The Nervous System: General Principles and Sensory Physiology Organization of the Nervous System Basic Functions of Synapses, and Neurotransmitters

General design of the nervous system The central nervous system Receives information from different sensory nerves and sensory organs then integrates all these to determine response to be made by the body

NEURON Is the basic functional unit Incoming signals enters neuron through synapses mainly located on the neuronal dendrites & also on the cell body

1.SENSORY PART OF THE CNS Sensory receptors Most activities of the CNS are initiated by sensory experiences that excite sensory receptors visual receptors in the eyes Auditory receptors in the ears Tactile receptors on the surface of the body Etc These sensory experiences can either cause immediate reactions from the brain memories of the experiences can be stored in the brain for minutes, weeks, months, years determine bodily reactions at some future date

Somatosensory axis of the nervous system Information enters the nervous system through the peripheral nervous And is conducted immediately to multiple sensory areas in the Spinal cord at all levels Reticular substance of the brain Medulla pons and mesencephalon Cerebellum Thalamus Areas of the cerebral cortex

2. Motor part of the central nervous system Effectors These are actual anatomical structures that perform the function dictated by nerve signals Muscles & glands The most eventual role of the CNS is to control various body activities This is achieved by controlling Contraction of appropriate skeletal muscles thru out the body Contraction of smooth muscles in the internal organs Secretion of active chemical substances by both exocrine & endocrine glands These activities are collectively called the motor actions of the nervous system

Skeletal motor Axis of the NS For controlling skeletal muscle contraction Levels of control includes The spinal cord The reticular substance of the medulla,pons and mesencephalon The basal ganglia The cerebellum The motor cortex

Skeletal motor Axis of the NS The lower regions concerned primarily with automatic instantaneous responses to sensory stimuli The higher regions concerned with deliberate complex muscle movements controlled by the thought process of the brain

3. Processing of Information Integrative Function of the Nervous System The incoming information is processed in such a way that appropriate mental and motor responses will occur More than 99% of all sensory information is discarded by the brain as irrelevant and unimportant Important sensory information that excites the mind is channeled into proper integrative and motor regions of the brain to cause a desired response Integration is the channeling and processing of information

Role of synapses in processing of the information Is a junction point from one neuron to the next Determine the directions that the nervous signals will spread through the nervous system Synaptic transmission Can also be controlled from other areas of the CNS Facilitatory- open up synapses for transmission Inhibitory-closes up the synapses In addition some postsynaptic neurons respond with Large numbers of out put impulses Few numbers of output impulses

Roles of synapses Thus synapses can Perform a selective function Often blocking weak signals while allowing strong signals to pass Select and amplify weak signals Channel signals in many directions rather than in only one direction

General Design of the Nervous System Storage of Information (Memory) Information stored for future control of motor activities and for use in the thinking process is stored in the cerebral cortex Facilitation-each time a synapse transfer info, the synapses become more and more capable

Major Levels of CNS Function 1. Spinal Cord Level A conduit for information to travel from the periphery of the body to the brain and vice versa Can cause walking movements Withdrawal reflexes Reflexes that stiffen the legs to support the body against gravity Reflexes that control local blood vessels, G.I. movements, and urinary excretion

Major Levels of CNS Function 2. Lower Brain or Subcortical Level Control of most of the “subconscious” activities Arterial pressure and respiration Control of equilibrium Feeding reflexes Many emotional patterns (anger, excitement, sexual response, reaction to pain and pleasure)

Major Levels of CNS Function 3. Higher Brain or Cortical Level Cerebral cortex is an extremely large memory storehouse Never functions alone but in association with lower centers of the nervous system c. Essential for most thought processes

CNS Synapses Types of Synapses Chemical Almost all of the synapses in the CNS 2. First neuron secretes a neurotransmitter 3. Neurotransmitter binds to receptors on the second neuron (excites, inhibits, or modifies its sensitivity

CNS Synapses (cont.) Types of Synapses b. Electrical Have direct open fluid channels that conduct electricity from one cell to the next Have gap junctions which allow the movement of ions Very few in the CNS but are the predominant type in the periphery of the body (i.e. smooth muscle and cardiac muscle contraction)

