Nervous System Part 3 Integration Control Nervous Tissue

Nervous System Part 3: Integration & Control

Nervous Tissue One of the 4 basic tissue types in the human body. n 4 Functions: Known as the 4 C’s n ¨ Communicates ¨ Commands ¨ Controls ¨ Coordinates

Types of Nervous Tissue Functions n Communication: Occurs through action potentials. ¨ n n n Action potentials: Electrical impulses triggered by ion flux. Sensory Functions: Conveying information from external and internal stimuli to the CNS via afferent neurons. Integrative Functions: Analyzing & storing sensory information; deciding on appropriate responses via interneurons. Motor Functions: Responding to integrative decisions by carrying impulses from the CNS to the effectors in the organs via efferent neurons.

Divisions of the Nervous System Central Nervous System: Consists of the brain and spinal cord. n Peripheral Nervous System: Consists of all nervous tissue outside the brain & spinal cord – 2 subdivisions: n ¨ Autonomic Nervous System (ANS) n Sympathetic Nervous System n Parasympathetic Nervous System n Enteric Nervous System ¨ Somatic Nervous System (SNS)

Central Nervous System (CNS) n Structures: ¨ Brain ¨ Spinal n Corn Both will be talked about in detail in Chapter 11!

Peripheral Nervous System (PNS) Consists of all nervous tissue located outside of the CNS. n Includes Ganglia, which are small masses of nervous tissue. n Subdivisions: n ¨ Autonomic Nervous System (ANS) n Sympathetic Nervous System n Parasympathetic Nervous System n Enteric Nervous System ¨ Somatic Nervous System (SNS)

Autonomic Nervous System (ANS) Consists of sensory neurons that pass information from the autonomic sensory receptors in the visceral organs & motor neurons to the CNS and back. n 2 Major Branches: n ¨ Sympathetic Nervous System: Triggers fight -or-flight response. ¨ Parasympathetic Nervous System: Returns body to homeostasis when threat has passed.

Enteric Nervous System (ENS) Considered a “small brain” since it has so many neurons. n Some debate about its classification is still continuing. Right now it is classified as a subdivision of the peripheral nervous system, but receives most of its innervations from the autonomic nervous system so is also classified as a subdivision of the ANS. n

Somatic Nervous System (SNS) n Consists of the… ¨ Sensory neurons passing information from receptors to the sensory areas of the CNS ¨ Motor neurons passing impulses from the SNS to the skeletal muscles.

Types of Neurons n 3 Basic Types of Neurons: ¨ Sensory Neurons aka Afferent Neurons: Detect stimuli & carry impulses from the cranial or spinal nerves to the CNS. ¨ Motor Neurons aka Efferent Neurons: Carry impulses away from the CNS from the brain to the spinal cord and PNS. ¨ Interneurons aka Association Neurons: Located inside the CNS and perform nervous system integrative functions.

Properties of Neurons n Properties shared by all neurons: ¨ Excitability: Ability to respond to stimuli & changes in the environment that excites a sensory receptor, neuron, or muscle fiber. ¨ Conductivity: The ability to produce and respond to electrical signals to aid in communication. ¨ Secrete: The ability to secrete chemical neurotransmitters to aid in communication.

The Nerve Cell n 3 Basic Parts: ¨ Dendrites ¨ Soma ¨ Axon

The Nerve Cell n Dendrites: Consists of many branches coming out of the soma of the cell. ¨ Function to receive information from the synaptic clef. ¨ Process: Fibers that run to dendrites and from axons.

The Nerve Cell n Soma: The cell body consisting of a central nucleus surrounded by cytoplasm & containing standard organelles. ¨ Nissl Bodies: Clusters of endoplasmic reticulum found in the soma.

The Nerve Cell n Axon: The long, single-stranded portion of the neuron exiting from the opposite end of the soma from the dendrites. ¨ Functions to transmit information to the synaptic clef. ¨ Axon Hillock: The point of connection between the axon to the soma. ¨ Axon Terminals: The tiny branches dividing off at the end of the axons. ¨ Axoplasm: The cytoplasm of the axon.

The Nerve Cell n Myelin Sheath: Thin layer of fat cells around the axon that greatly increases the speed of neural transmission. ¨ Produced by Schwann Cells and Oligodendrocytes. ¨ Nodes of Ranvier: Gaps in the myelin sheath that occur at regular intervals to help impulses “skip” between sections & therefore travel faster. n Salutatory Conduction: The process of signals “skipping” across the nodes of Ranvier to increase transmission speed.

Classification of Neurons n Based on myelin: ¨ Myelinated: Neurons that have a myelin sheath. ¨ Unmyelinated: Neurons that do not have a myelin sheath. n Based on number of axons & dendrites: ¨ Multipolar: Neurons that have a single axon and several dendrites – the most common! ¨ Bipolar: Neurons with a single axon and single dendrite – found in the eye, inner ear, and olfactory region of the brain. ¨ Unipolar: Neurons where the axon and dendrite have fused into a single process (axon) that branches in two directions. Primarily serves as a sensory neuron.

