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6 Nervous System.docx

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Western University
Physiology 2130
Anita Woods

 Nervous system consists of: o Central nervous system (CNS)  Brain and spinal cord o Peripheral Nervous system (PNS)  Nerves outside the CNS that go to muscles and organs  Somatomotor nervous system: nerves going to skeletal muscles  Autonomic nervous system: nerves going to organs The outside of the CNS that go to the muscles and organs →called PNS - PNS divided into 1. Somatomotor: going to skeletal muscle 2. Autonomic: going to other organs - Chemical synapse: the junction between two nerves Basic Structure of the Brain Structure Function Left hemisphere - Sends signals to activate muscles in the right side of the body - Sensory information from the right side of the body travels to the left hemisphere Right hemisphere - Sends signals to activate muscles in the left side of the body - Sensory information travels from left side of the body travels to right hemisphere Brain stem - Controls most basic functions of the body i.e. heart rate, respiration Made of: 1. The midbrain 2. Pons 3. Medulla oblongata which is continuous with the spinal cord Cerebellum - At posterior and just above the brain stem - Mainly responsible for coordinated movement Diencephalon - Consists of the thalamus and hypothalamus On the surface of the brain, there are: - These folds most prominent in humans  Gyri: bumps - ↑ SA of the brain - Locations of gyri and sulci are quite consistent between  Sulci: dips individuals (minor diff in shape and size) - Each hemisphere is divided up into four lobes - Within each lobe are regions that have specific functions Functional Structure of the Brain LATERAL Frontal lobe - Primary motor cortex - Processes input from skeletal muscles throughout the body, while the motor association area (premotor cortex) and the prefrontal cortex integrate movement information with other sensory inputs to generate perception (interpretation) of stimuli Temporal lobe - Primary auditory cortex and Auditory association areas, which receive and process signals from the auditory nerve and integrate them with other sensory inputs. - Other portions of the Temporal Lobe are involved in olfaction (smell) and in mediating short-term memory storage and recall. Parietal lobe - Primary somatosensory cortex - Receives input from the major sense organs (skin, musculoskeletal system, taste buds). The association areas of the Parietal Lobe integrate sensory information with other association areas of the cortex to form meaningful perceptions. Occipital lobe - Primary visual cortex - Receives input directly from the optic nerve, as well as visual association areas that further process visual information and integrate it with other sensory inputs - Area of the cerebral cortex responsible for vision Cerebellum - Processes sensory information and coordinates the execution of movement in the body - Structure with the largest number of neurons in the brain - Receives input from somatic receptors, receptors for equilibrium and balances and motor neurons from the cortex MEDIAL VENTRAL DORSAL Neurons: information transmitting and processing cells of the body - Small % of entire brain Glial cells: provide necessary environment for the neurons to function properly - Make up 90% of the brain Neurons Neurons found in mammals can be divided into three basic types based on the number of processes that emerge from the cell body: a) Bipolar neurons - Have 2 processes extending from the cell body - Specialized neurons found in the retina of the eye b) Unipolar neurons - Have 1 process extending from the cell’s body - Located in peripheral nerves outside the CNS - Sensory nature, transmit signals to and from the spinal cord - Cell body lies in the middle and off to one side of the axon c) Multipolar neurons - Contain many branching dendrites and 1 axon - Most common in CNS Glial Cells  The ―support‖ cells of the brains. T hey perform o A structural role (gluing things together) o Regulate the nutrients and specific interstitial environment of the brain. o Maintain delicate environment of the CNS  Approx.. 5 times as many glial cells than neurons  Regulate passage of substances between blood and brain’s interstitial space  Types of glial cells: - Astrocytes - Microglia - Oligodendrocytes → produce myelin Action potentials are the language of the nervous system  Information travels down axons in the form of action potentials  These action potentials are the language of the nervous system.  For example, how does your brain know if you have a light object in your hand or a heavy object? o In a situation like this, special receptors detect the pressure on the skin and send action potentials to the brain. o The weight of the object is "coded" into the action potential—the heavier the object, the more action potentials per second. This is called neural coding. o The information will have to be transmitted from the hand along several neurons to the brain. Synaptic Transmission: The Chemical Synapse  Nerve cells communicate with one another by a chemical synapse  At a chemical synapse, a presynaptic nerve will release a chemical called a neurotransmitter that affects a postsynaptic nerve Structure of chemical synapse 1. The axon terminal of the presynaptic cell containing a. voltage-gated calcium ion (Ca ) channels, b. synaptic vesicles containing the neurotransmitter, and c. mitochondria. 2. Synaptic cleft 3. The postsynaptic cell containing a. chemical receptors and b. chemically gated ion channels (also called ligand-gated ion channels). These open when a chemical attaches to them. In this case, the chemical is the neurotransmitter. Voltage gated Ca channels Chemically gated ion channels Sequence of Events at a Chemical Synapse 1. Presynaptic neurons synthesize neurotransmitters that are stored in synaptic vesicles. 2. An action potential in the presynaptic neuron depolarizes the membrane and activates voltage-gated Ca ++ channels; Ca flow into the axon terminal. ++ 3. Ca cause the synaptic vesicles to fuse to the wall of the synaptic terminal, causing exocytosis and the release of neurotransmitter (vesicles fuse to axon terminal). 4. Neurotransmitter diffuses across the cleft and acts on chemical receptors found on the postsynaptic cell membrane, causing a conformation change in the protein channel. 5. Receptors cause the opening of chemically gated ion channels allowing specific ion through. 