Weeks 4-5 Notes
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Date Created: 02/10/16
Biological Psychology WEEK 4 NOTES Early sensory experiences can have a profound effect on the organization of the nervous system Mice are highly dependent on their whiskers for survival as they are a major source of tactile stimulation and their whiskers are well represented by neurons in the brain If some of the whiskers are removed a few days after birth so that their corresponding neurons are not stimulated, the neurons fail to develop Depriving cats immediately after birth of visual stimulation by wearing goggles results in similar effects, the neurons in the visual system fail to develop Neurons die from not being stimulated during the critical period early in life, aging, injuries, and diseases Neurons, like all other cells, have a limited life span Neurons begin to die from the moment of birth Dead neurons are not replaced Neurons do not go through mitosis and they cannot reproduce Neurons need stimulation especially during the critical period, if not they die off Axonal sprouting – when damaged neurons work and bind to healthy neurons to “pick up the slack” Rehab helps other neurons to take over the jobs of dead neurons Interneuron – neurons in the CNS (brain and spinal cord) Sensory and motor neurons – neurons in the PNS Neurons have a unique capacity, the capacity to compensate for the loss of neighboring neurons by taking over their functions through a process known as the axonal sprouting We are born with far more neurons (70-100 billion in the brain) than we need But even the surplus pool can run out and it is possibly this situation that produces senility among old people When an axon is severed, the first part to go is its detached end Deprive of chemicals produced by Nissl substance, the detached end has no way of maintaining itself and soon dies This stage is known as Wallerian degeneration The next part of the neuron to break down is the part of the axon that remains attached to the soma This breakdown, known as retrograde degeneration usually coincides with the deterioration of Nissl substances in the soma and dendrites – a process known as chromatolysis 3 stages of degeneration: o Wallerian degeneration o Retrograde degeneration o Chromatolysis Retrograde and chromatolysis cannot be reversed in the CNS, but they can be reversed in the PNS with glial (Schwann) cell Astrocytes reside in the blood brain barrier Damage to interneurons in the CNS appears to be irreversible; damage to motor and sensory neurons in the PNS, the retrograde degeneration and chromatolysis can be reversed, and the axon can grow back to the original site The differences are thought to be related to differences in glial cells, rather than in the neurons themselves Neurons have the capacity to regenerate in both the CNS and the PNS In the CNS, they meet resistance from scars formed by the astrocytes In the PNS, they encounter no such resistance from glial cells, the Schwann cells can guide the regenerating axon back to its original connection Voltage allows electrical impulses to occur, the membrane separates + and – ions Ions – charged particles Neural impulse travels away from the soma Refractory period – inhibition to fire, a break between “fires” A neural impulse is slower than electricity and is not controlled by electrical impulses Stages of neural impulse: o Resting state – polarized (all balanced) 70 mili-volts o Firing state – depolarized o Refractory Period Absolute refractory period 1 mili-second (1/1000) Relative refractory period 2-4 mili-seconds Neuron may be able to re-fire now if the incoming stimulation is very strong (skips recovery state) o Recovery state – repolarized Ion channels – specific sites for different ions (K+, Na+, Cl-, Ca+, NaCl) Outside the membrane – Na+ (some K+) Inside the membrane – K+ (some Na+) Concentration gradient – Na+ wants to come in and K+ wants to go out via their specific ion channels Ion channels (more info): o Passive (always open) some channels for each Na+ and K+ o Gated channel (active channel) – closed during resting state, controlled by voltage specific channels for each ion There are selective channels in favor of K+ o For every 1 Na+ ion comes in, 50-75 K+ ions come in Neuron is stabilized by the Na+ - K+ pump which maintains the neuron at 70 mili-volts The neuron in the resting state registers a potential of -70 mili-volts (mv) A D-cell battery, by comparison have 1.5 volts = 1,500 mv The charge outside the neuron is (+) with respect to the charge inside the neuron (-) When a membrane of the neuron is in the resting state, it is