APK3110 Chapter 7: The Nervous System PDF

THE NERVOUS SYSTEM: STRUCTURE AND CONTROL OF MOVEMENT Chapter 7 Lecture Outline General Nervous System Functions. Organization of the Nervous System. Sensory Information and Reflexes. Muscle Chemoreceptors. Somatic Motor Function and Motor Neurons. Motor Control Functions of the Brain....

THE NERVOUS SYSTEM: STRUCTURE AND CONTROL OF MOVEMENT Chapter 7 Lecture Outline General Nervous System Functions. Organization of the Nervous System. Sensory Information and Reflexes. Muscle Chemoreceptors. Somatic Motor Function and Motor Neurons. Motor Control Functions of the Brain. Motor Functions of the Spinal Cord. Control of Motor Functions. Autonomic Nervous System. Exercise Enhances Brain Health. Central Nervous System (CNS) & Peripheral Nervous System Sensory nerve fibers, called afferent fibers conduct information from receptors to CNS Motor nerve fibers, called efferent fibers, conduct impulses from CNS to effector organs Anatomical divisions of the nervous system touch, pain, temperature, body position Visceral pain (angina, IBS, appendicitis) Vision, hearing, balance Skeletal muscle Glands, vascular smooth muscle, cardiac muscle  Structure of a neuron 2 1. Cell body (soma): center of operation 1 Contains the nucleus 2. Dendrites: receptive area 3 Conduct electrical impulses toward cell body 3. Axon (nerve fiber) Carries electrical impulse away from cell body towards another neuron or effector organ Nerve Axon Varies in length from a few mm to a meter Each neuron has only one axon but can be divided into collateral branches that terminate at other neurons, muscle cells or glands. In large nerve fibers like those that innervate skeletal muscle the axons are covered by Schwann cells Schwann cells form a discontinuous insulating layer (myelin sheath) along the length of axon Gaps or spaces between myelin segments along the axon called Nodes of Ranvier aid neural transduction Axons with large myelin sheath conduct impulses more rapidly than small nonmyelinated fibers. Damage of myelin results in nervous system dysfunction.  Electrical activity in neurons Neurons are an “excitable tissue” because of their properties of: Irritability: ability of dendrites and neuron cell body to respond to a stimulus and convert it to a neural impulse (=electrical signal). Conductivity: transmission of the impulse along the axon. Electrical signals are initiated by a stimulus that causes a change in normal electrical charge of the neuron. Resting membrane potential At rest the inside of cells is negatively charged relative to the charge on the exterior of the cell The negative charge is due to unequal distribution of charged ions (atoms) and it is called a resting membrane potential Negatively charged fixed ions (anions) trapped inside the cell (e.g., proteins, phosphate groups, nucleotides) and cannot penetrate the membrane These anions attract positively charged ions (cations) from the extracellular fluid Resting membrane potential Varies in cells between -5 to –100 mv In neurons it is -40 to –75 mv Magnitude of resting membrane potential is determined by the difference in ion concentrations across membrane Concentrations of Ions Across a Cell Membrane Magnitude of resting membrane potential is also determined by the permeability of plasma membrane to ions (Na+, K+) Illustration of Ion Channels Channels that regulate the passage of ions across the membrane are called ion channels. Ion channels are made of proteins that span the entire membrane from the inside to the outside surface. Ion passage is regulated by opening or closing of “gates” that serve as doors in the middle of the channel. Negative resting membrane potential in a neuron is due to primarily the diffusion of K+ out of the cell due to: 1. The concentration gradient for K+ from inside to outside of the cell 2. Higher permeability of the membrane to K+ than Na+ At rest all the Na+ channels are closed whereas a few K+ channels are open Resting membrane potential is maintained by the sodium-potassium pump The sodium-potassium pump Potassium tends to diffuse out of cell Na+/K+ pump moves 2 K+ in and 3 Na+ out  Action Potential Occurs when a stimulus of sufficient strength depolarizes the cell Opens Na+ channels, and Na+ diffuses into cell Inside becomes more positive Repolarization Return to resting membrane potential K+ leaves the cell rapidly Na+ channels close All-or-none law Once a nerve impulse is initiated, it will travel the length of the neuron without a decrease in