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Physiology 2130
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Physiology 2130 Module 6 Online Notes (Sec 6.1 to 6.54) Nervous System Introduction  Nervous system consists of the central nervous system (CNS) and peripheral nervous system (PNS)  CNS is made up of the brain and spinal cord  Nerves outside the CNS that go to muscles and organs (heart) are part of PNS  PNS can be divided into somatomotor (going to skeletal muscles) and autonomic (going to other organs) nervous systems Basic Structure of the Brain  Brain has several large anatomical features:  There are two cerebral hemispheres (left and right hemisphere)  Left hemisphere sends signals to activate muscles on right side of body  Similarly, sensory information from right side goes to left hemisphere  Right hemisphere and left side same thing but vice versa  Brain stem controls most of the basic functions of the body (heart rate, respiration, etc,)  The brain stem is made up of the midbrain, pons, and medulla oblongata  Medulla is continuous with the spinal cord  The back (or posterior region), just above the brain stem is the cerebellum, which is mainly responsible for coordinated movement  The diencephalon consists of the thalamus and hypothalamus  There are many bumps called gyri and dips called sulci on the surface of the brain as shown in the coloured picture  These folds are most prominent in humans and increase surface area of the brain  Locations of sulci and gyri are quite consistent between individuals (may very slightly in size and shape) and are so prominent that they have specific names  Each cerebral hemisphere can be divided up into four lobes based on these “landmarks”  Within each lobe are regions with very specific functions Functional Structure of the Brain Lateral Frontal Lobe – The primary motor cortex, processes input form 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 of stimuli Temporal Lobe –contains 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 smell and mediating short-term memory storage and recall Parietal Lobe – Contains primary somatosensory cortex which receives input form major sense organs (skin, taste buds). The association areas of parietal lobe integrate sensory info with other association areas of the cortex to make meaningful perceptions Occipital Lobe – area of cerebral cortex responsible for vision, contains primary visual cortex, which receives input directly from the optic nerve Cerebellum – process sensory information and coordinates the execution of movement in the body, has most number of neurons in the brain, receives input from somatic receptors, receptors for equilibrium and, balance and motor neurons from the cortex Medial Corpus Callosum – dense bundle of nerve fibers that serve as a pathway and connection between two cerebral hemispheres, which allows the brain to integrate sensory and motor information from both sides of the body Diencephalon – consists of thalamus and hypothalamus, thalamus receives sensory input as it travels from the spinal cord and integrates it before sending it to the cortex. The hypothalamus controls a variety of endocrine functions (body temp. thirst, food intake) through directing release of hormones Pituitary Glands – Primarily regulates the other endocrine organs, the anterior pituitary is derived from epithelial tissue of the pharynx while the posterior pituitary derives from neural tissue of the hypothalamus. Anterior pituitary hormones include LH, FSH, ACTH, TSH, GH and prolactin, the posterior pituitary releases the hormones vasopressin and oxytocin; pituitary function is regulated by hypothalamus Midbrain – bridges the lower brainstem with the diencephalon above, mainly controls eye movement and exerts control over auditory and visual motor reflexes Pons – act as a relay station for transferring info between the cerebellum and the cerebral cortex, also coordinates and controls breathing with control centers in the medulla Medulla – controls involuntary functions such as breathing, blood pressure and swallowing. Ventral Optic Nerves – optic nerves from each eye meet at the optic chiasma where they cross over and continue on as optic tracts to the lateral geniculate bodies of the thalamus. From there, axons extend to their respective hemisphere on the primary visual area of the occipital lobe Brain Stem – an extension of the spinal cord, and consists of three regions: the midbrain, pons then medulla (left to right when looking at picture) Dorsal Primary Motor Cortex – at posterior end of frontal lobe, processes information relating ton skeletal muscle movement. When electrically stimulated causes specific muscles to contract, arrangement of motor cortex is very specific Primary Somatosensory Cortex – at anterior end of parietal lobe, receives sensory information from opposite side of body such as pain, temperature, touch, etc. Language and Mathematical Area – often located in left hemisphere, serves as a general interpretive center, enabling a person to understand visual and auditory information and in turn to generate written and spoken responses Neurons and Glial Cells  Brain is made up of tens of billions of neurons and glial cells  Neurons are information transmitting and processing cells of the body, but they only make up a small percentage of the entire brain  Glial cells make up about 90% of the brain and provide the necessary environment for neurons to work properly 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 two processes extending from the cell body, are a form of specialized neurons that can be found in the retina of the eye b) Unipolar neurons have one process extending from the cell body, they are located in the peripheral nerves outside the CNS and are generally sensory in nature, transmitting signals to and from the spinal cord. Their cell body lies in the middle and off to one side of the axon c) Multipolar neurons, contain many branching dendrites and one axon, most common the in the CNS Glial Cells  Support cells of the brain, maintain the delicate internal environment of the CNS  Roughly five times as many glial cells as neurons  They also regulate nutrients and specific interstitial environment of the brain by regulating passage of substances between the blood and the brain’s interstitial space  Several types of glial cells including astrocytes, microglia, and oligodendrocytes (which produce myelin). The Language of the Nervous System and Neural Coding  As previously discussed, information travels down axons as action potentials  Action potentials are the language of the nervous system  How does your brain know if you have a light object in your hand or a heavy object?  