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Western University
Physiology 2130
Anita Woods

Nervous System Section 6.1 Objectives • Draw and label a diagram of the human brain, showing all the major regions, important gyri and sulci, and the major functional areas of movements, sensory, vision, hearing, speech, and so on. • Name two main types of brain cells. • Draw and label a chemical synapse. • Describe the events underlying synaptic transmission. • Name the four classes of neurotransmitters. • Name the main excitatory and inhibitory neurotransmitters in the brain. • Describe the ionic mechanisms and the changes in membrane potential associated with an excitatory postsynaptic potential (EPSP) and an inhibitory postsynaptic potential (IPSP). • Define spatial and temporal summation. • Draw and explain the arrangement of the motor system. • Define the motor cortex. • Draw a simple diagram of the corticospinal tract. • Draw a simplified diagram of a muscle spindle. • Draw a diagram of the reflex arc for the stretch reflex (for example, knee jerk reflex), and describe the sequence of events in this reflex. • Describe alpha-gamma coactivation. • Name three specific functions of the cerebellum. • Name seven behaviours influenced by the limbic system. • Name seven major functions of the hypothalamus. • List the two divisions of theAutonomic Nervous System (ANS). • Describe the pathways of the Parasympathetic NS (PSYN) and Sympathetic NS (SYN). • List the functions of the PNS and SNS. Section 6.2 Introduction • Nervous system:  Consists of the central nervous system (CNS) and peripheral nervous system (PNS). • The CNS is made up of the brain and spinal cord, while the nerves outside the CNS that go to muscles and organs like the heart are considered part of the PNS. We can therefore divide the PNS into somatomotor (going to skeletal muscles) and autonomic (going to other organs) nervous systems. Section 6.3 Introduction (cont’d) Section 6.4 Basic Structure of the Brain • The brain has several large anatomical features. • There are two cerebral hemispheres — a left and a right hemisphere. • The left hemisphere sends signals to activate muscles on the right side of the body; similarly, sensory information from the right side of the body travels to the left hemisphere (and vice versa). • The brain stem:  Controls some of the most basic functions of the body like heart rate and respiration  Made up of the midbrain, pons, and medulla oblongata.  The medulla:  Continuous with the spinal cord.  Cerebellum:  At the back, or posterior region, and just above the brain stem  Mainly responsible for coordinated movement.  Diencephalon:  Consists of the thalamus and hypothalamus. Section 6.5 Basic Structure of the Brain (cont’d) • On the surface of the brain, there are many bumps (called gyri) and dips (called sulci). • These folds are most prominent in humans and increase the surface area of the brain. • The locations of the sulci and gyri are quite consistent between individuals (with only minor differences in size and shape). • Each cerebral hemisphere can be divided up into four lobes based on these "landmarks." Section 6.6 Functional Structure of the Brain • Lateral: 1. Frontal Lobe:  The primary motor cortex processes input from skeletal muscles throughout the body.  The motor association area (premotor cortex) and the prefrontal cortex integrate movement information with other sensory inputs to generate perception (interpretation) of stimuli. 2. Temporal Lobe:  It contains the 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. 3. Parietal Lobe:  The Parietal Lobe contains the primary somatosensory cortex which receives input from the major sense organs (skin, musculoskeletal system and taste buds).  The association areas of the Parietal Lobe integrate sensory information with other association areas of the cortex to form meaningful perceptions. 4. Occipital Lobe: The OL is the area of the cerebral cortex responsible for vision. It contains the primary visual cortex which receives input directly from the optic nerve, as well as the visual association areas that further process visual information and integrate it with other sensory inputs. 5. Cerebellum:  It processes sensory information and coordinates the execution of movement in the body.  It is the structure in the brain that has the most neurons  It receives input from somatic receptors, receptors for equilibrium, and balance and motor neurons from the cortex. • Medial: 1. Corpus Callosum:  Adense bundle of nerve fibers that serve as a pathway and connection between the two cerebral hemispheres.  This connection allows the brain to integrate sensory and motor information from both sides of the body, and to coordinate whole- body movement and function. 2. Diencephalon:  Consists of two major areas: 1. The thalamus:  Receives sensory input as it travels from the spinal cord and integrates sensory information before sending it to the cortex. 2. The Hypothalamus:  Controls a variety of endocrine functions (body temperature, thirst, food intake, etc.), mainly through directing the release of hormones. 3. Pituitary Gland:  Regulates 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: LH, FSH,ACTH, TSH, GH and prolactin  The posterior pituitary releases the hormones vasopressin and oxytosin  Pituitary function is regulated by the hypothalamus 4. Midbrain:  The midbrain (or mesencephalon) bridges the lower brainstem with the diencephalon above.  