voltage Action potential Depolarization and repolarization of a nerve fiber Check your understanding ▪ The nervous system’s major divisions: ▪ (1) the _________ nervous system ▪ ________ ▪ ________ ▪ (2) the __________ nervous system ▪ _________________ ▪ ___________ send information to the CNS concerning changes in our environment (i.e., touch, pain, temperature, and chemical stimuli ). ▪ The peripheral nervous system is divided into ______________ and ______________ divisions. ▪ The afferent division is divided into the _____________, ____________ and _____________. ▪ The efferent division is divided into ______________ and ____________. Check your understanding ▪ Nerve cells (neurons) divided into: ▪ (1) _________ ▪ (2) _________ ▪ (3) _________ ▪ Axons are generally covered by _______ , with gaps between these cells called ______________. ▪ Neurons are specialized cells that respond to _______ or _______ changes in their environment. ▪ At rest, neurons are _________ charged in the interior with respect to the electrical charge outside the cell (resting membrane potential). Check your understanding ▪ A neuron “fires” when a stimulus changes the permeability of the membrane, allowing _______ to enter at a high rate, which ___________ the cell. ▪ When the depolarization reaches threshold, an __________ or nerve impulse is initiated. ▪ Repolarization occurs immediately following depolarization due to an increase in membrane permeability to _______ and a decreased permeability to _________. Synaptic transmission Neurons communicate with each other at synapses using neurotransmitters. synaptic cleft- small gap between presynaptic neuron and postsynaptic neuron For an impulse to cross from one neuron to another it must cross the synaptic cleft at a synapse. If sufficient amount of neurotransmitters is released then synaptic transmission occurs.  Neurotransmitters and synaptic transmission Neurotransmitter Chemical messenger released from presynaptic membrane Binds to receptor on postsynaptic membrane Can be excitatory or inhibitory Excitatory transmitters: cause ↑postsynaptic membrane permeability to sodium leading to depolarization If sufficient excitatory neurotransmitter → postsynaptic neuron is depolarized to threshold → an action potential is generated  EPSPs and IPSPs Excitatory postsynaptic potentials (EPSP): series of graded depolarizations in the dendrites and cell body of postsynaptic neuron Can bring postsynaptic neuron to threshold and generate an action potential by: 1. Temporal summation: summing several EPSPs from one presynaptic neuron that is active repeatedly over a short time 2. Spatial summation: summing EPSPs from several different presynaptic neurons that are active simultaneously Inhibitory postsynaptic potentials (IPSP): cause hyperpolarization of postsynaptic membrane (i.e., ↑ negative neuron resting potential so it resists depolarization)  EPSPs and IPSPs The ratio of EPSPs and IPSPs determines if a neuron reaches the threshold for an action potential to be generated If EPSPs = IPSPs then threshold is not reached, then no action potential If EPSPs > IPSPs then threshold is reached, then an action potential is generated  Acetylcholine -Acetylcholine (Ach) can be both excitatory and inhibitory depending on receptor -in skeletal muscle → depolarization (sodium enters cell) -in heart → hyperpolarization (potassium exits cell) -neurotransmitter is degraded -acetylcholinesterase breaks acetylcholine into acetate and choline  Check your understanding ▪ Neurons communicate with other neurons at junctions called __________. ▪ Synaptic transmission occurs when sufficient amounts of a specific neurotransmitter are released from the ___________ neuron. ▪ Upon release, the neurotransmitter binds to a receptor on the ____________ membrane. ▪ Neurotransmitters can be _________ or _______. ▪ An ___________ neurotransmitter increases neuronal permeability to sodium and results in _____________________________(EPSPs). ▪ An _______________ neurotransmitter causes the neuron to become more negative (hyperpolarized). This hyperpolarization of the membrane is called an ____________________________________(IPSP). ▪ _____________ summation: summing several EPSPs from one presynaptic neuron that is active repeatedly over a short time ▪ ______________ summation: summing EPSPs from several different presynaptic neurons that are active simultaneously  Sensory information Sensory receptors are “sense organs” “change” energy into nerve impulses transmitted by