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Fundamentals of the Nervous System and Nervous Tissue
CHAPTER 11
Nervous System - Structures
§Brain
§100 billion neuron
§12 pairs of cranial nerves
§Spinal cord
§connects to brain thru foramen magnum
§Enteric plexuses
§in walls of GI organs - “brain of the gut”
§Sensory receptors
§ nerve cells carry message from receptor to central nervous system
Nervous System - functions
§The master controlling and communicating system of the body - for homeostasis – shares with the endocrine system
§Functions
§Sensory input (afferent)– monitoring stimuli occurring inside and outside the body
§Integration – interpretation of sensory input
§Motor output (efferent) – response to stimuli by activating effector organs
Organization of the Nervous System
§Central nervous system (CNS)
§Brain and spinal cord
§Integration and command center
§Peripheral nervous system (PNS) – [largest]
§Paired spinal and cranial nerves
§Carries messages to and from the spinal cord and brain
Peripheral Nervous System (PNS): Two Functional Divisions
1. Sensory (afferent) division
§Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain
§Visceral afferent fibers – transmit impulses from visceral organs to the brain
2. Motor (efferent) division
§Transmits impulses from the CNS to effector organs
Motor Division: Two Main Parts
§Somatic nervous system
§Conscious control of skeletal muscles
§Autonomic nervous system (ANS)
§Regulates smooth muscle, cardiac muscle, and glands
§Divisions – sympathetic and parasympathetic
§Sympathetic nervous system – “flight or fight”
§Times of stress – emergency situations – stimulates for greater activity except for the digestive systems which stops or shuts down.
§Parasympathetic – “rest and digest”
Histology of Nerve Tissue
§The two principal cell types of the nervous system are:
§Neurons – excitable cells that transmit electrical signals
§Supporting cells – cells that surround and wrap neurons
Supporting Cells: Neuroglia
§The supporting cells (neuroglia or glial cells):
§Provide a supportive scaffolding for neurons
§Segregate and insulate neurons
§Guide young neurons to the proper connections
§Promote health and growth
Astrocytes
§Most abundant, versatile, and highly branched glial cells
§They cling to neurons and their synaptic endings, and cover capillaries
§Functionally, they:
§Support and brace neurons
§Anchor neurons to their nutrient supplies
§Guide migration of young neurons
§Control the chemical environment by buffering potassium and recapturing neurotransmitters.
Microglia and Ependymal Cells
§Microglia – small, ovoid cells with spiny processes
§Phagocytes that monitor the health of neurons
§Ependymal cells – range in shape from squamous to columnar
§They line the central cavities of the brain and spinal column
§ They are ciliated and play an active role in moving the cerebrospinal fluid
Oligodendrocytes, Schwann Cells, and Satellite Cells
§ Oligodendrocytes – branched cells that wrap CNS nerve fibers
§Produce myelin sheath (insulates) in CNS
§ Schwann cells (neurolemmocytes) – produce myelin sheaths which surround fibers of the PNS
§Neurilemma – outer, nucleated portion of Schwann cell
§ Satellite cells surround and support neuron cell bodies in PNS ganglia
§ Ganglia is group of nerve cell bodies outside the CNS - in the PNS
§ Nuclei is a group of nerve cell bodies in the CNS – most cell bodies are located in the CNS
Neurons (Nerve Cells) - Animation Link http://health.howstuffworks.com/adam-200011.htm
§Structural units of the nervous system
§Composed of a body, axon, and dendrites
§Long-lived, amitotic (do not divide) and have a very high metabolic rate
§Their plasma membrane functions in:
§Electrical signaling
§Cell-to-cell signaling during development
Nerve Cell Body (Perikaryon, Soma or Cyton)
§Contains the nucleus and a nucleolus
§Is the focal point for the outgrowth of neuronal processes
§Has no centrioles (hence its amitotic nature)
§Has well-developed Nissl bodies (rough ER)
§Contains an axon hillock – cone-shaped area from which axons arise
Collection of nerve cell bodies (somas) found in the CNS is a nucleus.
