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Exercise Physiology - Chapter 3

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Major divisions of the nervous system
- Central nervous system
- Peripheral nervous system
Components of the peripheral nervous system
- Sensory (affarent): incoming
- Motor (efferent): outgoing
Components of the motor neurons
- Somatic: voluntary, to skeletal muscle
- Autonomic: involuntary, to viscera
components of the autonomic
- sympathetic (fight or flight)
- parasympathetic (rest and digest)
Structure and function of the nervous system
- Basic structural unit of nervous system
- Has the same basic structure everywhere in the body
- Has three major regions
- Electrical signal for communication between periphery and brain
- Must be generated by a stimulus
- Must be propagated down an axon
- Must be transmitted to next cell in line
Three major regions of a neuron
- Cell body (soma)
- Dendrites
- Axon
Cell body
- contains nucleus
- cell processes radiate out
Dendrites
- Receiver cell processes
- Carry impulse toward cell body
Axon
- Sender cell process, starts at axon hillock
- End branches, axon terminals, neurotransmitters
Resting membrane potential
- Difference in electrical charges between the outside and inside of the cells (-70mV)
- Caused by uneven separation of charged ions
- Polarized
Why -70mV
- High [Na+] outside of cell, medium [K+] inside of cell
- Inside more negative relative to the outside
- Na+ channels closed (Na+ wants to enter the cell but can't, electrical and concentration gradients)
- K+ channels open (K+ leaves cell by a concentration gradient, offset by Na+--K+ pumps
Depolarization
- occurs when inside of cell becomes less negative
- more Na+ channels open, Na+ enters cell
- Required for nerve impulse to arise and travel
Hyperpolarization
- occurs when inside of cell becomes more negative
- more K+ channels open, K+ leaves cell
- makes it more difficult for nerve impulse to arise
Graded and Action potentials
- Depolarization and hyperpolarization contribute to nervous system function via graded and action potentials
Graded potentials
- help cell body decide whether to pass signal to axon
- can excite or inhibit a neuron
Action potentials
- Pass signal down axon
- only excitatory
Graded potentials
Localized changes in membrane potential
- Generated by incoming signals from dendrites
- Inhibitory signal = K+ efflux = hyperpolarization
- Excitatory signal = Na+ influx = depolarization
Strong GP --> AP
- Must polarize to threshold mV
- AP will be propagated down axon
- AP will be transmitted to next cell
Action potentials
- Rapid, substantial depolarization
- last 1ms
- begins as GP
AP: generating an AP
If GP reaches threshold mV, AP will occur
- -55mV
- all or none principle
-70mV--> +30mV --> -70mV
- -70mV to -55mV: depolarizing GP, Na+ influx
- -55mV to +30mV: depolarizing AP, Na+ influx
- +30mV to -70mV: repolarizing AP, K+ efflux
AP: refractory periods
- Absolute refractory period
- Relative refractory period
Absolute refractory period
- during depolarization
- neuron unable to respond to another stimulus
- Na+ channels already open, can't open more
Relative refractory period
- during repolarization
- neuron responds only to very strong stimulus
- K+ channels open (Na+ closed, could open again)
AP: propagation down axon
Myelin: speeds up propagation
- Fatty sheath around axon (schwann cells)
- not continuous (nodes of ranvier)
- saltatory conduction
- MS: degeneration of myelin
Axon diameter: larger=faster
Synapse: transmitting APs
- Junction or gap between neurons
- axon--> synapse--> dendrites
- AP can only move in one direction
- Axon terminals contain neurotransmitters
Junction or gap between neurons
- Site of neuron to neuron communication
- AP must jump across synapse
Axon--> synapse-->dendrites
- presynaptic cell-->synaptic cleft-->postsynpatic cell
- signal changes form across membrane
- electrical --> chemical --> electrical
Axon terminals contain neurotransmitters
- chemical messengers
- carry electrical AP signal across synaptic cleft
- bind to receptor on postsynaptic surface
- stimulate GPs in postsynaptic neuron
Neuromuscular Junction: A specialized synapse
- site of neuron to muscle communication
- postsynaptic cell=muscle fiber
Site of neuron to muscle communication
- uses acetylcholine (ACh) as its neurotransmitter
- excitatory: passes AP along to muscle