sensory neurons to CNS Proprioceptors Receptors that provide CNS with information about body position Located in joints and muscles Joint proprioceptors Free nerve endings Most abundant type of joint proprioceptors Sensitive to touch & pressure At beginning of movement they are strongly stimulated They adapt slightly at first, then transmit steady signal until movement is complete Golgi-type receptors Functionally similar to free nerve endings but less abundant Found in ligaments and around joints Joint proprioceptors Pacinian corpuscles Located in tissues around joints Detect rate of joint rotation Adapt rapidly following initiation of movement Muscle proprioceptors Provide sensory feedback to nervous system regarding Muscle length and rate of shortening - muscle spindles Force development by muscle - Golgi tendon organ  Muscle spindles Respond to changes in muscle length Assist with regulation of movement and body posture Large numbers in most human locomotor muscles. Highest density in muscles that require finest degree of control (i.e., hand muscles). In muscles responsible for gross movements there are relatively few spindles Consist of intrafusal fibers Run parallel to normal muscle fibers (extrafusal fibers) Primary endings respond to dynamic change in muscle length Secondary endings provide continuous information concerning static muscle length  Structure of muscle spindles Gamma motor neurons stimulate intrafusal fibers to contract with extrafusal fibers to prevent “slack” and maintain sensitivity  Golgi tendon organs (GTO) Monitor tension developed by muscle contraction “safety device”- prevents excessive force generation and muscle damage during muscle contraction Provides a finer control over skeletal movements Located within the tendon Stimulation results in reflex relaxation of muscle Excite inhibitory neurons that send IPSPs to muscle alpha motor neurons Amount of force produced may depend on ability to voluntarily oppose GTO inhibition Strength training may gradually reduce inhibition by GTOs → greater muscle force → better sport performance The Golgi tendon organ Muscle chemoreceptors Specialized nerve endings that are sensitive to changes in the chemical environment surrounding a muscle H+ ions, CO2, and K+ Provide information to CNS about metabolic rate of muscular activity Important in regulation of cardiovascular and pulmonary responses to exercise Reflexes Rapid, unconscious reaction to sensory stimuli Not dependent on activation of higher brain centers Order of events: Sensory nerve sends impulse to spinal column Interneurons are excited and stimulate motor neurons Motor neurons control movement of muscles  Stretch reflex Rapid muscle stretching causes reflex contraction Present in all muscles but most dramatic in extensors of limbs Knee-jerk reflex Blow by rubber mallet on patellar tendon stretches entire muscle Excites primary nerve endings located in muscle spindles These nerve endings synapse with alpha motor neuron at spinal cord level Muscle fibers contract Withdrawal and Crossed Extensor Reflex 1. During the withdrawal reflex, sensory neurons from pain receptors conduct action potentials to the spinal cord. 2. Sensory neurons synapse with excitatory interneurons that are part of the withdrawal reflex. 3. The excitatory interneurons that are part of the withdrawal reflex stimulate alpha motor neurons that innervate flexor muscles, causing withdrawal of the limb from the painful stimulus. 4. Collateral branches of the sensory neurons also synapse with excitatory neurons that cross to the opposite side of the spinal cord as part of the crossed extensor reflex 5. The excitatory interneurons that cross the spinal cord stimulate alpha motor neurons supplying extensor muscles in the opposite limb, causing them to contract and support body weight during the withdrawal reflex. Check your understanding ▪ _________________ are position receptors located in joint capsules, ligaments, and muscles. ▪ Joint proprioceptors include: ▪ ________________ ▪ ________________ ▪ ________________ ▪ Proprioceptors provide information on ____________ of body parts and the ____________ of limb movement. ▪ The ____________ functions as a length detector in muscle. ▪ ____________ continuously monitor the tension developed during muscular contraction (safety device: prevent excessive force during muscle contractions). Check your understanding ▪ ___________ _______________ ▪ are sensitive to changes in the chemical environment surrounding muscle ▪ send