Collection of nerve cell bodies (somas) found in the PNS is a ganglion.
Processes of neurons
§Armlike extensions from the soma
§ Bundle of nerve fibers are called tracts in the CNS and nerves in the PNS
No nerves in the CNS
§There are two types: axons and dendrites
Dendrites of Motor Neurons
§Short, tapering, and diffusely branched processes
§They are the receptive, or input, regions of the neuron
§Electrical signals are conveyed as graded potentials – electrical signals generated for short distances - (not action potentials)
§Not myelinated
Axons: Structure
§Slender processes of uniform diameter arising from the hillock
§Long axons are called nerve fibers
§Usually there is only one unbranched axon per neuron
§Rare branches, if present, are called axon collaterals
§Axonal terminal – branched terminus of an axon
Axons in PNS grow in adults at about 1 inch (2.4 cm) per month
Axons, which can be 1,000 or
10,000 times the length of the cell body, or soma, contain no ribosomes or means
of producing proteins, and so rely on axoplasmic transport for all their protein
needs.
Movement
along axons occurs in two ways
Anterograde — toward axonal terminal
Retrograde — away from axonal terminal
Axons: Function
§Generate and transmit action potentials
§ The part of the neuron that conducts impulses away from its cell body
§Secrete neurotransmitters from the axonal terminals
Neural Synapse (1 billionth of an inch)
§Synapse-junction of neuron to neuron or to effector cell
§Synaptic end bulbs or varicosities contain synaptic vesicles
Myelin Sheath
§Whitish color due to fatty content (protein-lipoid), segmented sheath around most long axons
§It functions to:
§Protect the axon
§Electrically insulate fibers from one another
§Increase the speed of nerve impulse transmission
Myelin Sheath and Neurilemma: Formation
§Formed by Schwann cells in the PNS and oligodendrocytes in CNS
§A Schwann cell:
§Envelopes an axon in a trough
§Encloses the axon with its plasma membrane
§Has concentric layers of membrane that make up the myelin sheath
§Neurilemma – outer, nucleated and cytoplasmic portion of a Schwann cell
Nodes of Ranvier (Neurofibral Nodes)
§Gaps in the myelin sheath between adjacent Schwann cells or oligodendrocytes
§They are the sites where axon collaterals can emerge
InterActive Physiology®: Nervous System I: Anatomy Review on your textbook CD
Axons of the CNS
§Both myelinated and unmyelinated fibers are present
§Myelin sheaths are formed by oligodendrocytes
§Nodes of Ranvier are more widely spaced than in PNS
§There is no neurilemma and therefore no regeneration
Regions of the Brain and Spinal Cord
§White matter – dense collections of myelinated fibers
§In brain-white matter is coated by gray matter
§In spinal cord- white matter surrounds the gray matter
§Gray matter – mostly soma and unmyelinated fibers
Neurological Definitions - Summary
Brain and spinal cord collectively is the central nervous system (CNS).
Spinal nerves and cranial nerves and ganglia are the peripheral nervous system (PNS).
A bundle of nerve processes in the CNS is a tract.
A bundle of nerve processes the PNS - is called a nerve.
Collection of nerve cell bodies (somas) found outside the CNS, thus in the PNS, is a ganglion.
Collection of nerve cell bodies (somas) found in the CNS is a nucleus.
Thus a ganglion is to the peripheral nervous system what a nucleus is to the central nervous system.
Neuron that conducts impulses away from the CNS to muscles and glands is an efferent neuron.
Neuron that conducts impulses toward the CNS from the body periphery is an afferent neuron.
Neuron serving as part of the conductivity pathway between sensory and motor neurons is an association neuron or an interneuron.
