Somatic nervous system and muscles

Isabelle Chartrand
Mind Map by , created about 6 years ago

Module 3 - Introductory Human Physiology

Isabelle Chartrand
Created by Isabelle Chartrand about 6 years ago
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Somatic nervous system and muscles
1 Autonomic nervous system controls the function of glands, smooth muscle, cardiac muscle, and the neurons of the GI tract. It is composed of two neurons in series that can either excite or inhibit the target organ.
2 Somatic nervous system contains single neurons that excite skeletal muscle. The movements controlled by the somatic nervous system can be voluntary of involuntary (reflexes)
3 Structure
3.1 Motor unit
3.1.1 Axons of motor neurons are myelinated and have large diameters for fast conduction of AP
3.1.2 Axon branches to form synapses with anywhere from three to one thousand muscle fibers
3.1.3 Consist of a neuron and all the muscle fibers it innervates
3.2 Motor end plate: portion of the skeletal muscle fiber plasma membrane that synapses with the axon terminal
3.3 Cell bodies of the neurons that innervate skeletal muscle of the body are found in the ventral horn of the spinal cord
3.4 Neurons that innervate the skeletal muscle of the head are in the brainstem
3.5 Excitation of motor neurons causes acetylcholine to be released at the neuromuscular junction causing contraction of the muscle
3.5.1 Upper motor neurons = brain controls
3.5.2 Lower motor neurons = spinal cord controls (movements such as walking) Alpha motor neurons: innervate extrafusal muscle fibers - control posture and movements Gamma motor neuron: innervates intrafusal muscle fibers in the muscle spindle
3.6 Sensory receptors
3.6.1 Muscle spindle provides information about muscle lenght and the rate of change of muscle lenght
3.6.2 Golgi tendon organs detect change in muscle tension
4 Management of movements
4.1 Action potential steps
4.1.1 1. AP arrives at axon terminal
4.1.2 2. Depolarization of the membrane opens voltage-gated Ca++ channels
4.1.3 3. Increase in intracellular Ca++ at the terminal causes release of acetylcholine vesicles into the neuromuscular junction
4.1.4 4. Acetylcholine binds nicotinic channels at the motor end plate which causes them to open and allow Na+ to enter
4.1.5 5. Na+ entry triggers voltage-gated Na+ channel near the motor end plate, initiating an AP which is propagated in all directions along the plasma membrane of the muscle fiber
4.2 Reflexes
4.2.1 Muscle stretch reflex If a muscle spindle is quickly stretched, the reflex causes contractions of the muscle and nearby muscle. Signals are send to the opposing muscle to cause it to relax
4.2.2 Golgi tendon reflex If the Golgi tendon organs of a muscle are stretched and stimulated, muscle contraction is inhibited. Opposing muscle is stimulated to contract
4.2.3 Withdrawal reflexes Occurs when a part of the body is subjected to a painful stimulus Flexor reflex causes contraction of one muscle and relaxation of opposing muscle to move that portion of body away The crossed extensor reflex allows the opposite side of the body to support the body's weight or to push the body out of the way of the painful stimulus Flexors contract to close the angle at the joint. Extensors contract to open the angle at the joint
4.3 Locomotion
4.3.1 Walking and running require legs to alternate between forward flexion (swing phase) and backward extension (stance phase)
4.3.2 Repetition of the pattern is synchronized with the other legs so the two legs remain in different phases
4.3.3 Accomplished through central pattern generators: oscillatory neural circuits of lower motor neurons in the spinal cord
4.3.4 Running occurs when the brain causes the CPG to shorten the stance phase
5 Muscle types
5.1 Skeletal muscle fibers
5.1.1 Large
5.1.2 Multinuceated syncytium
5.1.3 Mediate voluntary movements of the skeleton
5.1.4 Maintain body position and posture
5.1.5 Contraction controlled by the somatic nervous system
5.1.6 Types 1. fast, glycolytic fibers fatigue quickly Used for burst of strong force, used to jump 2. fast, oxydative fibers resist fatigue Muscles used in walking 3. slow, oxidative fibers resist fatigue Muscles used for posture
5.2 Cardiac muscle fibers
5.2.1 Small
5.2.2 One or two nuclei
5.2.3 Connected to each other through gap junctions
5.2.4 Form a functional coordinated unit
5.2.5 Coordinated by the autonomic (sympathetic and parasympathetic) nervous system
5.3 Smooth muscle cells
5.3.1 Small
5.3.2 Found as bundle or sheets in the wall of blood vessels, GI tract or uterus
5.3.3 Where cells are connected by gap junctions, the bundle or sheet acts as a single coordinated unit
5.3.4 Regulated by the autonomic nervous system
5.4 Contractile proteins
5.4.1 Actin Thin filament Only actin = I bands
5.4.2 Myosin Thick filament Anywhere there is myosin = A band
5.4.3 Principles Sliding filament mechanism in which myosin filaments bind to and pull actin filament as a basis for shortening Causes the Z lines of sarcomeres to move toward the M line (center of sarcomere) ATP is required to break myosin/actin bridge. In the absence of ATP, cross bridges remain attached to actin in a state called rigor Regulation of contractile proteins by Ca++ Changes in membrane potential lead to a rise in intracellular Ca++ resulting in contraction Organized into a serie of repeating functional units called sarcomeres
