12-The Nervous System. Taft College Human Physiology

Transcription

1 12-The Nervous System Taft College Human Physiology

2 Introduction To The Nervous System The nervous system is a wired system with discrete pathways (nerves) and local actions. The effects of nervous stimulation are usually immediate and short lived. E.g. muscle movement The nervous system in conjunction with the endocrine system is responsible for coordination of all of the other human body systems.

3 Introduction To The Nervous System The endocrine system is specialized to control activities that require duration (time) not speed. Hormones are secreted into the bloodstream and have wide ranging effects on target cells that contain hormone receptors. E.g. growth patterns, reproduction cycles, metabolism, water balance

4 Functions of the Nervous System 1. Sensory: via afferent neurons Millions of sensory (afferent) receptors monitor changes both inside and outside the body. The changes are called stimuli and the information gathered sensory input. Examples: Internal: stretching of stomach, ph changes in blood. External: Smells, sights, sounds, pressure, pain. 2. Integration: via interneurons It analyzes sensory inputs, stores some information, and makes decisions about what should be done based on the information. 3. Motor: via efferent neurons It may cause a response by activating effector (target )organs: causing muscles to contract and glands to secrete. This response would be called motor output.

5 Afferent (sensory) Efferent (motor) Interneurons ( linker ) 3 Types of Neurons

6 Functions of the Nervous System 4. Conceptual thought Capacity to record, store, and relate information received and use it at a later date. Like on Exam 2 coming up shortly!!! A high level of self awareness. The human brain represents the peak of development of animal brains. This trend has enabled such adaptive capacities as learning, planning, speech, and language.

7 Classification of the Nervous System It is very important that you understand which divisions of the nervous system are anatomical structures (i.e. a structure you would actually see during the course of a dissection or operation) and which nervous system terms are based on function that is, how it works.

8 Anatomical Classification of the N.S. Nervous System Central Nervous System Peripheral Nervous System 1. Brain 1. Nerves 2. Spinal Cord 2. Ganglia 3. Sensory Receptors

9 Anatomical Classification of the N.S. From an anatomical perspective the nervous system has 2 major divisions: 1. The Central Nervous System = CNS. 2. The Peripheral Nervous System =PNS (drawing on board) The Central Nervous system consists of the brain and the spinal cord and nothing else! Any other nervous system structure that connects to the brain or spinal cord would be a part of the Peripheral Nervous system.

10 Anatomical Classification of the N.S. The 3 major components of the peripheral nervous system are: 1. Nerves 2. Ganglia 3. Sensory receptors

11 3 Major Components of the Peripheral N.S. 1. Nerves A nerve is a bundle of neurons = nerve cells. Nerves can be motor, sensory or mixed. 2 important categories of nerves that are a part of the PNS: Cranial nerves (12 pr.) that branch to and from the brain.» You need to know these for exam 2. See the detail in the course outline.» For each nerve, know number, name, whether it is sensory or motor or both (mixed), and it s function. Spinal nerves (31 pr.) that exit the spinal cord bilaterally from between each vertebrae.

12 Cranial nerves For each cranial nerve you must know the Roman numeral, name, whether it is sensory (S) or motor (M) or both (B) (mixed), and function. # Name S,M, or B Function I Olfactory S smell II Optic S vision III Oculomotor M eye muscles IV Trochlear M eye muscles V Trigeminal B S- face & scalp M- muscles of mastication VI Abducent M eye muscles VII Facial B S- tongue & taste M- facial muscles VIII Vestibulocochlear S hearing & equilibrium IX Glossopharyngeal B S- tongue & taste General sensation of pharynx M- pharyngeal muscles(swallowing) X Vagus B S- visceral sensation M- visceral movement XI Accessory M swallowing, head & shoulder movement XII Hypoglossal M tongue- speech & swallowing

13 3 Major Components of the Peripheral N.S. 1. Nerves 2. Ganglia aggregations of nerve cell bodies. E.g. Dorsal root ganglia. 3. Sensory receptors (sense organs) Serve to give the body information about the immediate environment, both internal and external. They include the special sense organs involved in your sense of taste, touch, sight, hearing, or smell.

14 Functional Classification of the NS Nervous System CNS Integration Afferent Division Efferent Division (sensory division) (motor division) Involuntary or Voluntary Control Autonomic Somatic (involuntary) (voluntary) Sympathetic Parasympathetic Stress Calm Afferent: brain Efferent: brain

15 Afferent Division The afferent division of the nervous system is responsible for carrying information toward the brain. The afferent division is also called the sensory division as it picks up information about the environment and takes that information to the brain (CNS). Efferent Division The efferent division of the nervous system is responsible for carrying information out and away from the brain. The efferent division is also called the motor division The efferent division is further divided into 2 parts based on whether the information coming from the brain is under voluntary or involuntary control.

16 1. Somatic Nervous System = Somatic Division The somatic portion of the efferent division is under voluntary control and is composed of somatic motor neurons. It is the part of the nervous system that serves to innervate skeletal muscle tissue. 2. Autonomic Nervous System (ANS) You do not have control over the information that passes through the ANS and is sometimes referred to as the automatic or involuntary division. The specific tissues that are innervated by visceral motor nerves of the autonomic nervous system are: E.g. Smooth muscle, cardiac muscle, and glands.

17 2 Divisions of the Autonomic Nervous System The 2 divisions of the autonomic nervous system are: a. The Parasympathetic division. b. The Sympathetic division.

18 2 Divisions of the ANS Parasympathetic Division The parasympathetic division attempts to conserve body resources. Restore after exertion. It is primarily dominant under calm conditions rest and digest. SLUDD = salivation, lacrimation, urination, digestion, defecation. The 3 Decreases: heart rate, diameter of airway, diameter of pupils.