Neuron Classification
§Structural:
§Multipolar — three or more processes (axon and several dendrites)-brain and spinal cord - motor
§Bipolar — two processes (axon and dendrite)-retina, inner ear, olfactory area of brain
§Unipolar — single, short process (axon and dendrite fused)
The Unipolar and Multipolar Neurons
http://www.wisc-online.com/objects/index.asp?objID=AP11804
Neuron Classification
§Functional:
§Sensory (afferent) — transmit impulses toward the CNS
§Motor (efferent) — carry impulses away from the CNS
§Interneurons (association neurons) — connects other neurons - shuttle signals through CNS pathways (most numerous neuron - 99%)
Neurophysiology
§Neurons are highly irritable – ability of a neuron to sense change in environment and be able to respond
§Action potentials, or nerve impulses, are:
§Electrical impulses carried along the length of axons
§Always the same regardless of stimulus
§The underlying functional feature of the nervous system
Electricity Definitions
§Voltage (V) – measure of potential energy generated by separated charge
§(1 millivolt [mV] = 0.001V)
§Potential difference – voltage measured between two points
§Current (I) – the flow of electrical charge between two points
§Resistance (R) – hindrance to charge flow
§Insulator – substance with high electrical resistance
§Conductor – substance with low electrical resistance
Electrical Current and the Body
§Reflects the flow of ions rather than electrons
§There is a potential on either side of membranes when:
§The number of ions is different across the membrane (charge inside does NOT equal charge outside) – separation of electrical charges represents potential energy – it takes work to bring about the separation.
§The membrane provides a resistance to ion flow
Role of Ion Channels – made up of membrane proteins
§Ion Channel-flow from positive to negative and vice versa
§Types of plasma membrane ion channels:
§Leakage channels
§Always open
§Gated channels
§Open and close
Ion Channels - Leakage Channels- Passive
§Passive, or leakage, channels
§More K than Na ions
Ion Channels- Gated Channels
§Voltage-gated channels – open and close in response to membrane potential (voltage changes)
§Chemically (ligand) gated channels – open with binding of a specific neurotransmitter
§Mechanically gated channels – open and close in response to physical deformation of receptors (vibration, pressure, stretching)
Gated Channels
§When gated channels are open:
§Ions move quickly across the membrane
§Movement is along their electrochemical gradients
§An electrical current is created
§Voltage changes across the membrane
Operation of a Gated Channel
§Example: Na+-K+ gated channel
§Closed when a neurotransmitter is not bound to the extracellular receptor
§Na+ cannot enter the cell and K+ cannot exit the cell
§Open when a neurotransmitter is attached to the receptor
§Na+ enters the cell and K+ exits the cell
Operation of a Voltage-Gated Channel
§Example: Na+ channel
§Closed when the intracellular environment is negative
§Na+ cannot enter the cell
§Open when the intracellular environment is positive
§Na+ can enter the cell
InterActive Physiology®: Nervous System I: Ion Channels on your textbook CD
Electrochemical Gradient
§Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration
§Ions flow along their electrical gradient when they move toward an area of opposite charge
§Electrochemical gradient – the electrical and chemical gradients taken together
Resting Membrane Potential (Vr)
§The potential difference (–70 mV) across the membrane of a resting neuron
§It is generated by different concentrations of Na+, K+, Cl-, and protein anions (A-)
§Cytosol has more K and less Na than outside
§Ionic differences are the consequence of:
§Differential permeability of the neurilemma to Na+ and K+
§Operation of the sodium-potassium pump
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Nervous System I: Membrane Potential
Membrane Potentials: Signals
§Used to integrate, send, and receive information
§Membrane potential changes are produced by:
§Changes in membrane permeability to ions
§Alterations of ion concentrations across the membrane
§Types of signals – graded potentials and action potentials
Changes in Membrane Potential
§Changes are caused by three events
§Depolarization – the inside of the membrane becomes less negative – increases probability of impulse
§Repolarization – the membrane returns to its resting membrane potential
§Hyperpolarization – the inside of the membrane becomes more negative than the resting potential-decreases probability of impulse
| The diagram above represents an axon
terminal on the left and a dendritic process on the right seperated by a
synaptic cleft. When an impulse reaches the end of an axon, it triggers the
formation of synaptic vessicles at that terminal. Synaptic vessicles are
specialized vacuoles that contain neurotransmitters such as acetylcholine.
