Chapter 7. The Nervous System: Structure and Control of Movement

Transcription

1 Chapter 7 The Nervous System: Structure and Control of Movement

2 Objectives Discuss the general organization of the nervous system Describe the structure & function of a nerve Draw and label the pathways involved in a withdraw reflex Define depolarization, action potential, and repolarization

3 Objectives Discuss the role of position receptors in the control of movement Discuss the brain centers involved in voluntary control of movement Describe the structure and function of the autonomic nervous system

4 General Nervous System Functions 1. Control of the internal environment Nervous system works with endocrine system 2. Programming spinal cord reflexes 3. Voluntary control of movement 4. Assimilation of experiences necessary for memory and learning

5 Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Peripheral nervous system (PNS) Neurons outside the CNS Sensory division Motor division Fig 7.1

6 Organization of the Nervous Sensory division System Afferent fibers transmit impulses from receptors to CNS Sensation-heat, light, touch, smell, pressure Blood and lymph vessels, internal organs Skin, muscles, tendons Motor division Efferent fibers transmit impulses from CNS to effector organs Results in appropriate responses Muscle contraction Reflex activity

7 Divisions of the Nervous System Fig 7.1

8 Relationship Between PNS and CNS Fig 7.2

9 Structure of a Neuron Cell body - Soma Dendrites: receptor areas, conduct impulses toward cell body Axon-nerve nerve fiber Carries electrical impulse away from cell body toward another neuron, organ Length few millimeters to a meter

10 Basic Nerve Structure Neuron Cell body (soma) Dendrites Axon A. Motor neuron Away from soma B. Sensory neuron Toward soma

11 Basic Nerve Structure Large nerve fibers Skeletal muscles Myelin sheath Composed of fat and protein Segments separated by spaces-nodes of Ranvier

12 Structure of a Neuron Axon-nerve nerve fiber May be covered by Schwann cells Forms discontinuous myelin sheath along length of axon Gaps Nodes of Ranvier Nerve impulse is propagated-bounces from node to node Saltatory conduction-increases increases conduction velocity Large myelinated fibers-skeletal skeletal muscle 60 to 100 m/sec (135 to 225 miles/hr) Non-myelinated fibers-mixed in with myelinated 6 to 10 m/sec

13 Structure of a Neuron

14 Electrical Activity in Neurons Neurons are Excitable Tissue Irritability: : ability to respond to a stimulus and convert it to a neural impulse Conductivity: transmission of the impulse along the axon

15 Resting Membrane Potential All cells Negatively charged inside cell Negative charges inside cell (anions) Proteins, phosphate groups, nucleotides Attract positively charged ions (cations( cations) ) ECF Positively charged outside cell in relation to inside Most important Na + K + Cl -

16 Ion Concentrations Ion ECF Sodium mmoles/l ICF /L 15 Chloride Potassium Ion [ ] set up potential difference across membrane Uneven distribution of charged ions Resting membrane potential

17 Resting Membrane Potential Membrane is semipermeable Gates in membrane regulate entry and exit Changes in concentration brought about by change in permeability to anions Change in permeability Concentration gradient high to low Na in and K out

18 Resting Membrane Potential Resting membrane potential A few K gates are open, Na gates closed K leaks out Maintains negative charge inside cell RMP maintained by Na/K pump (3 Na out/2 K in)

19 Action Potential Sufficient stimulus Opens Na + gates Depolarization - inside becomes more + Threshold Opens Na + gates wider Starts action potential Nerve impulse Propagated by ion exchanges along axon

20 Action Potential Brief increase in K + permeability-out Inside becomes more negative Na + gates close No more + entering cell Repolarization Return to resting

21 Electrical Activity in Neurons Resting membrane potential Neural message Stimulus Raises RMP to threshold which opens Na + gates

22 An Action Potential Action potential Permeability of the membrane changes, Na + gates open interior positively charged Action potential (nerve impulse) occurs Propagated

23 Depolarization

24 Repolarization Repolarization Brief increase in K + permeability K + leaves cell-returning inside to negative Stimulus removed Na + gates close-entry entry into cell is slowed RMP restored

25 Repolarization Fig 7.6b

26 Synaptic Transmission Neurons communicate across synapses using neurotransmitters Released from presynaptic membrane Binds to receptor on post synaptic membrane

27 Synaptic Transmission Synapse: Contact points between axon of one neuron and dendrite of another neuron Nerve to nerve Nerve to muscle- Neuromuscular junction Myoneural junction Motor end plate

28 Synaptic Transmission

29 Basic Structure of a Chemical Synapse

30 Synapse-two neurons Axon terminals of presynaptic neuron Synaptic cleft-space Receptors of second neuron Vesicles release transmitter substance NT diffuses across cleft and impulse continues Response?

