomere shortening in muscle fibers within the whole muscle leads to overall build up of tension in the whole muscle Muscle fiber contraction leads to Tension production  How do you get STRONGER contractions in a muscle? 1) Increase the tension (strength) of a single muscle fiber 1) optimum length (stretch) 2) maximum calcium level (stimulation) 2) Activate more muscle fibers in the whole muscle 1) maximum number muscle fibers active (recruitment) 2) more muscle fibers added to muscle (increased size)  Calcium Levels The level of calcium in a muscle is determined by release (increase) and balanced by re- uptake (decrease) of calcium inside the muscle electrical stimulation  calcium released from SR time to relax  calcium pumped back into SR Remember: calcium binds to troponin to release tropomyosin from actin binding sites, so this is also about crossbridge formation  Larger Muscles are Stronger The size of muscle can increase strength by: – Increased number of muscle fibers per motor unit determined by muscle development more fibers per motor neuron means the same amount of stimulation will give a stronger response – Increased size of individual muscle fibers determined by training: fibers produce more myofilaments in response to demands placed on them (ex: athletic training) more myofilaments means a stronger response to the same stimulus  Isotonic & Isometric Contraction The load that a muscle is trying to move is heavy and creates resistance. The force that the muscle applies creates tension, and must be greater than the resistance in order to move the load. – isotonic contraction: moves the load with equal- steady force, load moves – isometric: changing force, but load does not move isometric contraction: tensionresistance load is moved The End 79