said to be polarized A neuron generates impulses through a temporary breakdown in the membrane The initial flow of current that follows this breakdown is known as firing Once the neuron has fired, we say that it is depolarized After current is produced, a neuron has the capacity to separate the charges and to recharge itself o After the firing, the neuron is able to repolarize This sequence of electrical events – firing and repolarization is known as the action potential The 3 basic condition govern electrical activity in the neuron are: o 1. The resting state – the neuron is polarized o 2. The firing state – the neuron is depolarized; and o 3. The recovery state – the neuron is repolarized WEEK 5 NOTES Immediately after stimulation, when the neuron fires, the membrane becomes more permeable to Na+ and less permeable to K+ The Na+ ions accumulate inside the neuron to such a degree that the electrical charge is reversed It becomes positive on the inside and negative on the outside The potential now measures +30 mv (inside positive) The reloading occurs after the firing, the membrane’s permeability to K+ to the rise and the K+ ions to move outward The permeability to Na+ declines, the Na-K pump swings back into action, and the ionic balance gradually returns to its resting level (-70 mv) K-Na pump keeps the neuron depolarized at resting state -90 mv = hyperpolarization The firing process occurs quickly but recovery takes more time For about .5 mili-seconds after the neuron has fired, it is in a completely insensitive state and cannot be re-excited – is called absolute refractory period, coincides with the inrush of Na+ Then for approximately for 2-4 msec, as the neuron begins to regain its sensitivity, it is capable of being re-excited, but only by intense stimulation – it is called relative refractory period, coincides with the outrush of K+ Speed of a neural impulse: slowest 10 meters/second & fastest 120 meters/second Ion channels are microscopic pores that penetrate the membrane and govern the flow of ions into and out of the neuron during the resting and action potentials There are 2 types of ion channels: o Active (gated) and passive (always open) Each channel, whether active or passive, is selective for a specific ion – such as Na+, K+, and Cl- Passive channels determine membrane permeability during the resting potential There are more passive channels for K+ than for Na+ and hence the membrane is more permeable to K+ than to Na+ (50 – 70 times) Active channels, on the other hand, determine membrane permeability during the action potential The gates of the active channels are controlled by the electrical charge (voltage) across the membrane During the resting potential (-70 mv) – the gates are closed During the action potential (+30 mv) – the gates are open 1. The gates of the Na+ channels open, accounting for the rush of Na+ into the neuron and the subsequent firing phase of the action potential; 2. Then, roughly 1.0 msec later, the gates of the Na+ channels close and the gates of the K+ channels open for the next 2-4 msec The resulting outward flow of K+ repolarize the membrane back to -70 mv There are approximately 500 Na+ channel per square micrometer of membrane surface in the giant axon of the squid, the channels are separated by a distance of roughly 450 angstroms Sequential depolarization – a stimulus initiates a succession of action potentials that propagate down the membrane and produce the neural impulse Even though the Na+ rush occurs only in the section of the neural membrane that receives stimulation, the neighboring areas are affected as well They produce a charge in membrane permeability and allow for additional Na+ flow The domino effect that occurs in unmyelinated axons: Push the first domino and it will knock down the second, which will knock down the third, and so forth Changes in permeability along the membrane take place in a sequential pattern and produce the Na+ movement point by point along the membrane The neural impulse is a wave of depolarization along the neural membrane The neural impulses occur in unmyelinated axons are slow (10 meters per second) Saltatory conduction that occurs in myelinated axons: Myelin sheath covers the axon, and speeds up conduction along the axon The myelin sheath, along with the nodes of Ranvier, plays a crucial function in the conduction of the neural impulse Myelin makes neural impulse jump from node to node, instead of moving sequentially along every part of the neural membrane Depolarization in a myelinated axon gives rise to a phenomenon called saltatory conduction which can speed up to 120 meters per