SPE601 - Topic 2 - Peripheral Nervous System PDF

Peripheral Nervous System PNS is divided into two systems: ○ Somatic - controls voluntary(striated) muscles ○ Autonomic - controls involuntary(smooth) muscles is divided into: Sympathetic branch which controls the “fight or flight” reaction; to exp...

Peripheral Nervous System PNS is divided into two systems: ○ Somatic - controls voluntary(striated) muscles ○ Autonomic - controls involuntary(smooth) muscles is divided into: Sympathetic branch which controls the “fight or flight” reaction; to expend energy. Events causing arousal such as freight or drugs (cocaine) Pupils dilate(make more light,heart rate increases, more saliva production Parasympathetic branch which is responsible for maintenance of everyday activities and brings you back down to normal from a sympathetic response Resting activity: low heart rate, respiratory. ○ Somatic neurons - involved with voluntary events and includes the production and reception of environmental events Production meaning motor, such as arms, legs Utilized striated muscles ○ Autonomic neurons - involuntary and help to maintain a healthy internal environment Responsible for production and reception from internal organs, blood vessels and glands Utilizes smooth muscle, heart, diaphragm, intestines Neuroglia - Neurons are supported by several non excitable cells called neuroglia - Support cells of the nervous system - Glial refers to glue, supports and protects the nerve cells - Generally smaller than neurons and outnumber them 5-10xs - Regulates ionic concentration within the extracellular fluid involving the brain and spinal cord - Ions such as potassium, sodium etc. - Outside of the cell - Cannot form synapses, don't create or transmit synapses - Do not generate or transmit nerve impulses - Keeps neurons together  - Includes astrocytes, oligodendroglia. Schwen cells, microglia and ependymal glia 1. Astrocytes Most common type of in CNS - star shaped cells that nourish neurons and help maintain environment Function as connective tissue, providing skeletal support for brain cells and their processes Transport nutrients from capillaries Contributes to blood-brain barrier by contacting capillary surfaces with their end feet restricting movement of certain substances from the blood to the brain - Very often multiple in pathological conditions such as tumor, very commonly an astrocytoma - very difficult to remove from brain because the “tentacles” of star shape wrapped key portions of brain such as brain stem and medulla - Removal can damage health cells(cranial nerves) causing motor dysfunction,, swallowing, dysarthria, vision difficulty - Different types of astrocytes: - Protoplasmic - found in gray matter and have numerous branching processes - Fibrous - found in white matter and long thin unbranched processes - Grey matter is gray because it is made up of unmyelinated cell bodies Oligodendroglia and Schwann Cells - One oligodendroglia can myelinate many axons - Medication to treat infection is an antibiotic which activates microglia to destroy the bacteria. - Cerebrospinal fluid(produced in the brain) provides protection to the brain and nourishment to the brain and spinal cord. - Satellite cells are in the PNS - Spinal ganglia is and collection cell body and axons located outside of the spinal cord Produce and coat axons with myelin Oligodendroglia perform this function - CNS Schwann cells - PNS  The uncoated areas, called Nodes of Ranvier help facilitate the propagation of electrical signals down the axon due to jumping from one node to the next Microglia - work in response to injury, disease and infection. They are phagocytic(scavenger, eating bacteria), destroy bacteria Ependymal - line cavities of brain and spinal card and contribute to the flow of cerebrospinal fluid Neuroglia in the peripheral nervous system ○ Satellite cells - astrocytes of the PNS - surround neurons to support and help nourish them Types of Neuroglia Cells 1. Astrocytes: bind blood vessels to nerves. Form blood-brain barrier. Star shaped with numerous arms attached to blood vessels and neurons. In CNS. 2. Ependymal cells: form and circulate cerebrospinal fluid. Columnar cells with cilia. In CNS. 3. Microglia: mobile, phagocytic cells. Protect neurons by engulfing bacteria and other invaders. Small unattached cells with numerous arms. In CNS 4. Oligodendrocytes: form myelin sheath around axons in the CNS. Small cells with a few long arms attached to myelin sheath Neuronal Potential, Action Potential, and Intercellular communication Electrochemical Activity of Cells - Brain contains electrical cavity Neural process  - Efferent is motor, it originates