Types of Matter n Gray Matter: Contains mainly nerve cell bodies and unmyelinated fibers. ¨ Found in the outer layer of the brain. ¨ Found in the inner layer of the spinal cord. n White Matter: Contains mainly myelinated axons and appears to be white. ¨ Found on the inside of the brain. ¨ Found on in the outer later of the spinal cord. Cerebeluum

Synapses Synapse: The point at which one neuron communicates with another. n Synaptic Cleft: The slight gap between two neurons where the synapse occurs. n Presynaptic Neuron: The neuron transmitting the signal through its axon. n Postsynaptic Neuron: The neuron receiving the signal through its dendrites. n

Synapses n n Two types of synapses exist, named for their method of communication transmission. Electrical Synapses: Electrical signals are directly transmitted between cells. ¨ Gap Junctions convey electrical signals directly between adjacent cells. ¨ Provide faster communication & can be produced in unison by a large number of neurons & muscle fibers. ¨ More common in smooth muscle, cardiac muscle, and in embryos.

Synapses n Chemical Synapses: Membranes do not touch, so signals do not pass directly. ¨ Cells respond to neurotransmitters released from synaptic knobs into the synaptic cleft binding to receptor sites. ¨ Steps: n n The presynaptic cell translates the electrical impulse into a chemical signal. The postsynaptic cell translates the chemical signal back into an electrical impulse. ¨ Steps cause a synaptic delay close to 0. 5 microseconds.

Neuraglia n Neuroglia: Support cells of the nervous system. ¨ n Cells smaller than neurons and outnumbering them 50 to 1. They do not generate impulses but can divide & multiply. Types of Neuroglia: ¨ Schwann Cells: Cells located in the PNS that produce myelin. n Neurolemma aka Sheath of Schwann: Outer cytoplasmic layer of the Schwann cells containing its own nucleus. Oligodendrocytes: Cells located in the CNS that produce myelin. ¨ Astrocytes: Star-shaped cells that. . . ¨ n n Provide nutrients Help regulate the chemical environment Absorb excess neurotransmitters Assist in forming the blood-brain barrier

Neuroglia n Types of Neuroglia: ¨ Microglia: Macrophage cells that act as an immune response to protect the CNS from disease. ¨ Ependymal Cells: Cells that line the brain. n Help circulate the cerebrospinal fluid in the CNS using cilia on the outside of the cells. ¨ Satellite the PNS. Cells: Help support the other cells of

Electrophysiology Electrical Potential: The gradient difference in the concentration of charged particles between points. n Polarized: The term used to describe cells that have an electrical potential. n Depolarized: The term used to describe cells that do not have an electrical potential. n

Electrophysiology n Membrane Potential: The potential electrical voltage difference stored in the resting membrane. ¨ Works n the same way a car battery does! Resting Membrane Potential: The amount of electrical charge built up as a result of…. ¨ Negative ions lined up along the inside of the cell membrane ¨ Positive ions lined up along the outside of the ¨ Potassium (K+) ions leaking outward at a faster rate than Sodium (Na+) ions permeating inward n Resting Membrane Potential of a polarized neuron is -70 m. V (minivolts).

Neural Communication Neurons communicate through action potentials brought on by polarization & depolarization. n Ion Channels: Channels in the cell membrane that allow ions to move in and out of the cell to create the resting membrane potential. n ¨ Gates: The method of controlling ion passage.

Neural Communication n 4 Types of Ion Channels: ¨ Leakage Channels: Gates open and close randomly – regulated by normal permeability. ¨ Voltage-Gated Channels: Open in response to a change in the membrane potential. n Voltage-gated sodium channels are blocked with local anesthetics, which prevents the pain signal from reaching the CNS. ¨ Ligand-Gated Channels: Open and close in response to chemical stimuli – primarily regulated by neurotransmitters and hormones. ¨ Mechanically Gated Channels: Open and close in response to outside mechanical stimulation. n Example: Auditory receptors in the ears stimulated by sound waves or physical pressure.

Neural Communication n Local Potential or Graded Potential: Occurs when only a small deviation is found in the resting potential. ¨ Only occurs due to the opening of a ligandgated or mechanically-gated channel. ¨ Potentials vary in size, are only effective for a short distance, and can be excitatory, inhibitory, & reversible.

Action Potentials n Action Potentials aka Impulses: Generated along voltage-gated ion channels. ¨ Consist of rapid changes in membrane voltage. ¨ Triggered by depolarization of the cellular membrane.