second The neural impulse in the axon follows all or non principle In order to generate electrical activity in the axon, the stimulus must reach a certain level of intensity – it must pass a threshold of .55 mv When the threshold is reached, the resting potential in the neuron goes from -70 mv to -55 mv and axonal firing occurs Once the intensity of the stimulus has passed the threshold, the magnitude of the impulse remains the same (+30 mv) no matter how intense the stimulus becomes This is an all or none effect in the neural firing of the axon The impulse does not decrease its strength as it travels farther from the point of stimulation – is called nondecremental impulse A mild stimulus applied to either the dendrites of the soma of a neuron produces an impulse Nondecremental – no decrease If a neural impulse is to influence behavior, it cannot stay within the neuron; it must travel from neuron to neuron or from neuron to muscle Before information can pass from one neuron to another (or to a muscle) a series of complex chemical events must take place across the microscopic gap separating the neurons The gap is called the synaptic gap, or synapse (Greek word for fasten together) it is about 100 to 200 angstroms (1/10 nanometer) wide The chemical substances that govern the transmission of neural impulses from one neuron to the next are called neurotransmitters Graded – degree on intensity of stimulus on dendrites or soma determines impulse Excitation vs. inhibition ( + vs. - ) Summation – multiple stimuli to crease neural impulse, two types: o Spatial – different spot stimulation o Temporal – same spot over and over stimulation A neurotransmitter is capable of changing their membrane permeability of the neuron of muscle it has reached Thus, it triggers or inhibits the neuron impulse or muscle contraction Up to now, 100 or more neurotransmitters have been discovered in the human nervous system The production of most neurotransmitters takes place in the axonal endings of the neuron The chemical reactions that produce neurotransmitters are usually directed by enzymes The enzymes themselves are produced in the soma of the neuron Then they move down the axon to the axonal endings through structures known as neurotubules (microtubules) The process of the movement of substance from soma to axonal endings is called axonal transport After the neurotransmitters are produced in the axonal endings, they are stored in tiny sacs, called synaptic vesicles Synaptic vesicles, sometimes described as neurotransmitter warehouses, are formed in the soma and transported to the axonal endings by axonal transport Axonal flow involves more than the flow from soma to the axonal endings of synaptic vesicles and of the enzymes needed to produce neurotransmitters (axonal transport) Transport from axonal endings to soma is called retrograde transport, the axonal flow is a two-way traffic The neuron that released the neurotransmitters into the synaptic gap is called the presynaptic neuron The neuron that receives the neurotransmitters is called the postsynaptic neuron Pre-synaptic neuron – neurotransmitters Post-synaptic neuron – receptor sites Permeability to Na+ and Cl Na+, K+ goes up = + Cl-, K+ goes up = - Axonal transport ----> Retrograde transport <---- Modulation neuron releases neuromodulators which control how much calcium ions are released to enter end of pre-synaptic neuron Nissl substances – mitochondrion, lysosomes, ribosomes, etc. Auto-receptors on membrane of axonal endings monitors how many neurotransmitters are released into the synapse and stops the release when there is enough (regulation mechanism) Exocytosis – when Ca+ comes into axon endings after neural impulse, synaptic vesicles move to the membrane and fuse with inner membrane after neural impulse at (axonal ending), then membrane ruptures so neurotransmitters are released into the synapse, neurotransmitters look to bind to receptor sites o 1. Move to o 2. Fuse o 3. Rupture Sub-sensitivity of post synaptic neuron, opening more receptor sites to increase binding of neurotransmitters, post-synaptic neuron, super sensitivity when there are not enough neurotransmitters (defense mechanism) 100 kinds of neurotransmitters o Excitatory EPSP – excitatory post-synaptic potential Caused by binding to neurotransmitters + Change membrane permeability to allow more Na+ Summation – depolarization 30 mv o Inhibitory IPSP – inhibitory post-synaptic potential Caused by binding to neurotransmitters - Change membrane permeability to allow more Cl- Summation – hyperpolarization -90 mv QUIZ 2
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