Biopsychology. Neurons

Transcription

1 Biopsychology What is Biopsychology? The study of the physiological, evolutionary, and developmental mechanisms of behavior and experience (Kalat) Primarily focused on brain activity especially as it relates to behavior Alternate Terms: biological psychology, psychobiology, physiological psychology, behavioral neuroscience Associated Fields of Specialization Neurons Neurons billion throughout the CNS (Approximately 20 billion in neocortex alone Most all are present at birth Can move and grow No replication of cells Cells receive 100,000 and more contacts We naturally lose 10,000/day (although some experts suggest up to 100,000/day) Utilize and electrochemical process for transmission of signals Expend energy and electrical/magnetic fields (glucose, oxygen, blood flow) forms the basis of CT, MRI, and PET technology Neuron Structure Cell Body Cell Nucleus Dendrites Axon Myelin Sheath Nodes of Ranvier Schwann Cell Axon terminals Neuron Organization Nerve bundle of neurons outside of CNS Tract bundle of neurons inside the CNS Ganglion group of cell bodies outside of CNS Nucleus group of cell bodies inside the CNS Neuron Classes Type 1 o Long, myelinated o Fast o Bipolar, unipolar

2 Type 2 o Short, unmyelinated o Slow o Multipolar Neuron Types Motor o Type 2 (multipolar throughout nervous system) o Projects messages from brain/spinal cord to muscles and organs Sensory o Type 1 (unipolar outside brain or bipolar outside of brain/snail cord) o Carries sensory information from body to brain Interneuron o Type 2 (multipolar throughout the brain and spinal cord) o Conducts information among neurons in same area Neuron Shape Glial Cells Likely >100 billion in neocortex (2-3 times more than the number of neurons there) Provide support to neurons Do not transmit information They play a role in synaptic functioning and neural signaling (astrocytes) Provide o Structural support o Nutritional and scavenger functions o Release of growth factors Three types o Astrocytes in brain and spinal cord maintain the appropriate chemical environment and support the endothelial cells that form the blood-brain barrier o Oligodendrocytes in the CNS (develop myelin)/schwann cells in PNS (same job) o Microglial cells scavengers that remove cellular debris after cell turnover or damage Blood-Brain Barrier Formed by capillary endothelial cells that are linked by tight junctions Protects the brain from being invaded by large-molecule organisms including many common bacterial infections (and also by many antibodies/antibiotics The BBB becomes more permeable during inflammation Cells in the Nervous System Myelination Clinical Applications Multiple Sclerosis

3 Degeneration of Myelin Sheath Individual experience loss of muscle function Primary Symptoms of MS Numbness/Weakness/Tingling in limbs/trunk Visual loss (occasionally accompanied by eye movement pain) Double or blurred vision Electric shock sensations with neck movement Tremor, lack of coordination, unsteady gait Fatigue Slurred speech Dizziness Neural Communication The Neural Membrane Most critical factors in the neuron s ability to communicate Comprised of lipid (with head and tail) and fat Membrane control entry to the cell Protein Channels open and close under specific circumstances (selective permeability) Polarization Electrical Gradient The most fundamental characteristic of neruons Definition (Kalat, page 26) difference of electrical charge between the inside and outside of the cell Resting Potential The difference in charge between the inside and outside of the neural membrane at rest Range -40 to -80 mv (approximately -70 average) Due to the unequal distribution of electrical charges on the two sides of the membrane This charge comes from ions External NaCl Internal KA (where A conjugate bas of an acid, or anion) Force of Diffusion Molecules tent to move from high concentration to low concentration Electrostatic Pressure Ions are repelled from the side similarly charged and attracted to oppositely charged side The Key Player ions are K and Na o K outward o Na Inward

