02.Neural Lectures Prof.wilm.Neural Lectures_Prof.Wilmoreore

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NEUROLOGICAL CONTROLOF MOVEMENT

NEUROLOGICAL CONTROLOF MOVEMENT

CHAPTER 2 CHAPTER 2

Prof. Jack Wilmore

The Nervous System

Two major control systems

Neural - quick acting, short lived Hormonal - slow acting, long lived Major purposes of the nervous system

Means of communication Link to inside and outside environment

sensory receptors Activator of movement motor response


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Overview of the Nervous System

Central Nervous SystemBrain and Spinal Cord

Peripheral Nervous System

Motor DivisionSensory Division

Autonomic Somatic

Sympathetic Parasympathetic

THE STRUCTUREOF A NEURON


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An electrical charge that travels the length of the axon andpasses from one neuron to the next and/or to an end organ,such as a group of muscle fibers.

Nerve Impulse

w Difference between the electrical charges inside andoutside a cell, caused by separation of charges across amembrane to maintain a negative charge inside the cell

w High concentration of K + inside the neuron and Na + outside the neuron

w K+ ions can move freely, even outside the cell to helpmaintain imbalance

Resting Membrane Potential (RMP)

w Sodium-potassium pump actively transports 3 Na + out ofthe cell for every 2 K + into the cell to maintain imbalance

w The constant imbalance keeps the RMP at 70mV insidethe cell


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Depolarization inside of cell becomes less negativerelative to outside (> 70 mV, e.g. 55 mV)

Hyperpolarization inside of cell becomes more negativerelative to outside (< 70 mV, e.g. 80 mV, less responsive )

Graded potentials localized changes in membranepotential (either depolarization or hyperpolarization)

Changes in Membrane Potential

Action potentials rapid, substantial depolarization of the

membrane (

70 mV to +30 mV to

70 mV all in 1 ms).Results from a graded potential of at least 15 to 20 mV,depolarizing the RMP to 50 to 55 mV.

w Starts as a graded potentialw Requires depolarization greater than the threshold value

of 15 mV to 20 mV, e.g.,from 70 down to -50 to 55 mVw Once threshold is met or exceeded, the all-or-none

principle applies

What Is an Action Potential?


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RESTING STATE

AN ACTION POTENTIAL


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w Period of repolarization.

w The refractory period limits a motor unit's firing frequency.

Refractory Period

w The muscle fiber is unable to respond to any furtherstimulation.

1. The resting state

2. Depolarization

3. Propagation of an action potential

Events During an Action Potential

4. Repolarization

5. Return to the resting state with the help of the sodium- potassium pump


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Myelinated fibers w Saltatory conduction action potential travels quickly from

one break in myelin (i.e., Nodes of Ranvier), to the next.w Action potential is 5 to 50 times faster in myelinated

compared to unmyelinated axons.

The Velocity of an Action Potential

Diameter of the neuron w Larger diameter neurons conduct nerve impulses fasterw Larger diameter neurons present less resistance to

current flow ( remember FT muscle fibers !)

THE STRUCTUREOF A NEURON


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Key Points

w A neuron's RMP of 70 mV is maintainedby the sodium-potassium pump.

w Changes in membrane potential occurwhen ion gates in the membrane open,permitting ions to move from one side tothe other.

The Nerve Impulse

(continued)

w If the membrane potential depolarizes by15 mV to 20 mV the threshold is reached,resulting in an action potential.

Key Points

w Impulses travel faster in myelinated axonsand in neurons with larger diameters.

w Saltatory conduction refers to an impulsetraveling along a myelinated fiber by

jumping from one node of Ranvier to thenext.

The Nerve Impulse


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w The junction is a site where a motor neuroncommunicates with a muscle fiber.

w Motor axon terminal releases neurotransmitters (such asacetylcholine) to receptors on a muscle fiber.

w This binding causes depolarization, thus possibly causingan action potential, providing sufficient ACh is released .

The Neuromuscular Junction

w The action potential spreads across the sarcolemmacausing the muscle fiber to contract.

THE NEUROMUSCULAR JUNCTION


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Key Points

w Neurons communicate with one anotherby releasing neurotransmitters acrosssynapses.

w Synapses involve a presynaptic axonterminal, a postsynaptic receptor,neurotransmitters, and the space betweenthem.

Synapses

w Neurotransmitters bind to the receptorsand cause depolarization ( excitation ) orhyperpolarization ( inhibition ) dependingon the specific neurotransmitter and thesite to which it binds.