31 Neuron Response Type of transmitter released Excitatory-cause cause depolarization of membranes Acetylcholine-neuron neuron to muscle Norepinephrine-peripheral peripheral sympathetic nerves Adrenergic receptors Acetylcholine-peripheral parasympathetic nerves Cholinergic receptors Inhibitory-cause hyperpolarization of membranes Gamma-aminobutyric aminobutyric acid (GABA)-spinal cord/brain Glycine-spinal cord

32 Synaptic Transmission Excitatory postsynaptic potentials (EPSP) Causes depolarization which may or may not reach threshold Sufficient amounts of NT causes depolarization to threshold Action potential is generated

33 Synaptic Transmission Excitatory postsynaptic potentials (EPSP) Temporal summation: Timing is important Successive discharges from the same terminal Summing several EPSPs from one presynaptic neuron

34 Synaptic Transmission Spatial summation: summing from several different presynaptic neurons Additive effect of several stimuli

35 Stimulus Inhibitory postsynpatic potentials (IPSP) Hyperpolarization NT inhibits response Inhibitory postsynaptic potential (IPSP)

36 Sensory Information Proprioceptors Proprioception: ability to determine position of joint Kinesthesia: sensation of joint motion or acceleration (rate of movement) Muscle Chemoreceptors Sensitive to changes in the chemical environment surrounding a muscle

37 Proprioceptors Provide CNS with information about body position and joint angle Free nerve endings touch & pressure Golgi-type receptors in ligaments & joints Pacinian corpuscles in tissues around joints Strongly stimulated at beginning of movement then adapt Steady signal until move is completed

38 Proprioceptors Joint receptors Provide body with information of orientation of body parts Feedback about rates of limb movement

39 Muscle Chemoreceptors Provide CNS with information regarding the metabolic rate of muscular activity Hydrogen ion concentration Carbon dioxide (CO 2 ) Potassium (K + ) Chemoreceptor feedback may regulate cardiovascular and pulmonary exercise responses

40 Reflexes Rapid, unconscious means of reacting to stimuli Not dependent on higher brain centers Rapid removal from pain source

41 Reflexes Pathway for neural reflex: 1.Sensory nerve sends impulse to spinal column 2.Interneurons activate motor neurons 3.Motor neurons control movement of muscles Reciprocal inhibition EPSPs to muscles to withdraw from stimulus IPSPs to antagonistic muscles

42 A Reflex Arc Illustrating Reciprocal Inhibition

43 Somatic Motor Function Somatic (outer, nonvisceral) ) motor neurons of PNS Responsible for carrying neural messages from spinal cord to skeletal muscles Alpha motor neurons Cell body in spinal cord Axon splits into collateral branches, each innervates muscle fiber - Neuromuscular junction Fig 7.9

44 Somatic Motor Function Motor unit Motor neuron and all the muscle fibers it innervates Activation of single motor neuron All or none law All muscle fibers contract

45 Somatic Motor Function Innervation ratio Number of muscle fibers per motor neuron Low ratio Fine motor control Eye movement High ratio Gross motor control Leg muscles

46 Illustration of a Motor Unit Fig 7.9

47 Vestibular Apparatus and Equilibrium Located in the inner ear (Semi-circular canals) Responsible for maintaining general equilibrium and balance Sensitive to changes in linear and angular acceleration

48 Motor Control Functions of the Brain Stem Medulla Pons Midbrain Reticular formation Cerebrum Cerebral cortex outer layer Cerebellum Brain

49 Motor Control Functions of the Cerebrum Cerebral cortex Brain Organization of complex movement Storage of learned experiences Reception of sensory information Motor cortex Most concerned with voluntary movement

50 Motor Control Functions of the Cerebellum Brain - Monitors complex movement - Aid in control of movement in response to feedback from proprioceptors

51 Autonomic Nervous System Responsible for maintaining internal environment Effector organs not under voluntary control Smooth muscle, cardiac muscle, glands Sympathetic division Releases norepinephrine (NE) Excites an effector organ Parasympathetic division Releases acetylcholine (ACh) Inhibits effector organ

52 Motor Integration