Action Potentials Stimuli triggers the start of the neural process. n Depolarization occurs as the resting membrane voltage changes toward threshold level. n Threshold is reached when the voltage rises enough to open the voltage-gated ion channels. n ¨ Typically neurons. occurs at -55 m. V (minivolts) in

Action Potentials n Gates Open: ¨ Sodium Gates open quickly, allowing an inrush of sodium ions. ¨ Potassium Gates open slowly, causing a trickle of potassium ions into the cell. n Sodium Influx only lasts a few 10, 000’s of a second and causes: ¨ Membrane to reach +30 m. V (minivolts) ¨ Sodium gates to close ¨ Potassium gates to fully open n Repolarization occurs as the potassium levels rapidly drop, causing the voltage to drop back to -55 m. V (minivolts).

Action Potentials n n n Refractory Period: The amount of time necessary for the cell to return to its resting state via repolarization. Absolute Refractory Period: The period when the repolarization is less than one-third complete and cannot generate another action potential, no mater what stimuli occur. Relative Refractory Period: Once repolarization is one-third complete, stimuli considerably stronger than the threshold stimulus can produce an action potential.

Action Potentials n All-Or-Nothing Law: An action potential will occur once the voltage level reaches threshold, no matter what. ¨ It will always be the same size ¨ The signal will always travel down the entire length of the neuron ¨ The action potential will not fire until it reaches threshold, and will always fire once it does.

Methods of Conduction Continuous Conduction: When an impulse travels around 2 m/sec (meters per second) down an unmyelinated neuron. n Saltatory Conduction: When an impulse travels around 120 m/sec down a myelinated neuron. n ¨ The impulse “jumps” from new action potentials generated at each node of Ranvier.

Neurotransmitters n Neurotransmitters: Chemical signals transmitted and interpreted by the nervous system. ¨ Neuroreceptors: Sites located on the postsynaptic neuron that binds to neurotransmitters. n Binding causes an influx of Sodium (Na+) which depolarizes the neuron and fires of an action potential. ¨ Synaptic Vesicles: Sacs on the knobs on the end of the axons that are used to store neurotransmitters.

Neural Integration n Excitatory Postsynaptic Potential (EPSP) aka Excitatory Neurotransmitters: Brings the charge closer to threshold being reached by depolarizing the membrane. ¨ The larger the number of EPSPs the higher the change that the threshold will be reached. ¨ Effect of EPSPs must be grater than the effect of the IPSPs. n Inhibitory Postsynaptic Potential (IPSP) aka Inhibitory Neurotransmitters: Takes the charge farther away from threshold by hyperpolarizing the membrane.

Neural Integration n Summation: The process of a neuron integrating the information from thousands of synapses providing incoming information at a time. ¨ n Trigger Zone: The point of reception for the summation of incoming signals. Two forms of summation: Temporal Summation: A single presynaptic neuron generates enough EPSPs in short intervals, allowing signals to build up before the first ones decay. ¨ Spatial Summation: EPSPs from several different postsynaptic neurons are added up to reach the threshold. ¨ n n Facilitation: When neurons enhance the effects of other neurons. Inhibition: When neurons inhibit or suppress the effects of other neurons.

Neurotransmitters n Over 100 neurotransmitters found in the body & categorized into 3 basic categories of small-molecule neurotransmitters: ¨ Acetylcholine ¨ Amino acids ¨ Biogenic amines

Acetylcholine ACh is the most studied neurtransmitter n Acts as an excitatory or inhibitory neurotransmitter depending on the synapse it is in. n Ionotropif Effect: ACh binding to ligandgated channels causes enough of a local action potential to fire the neuron. n Acetylcholinesterase (ACh. E): A second neurotransmitter released to deactivate Ach when it is no longer needed. n

Amino Acids Act as neurotransmitters in the CNS. n Two important amino acids: n ¨ Glutamate: n Excitatory neurotransmitter. Excitotoxisity: Occurs when too high a level of glutamate is present in the NS – leads to neural death. Due to lack of oxygen or inadequate blood supply. ¨ Seen in stroke victims. ¨ ¨ Aspartate: Another excitatory neurotransmitter linked to glutamate.

Biogenic Amines n n Modified amino acids that are both excitatory & inhibitory. Catecholamine: An amino acid with a catechol ring (6 carbons and 2 hydroxyl groups). ¨ n Biogenic amines are typically made of these. This category includes: Norepinephrine: Catecholamine responsible for arousal, dreaming & mood. ¨ Dopamine: Catecholamine that contributes to emotions, addictive behaviors, pleasures, & muscle tone. ¨ Serotonin: NOT a catecholamine – involved in sensory perception, mood control, appetite, temperature regulation, and sleep. ¨

Prozac Nation Selective Serotonin Reuptake Inhibiter (SSRI): Stops the uptake of the neurotransmitter serotonin & increases the “good mood” by allowing the serotonin to stick around the synapse longer. n Prozac! n

Neuropeptides Neuropeptide: A string of 3 -40 amino acids linked by peptide bonds that function as neurotransmitters. n These include: n ¨ Enkephalins: Have a pain relieving effect 200 times as strong as morphone. ¨ Endorphins: Block pain and play a role in emotions, sexual activity, and memory. ¨ Dynorphins: Block pain and play a role in emotions, sexual activity, and memory.