CNS Synapses (cont.) characteristics “One-Way Conduction at Chemical Synapses Always transmit signals in one direction (from the pre-synaptic neuron (releases neurotransmitter) to the post-synaptic neuron Called the principle of one-way conduction Allows signals to be directed toward specific goals

Physiologic Anatomy of the Synapse CNS Synapses (cont.) Physiologic Anatomy of the Synapse Fig. 45.5 Typical anterior motor neuron, showing pre-synaptic terminals on the neuronal soma and dendrites

CNS Synapses (cont.) Physiologic Anatomy of the Synapse Presynaptic terminals may be either stimulatory or inhibitory (Fig. 45.5) Neurons in other parts of the spinal cord and brain differ from the anterior motor neuron in: Size of the cell body Length, number, and size of the dendrites Length and size of the axon The number of presynaptic terminals

Presynaptic Terminals CNS Synapses (cont.) Presynaptic Terminals Fig. 45.6 Physiologic anatomy of the synapse

CNS Synapses (cont.) Neurotransmitter Release From the Presynaptic Terminal The membrane of the presynaptic terminal contains large numbers of voltage gated Ca channels When the membrane depolarizes, the channels open and Ca ions flow into the terminal Quantity of transmitter released is directly related to the amount of Ca that enters Ca binds with special proteins called release sites which open and allow the transmitter to diffuse into the synaptic cleft

CNS Synapses (cont.) Action of the Neurotransmitter The postsynaptic membrane contains receptor proteins that have two components: A binding part that protrudes outward and binds the neurotransmitter, and An ionophore part that passes through to the interior of the postsynaptic neuron The ionophore is either an ion channel or a second messenger activator

CNS Synapses (cont.) Ion Channels- two types Cation- most often allow Na ions to pass, but sometimes K, and Ca also; lined with negative charges which attract cations but repel anions; opened by excitatory transmitters Anion- when channels are large enough, Cl ions pass through (cations are hydrated and too large); opened by inhibitory transmitters

Second Messenger Systems CNS Synapses (cont.) Second Messenger Systems Fig. 45.7 Second messenger systems

CNS Synapses (cont.) Second Messenger Systems- the alpha component of the G protein performs one of four functions: Opening specific ion channels through the post- synaptic membrane Activation of cAMP or cGMP Activation of one or more cellular enzymes Activation of gene transcription

CNS Synapses (cont.) Excitatory Receptors in the Postsynaptic Membrane In excitation: the opening of Na channels to allow large numbers of + electrical charges to flow to the interior. This raises the membrane potential toward threshold (most widely used method of excitation) In excitation: depressed conduction through chloride or potassium channels or both; decreases the diffusion of Cl to the inside or K to the outside which makes the membrane potential more positive Metabolic changes to excite cell activity, increase excitatory receptors or decrease inhibitory receptors

CNS Synapses (cont.) Inhibitory Receptors in the Postsynaptic Membrane Opening of chloride channels allowing the rapid influx of ions which causes the membrane potential to become more negative, and therefore inhibitory Increase in conductance of potassium ions out of the neuron allowing positive ions to diffuse to the outside causing increased negativitiy, and therefore inhibitory Activation of receptor enzymes that inhibit metabolic functions or increase the number of inhibitory receptors or decrease the number of excitatory receptors

Types of Neurotransmitters Small Molecule, Rapidly Acting Transmitters Table 45.1 Class I Class II: The Amines Class III: Amino Acids Class IV Acetylcholine Norepinephrine GABA Nitric Oxide Epinephrine Glycine Dopamine Glutamate Serotonin Aspartate Histamine

Types of Neurotransmitters Neuropeptide, Slow Acting Transmitters or Growth Factors Hypothalamic Releasing Hormones Pituitary Peptides Peptides-Act on Gut and Brain Peptides- Act on Gut and Brain From Other Tissues Thyrotropin RH ACTH Leucine enkephalin Insulin Angiotensin II Leutinizing HRH Beta-endorphin Methionine enkephalin Glucagon Bradykinin Somatostatin Alpha-MSH Substance P Carnosine Prolactin Gastrin Sleep peptides LH CCK Calcitonin Thyrotropin VIP GH Nerve growth factor Vasopressin Brain derived neurotropic factor Oxytocin Neurotensin Table 45.2