information back to the CNS about the ___________ rate of muscular activity. This information play a role in the regulating the cardiovascular and pulmonary response to exercise. ▪ _________ provide the body with a rapid, unconscious means of reacting to some stimuli (e.g., painful stimuli). Somatic motor function Somatic motor neurons of PNS Carry neural signals from spinal cord to skeletal muscles to contract Somatic motor function Motor neuron: also called an alpha motor neuron is the somatic neuron that innervates skeletal muscle fibers The cell body of motor neurons is located in the spinal cord The axon leaves the spinal cord and splits into collateral branches; each branch innervates a single muscle fiber. Motor unit: motor neuron and all the muscle fibers it innervates Innervation ratio Innervation ratio: number of muscle fibers/motor neuron This ratio varies from muscle to muscle Low ratio in muscles that require fine motor control 23/1 in extraocular muscles responsible for eye movement Higher ratio in other muscles 1,000/1 or greater in large muscles (e.g., leg muscles)  Motor unit recruitment and size principle Activation of a single motor neuron leads to contraction of all the muscle fibers it innervates. Activation of a single motor unit results in a weak muscle contraction (i.e., limited force production). To increase muscle force production more motor units must be recruited. Motor unit recruitment: progressive activation of more and more muscle fibers by the successive recruitment of additional motor units. Size principle: orderly and sequential motor unit recruitment. Smallest motor units recruited first.  Main types of motor units Type S (slow): small motor neurons innervate slow and high oxidative muscle (type I) fibers → smallest motor units Type FR (fast, fatigue resistant): larger motor neurons innervate the intermediate muscle fibers (type IIa) → intermediate motor units Type FF (fast, fatigable): largest motor neurons innervate the fast muscle fibers (type IIx) → largest motor units Incremental tests First stage low level muscle force production needed slow type S motor units recruited As test progresses, to produce more muscle force, more and more type S motor units are recruited and eventually type FR motor units are recruited. As the test becomes more difficult, to increase muscle force production, type FF motor units are recruited.  Brain stem Located inside the base of scull just above the spinal cord Major structures: Midbrain Pons Medulla oblongata Reticular formation (neurons scattered throughout the brain stem) Receives & integrates information from all regions of the CNS Works with higher brain centers in controlling muscular activity Responsible for: Many metabolic functions Cardiorespiratory control Complex reflexes Control of eye movement and muscle tone, equilibrium, maintenance of upright posture Damage of brain stem results in impaired movement control Cerebrum Cerebral cortex Stores learned experiences Receives sensory information Organizes complex movement  Motor cortex Portion of cerebral cortex that is most concerned with: Motor control Voluntary movement Final relay point Receives information from subcortical structures (e.g., cerebellum) Information is summed Final movement plan is formulated Motor commands are sent to spinal cord Movement plan can be modified by subcortical and spinal centers which supervise the fine details of the movement  Cerebellum Coordinates and monitors complex movement Incorporates feedback from proprioceptors cerebellum Has connections to: Motor cortex Brain stem Spinal cord May initiate fast, ballistic movements Damage to cerebellum results in: Poor motor control Muscular tremor (especially during rapid movements) Motor functions of the spinal cord Details of movement are refined in spinal cord via interaction of spinal neurons with higher brain centers. Spinal tuning: spinal mechanism by which a voluntary movement is translated into appropriate muscle action. Control of motor function Motor cortex does not give initial signal to move First step of voluntary movement occurs in subcortical and cortical motivation areas Association areas of cortex form a “rough draft” of the movement Cerebellum and basal ganglia (nuclei) Convert “rough draft” into movement plan Cerebellum: fast movements Basal ganglia: slow, deliberate movements Movement plan send to motor cortex through thalamus Motor cortex forwards message down spinal neurons for “spinal tuning” and finally to muscles Feedback from