Controlling eye movements  Exerts control over auditory and visual motor reflexes. 5. Pons:  Act as a relay station for transferring information between the cerebellum and the cerebral cortex.  Coordinates and controls breathing. 6. Medulla:  The portion of the brainstem that has primary control over involuntary functions such as breathing, blood pressure and swallowing.  It is also here that fibers from the corticospinal tract, which originate in the motor cortex, cross over to the opposite side of the spinal cord to innervate muscles on the opposite side of the body. 7. Cerebellum:  It processes sensory information and coordinates the execution of movement in the body.  It is the structure in the brain that has the most neurons  It receives input from somatic receptors, receptors for equilibrium, and balance and motor neurons from the cortex. • Ventral: 1. 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 2. Brain Stem:  An extension of the spinal cord and from left to right on the diagram, consists of three regions:  The midbrain  Pons  Medulla  The brain stem is a center form many involuntary functions (ex. breathing)  Incorporates 9 cranial nerves. 3. Cerebellum:  It processes sensory information and coordinates the execution of movement in the body.  It is the structure in the brain that has the most neurons  It receives input from somatic receptors, receptors for equilibrium, and balance and motor neurons from the cortex. • Dorsal: 1. Primary Motor Cortex:  At the posterior end of the frontal lobe, the primary motor cortices process info relating to skeletal muscle movement.  When electrically stimulated, it will cause a specific muscle to contract.  The arrangement of the motor cortex is very specific. 2. Primary Somatosensory Cortex:  At the anterior end of the parietal lobe, the PSC receives sensory info from the opposite side of the body.  The sensations of pain, temp, touch and vibration are processed here. 3. Language and Mathematical Area:  Mostly located in the left hemisphere (even for left-handed ppl)  Serves as a general interpretive center, enabling a person to understand visual and auditory information and in turn, to generate written and spoken responses. Section 6.7 Neurons and Glial Cells • The brain is made up of tens of billions of neurons and glial cells. • As we have seen, neurons are the information transmitting and processing cells of the body, yet they constitute only a small percentage of the entire brain. • Glial cells, on the other hand, make up about 90% of the brain and provide the necessary environment for the neurons to function properly. Section 6.8 Neurons • Neurons found in mammals can be divided into three basic types based on the number of processes that emerge from the cell body: 1. Bipolar neurons:  Have two processes extending from the cell body  Aform of specialized neurons that can be found in the retina of the eye. 2. Unipolar neurons:  Have one process extending from the cell's body  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. 3. Multipolar neurons:  Many branching dendrites and one axon  Most common in the CNS.  In the nerve cell Section 6.9 Glial Cells • The "support" cells of the brain, as they maintain the delicate internal environment of the CNS. • There are roughly five times as many glial cells as neurons. • Not only do they perform a structural role (gluing things together), but they also regulate the nutrients and specific interstitial environment of the brain. • They perform this function by regulating the passage of substances between the blood and the brain's interstitial space. • There are several types of glial cells, including astrocytes, microglia, and oligodendrocytes (which produce myelin). Section 6.10 The Language of the Nervous System and Neural Coding • Information travels down axons in the form of an action potential. 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? In a situation like this, 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 called neural coding. • The information will have to be transmitted from the hand along several neurons to the brain. This requires that each neuron communicate with one another. Let's see how this is done. Section 6.11 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. Section 6.12 Structure of a Chemical Synapse • The structure of the chemical synapse includes the following: 1. The axon terminal of the presynaptic cell containing: a) voltage-gated calcium ion (Ca ) channels b) synaptic vesicles containing the neurotransmitter c) mitochondria 2. Synaptic Cleft 3. The postsynaptic cell containing: a) Chemical receptors 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. • Video: The chemical synapse is found between the axon terminal of a presynaptic nerve and the dendrites of a second or post synaptic nerve. The synaptic cleft forms the gap between these two nerves. Specific protein channels are located on the membranes of each nerve; each performing a specific function. Within the axon terminal are mitochondria for the production ofATP and synaptic vesicles containing a specific neurotransmitter. The neurotransmitter is the chemical that will be released by the presynaptic nerve and will have an effect on the postsynaptic cell Section 6.13 Sequence of Events at a Chemical Synapse • The sequence of events at the chemical synapse follows: 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 (the white molecules in the animation) 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. 