in the primary motor cortex aka (in front of)precentral gyrus - Afferent = bottom up and is sensory, ascending communication from body to brain - Efferent = top down and motor, descending communication from brain to the body - Overally, the communication of neurons involves 2 phases: - Electrical phases - dendrites, soma and axon - Chemical phase - synaptic cleft and neurotransmitters - Communication of neurons - electrochemical in nature - Indentation of brain is a sulci or sulcus - A gyrus is the raised portion of the brain - Information goes to the thalamus(stimulus is identified) and eventually to the primary sensory cortex(post central gyrus) - Some neurotransmitters that cause behavior and others diminish a behavior - Cellular and molecular activities enable neurons to interact and therefore “behavior” occurs - Disorders of the nervous system are therefore a result of problems with the cellular and molecular activities within a neuron - Alzhiemer’s dementia is a result of neuron in the cognitive portion of brain become entangled and can no longer transmit impulses causing synapses(acetylcholine) - Many types and causes of dementia How does a neuron fire? - Neurons are firing all the time - Trigger is when an impulse is created causing a change in that neuron - If the impulse is intense enough it will cause a change in a permeability of the cell body’s membrane How does a neuron work? - Transport - Active - energy is used to move something from point A to point B - Passive - no energy is used to move something from point B to point A - Gradient - Sloping or imbalance  - Examples - a mountain roads has a grade or gradient to it - Membranes like the ones in neurons are semi-permeable (allow some substances in and keep out others. - Because of semi-permeable natures, 2 imbalances occur - concentration gradients and charge (or electrical) gradient Electrical activity of neurons - Fluid is inside and outside of cells - Intracellular fluid - Fluid inside the neuron - Neutral except that which is close to the membrane, this has a negative charge - Large amounts of potassium (K) and lesser amounts of sodium (Na) - Extracellular fluid: - Fluid outside the neuron - Most is neutral except that which is closest to the membrane, this maintains a positive charge - Large amounts of sodium (Na) and small levels of potassium (K) - Cytoplasm has electrical charge and increases near the membrane - Neutral when far from membrane - Fluid inside cell is neural expect closer to membrane is charged - Cell contain potassium and sodium and chloride - Potassium inside the cell is negative - Less sodium inside cell than outside - Sodium inside cell is negative - Less potassium outside of the cell - Potassium outside of cell is positive - Sodium outside of cell is positive - Polarization of cell, negative inside and positive outside - Ions are constantly moving inside and outside of cell - Opposites are attracted to themselves  - Along membrane is a protein that ensure cells are moving in and out of cell called sodium-potassium pump, this is the “resting state” of the cell - Proteins, A- and Chloride - Measured in microvolts, resting state is about 50 to 60 microvolts - Resting state = -50 to -70mv - Creation of impulse is an action potential The Firing of Neuron: Analogy of a Gun 1. Gun must be loaded 2. Something must trigger gun to fire 3. Gun is reloaded in order to be used again The loaded Neuron (polarization) Cell Membrane at Rest In a state of rest,neuron is “polarized” which means there are two imbalances: ○ Imbalance of sodium(Na+)with a large concentration of it on the outside of the neuron(extracellular fluid) and a small concentration on the inside (sodium ion). Ions are atoms that have either gained or lost an electron(subatomic particle - negative charge of electricity), causing them to have a positive or negative charge ○ Large concentration of potassium(K+) inside(intercellular fluid) of the membrane, smaller amount on outside ○ Positive ions are attracted to negative ions and vise versa Therefore - sodium(Na+)(positive - extracellular) come into cell due to the attraction to the negative charge of the intracellular fluid Potassium (K+) flows out of the cell due to the attraction to the lower concentration on the outside (positive) Inflow of sodium and the outflow of potassium can change the charge of the fluid. Next imbalance - there is an electrical charge with the interior of the neuron being more negative than the outside of the neuron. This resting potential is(-50 mV to -70 mV) this imbalance is due to a large concentration of negatively charged chloride (Cl-) inside the neuron  Semi-permeable membrane