The vessicles transport the neurotransmitters to the end of the axon and
release them into the synaptic cleft. These neurotransmitters attach to
receptor sites on the cell membrane of the receiving neuron. When enough
receptor sites are filled, the firing threshold of the receiving neuron is
reached and a depolarization event is triggered.
Before the neuron depolarizes, it is held steady in its resting potential. This potential, which is achieved by maintaining a relatively high concentration of sodium ions outside of the cell membrane, represents an approximately -70 millivolt discrepancy between the negatively charged interior and positively charged exterior. As neurotransmitters attach to the receptor sites and overcome the firing threshold, small molecular gates open along the cell membrane allowing the sodium ions to rapidly flood the neuron. This sudden change in polarity from the influx of positive ions triggers an action potential that moves like a wave down the axon triggering another nerve, muscle cell, etc. At this point, a different series of molecular gates open which allows potassium ions to rush out of the neuron. The potassium ions, which have a positive charge as well, create a negatively charged cell interior by their absence. This event stops the depolarization process. The sodium ions are pumped more slowly to the cell exterior by active transport, resulting in the fully restored resting potential once again. |
Graded Potentials
§Short-lived, local changes in membrane potential
§Involve chemical (ligand) and mechanically gated channels (primarily in dendrites and cell bodies)
§Decrease in intensity with distance
§Their intensity varies directly with the strength of the stimulus
§Sufficiently strong graded potentials can initiate action potentials
Graded Potentials
§Voltage changes in graded potentials are decremental (it dies out within a few millimeters of its origin)
§Current is quickly dissipated due to the leaky plasma membrane
§Can only travel over short distances
Action Potentials (APs)
§A brief reversal of membrane potential with a total amplitude of 100 mV
§Action potentials are only generated by muscle cells and neurons
§They do not decrease in strength over distance
§They are the principal means of neural communication
§An action potential in the axon of a neuron is a nerve impulse
Action Potential: Resting State
§Na+ and K+ channels are closed
§Leakage accounts for small movements of Na+ and K+
§Each Na+ channel has two voltage-regulated gates
§Activation gates – closed in the resting state
§Inactivation gates – open in the resting state
Action Potential: Depolarization Phase
§Na+ permeability increases; membrane potential reverses
§Na+ gates are opened; K+ gates are closed
§Threshold – a critical level of depolarization (-55 to -50 mV)
§At threshold, depolarization becomes self-generating
Action Potential: Repolarization Phase
§Sodium inactivation gates close
§Membrane permeability to Na+ declines to resting levels
§As sodium gates close, voltage-sensitive K+ gates open
§K+
exits the cell and internal negativity of the resting neuron
is restored
Action Potential: Hyperpolarization
§Potassium gates remain open (slow closure), causing an excessive efflux of K+
§This efflux causes hyperpolarization of the membrane (undershoot)
§The neuron is insensitive to stimulus and depolarization during this time
Action Potential:
Role of the Sodium-Potassium Pump
§Repolarization
§Restores the resting electrical conditions of the neuron
§Does not restore the resting ionic conditions
§Ionic redistribution back to resting conditions is restored by the sodium-potassium pump
§ Na is actively transported through the cell membrane to establish a resting potential
Phases of the Action Potential
§1 – resting state
§2 – depolarization phase
§3 – repolarization phase
§4 – hyperpolarization
Anesthetics
§Local anethestics work by blocking the opening of voltage gated sodium channels. ( alcohol, sedatives, injected ‘caines)
Threshold and Action Potentials
§Threshold – depolarization level required to get an action potential. Membrane is depolarized by 15 to 20 mV
§Established by the total amount of current flowing through the membrane