Neuromodulators n Neuromodulators: Neuropeptides, hormones, or chemicals that alter synaptic transmission. ¨ Work by raising or lowering the number of receptors or altering the production, release, & reuptake of neurotransmitters.

Neuromodulators n Examples: ¨ Drugs that stimulate or inhibit neurotransmitter synthesis. n ¨ L-Dopa increases dopamine production & is used for Parkinson’s disease. Drugs or toxins that increase or decrease the release of neurotransmittesr n n Amphetamines increase the release of dopamine and norepimepherine to increase energy. Botulinum toxin blocks the acetycholine release from motor neurons resulting in paralysis. Agonists: Agents that activate neurotransmitter receptors. ¨ Antagonists: Agents that block neurotransmitter receptors. ¨ Agents that increase or decrease the removal of neurotransmitters. ¨ n Selective Serotonin Reuptake Inhibitors (SSRIs) for depression

Neural Circuits: Complex networks of billions of neurons that are processing a specific kind of information. n Labeled according to arrangement & branching of the neurons. n

Types of Neural Circuits n Simple Series Circuits: Consist of one presynaptic neuron stimulating only one postsynaptic neuron. ¨ Neurons form a chain of communication.

Types of Neural Circuits n Diverging Circuits: One presynaptic nerve sends impulses to an increasing number of postsynaptic nerves along the circuit. ¨ Example: The relatively small number of neurons in the brain that diverge to serve the large number of neurons in the spinal cord & beyond.

Types of Neural Circuits n Converging Circuits: One postsynaptic neuron receives signals from several sources. ¨ Example: A single motor neuron that receives input from several brain pathways.

Types of Neural Circuits n Reverberating Circuits: Send impulses in cycles & repeatedly through the circuit. ¨ Presynaptic neuron is stimulated ¨ Postsynaptic neuron fires a series of signals ¨ Example: Breathing is stimulated by a series of signals to contract and relax the diaphragm muscles.

Types of Neural Circuits n Parallel After-Discharge Circuits: One presynaptic cell stimulates a group of neurons, which then converge upon one common postsynaptic cell. ¨ Results: The postsynaptic cell receives a series of impulses in rapid succession & reaches threshold faster.

Synaptic Plasticity n Synaptic Plasticity: The ability of neurons to create new synapses or modify existing ones. ¨ Allows the brain to change with experience. ¨ New dendrites can grow ¨ New proteins can form ¨ Synapses between neurons can change. n This is critical to neural repair and memory!

Neuron Injury & Repair n Injury of a neuron can occur due to… Hypoxia: Lack of oxygen ¨ Trauma: Physical injury or bruising due to head injury. ¨ Burns: 3 rd degree burns will destroy the sensory neurons in the area! ¨ n Neurogenesis: The development of neurons from undifferentiated stem cells. DOES NOT OCCUR IN ADULTS! ¨ n Neurons do not undergo mitosis past adolescence! Fortunately the brain is highly adaptive! ¨ Unused or underused areas of the brain will take over the jobs of areas that are damaged or destroyed. It may take a while, but it is possible to regain the functions lost to brain damage due to synaptic plasticity!

Memory Engram: The neural pathway through which memories are traced. n Types of Memory: n ¨ Immediate Memory ¨ Short-Term Memory ¨ Long-Term Memory

Immediate Memory n Immediate memory only lasts for a few seconds. ¨ Does not become stored as permanent memory without utilizing a learning technique such as rehearsal. ¨ Only VERY small amounts of information can be stored.

Short-Term Memory n Short-Term memory aka Working Memory only lasts for a for several seconds to several hours. ¨ LIMITED capacity controlled by Miller’s Magic Number ¨ Miller’s Magic Number: 7+/-2 This means we can only hold between 5 and 9 (average of 7) pieces of information in short-term memory at a time. n This can be numbers, letters, objects, or short sentences. n

Long-Term Memory n Long-term memory can last a lifetime, but may become difficult to retrieve, or bring back to conscious thought. ¨ Practically n unlimited capacity. Two types of long-term memory: ¨ Declarative Memory: Memories formed of events and facts. n Who, what, when & where. ¨ Procedural n How-to! Memory: The ability to retain skills.

Multiple Sclerosis (MS) n Multiple Sclerosis (MS): An autoimmune disease that causes marked degeneration of the myelin sheaths in the CNS. ¨ Symptoms: Progressive loss of nerve control. ¨ Approximately 2 million affected worldwide.