is responsible for maintaining these imbalances and allowing ions to travel between the fluids - Chloride is negatively charged which adds to the negative environment within that cell - Chloride styes in cell - Channels/gate run along cell though sodium or potassium channels Stimulation of neuron The concentration and charge gradients put the neuron in a position of being ready to fire Changes in cell occur when neuron is stimulated by an impulse ○ If stimulus is weak - small change in electrical charge of intracellular fluid - nothing happens ○ Stimulus is strong - right circumstance need for the trigger to be pulled and the neuron to fire ○ Now, there is potential for action to occur…action potential When a cell is in its resting state, it is getting ready to fire, ripping environment to get ready for something to happen Changes in cell are the result of the stimulus If stimulus is appropriate and strong enough, creates action potential Synapse - Neurons connect with each other to pass signals - places of connection are called synapses - Transferring information from one neuron to another neuron, muscle or gland - When neurons connect to carry out an intended behavior this is called a synapse - This transmission is called synaptic transmission - Signals are passed from one neuron to the other through the chemical process (neurotransmitters) - Neurotransmitters mediate transmission between neurons by exciting(starting) or inhibiting(stopping) postsynaptic action potentials  - When excitatory impulse of sufficient strength reaches the telodendria , the synaptic vesicles fuse with the axonal membrane and open leading to the depolarization of the pot-synaptic membrane - If the opened channels are inhibitory channels, the postsynaptic membrane will be hyperpolarized and impulse arrested - EPSP - excitatory postsynaptic potential - increasing or starting something (epinephrine) - IPSP -Inhibitory postsynaptic potential - causes less of or stopping something(GABA) There are different types of synapses which can occur within the CNS and PNS - Dendrodentritic :Dendrite to dendrite - Identify a sensory stimulus - Axodentric: axon to dendrite(most often) - Works much faster, increase heart rate - Axosomatic: axon to cell body(soma) - Axoaxonic: axon to axon 2A. The chemical firing of the neuron (chemical transmission) - Occurs at the synapse between neuron and another neuron’s dendrites, some or axon(OR MUSCLE/GLAND) - Neurotransmitters are released from synaptic vesicles from presynaptic membrane, travel into synaptic cleft and attach to receptors on the postsynaptic membrane - Neurotransmitter acts like a key and the receptor on the postsynaptic membrane is like a lock - If key unlocks the lock, the action signal is transmitted to the receiving neuron - The postsynaptic membrane is the receptor site - If the neurotransmitter being used is the correct stimulus, something will happen if not, the intended behavior won't occur? - Synaptic vesicles contain neurotransmitters in a synapse - Another chemical is introduced into the synaptic space - Divalent calcium - to ensure that synapse takes place - Calcium channel blocker(IPSP) - slows down synapse Action potential - electrical firing of the neuron(depolarization)  - Change in the permeability of of the membrane causes sodium channels to open - Sodium (-) flows in & the negative environment is reversed to +30mv(previously -70mv) - Sudden change in polarity from influx of +sodium ions triggers an action potential(a rapid change in membrane polarity which moves like a wave down an axon - Occurs at the nodes of ranvier - signal jumps from node to node which speeds the transmission signal down axon ★ Zemlin was a speech scientist, his book was the bible of “speech science” ★ Action potential(neural impulse) - a brief reversal of membrane potential with a total amplitude or change in voltage from - very negative to very positive The repolarization - reloading of the neuron Sodium channels close and potassium channels open allowing no more Na+ to flow inward and allowing K+ ions to flow out of neurons These ions are also positively charged, their absence causes the inferior of the axon to become negative again, stopping the depolarization process Membrane potential returns to - 70mV(repolarization) K+ channels close and -70mV resting potential is achieved Sodium ions are pumped more slowly outside of the neuron by the SODIUM POTASSIUM PUMP The distribution of sodium and potassium ions across the cell membrane is adjusted constantly by the sodium-potassium pump to help maintain the resting potential - (50-70mV) of the ionic makeup Neuron is put back into a polarized state