§Weak (subthreshold) stimuli are not relayed into action potentials
§Strong (threshold) stimuli are relayed into action potentials
§All-or-none phenomenon – action potentials either happen completely, or not at all
Coding for Stimulus Intensity
§All action potentials are alike and are independent of stimulus intensity
§Strong stimuli can generate an action potential more often than weaker stimuli
§The CNS determines stimulus intensity by the frequency of impulse transmission
Refractory Period
§ The period of time during which an excitable cell cannot generate another action potential (Does not apply to graded potentials)
§Only with action potential
§Large nerve – very short
§Small nerves - longer
Conduction Velocities of Axons
§Conduction velocities vary widely among neurons
§Rate of impulse propagation is determined by:
§Axon diameter – the larger the diameter, the faster the impulse
§Presence of a myelin sheath – myelination dramatically increases impulse speed
InterActive Physiology®:
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Nervous System I: Action Potential
Saltatory Conduction
§Current passes through a myelinated axon only at the nodes of Ranvier
§Voltage-gated Na+ channels are concentrated at these nodes
§Action potentials are triggered only at the nodes and jump from one node to the next
§Much faster than conduction along unmyelinated axons
Unmyelinated axons conduct nerve impulses at about 2 miles per hour whereas myelinated axons conduct the impulses at 300 miles per hour.
Axon Nerve Impulse Conduction Speed
Myelinated - 300MPH Unmyleinated - 2 MPH
Multiple Sclerosis (MS)
§An autoimmune disease that mainly affects young adults - body attacks the myelin sheaths
§Symptoms include visual disturbances, weakness, loss of muscular control, and urinary incontinence
§Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses
§Shunting and short-circuiting of nerve impulses occurs
Synapses - The point at which an impulse from one nerve cell is communicated to another nerve cell(1 millionth of an inch)
§A junction that mediates information transfer from one neuron:
§To another neuron
§To an effector cell
§Presynaptic neuron – conducts impulses toward the synapse
§Postsynaptic neuron – transmits impulses away from the synapse
Types of Synapses
§Axodendritic – synapses between the axon of one neuron and the dendrite of another
§Axosomatic – synapses between the axon of one neuron and the soma (body) of another
§Other types of synapses include:
§Axoaxonic (axon to axon)
§Dendrodendritic (dendrite to dendrite)
§Dendrosomatic (dendrites to soma)
Electrical Synapses
§Most efficient – impulse travels cell to cell via gap junctions
§Are uni or bi-directional
§Are less common than chemical synapses
§Are important in the CNS in:
§Arousal from sleep
§Mental attention
§Emotions and memory
§Ion and water homeostasis
Chemical Synapses
§Unidirectional
§Specialized for the release and reception of neurotransmitters
§Typically composed of two parts:
§Axonal terminal of the presynaptic neuron, which contains synaptic vesicles
§Receptor region on the dendrite(s) or soma of the postsynaptic neuron
Synaptic Cleft – not a structural feature of a neuron
§Fluid-filled space separating the presynaptic and postsynaptic neurons
§Prevents nerve impulses from directly passing from one neuron to the next
§Transmission across the synaptic cleft:
§Is a chemical event (as opposed to an electrical one)
§Ensures unidirectional communication between neurons
Synaptic Cleft: Information Transfer
§Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca2+ channels
§Neurotransmitter is released into the synaptic cleft via exocytosis in response to synaptotagmin
§Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron
§Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect
Termination of Neurotransmitter Effects
§Neurotransmitter bound to a postsynaptic neuron:
§Produces a continuous postsynaptic effect
§Blocks reception of additional “messages”
§Must be removed from its receptor
§Removal of neurotransmitters occurs when they:
§Are degraded by enzymes
§Are reabsorbed by astrocytes or the presynaptic terminals
§Diffuse from the synaptic cleft
Physiological Events at the Neural Synapse