Electrical Events During Excitation Resting Membrane Potential (-65 mV for a spinal motor neuron) Fig. 45.8

Electrical Events During Excitation Concentration Difference of Ions Fig. 45.8

Electrical Events During Excitation Uniform Distribution of Electrical Potential Inside the Soma Effect of Synaptic Excitation on the Postsynaptic Membrane—Excitatory Postsynaptic Potential Effect of inhibitory Synapses on the Postsynaptic Membrane—inhibitory Postsynaptic

Electrical Events During Excitation Fig. 45.9 Three states of a neuron

Electrical Events During Excitation Generation of APs in the Initial Segment Axon hillock The membrane has 7x the voltage gated Na channels as does the membrane of the soma c. Threshold is about -45 mv (Fig. 45.9)

Electrical Events During Inhibition Effect of Inhibitory Synapses on the Postsynaptic Membrane—Inhibitory Postsynaptic Potential Inhibitory synapses open mostly Cl channels As the chloride ions enter, the membrane potential becomes more negative (toward -70 mV) Opening K channels allows the positive ions to move out; with the Cl, this causes a hyperpolarization d. Causes an IPSP (inhibitory postsynaptic potential)

Electrical Events During Inhibition Presynaptic Inhibition Release of an inhibitory substance onto the outside of the presynaptic nerve fibrils (usually GABA) Opens anion channels, allows Cl to diffuse inward Negative charges cancel much of the excitatory effect Occurs in many sensory pathways

Electrical Events During Inhibition Time Course of Postsynaptic Potentials Fig. 45.10 EPSPs

Electrical Events During Inhibition Spatial Summation- stimulation of many presynaptic terminals; the effects can summate until neuronal excitation occurs (Fig. 45.10) Temporal Summation- successive discharges from a single presynaptic terminal; if they occur rapidly enough, they also summate

Electrical Events During Inhibition Simultaneous Summation of IPSPs and EPSPs- the two effects either completely or partially nullify each other Facilitation of Neurons Occurs when the summated postsynaptic potential is excitatory but has not reached the threshold Another excitatory signal can then excite the membrane quite easily

Electrical Events During Inhibition Special Functions of Dendrites for Exciting Neurons Large spatial field of excitation of the dendrites- 80-95% of all presynaptic terminals of the anterior motor neuron terminate on dendrites Most dendrites cannot transmit APs but they can transmit signals by ion conduction of the fluids in cytoplasm

Electrical Events During Inhibition Decrement of Electrotonic Conduction in the Dendrites- Greater Excitatory or Inhibitory Effect by Synapses Located Near the Soma Fig. 45.11

Electrical Events During Inhibition Summation of Excitation and Inhibition in Dendrites Fig. 45.11

Electrical Events During Inhibition Relation of State of Excitation of the Neuron to Rate of Firing- excitatory state is the summated degree of excitatory drive to the neuron Fig. 45.12 Response characteristics of different types of neurons to different levels of excitatory state

Special Characteristics of Synaptic Transmission Fatigue of Synaptic Transmission Decrease response with increased stimulation Protection against over-excitation Exhaustion of neurotransmitter stores Progressive inactivation of postsynaptic membrane receptors Slow development of abnormal conc. Of ions inside the post synaptic neuronal cell Effect of Acidosis or Alkalosis Alkalosis-increase neuronal excitability Acidosis –depresses neuronal activity

Special Characteristics of Synaptic Transmission Effect of Hypoxia Decrease excitability Brain -unconscious Effects of Drugs Increases excitability Caffeine, theophylline, theobromine Reduce threshold of excitation Anaesthetics Increase neuronal membrane threshold for excitation

Special Characteristics of Synaptic Transmission Synaptic Delay Time consumed during transmission from presynaptic to post synaptic neuron Discharge of transmitter substance by the presynaptic terminal Diffusion of transmitter to the postsynaptic membrane Action of the NT on the membrane receptor Action of the receptor to increase the membrane permeability Inward diffusion of sodium to raise the excitatory postsynaptic potential to high enough level to elicit an action potential