19 2 Divisions of the ANS Sympathetic Division The sympathetic division dominates under conditions of stress. It serves to prepare for the quick utilization of body resources. The sympathetic division is said to be the division that prepares you for fight or flight. Supports vigorous physical activity and ATP production. E situations: exercise, excitement, emergency, embarrassment. Aided by hormone (E and NE) release from adrenal medulla. Dilates eyes, increases heart rate and contractions, increase BP, dilate airway, increase blood to skeletal and cardiac muscle, digestion inhibited.

20 Electrical Properties of Cells If you were to stick an electrode into any cell of the body and compare it to the outside of the cell you would be able to measure an electrical potential. ECF = extracellular fluid ICF = intracellular fluid ECF ICF Volt meter - 70 mv This electrical potential is produced by differences in concentration of ions (ion imbalance) on either side of the cell membrane. Your book calls it a membrane potential.

21 The resting potential is determined mainly by 2 cations: Na+ and K+. If the positive and negative ions are in balance on each side of the membrane, the voltage is zero (0). Since such small amounts of electricity are involved the voltage is measured in millivolts = 1/1000th of a volt. At rest there are more positive ions outside the cell than inside so the resting potential of the cell is given as negative voltage ( average -70 mv, range 40 to 90 mv). Voltage mv zero -70 Resting Potential By convention the voltage inside the cell (intracellular fluid ICF) is compared to the voltage out the cell (extracellular fluid ECF) therefore, negative ( ) voltage.

22 Mechanisms of Establishment of Membrane Potentials There are 2 basic processes that may lead to an unequal distribution of charges (ionic imbalance) on either side of the membrane. 1. Differential permeability of membranes to specific ions moving by diffusion. 2. Active transport (Na + /K + pump) (Na + pumped out & K + pumped in)

23 Mechanisms of Establishment of Membrane Potentials 1. Differential Permeability of Membrane. There is leakage of ions through protein channels in the membrane that are always open. Since they are always open they are called leakage or nongated ion channels. The major cations involved here are sodium (Na+) and potassium (K+). The cell membrane has more K+ than Na+ leakage channels and is therefore more permeable to K+ than Na+ (permeability of K x >Na+). Therefore, Na+ ions are found primarily outside the cell. K+ ions are found primarily inside the cell but tend to leak out due to diffusion through the leaky membrane. It is due to the leakyness of the K+ ions that the inside of the cell is less positive and said to be slightly negative. Positive outside Higher Na Higher K+ Cell Less positive or negative inside cell

24 Mechanisms of Establishment of Membrane Potentials K+ ions can t leak out forever. There must be something that serves to replace them. 2. Active Transport, the Na+/K+/ATPase pump. Through active transport there is a K+ pump/ Na+ pump. The Na+/K+ pump serves to move K+ ions into the cell and to pump Na+ ions out of the cell. It requires ATP to provide energy for the pump. For every 3 Na+ ions moved out, 2 K+ ions are moved in. Since K+ diffuses out of the cell easily, the net effect of the pump is to remove Na+ from the cell.

25 Although many different ions are found in the ECF and ICF, the resting potential is determined mainly by 2 cations: K + and Na +. The lipid bilayer acts as an effective insulator. Cell membrane is more permeable to K + than to Na + Permeability K + > Na + Na + found primarily outside of cell K + found primarily inside cell but tend to leak out due to permeability which makes the inside of the membrane slightly negative.

26 Mechanisms of Establishment of Membrane Potentials The combination of the passive forces of diffusion through a semipermeable membrane (leaky channels) and the active force of active transport Na+/K+ pump), there is an unequal distribution of ions leading to a membrane potential. This type of membrane potential is called a resting potential as the nerve cell is at rest. It remains this way as long as there is no stimulation. The membrane is said to be polarized Resting nerve cell or fiber, with polarized membrane. Let s compare a resting nerve fiber with a stimulated nerve fiber.

27 Nerve Impulse & Action Potential What happens when membrane is stimulated (say during nerve excitation)? Step 1: becomes highly permeable to Na + Na + rushes in due to the concentration gradient & membrane is depolarized = action potential _ + _ + _ + _ + _ + _ + _ + _ + + _ + _ + _ + _ + _ + _ + _ Stimulated nerve fiber. Shows (+) ion attracted to consecutive ( ) charges and moves along fiber = impulse

28 Nerve Impulse & Action Potential Step 2: membrane regains original permeability K + renews outward movement and resting potential is restored repolarization (in that area of the membrane _ Resting potential is restored but (+) charge will depolarize adjacent part of membrane and action potential will take place along entire fiber step-by-step = nerve impulse. Movement of the nerve impulse along the nerve= Propagation

29 Action potential can be graphed as follows: An action potential or impulse consists of depolarization and repolarization. It is a sequence of events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). Na+ In K+ out Step 1 Step 2

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32 Resting Potential = the state of a neuron when it is not conducting an impulse. Action Potential = the state of a neuron when it is conducting an impulse. Action potential consists of depolarization followed by repolarization.

33 Effects of Chemicals and Drugs on Nerve Cell Membranes DDT: One of the reasons the pesticide DDT is so dangerous is that it increases the nerve cell membrane s permeability to Na+ ions. This causes spontaneous action potentials to occur all of the time. This seriously disrupts nerve cell transmission of information. This is how it kills insects! In humans, too much DDT effects the diaphragm and results in respiratory arrest. Local Anesthetics: Lidocaine and Novacaine have the opposite effect of DDT. They serve to and decrease the permeability of the membrane to Na+ and prevent action potentials. This serves then to numb the localized area.