http://www.wisc-online.com/objects/index.asp?objID=AP1201
Postsynaptic Potentials
§Neurotransmitter receptors mediate changes in membrane potential according to:
§The amount of neurotransmitter released
§The amount of time the neurotransmitter is bound to receptors
§The two types of postsynaptic potentials are:
§EPSP – excitatory postsynaptic potentials
§IPSP – inhibitory postsynaptic potentials
Excitatory Postsynaptic Potentials
§EPSPs are graded potentials that can initiate an action potential in an axon
§Use only chemically gated channels
§Na+ and K+ flow in opposite directions at the same time
§Postsynaptic membranes do not generate action potentials (partial depolarization but no impulse sent)
§More easily stimulated by next input
Inhibitory Synapses and IPSPs
§Neurotransmitter binding to a receptor at inhibitory synapses:
§Causes the membrane to become more permeable to potassium and chloride ions
§Leaves the charge on the inner surface negative
§Reduces the postsynaptic neuron’s ability to produce an action potential (get hyperpolarization-no impulse)
Potential Summation
§A single EPSP cannot induce an action potential-input accumulated and gets integrated at the trigger zone
§Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the same time
§Temporal summation – presynaptic neurons transmit impulses in rapid-fire order
§IPSPs can also summate with EPSPs, canceling each other out
InterActive Physiology®:
Nervous System II: Synaptic Potentials
Neurotransmitters
§Chemicals used for neuronal communication with the body and the brain
§50 different neurotransmitters have been identified
§Classified chemically and functionally
Neurotransmitter control of hormones
§Many neurotransmitters are hormone controls
§Synthesis can be stimulated or inhibited
§Release can be stimulated or inhibited
§Removal can be stimulated or inhibited
§Receptor site can be blocked or activated
§Agonist – enhances synaptic transmission
§Antagonist – blocks synaptic transmission
Chemical Neurotransmitters
§Acetylcholine (ACh)
§Biogenic amines
§Amino acids
§Peptides
§Novel messengers: ATP and dissolved gases NO and CO (carbon monoxide)
Neurotransmitters: Acetylcholine
§First neurotransmitter identified, and best understood
§Released at the neuromuscular junction by motor neurons innervating skeletal muscle
§Synthesized and enclosed in synaptic vesicles
§Degraded by the enzyme acetylcholinesterase (AChE)
§Released by:
§All neurons that stimulate skeletal muscle
§Some neurons in the autonomic nervous system
Neurotransmitters: Biogenic Amines
§Include:
§Catecholamines – include dopamine, norepinephrine (NE) which is also known as noradrenaline, and epinephrine which is also known as adrenaline.
§Indolamines – serotonin and histamine
§Broadly distributed in the brain
§Play roles in emotional behaviors and our biological clock
Synthesis of Catecholamines
§Enzymes present in the cell determine length of biosynthetic pathway
§Norepinephrine and dopamine are synthesized in axonal terminals
§Epinephrine (also known as adrenaline) is released by the adrenal medulla
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Synthesis of the catecholamines from tyrosine. |
Tryptophan-Derived Neurotransmitters
Tryptopan serves as the precursor for the synthesis of serotonin (5-hydroxytryptamine, 5-HT, see also Biochemistry of Nerve Transmission) and melatonin (N-acetyl-5-methoxytryptamine).
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Pathway for serotonin and melatonin synthesis from tryptophan.
Neurotransmitters: Amino Acids
§Include:
§GABA – Gamma (g)-aminobutyric acid
§Glycine
§Aspartate
§Glutamate
§Found only in the CNS
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Nervous System II: Synaptic Transmission
Neurotransmitters: Peptides
§Include:
§Substance P – mediator of pain signals
§Beta endorphin, dynorphin, and enkephalins
§Act as natural opiates, reducing our perception of pain
§Bind to the same receptors as opiates and morphine
§Gut-brain peptides – somatostatin, and cholecystokinin
Gut-brain peptides – somatostatin, and cholecystokinin
Cholecystokinin (CCK) (kō"lu-sis"tu-kī'nin)
In just the last few years, over 1400 papers have been published on CCK.