4. Neurotransmitter diffuses across the cleft and acts on chemical receptors found on the postsynaptic cell membrane. 5. Receptors cause the opening of chemically gated ion channels. 6. The postsynaptic membrane potential changes, causing a depolarization or hyperpolarization depending on the type of neurotransmitter. • Adepolarization increases the probability of an action potential on the postsynaptic neuron, while a hyperpolarization decreases the likelihood. • Video: You should remember that Na and Ca are found in high [ ]s outside these cells while the [ ] of K is high on the inside of these nerves. When the action potential which has been travelling down the axon, reaches the nerve terminal, it causes the opening of special Ca voltage gated channels. Ca will be allowed to flow into the cell down its electrochemical gradient. The influx of Ca causes a complex set of reactions within the nerve terminal that ultimately causes the synaptic vesicles to fuse to the axon terminal. When the vesicles fuse to the membrane they release the neurotransmitter which diffuses across the synaptic cleft. The neurotransmitter then binds to special receptors located on the membrane of the postsynaptic nerve. The binding of the neurotransmitter causes a conformational change in the protein channel associated with the receptor. This change causes the channels to open and allow only a specific ion to flow through. Depending on the ion, the postsynaptic cell will either depolarize or hyperpolarize. The protein channels with then close and the neurotransmitter will then be broken down and taken back up by the presynaptic cell to be recycled and used again. Section 6.14 Neurotransmitters • Neurotransmitters:  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. Section 6.15 Neurotransmitters (cont’d) • There are four different groups of neurotransmitters classified according to their chemical makeup. • The main neurotransmitters are:  Acetylcholine (which we have seen before at the neuromuscular junction)  Biogenic amines  Amino acids  Neuropeptides • The most common excitatory neurotransmitter = glutamate. • The most common inhibitory neurotransmitter = gamma-amino-butyric acid (GABA). • Remember, an excitatory neurotransmitter excites or "turns on" a neuron, while an inhibitory neurotransmitter shuts off the nerve cell. Section 6.16 Neurotransmitters (cont’d) • You have seen that the chemical synapse is very similar in structure and functions to the neuromuscular junction, or NMJ. • However, there is one very important difference between the two:  At the NMJ, a single action potential in the motor neuron produced a single action potential in the muscle cell, causing the muscle to contract.  At the chemical synapse, however, a single action potential on a presynaptic neuron will not produce an action potential on a postsynaptic neuron! • How, then, do you generate an action potential on a postsynaptic nerve cell? Let's have a look… Section 6.17 Ionic Basis of the Postsynaptic Potentials—EPSPs and IPSPs • As we have seen, the neurotransmitter will cause a local change in the membrane potential of the postsynaptic cell. • The potential can either be excitatory or inhibitory, depending on the type of neurotransmitter released. • An 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. • This will cause a local depolarization of the membrane called an excitatory postsynaptic potential (EPSP). • 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. Section 6.18 Ionic Basis of the Postsynaptic Potentials—EPSPs (cont’d) + • The influx of Na will depolarize the region of the dendrite, but it will not fire an action potential. Why not? Because, there are no voltage-gated channels on the dendrites or cell body of the neuron! • 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. • Thus, in order to generate the action potential, the EPSP must depolarize the axon hillock. Section 6.19 EPSPs • EPSPs get smaller with the distance it has to travel. • 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. • Aquestion still remains: How do you make the EPSP strong enough to reach the axon hillock? Let's have a look. Section 6.20 Spatial and Temporal Summation of Synaptic Potentials • The strength of an EPSP can be increased in two ways:  By spatial summation of EPSPs  By temporal summation of EPSPs. • Spatial summation of EPSPs:  It’s 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 ESPSs:  It’s 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. • It is important to distinguish between an EPSP and an action potential.  The EPSP, which occurs only on the dendrites and cell body, will decrease with time and distance from its point of origin, while the action potential is all-or-nothing and is usually only found on the axon.  Also, EPSPs can be added one on top of the other while the action potential cannot. Section 6.21 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. Section 6.22 Temporal Summation • Temporal summation:  It’s 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. Section 6.33 Inhibitory Postsynaptic Potentials – IPSPs • As mentioned previously, 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 (or IPSP). Section 6.24 IPSPs (cont’d) • 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 pot
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