Key Points

w Binding of a neurotransmitter causes agraded action potential in the postsynapticmembrane.

w An excitatory impulse causeshypopolarization or depolarization.

The Postsynaptic Response

w An inhibitory impulse causeshyperpolarization.

w The axon hillock keeps a running total ofthe neuron's responses to incomingimpulses.

w A summation of impulses is necessary togenerate an action potential.


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Key Points

w Neurons communicate with muscle cells atneuromuscular junctions, which functionmuch like a neural synapse.

w The refractory period is the time it takesthe muscle fiber to repolarize before thefiber can respond to another stimulus.

Neuromuscular Junctions

w Acetylcholine and norepinephrine are theneurotransmitters most important inregulating exercise.

Key Points

w The peripheral nervous system contains43 pairs of nerves and is divided intosensory and motor divisions.

w The motor division includes the autonomicnervous system.

The Peripheral Nervous System

w The sensory division carries informationfrom the sensory receptors to the CNS.

(continued)

w The motor division carries impulses fromthe CNS to the muscles or target organs.


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Key Points

w The autonomic nervous system includesthe sympathetic and parasympatheticnervous systems.

w The sympathetic nervous system preparesthe body for an acute response.

The Peripheral Nervous System

w The parasympathetic nervous systemcarries out processes such as digestionand urination.

w The sympathetic and parasympatheticsystems are opposing systems thatfunction together.

Sympathetic Nervous System

Fight-or-flight prepares you for acute stress or physicalactivity

Facilitates your response with increases inw Heart rate and strength of heart contractionw Blood supply to the heart and active musclesw Metabolic rate and release of glucose by the liverw Blood pressurew Rate of gas exchange between lungs and bloodw Mental activity and quickness of response


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Parasympathetic Nervous System

Housekeeping digestion, urination, glandular secretion,and energy conservation

Actions oppose those of the sympathetic systemw Decreases heart ratew Constricts coronary vesselsw Constricts tissues in the lungs

Spinal cord simple motor reflexes such as pulling yourhand away after touching something hot.

Lower brain stem more complex subconscious motorreactions such as postural control.Cerebellum subconscious control of movement such asthat needed to coordinate multiple movements.

Integration Centers

Thalamus conscious distinction among sensations suchas feeling hot or cold.

Cerebral cortex conscious awareness of a signal andthe location within body of the signal.


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Types of Sensory Receptors

Mechanoreceptors respond to mechanical forces suchas pressure, touch, vibration, or stretch.

Thermoreceptors respond to changes in temperature.

Nociceptors respond to painful stimuli.

Photoreceptors respond to light to allow vision.

Chemoreceptors respond to chemical stimuli from foods,odors, and changes in blood concentrations.

Muscle and Joint Nerve Endings

w Joint kinesthetic receptors in joint capsules sense theposition and movement of joints.

w Muscle spindles sense how much a muscle is stretched.w Golgi tendon organs detect the tension of a muscle on

its tendon, providing information about the strength ofmuscle contraction.


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Muscle Spindles

w A group of 4 to 20 small muscle fibers (intrafusal) withsensory and motor nerve endings, covered by aconnective tissue sheath, and connected to extrafusal (orregular) muscle fibers.

w The middle of the spindle can stretch, but cannot contractas it contains little or no actin and myosin.

w When extrafusal fibers attached to the spindle arestretched, sensory neurons on the spindle transmitinformation to the CNS about the muscle's length.

w Reflexive muscle contraction is triggered through thealpha motor neuron to resist further stretching.

w Gamma motor neurons activate intrafusal fibers, causingthe middle of the spindle to stretch, making the spindlesensitive to small degrees of stretch.

MUSCLE SPINDLE


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GOLGI TENDON ORGAN

Did You Know?

Muscles controlling fine movements, such as thosecontrolling the eyes, have a small number of musclefibers per motor neuron (about 1 neuron for every 15

muscle fibers). Muscles with more general function, suchas those controlling the calf muscle in the leg, have manyfibers per motor neuron (about 1 neuron for every 2,000muscle fibers).


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Key Points

w Each muscle fiber is innervated by onlyone neuron, but one neuron may innervateup to several thousand muscle fibers.

w All muscle fibers within a motor unit are ofthe same fiber type.

The Motor Response

w Motor units are recruited in an orderlymanner. Thus, specific units are called oneach time a specific activity is performed;

the more force needed, the more unitsrecruited.w Motor units with smaller neurons (ST units)

are called on before those with largerneurons (FT units).

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02.Neural Lectures Prof.wilm.Neural Lectures_Prof.Wilmoreore