CCK is secreted by the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively.
CCK acts as a hunger suppressant.
Recent evidence has suggested that it also plays a major role in inducing drug tolerance to opioids like morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal.
Neurotransmitters: Novel Messengers
§ATP
§Is found in both the CNS and PNS
§Produces excitatory or inhibitory responses depending on receptor type
§Induces Ca2+ wave propagation in astrocytes
§Provokes pain sensation
Neurotransmitters: Novel Messengers
§Nitric oxide (NO)
§Activates the intracellular receptor guanylyl cyclase
§Is involved in learning and memory
§Carbon monoxide (CO) is a main regulator of cGMP in the brain
Neurotransmitter Receptors
§Channel-linked (open channels)
§Excitatory (nicotinic Ach, glutamate, aspartate, ATP)
§Inhibitory (GABA, glycine)
§G-protein linked
§Generate internal 2nd messengers
Channel-Linked Receptors
§Composed of integral membrane protein
§Mediate direct neurotransmitter action
§Action is immediate, brief, simple, and highly localized
§Ligand binds the receptor, and ions enter the cells
§Excitatory receptors depolarize membranes
§Inhibitory receptors hyperpolarize membranes
Functional Classification of Neurotransmitters
§Two classifications: excitatory and inhibitory
§Excitatory
neurotransmitters cause depolarizations
(e.g., glutamate, aspartate, ATP)
§Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)
Functional Classification of Neurotransmitters
§Some neurotransmitters have both excitatory and inhibitory effects
§Determined by the receptor type of the postsynaptic neuron
§Example: acetylcholine
§Excitatory at neuromuscular junctions with skeletal muscle
§Inhibitory in cardiac muscle
What's Love Got to Do With This? Love has many meanings but here is one view that is on topic.
1)
Lust
Lust is driven by initial physical attraction and flirting. This stage can
depend on characteristics such as a symmetrical face and proportionate body
dimensions. Flirting can include gazing into the eyes, touching, and mirroring
in body language. The two chemicals that surface during this stage are the sex
hormones (testosterone and estrogen) and pheromones.
In the animal world, PHEROMONES are individual scent "prints" found in urine or sweat that dictate sexual behavior and attract the opposite sex. The existence of human pheromones was discovered in 1986 by scientists at the Chemical Senses Center in Philadelphia and its counterpart in France. They found these chemicals in human sweat.
2) Falling in love - Attraction
The romantic or passionate love is characterized by euphoria when things are
going well, and terrible mood swings when they're not. When you fall in love
you may have many physical symptoms: lose of appetite, can't sleep, can't
concentrate, palms sweat, butterflies in stomach... This is due to surging
brain chemicals called monoamines:
- DOPAMINE: it's commonly associated with the pleasure system of the brain, providing feelings of enjoyment and reinforcement to motivate us to do certain activities. It's released by naturally-rewarding experiences, such as sex or food. Some research studies show that when female rodents were injected dopamine in the presence of a male rodent, the female will pick him out of a crowd later.
- PHENYLETHYLAMINE: It's a natural amphetamine like the known drug and can cause the same stimulation effects. It contributes to that on-top-of-the-world feeling that attraction can bring, and gives you the energy to stay up day and night with a new love.
- SEROTONIN: it controls impulses, unruly passions, obsessive behavior, aiding the sense of "being in control".
- NOREPINEPHRINE is another neurotransmitter which induces euphoria in your brain, exciting the body by giving it a booster dose of natural adrenaline. This causes the heart to beat faster and blood pressure to rise. That's why you can experience a pounding heart or sweaty palms when you see someone you're attracted to.
3) Attachment - Staying together
There is a sense of calm and stability that we feel with a long-term partner,
a sort of bond that keeps couples together. This kind of love is driven these
hormones:
- OXYTOCIN: it's sometimes known as "the cuddle chemical." It's the hormone best known for its role in inducing labor by stimulating contractions. But recently it has been observed that it may influence our ability to bond with others, as both genders release this hormone when touching and cuddling, with the oxytocin level peaking during orgasm.