again and ready to fire when needed (go back to polarization) Refractory period( hyperpolarization, below -70) - For a brief period following excitation, the axon can no longer be stimulated no matter how strong the stimulus  - Sodium channels are slow to open and potassium channels are still opened - Impossible for sodium inflow to exceed potassium outflow to reach threshold Action potential 1. Resting membrane potential 2. Some of the voltage-gated Na-channels open and enters the cell (threshold potential) 3. Opening of more voltage gated Na-channels and further depolarization (rapid upstroke) 4. Reaches to peak level 5. Direction of electrical gradient for Na is reversed + Na-channels rapidly enters enter a closed state “inactivated state” + voltage - K-channel open (start of repolarization) 6. Return to the resting membrane potential Action Potential An action potential occurs when a neuron is conducting a nerve impulse In order for an action potential to occur, the neuron must receive sufficient stimulation to open enough Na gates to reach the threshold level If sufficient Na gates are opened to reach the threshold level, other Na and K gates will be stimulated to open This results in a self-propagating wave of action potentials and Na and K gates opening along the entire length of the neuron and an action potential and nerve impulse occur Since an action potential will only if the membrane threshold level is reached, an action potential can also be described as an all or none response action potential can be divided into 2 phases: depolarization and repolarization Action potential leading to impulse - Axon synapsing on the dendrite - Impulse has been created - Interruption on impulse - One synapse has been created, the neurotransmitter is re-absorb into axon is reuptake - Introduction of enzyme: destroys the neurotransmitter - acetylcholin(esterase), esterase = enzyme Impulse vs. Synapse In order to have a synapse you need an impulse - Impulse is generated at axon hillock - Impulse travels down the axon to the ending - Synapse is the connection between two neuron at synaptic cleft If receptor site is an adrenal gland, the purpose of the synapse would be releasing/production of a hormone Sodium-potassium pump is a protein membrane which ensures the resting potential of that neuron Action potential(peak level at +40) has a refractory period Synaptic vesicle hold neurotransmitter(chemical) Most common Neurotransmitters: - Synapse between neurons and muscles use Acetylcholine (ACh) as their neurotransmitter and are always excitatory - Synapses between motor neurons and glands use norepinephrine(inhibitory) or Acetylcholine(excitatory) - After a message is sent, neurotransmitters are either destroyed by enzymes or picked up and stored in the terminal bouton. - Neurotransmitters are important to understand because absence or excess production of some play a role in brain disease disease and behavioral disorder. The brain’s reaction to drugs also depends on the synaptic transfer system. Acetylcholine(motor) - Major neurotransmitters of the PNS (rapid fire) cause muscle tissue to contract - Carries messages controlling voluntary muscle movement - Anticholinergic medication may be prescribed to prevent over salivation’ - Neurons that release ACh are called cholinergic neurons - Restricted role in CNS - After messages transmitted, ACh is broken down in the synaptic cleft by an enzyme (acetylcholinesterase) Acetylcholine(sensory)  - Regulates CNS neuronal activity - alertness, attention, memory and learning - Degeneration of cholinergic neurons - Alzheimer’s disease - Different forms/causes of dementia: alzheimer's is only one cause of dementia - Insufficient supply of ACh causes a condition - Myasthenia Gravis. In this disorder, the strength of neural impulses is diminished, causing voluntary muscles to be weak, including those involved with articulation, voicing and respiration. - Tired, droopy eyelids(first sign), some trouble with speech, excessive fatigue with little effort(walking stairs) - Ptosis - drooping eyelids, first sign of myasthenia gravis - Tensilon injection is a medication to test for MG, will cause increased strength temporarily - Now treatable Glutamate - The acetylcholine if the CNS - major excitatory chemical of synaptic activity and involved in both learning and memory - Evidence - plays a role in schizophrenia Gamma-Aminobutyric Acid (GABA) - Main inhibitory neurotransmitters of the CNS - Binds postsynaptic receptor sites, blocking action of other neurotransmitters - Neurons that contain GABA - GABAnergic neurons - Plays a role in the sleep-wake cycle - Low levels - depression and insomnia