muscle proprioceptors allows fine tuning and improvement of motor program  Check your understanding ▪ The brain can be subdivided into three parts: (1) _______________, (2) _____________, and (3) _____________. ▪ The _________ cortex controls motor activity with the aid of input from ___________ areas. ▪ Evidence exists that the spinal cord plays an important role in voluntary movement with groups of neurons controlling certain aspects of motor activity. ▪ The spinal mechanism by which a voluntary movement is translated into appropriate muscle action is called ____________. Check your understanding ▪ Control of _________________ is complex and requires the cooperation of many areas of the brain as well as several subcortical areas. ▪ The first step in performing a voluntary movement occurs in __________ and ____________ motivational areas, which send signals to the ______________, which forms a “rough draft” of the planned movement. ▪ The movement plan is then sent to both the ___________ and the ______________. These structures cooperate to convert the “rough draft” into precise temporal and spatial excitation programs. Check your understanding ▪ The __________ is important for making fast movements, while the _____________ are more responsible for slow or deliberate movements. ▪ From the ___________ and ___________, the precise program is sent through the thalamus to the ___________, which forwards the message down to spinal neurons for “_____________” and finally to skeletal muscle. ▪ Feedback to the CNS from the _________ proprioceptors allows the modification of motor programs, if necessary. Autonomic nervous system Responsible for maintaining internal environment Innervates effector organs not under voluntary control Smooth muscle in blood vessels/airways/gut, cardiac muscle, and glands Sympathetic division Releases norepinephrine (NE) Tends to activate an effector organ (e.g., increases heart rate) Parasympathetic division Releases acetylcholine (ACh) Tends to inhibit an effector organ (e.g., slows heart rate) Most organs receive dual innervation from both sympathetic and parasympathetic branches Activity of an organ is regulated by the ratio of sympathetic/parasympathetic impulses to the tissue During exercise, activity of PNS decreases and SNS increases Sympathetic nervous system Ganglia: group of cell bodies outside of the CNS Sympathetic division: Cell bodies of sympathetic preganglionic neurons are located in thoracic and lumbar regions of the spinal cord Preganglionic fibers leave spinal cord and enter sympathetic ganglia Preganglionic fibers release acetylcholine Postganglionic fibers leave sympathetic ganglia and innervate effector tissues Postganglionic fibers release norepinephrine which binds on alpha and beta adrenergic receptors on the membrane of target organs. Following stimulation norepinephrine is removed from synapse: taken up by the postganglionic fiber broken down into inactive byproducts by enzymes (e.g., monoamine oxidase)  Parasympathetic nervous system Cell bodies of parasympathetic preganglionic neurons are located within the: Brain stem Sacral portion of spinal cord Parasympathetic preganglionic fibers leave brain stem and spinal cord and enter parasympathetic ganglia Both preganglionic and postganglionic fibers release acetylcholine After parasympathetic nerve stimulation acetylcholine is released and rapidly degraded by the enzyme acetyl-cholinesterase.  Exercise enhances brain health Exercise improves brain function and ↓ the risk of impairment with aging Regular exercise can protect the brain against: Disease (Alzheimer’s) Certain types of brain injury (stroke) How does exercise enhance brain health? Enhances learning and memory Stimulates formation of new neurons Improves brain vascular function and blood flow Attenuates depression Reduces peripheral factors for cognitive decline Inflammation, hypertension, and insulin resistance Check your understanding ▪ The __________________________ is responsible for maintaining the constancy of the body’s internal environment. ▪ Anatomically and functionally, the autonomic nervous system can be divided into two divisions: (1) the _______________ division and (2) the ________________ division. ▪ In general, the _____________ portion (releasing ______________) tends to excite an organ, while the ________________ portion (releasing ______________) tends to inhibit the same organ. ▪ Research indicates that exercise can improve __________ function, particularly in older individuals.