6 Mechanisms of contraction
6.1 Contraction is the activation of the force generating sites. Does not necessarily mean shortening. Force generated is called tension.
6.2 Relaxation is the cessation of force generating activity and a decline in tension
6.3 In skeletal muscle, regulation of contraction occurs on the thin filament by a complex of two proteins, tropomyosin and troponin.
6.4 Binding of Ca++ to troponin causes tropomyosin to shift thereby exposing the myosin binding site on actin.
6.5 Contraction will continue in the presence of ATP and Ca++.
6.6 Relaxation occurs when Ca++ is removed
6.7 In skeletal muscles, Ca++ is stored in the sarcoplasmic reticulum - Ca++ is released in the cytoplasm in response to AP
6.8 During contraction SR CaATPase removes Ca++ from the cytoplasm and will restor cystosolic Ca++ levels in less than 30msec after the nerve impulse stops
6.9 Twitch: mechanical response of a single muscle fiber to a single action potential
6.10 Repeated stimulation of skeletal muscle will cause summation of the contractions until there is no relaxation and fused tetanus is reached
6.11 Tension and load
6.11.1 Maximal velocity of shortening occurs with no opposing load.
6.11.2 Isotonic contractions: muscle shortens and moves the load
6.11.3 Isometric contraction: muscle develops tension but does not shorten or lenghten because the opposing load equals or exceeds the force generation of the muscle
6.11.4 Maximum amount of force (tension) a muscle can generate is determined by the degree of overlap of the thick and thin filaments. Because the muscle is anchored to bone within the body, condition of excessive stretch and excessive contraction are avoided
6.12 Skeletal muscle metabolism
6.12.1 Creatine phosphate Used up in the first 10 seconds High intensity activities like 100 meters dash Stores are limited
6.12.2 Anaerobic metabolism Burns glucose and glycogen Used in the first 1.5 minutes Activities like the 400 meters dash
6.12.3 Aerobic metabolism Uses glycogen, blood glucose and fatty acids Endurance activities like marathon
6.13 Recruitment is the process of activating different types of muscle fibers within a fascicle in response to need
7 Smooth muscles
7.1 Thick filament (myosin light chain kinase) controls the contraction in smooth muscles
7.2 Hydrolyzes ATP at about 10% of the rate observed in skeletal muscle. Consequently, smooth muscle produces slow, sustained contractions
7.3 Contraction is dependant on a rise of cytosolic Ca++ due to changes in plasma membrane. Ca++ enters from the ECF by diffusion through Ca++ gated channels (voltage, ligand, mechanical)
7.4 Inputs that regulate contraction
7.4.1 Autonomic NS (parasympathetic, sympathetic and enteric) via voltage-gated Ca++ channels
7.4.2 Hormones via ligand-gated Ca++ channels
7.4.3 Stretch via mechano-gated Ca++ channels
7.5 At any one time, multiple inputs, some excitatory and others inhibitory, can be activated in a single cell
7.6 Spontaneous pacemaker potentials
7.6.1 Spontaneous contractile activity in the absence of either nerve or hormonal stimuli
7.6.2 Resting membrane potential gradually drift toward threshold where it triggers AP
7.6.3 Following repolarization, the membrane again begins to depolarize
7.6.4 Found in the GI tract
7.7 Multi-unit fibers
7.7.1 Each fiber innerved separately
7.7.2 Depolarization of one fiber = contraction of that fiber only
7.7.3 Contraction and relaxation - no stretch
7.8 Single-unit fibers
7.8.1 Depolarization of one fiber triggers synchronous depolarization throughout the bundle followed by contraction of the bundle
7.8.2 Found in small blood vessels, GI tract and uterus where stretching creates a coordinated contraction
8 Cardiac muscle
8.1 Striated fiber containing the same arrangements of contractile filaments as skeletal muscle
8.2 Conducting CM cells
8.2.1 1% of all CM cells
8.2.2 Specialized for excitation
8.2.3 Intrinsic pacemakers
8.3 Contractile CM cells
8.3.1 Slow oxydative muscle fibers
8.3.2 Form the wall of the heart
8.3.3 Shorten and produce tension
8.4 Contraction dependant on the entry of Ca++ as in skeletal muscle
8.4.1 CaATPase uses ATP to actively pump Ca++ out
8.4.2 Na+/Ca++ exchanger transport Na+ into the cell for each Ca++ moved out of the cell
8.5 AP phases
8.5.1 0: voltage-gated Na+ channels open
8.5.2 1: voltage-gated Na+ channels inactivate and voltage-gated K+ channels open
8.5.3 2: voltage-gated Ca++ channels open and voltage-gated K+ channels remain open
8.5.4 3: only K+ channels remain open and cell repolarize
8.5.5 4: All voltage-gated channels are closed and the resting membrane potential is restored by the Na+/K+ ATPase
8.6 Absolute refractory period: until Na+ channels pass from inactivated to closed, about 180msec
8.6.1 No amount of stimulus can cause an AP during that period
8.7 Relative refractory period: some Na+ channels are closed, from 180msec to 200msec
8.7.1 AP can be fired but requires greater stimulus
8.8 Refractory period = contractions cannot sum = no fused tetanus possible

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