Special receptors detect the pressure on the skin and send action potentials to the brain  The weight of the object is “coded” into the action potential – the heavier the object the more action potentials per second  This is referred to as neural coding  The information has to be transmitted from the hand along several neurons to the brain  This requires that each neuron communicate with one another 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 will affect a postsynaptic nerve  The structure and process is very similar to the neuromuscular junction but there are many important differences aswell Structure of a Chemical Synapse Structure of a chemical synapse includes the following: 1) Axon terminal of the presynaptic cell containing a. Voltage gated calcium ion channels b. Synaptic Vesicles containing the neurotransmitter c. Mitochondria 2) Synaptic Cleft 3) Postsynaptic cell containing a) Chemical receptors b) Chemically gated ion channels (aka ligand-gated ion channels) open when a chemical attaches to them, in this case the chemical is the neurotransmitter Sequence of Events at a Chemical Synapse Events are as follows: 1. Presynaptic neurons synthesize neurotransmitters that are stored in the synaptic vesicles 2. An action potential in the presynaptic neuron depolarizes the membrane and activates voltage-gated Ca++ channels; Ca++ (the white molecules in the animation) flow into the axon terminal. 3. 3. Ca++ causes the synaptic vesicles to fuse to the wall of the synaptic terminal, causing exocytosis and the release of neurotransmitter. 4. 4. Neurotransmitter diffuses across the cleft and acts on chemical receptors found on the postsynaptic cell membrane. 5. 5. Receptors cause the opening of chemically gated ion channels. 6. 6. The postsynaptic membrane potential changes, causing a depolarization or hyperpolarization depending on the type of neurotransmitter. A depolarization increases the probability of an action potential on the postsynaptic neuron, while a hyperpolarization decreases the likelihood. Neurotransmitters  Neurotransmitters are chemicals released by neurons at their axon terminals  Synthesized within neuron and are stored in synaptic vesicles to be released in response to an action potential  After they are released, they diffuse across the synaptic cleft and produces a response in the postsynaptic neuron  Depending on which neurotransmitter this is, the response may be excitatory, leading to a depolarization of the postsynaptic cell, and if strong enough may fire an action potential. Or the neurotransmitter may 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  The main groups are: o Acetylcholine o Biogenic Amines o Amino Acids o Neuropeptides  The most common excitatory neurotransmitter is glutamate and the most common inhibitory neurotransmitter is gamma-amino-butyric acid (GABA)  Note, excitatory neurotransmitters excites or “turns on” a neuron while an inhibitory neurotransmitter “shuts off” the nerve cell  As seen earlier, the chemical synapse is very similar in structure and functions to the neuromuscular junction  However, there is one important difference between the two  At the NMJ a single action potential in the motor neuron produces a single action potential in the muscle cell causing it to contract  But, at the chemical synapse, a single action potential on a presynaptic neuron will not produce an action potential on a postsynaptic neuron! Ionic Basis of the Postsynaptic Potentials – EPSPs and IPSPs  How do you generate an action potential on a postsynaptic nerve cell?  The neurotransmitter will cause a local change in the membrane potential of the postsynaptic cell (either excitatory or inhibitory)  An excitatory neurotransmitter causes the opening of chemically gated channels, these gates are selective for only positive ions and will allow the influx of predominantly sodium ions into the cell o This will cause a local depolarization of the membrane called an excitatory postsynaptic potential (EPSP) o The EPSP is a very local event that diminishes with time and distance from its point of origin and as a result, is also called a graded potential  The influx of the sodium ions will depolarize the region of the dendrite, but it will not fire an action potential  This is because there are no voltage-gated channels on the dendrites or cell body of the neuron!  Voltage gated channels are essential for the production of an action potential, and an action potential begins at the axon hillock (region with the highest concentration of voltage-gated channels  Therefore, in order to generate the action potential, the EPSP must depolarize the axon hillock! EPSPs  EPSP gets smaller with the distance it has to travel  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  But how exactly do you make the EPSP strong enough to reach the axon hillock? Spatial and Temporal Summation of Synaptic Potentials  The strength of an EPSP can be increased in two ways o Spatial summation of EPSPs o Temporal summation of EPSPs  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 been generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron It is important to distinguish between an EPSP and an action potential. The EPSP, occurs only on the dendrites and cell body, and decrease with time and distance from the point of origin. Meanwhile the action potential is an all-or-nothing response and is usually only found on the axon. EPSPs can be added on top of the other while the action potential cannot. Spatial Summation  Each postsynaptic neuron can receive thousands of synapses from other nerve cells, so many EPSPs occurring simultaneously can be added together to produce a large depolarization  When this depolarization reaches the axon hillock, it opens a sufficient number of voltage-gated channels to reach threshold and to fire the action potential Temporal Summation  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 open a sufficient number of voltage-gated channels to reach threshold and fire the action potential IPSPs – Inhibitory Postsynaptic Potentials There are also inhibitory neurotransmitters whose effects are to shut off nerve cells, the neurotransmitters in this situation create a hyperpolarization called an inhibitory postsynaptic potential  Inhibitory neurotransmitters produce a hyperpolarization by opening different chemically gated channels  These channels, depending on the type of neurotransmitter, let either chloride ions (Cl ) into the cell (adding negative charge) or let potassium ions (K ) out
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