- VASOPRESSIN: also called as "the monogamy chemical". Researchers have found that suppression of vasopressin can cause males to abandon their love nest and seek new mates.
- ENDORPHINS: they are biochemical compounds that enhance our immune system, block the lesion of blood vessel, have anti-aging, anti-stress and pain-relieving effect, and also help to improve your memory.
High levels of oxytocin and vasopressin may interfere with dopamine and norepinephrine pathways, which may explain why with the time attachment grows as mad passionate love fades.
Well, as you can see, there is real chemistry taking place in our body when we are in love! This doesn't mean that love is only chemistry, but at least now you can understand this feeling from a different point of view, don't you?
Neural Integration: Neuronal Pools
§Functional groups of neurons that:
§Integrate incoming information
§Forward the processed information to its appropriate destination
Neurotransmitter Receptor Mechanisms
§Direct: neurotransmitters that open ion channels
§Promote rapid responses
§Examples: ACh and amino acids
§Indirect: neurotransmitters that act through second messengers that operate the gate (hormones)
§Promote long-lasting effects
§Examples: biogenic amines, peptides, and dissolved gases
Neural Integration: Neuronal Pools
§Simple neuronal pool
§Input fiber – presynaptic fiber
§Discharge zone – neurons most closely associated with the incoming fiber
§Facilitated zone – neurons farther away from incoming fiber
Types of Circuits in Neuronal Pools
§Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits. From CNS outward.
§Convergent – opposite of divergent circuits, resulting in either strong stimulation or inhibition
§Reverberating – chain of neurons containing collateral synapses with previous neurons in the chain
§Parallel – after discharge a pre-stimulates several posts which in turn all affect the same common post-synaptic cell
Patterns of Neural Processing
§Serial Processing
§Input travels along one pathway to a specific destination
§Works in an all-or-none manner
§Example: spinal reflexes
Patterns of Neural Processing
§Parallel Processing
§Input travels along several pathways
§Pathways are integrated in different CNS systems
§One stimulus promotes numerous responses
§Example: a smell may remind one of the odor and associated experiences
Neurotransmitter Clinical Highlights:
o Dopamine
o A “feel good” neurotransmitter
o Release enhanced by L-dopa and amphetamines
o Made from the amino acid tyrosine
o When not balanced by proper amounts of serotonin can cause addictive, compulsive behavior: “got to have it”
§ Drug abuse, compulsive shopping, eating, sex
o May be involved in schizophrenia
o Deficient in Parkinson’s disease
o GABA
o Principle inhibitory neurotransmitter
o Inhibitory effects augmented by alcohol
o Valium (benzodiazepine) increase GABA
o Valerian Root (herb) and B6 increase GABA
o Glutamate
o Important in learning and memory
o Norepinephrine
o A “feel good” neurotransmitter
o Release enhanced by amphetamines,
o Removal from synapse blocked by tricyclic anti-depressants and cocaine
Synthesis of catecholamines begins with the amino acid tyrosine, which is taken up by chromaffin cells in the medulla and converted to norepinephrine and epinephrine through the following steps:
o Serotonin
o “Who cares” neurotransmitter
o Balances dopamine to offset addictive/compulsive behavior
o Low levels of serotonin have been associated with uneasiness, carbohydrate cravings and weight gain, mood, sleep disorders and substance dependence.
o Drugs that block its uptake (selective serotonin reuptake inhibitors – SSRI) such Prozac, raise serotonin levels
o L-Tryptophan is used by the body to produce serotonin. The best dietary sources for Tryptophan are cottage cheese, brown rice, meat, peanut and soy protein.
o 5-HTP (5-Hydroxytryptophan) L-Tryptophan is an essential amino acid that is converted into 5-HTP in the body. 5HTP is then converted into serotonin and melatonin.
Man's Brain Rewires Itself - News Article
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