4 Sodium-Potassium Pump Helps maintain established equilibrium by moving straying ions back home via large protein molecules Exchange rate approximately 3 Na for 2 K This helps keep the interior more negative than the outside A metabolic process accounts for approximately 40% of neural energy use Resting Potential Reviewed Concentration forces Selective Permeability Electrical Forces o Na + o K +, Cl -, A - = Anion Active transport: Sodium-Potassium Pump Action Potential (AP) Neural Impulse A brief electrical change that travel down an axon Generated by the movement of charge atoms through the neural membrane An abrupt depolarization of the membrane that allows the neuron to communicate over long distances (Garrett, Page 31) How AP Works An excitatory signal causes a partial depolarization (polarity in a small area of membrane is shifted toward zero) This disturbs the ion balance in adjacent membrane, causing disturbance down the dendrites and cross the membrane This partial depolarization is called Ion potential If the Local Potential is greater that the activation threshold (10mV or more), action potential occurs The accumulation of LP s occurs at the Axon Hillock once the threshold is reached, the neuron fires At AP peak, the neuron begins recovery toward resting potential by closing Na channels and opening K channels (K moves out) Synapses Synapses Specialized gaps (junctions) between neurons o Promote neural communication o Signals transmitted chemically Ramon y Cajal o Demonstrated separation of neurons Charles S. Sherrington (1906) o Identified synaptic communication via reflex arc studies

5 Reflex Arc for Leg Flexion (Figure 2.1) Synaptic Delay Speed of conduction along an axon is about 40m/s Conduction through a reflex arc is slower and more variable, sometimes 15 m/s or less Temporal Summation Sherrington observed that several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus Temporal Summation o Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak Spatial Summation Sherrington also noticed that several small stimuli in a similar location produces a reflex when a single stimuli did not Spatial Summation o Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse Directional Effects of Summation Excitatory Postsynaptic Potential (EPSP) Presynaptic Neuron Neuron that delivers the synaptic transmission Postsynaptic Neuron Neuron that receives the message Excitatory Postsynaptic Potential Graded potential that decays over time and space The cumulative effect of EPSPs form the basis for temporal and spatial summation Summation Summary Spatial Summation is critical to brain functioning Each neuron receives signals from many incoming axons that frequently produce synchronized responses The order of a series of axons influences the results Temporal Summation and Spatial Summation ordinarily occur together Inhibitory Synapse Sherrington noticed that the leg of a dog that was pinched retracted while the other three legs were extended And interneuron in the spinal cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs Inhibitory Postsynaptic Potential (IPSP) ISPS Defined the temporary hyperpolarization of a membrane

6 o Occurs when synaptic input selectively opens the gates for a positively charged potassium ions to leave the cell, or negatively charged chloride ions to enter the cells Serves as an active brake that suppresses excitation Relationship Among EPSP, IPSP, and Action Potentials Sherrington assumed that synapses produce on/off responses Synapses vary enormously in their duration of effects o The effect of two synapses at the same time can have greater than 2 times the effect of either one, or less than double Spontaneous Firing Rate Defined The periodic production of action potentials despite synaptic input o EPSPs increase the number of action potentials above the spontaneous firing rate o IPSPs decrease the number of action potentials below the spontaneous firing rate

7 Neurotransmitters Three Major Classes Cholinergic Monoamine Cholinergic Neurotransmitters Acetylcholine (ACh) Location Peripheral Nervous System and Ganglion o Cranial nerves, spinal nerves, and autonomic nervous system Functions o Parasympathetic (uses ACh almost exclusively) o Sympathetic o Stimulates receptors at neuromuscular junction of skeletal muscles (enabling muscle contraction) o Involved in learning ACh is broken down by an enzyme called Acetylcholinesterase (Acetate and Choline) o This enzyme plays a key role in the development of Neurocognitive Disorder (Dementia) due to Alzheimer s Disease Two Types of Cholinergic Receptors Muscarinic (metabotropic) o Mediate hyperpolarization (IPSP) and slow depolarization (Slow EPSP); these represent recovery of the postganglionic neuron Nicotinic (ionotropic) o Responsible for initial fast depolarization (Fast EPSP) of postganglionic neuron Myasthenia Gravis Related to ACh An autoimmune neuromuscular disorder associated with decreased synaptic ACh receptors at the neuromuscular junction Key Symptoms o Drooping eyelids o Double vision o Impaired speech and swallowing o Fatigue (extreme) Treatment o Long-term administration of acetylcholinesterase inhibitors (Aricept), to increase neuromuscular transmission o Thymectomy (resection of thymus; uncertain reason)