Postsynaptic cell=muscle fiber
- ACh binds to receptor at motor end plate
- causes depolarization
- AP moves along plasmalemma, down T-tubules
- Repolarization, refractory period
Neurotransmitters
-50+ known suspected
- two major categories
- ACh and norepinephrine (NE) govern exercise
Two major categories
- small molecule, rapid acting
- large molecule neuropeptides, slow acting
ACh and norepinephrine (NE) govern exercise
- ACh stimulates skeletal muscle contraction, meditates parasympathetic nervous system effects
- NE mediates sympathetic nervous system effects
Postsynaptic response
- Neurotransmitters trigger GPs on new cell
- Excitatory postsynaptic potential (EPSP)
- Inhibitory postsynaptic potential (IPSP)
Excitatory postsynaptic potential (EPSP)
- Depolatizing, excitatory, promotes AP
- Summation: multiple EPSPs = more depolarizing
- Reach threshold depolarization --> AP will occur
Inhibitory postsynaptic potential (IPSP)
- hyperpolarizing, inhibitory, prevents AP
- summation: multiple IPSPs = more hyperpolarizing
Central Nervous system
- brain
- spinal cord
Brain
- cerebrum
- diencephalon
- cerebellum
- brain stem
Cerebrum
-Left and right hemisphere
- cerebral cortex
Left and right hemisphere
- connected by corpus callosum, which alows interhemisphere communication
cerebral cortex
- outermost layer of cerebrum
- gray matter (non myelinated)
- conscious brain (mind, intellect, awareness)
Diencephalon
Hypothalamus
Hypothalamus
- Maintains homeostasis, regulates internal environment
-- Neuroendocrine control
-- Appetite, food intake, thirst/fluid balance, sleep
-- Blood pressure, heart rate, breathing, body temperature
Cerebellum
- Controls rapid, complex movements
- Coordinates timing, sequence of movements
- Compares actual to intended movements and initiates correction
- Accounts for body position, muscle status
- Receives input from primary motor complex, helps execute and refine movements
Sensory division: special families of sensory receptors
- Joint kinesthetic receptors
- Muscle spindles
- Golgi tendon organs
Joint kinesthetic receptors
- sensitive to joint angles, rate of angle change
- sense joint position, movement
Muscle spindles
- sensitive to muscle length, rate of length change
- sense muscle stretch
Golgi tendon organs
- sensitive to tension in tendon
- sense strength of contraction
ANS
- controls involuntary internal functions
- Exercise-related autonomic regulation
(heart rate, blood pressure, lung function)
- Two complementary divisions
Two complementary division of ANS
- Sympathetic nervous system
- Parasympathetic nervous system
ANS - sympathetic
- fight or flight: prepares body for exercise
Sympathetic stimulations
- Increase heart rate, blood pressure, blood flow to muscle, airway diameter (bronchodilation), metabolic rate, glucose levels, FFA levels, mental activity
ANS - Parasympathetic
- rest and digest (active at rest, opposes sympathetic effects)
parasympathetic stimulation
- increase digestion, urination
- conservation of energy
- decrease heart rate
- decrease diameter of vessels and airways
Sensory Motor Integration (SMI): Muscle spindles
- Specialized intrafusal muscle fibers
- When stretched , muscle spindle sensory neuron
Specialized intrafusal muscle fibers
- Different from normal (extrafusal) muscle fibers
- Innervated by y-motor neurons
- Sensory receptors for muscle fiber stretch
When stretched , muscle spindle sensory neuron
- Synapses in spinal cord with an alpha motor neuron
- Triggers reflex muscle contraction
- Prevents further (damaging) stretch
- Stretch reflex
SMI: golgi tendon organs
- Sensory receptor embedded in tendon
- When stimulated by excessive tension, golgi tendon organs
Sensory receptor embedded in tendon
- Associated with 5-25 muscle fibers
- Sensitive to tension in tendon (strain gauge)
When stimulated by excessive tension, golgi tendon organs
-Inhibit agonists, excite antagonists
- Prevent excessive tension in muscle/tendon
- Reduce potential for injury
Motor response
- alpha motor neuron carries AP to muscle
- AP spreads to muscle fibers of motor unit
- Homogeneity of motor units
AP spreads to muscle fibers of motor unit
- Fine motor control: fewer fibers per motor unit
- Gross motor control: more fibers per motor unit
Homogeneity of motor units
- Fiber types not mixed within a given motor unit
- Either type I fibers or type II fibers
- Motor neuron may actually determine fiber type