6. The postsynaptic membrane potential changes, causing a depolarization or hyperpolarization depending on the type of neurotransmitter (depending on the ion). 7. Protein channels will then close, and the neurotransmitter will be broken down and taken back up by the presynaptic cell to be recycled and used again. A depolarization increases the probability of an action potential on the postsynaptic neuron, while a hyperpolarization decreases the likelihood.  ↑[Na] and ↑[Ca] outside the cell, ↑[K] inside nerves  Depolarization ↑ action potential on the postsynaptic neuron Probability of an action potential - Excitatory response…‖turns on‖ a neuron  Hyperpolarization ↓ action potential on the postsynaptic neuron - Inhibitory response…‖turns off‖ a neuron  Neurotransmitters are chemicals released by neurons at their axon terminals.  They are synthesized within the neuron and are stored in synaptic vesicles to be released in response to an action potential.  After being released, the neurotransmitter diffuses across the synaptic cleft and produces a response in the postsynaptic neuron.  Depending on the type of neurotransmitter, this response may be excitatory, leading to a depolarization of the postsynaptic cell. If the depolarization is strong enough, it may fire an action potential.  On the other hand, the neurotransmitter could produce an inhibitory response leading to a hyperpolarization of the postsynaptic membrane and making it harder to generate an action potential.  There are four different groups of neurotransmitters classified according to their chemical makeup. The main neurotransmitters are o Acetylcholine (which we have seen before at the neuromuscular junction) o Biogenic amines o Amino acids o Neuropeptides.  The most common excitatory neurotransmitter is glutamate.  The most common inhibitory neurotransmitter is gamma-amino-butyric acid (GABA).  Remember, an excitatory neurotransmitter excites or "turns on" a neuron, while an inhibitory neurotransmitter shuts off the nerve cell. Most common  The chemical synapse is very similar to the structure and function of the neuromuscular junction  Important difference is that NMJ, a single action potential in the motor neuron produces a single action potential in the muscle cell causing muscle to contract  In a chemical synapse, a single action potential on a presynaptic neuron will not produce an action potential on a postsynaptic neuron! Ionic basis of postsynaptic potentials – EPSPs and IPSPs Excitatory Postsynaptic Potential - EPSP  Excitatory neurotransmitter will cause the opening of chemically gated channels - These gates are selective for only positive ions and will allow the influx of predominantly sodium ions (Na ) into the cell - Will cause a local depolarization of the membrane called an excitatory postsynaptic potential (EPSP) - EPSP is a very local event that diminishes with time and distance from its point of origin, is also called a graded potential + - The influx of Na will depolarize the region of the dendrite, but it will not fire an action potential. Why not? o Because, there are no voltage-gated channels on the dendrites or cell body of the neuron! o Remember: Voltage-gated channels are essential for the production of an action potential, and the action potential begins at the axon hillock where there is the highest concentration of voltage-gated channels. o Thus, in order to generate the action potential, the EPSP must depolarize the axon hillock. EPSPs  One of the characteristics of EPSP that we mentioned earlier is that it gets smaller with the distance it has to travel (graded potential)  Therefore, in order to cause a sufficient depolarization to open the voltage-gated sodium channels located at the axon hillock, the positive current of the EPSP must be strong enough to spread all the way from the synapse where it originated to the axon hillock. Now you can have an action potential.  A question still remains: How do you make the EPSP strong enough to reach the axon hillock? Strength of EPSP increases by 2 ways: 1. Spatial summation of EPSPs 2. Temporal summation of EPSPs  Additive effect Spatial summation of EPSPs is the additive effect produced by many EPSPs that have been generated at many different synapses on the same postsynaptic neuron at the same time. Temporal summation of EPSPs is the additive effect produced by many EPSPs that have been generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron. EPSP Action Potential - Usually found only on the axon - Occurs only in the dendrites and cell body - Decrease with time and distance from its - All or nothing effect point of origin - Cannot be added on top of each other - Can be added one on top of the other Spatial summation Since each postsynaptic neuron can receive thousands of synapses from other nerve cells, many EPSPs occurring simultaneously at many different synapses can be added together to produce a large depolarization. When this depolarization reaches the axon hillock, it will open a sufficient number of voltage-gated channels to reach threshold and to fire the action potential. Temporal Summation Temporal summation is the summing of a series of consecutive EPSPs that were generated by a set of high-frequency action potentials at the same synapse over a short period of time. Like spatial summation, when this depolarization reaches the axon hillock, it will open a sufficient number of voltage- gated channels to reach threshold and to fire the action potential. Inhibitory Postsynaptic Potentials – IPSPs  There are also inhibitory neurotransmitters whose effects are to shut off nerve cells.  Neurotransmitters in this situation create a hyperpolarization called an inhibitory postsynaptic potential (or IPSP)  Inhibitory neurotransmitters produce a hyperpolarization by opening different chemically gated channels.  These channels, depending on the type of neurotransmitter, will either let chloride ions (Cl ) + into the cell (adding negative charge) or let potassium ions (K ) out (removing positive charge).  The overall effect is the same—that is to make the membrane potential more negative, creating a hyperpolarization.  This local hyperpolarization is called an inhibitory postsynaptic potential (IPSP).  The hyperpolarization will move the membrane potential further away from threshold, making it less likely to fire an action potential; this will, essentially, shut off the nerve cell.  It should be noted that spatial and temporal summation can occur with
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