Neuronal Integration Lecture Notes PDF

Lecture 2 - Neuronal Integration (LSR) Outline 1. Structure of neurons 2. Groups of neurons & axons 3. Glia 4. Classifications by: o Process o Target o Direction of conduction o Shape o Transmitter o Effect 5. Electrical properties of neurons 6. Axonal conduction & myelin * Notes...

Lecture 2 - Neuronal Integration (LSR) Outline 1. Structure of neurons 2. Groups of neurons & axons 3. Glia 4. Classifications by: o Process o Target o Direction of conduction o Shape o Transmitter o Effect 5. Electrical properties of neurons 6. Axonal conduction & myelin * Notes are put together by your lecturers as a resource, but they do not cover everything discussed in class and should not be used as an alternative to taking notes during lectures * ** The below figure numbers are referring to your Nolte textbook ** The Structure of Neurons: Be able to define and describe the structure and function of various parts of the neuron including cell body (soma, perikaryon), dendrite, axon, axon terminal, and synapse. (Fig. 1-3) Neuronal structure - information normally flows from synapse on dendrite to soma to axon to next synapse at axon terminal. Dendrite - highly branched structure, receives input from other neurons at synapses (neurotransmitters), often has dendritic spines (site of some synapses). Generally range from hundreds of micrometers to about a millimeter in length. Soma (= cell body, perikaryon) - metabolic center of the cell (e.g., has mitochondria for energy metabolism and ribosomes for protein synthesis) and often integrates electrical information. Usually 5-100 micrometers in diameter. Axon - cylindrical structure with some branches (may have collateral branches going to different targets, within a target will have several branches near the terminals, where transmitter is released), conducts electrical signals in the form of action potentials, often insulted with myelin for faster conduction. Can be hundreds of micrometers to over a meter in length. Synaptic terminal - enlarged endings at the tips of axons which contain synaptic vesicles. These vesicles contain the neurotransmitter. When an action potential depolarizes the nerve terminal, synaptic vesicles fuse with the presynaptic membrane and release the neurotransmitter onto the postsynaptic cell. II. Groups of Neurons and their Components: Be able to use appropriate terms to describe groups or populations of neurons. Central Nervous System (CNS) Groups of functionally related nerve cell bodies nucleus - a group of neurons column - a columnar-shaped group of neurons Other groups of nerve cell bodies (not necessarily functionally related) grey matter, cortex, layer, lamina, or stratum - a group that forms a layer parallel to the surface of the structure. Groups of parallel axons tract = a bundle of axons that are functionally related fasiculus, or lemniscus = a bundle of axons, can be functionally related white matter, funiculus = a group of several tracts or fasiculi (often carries multiple modalities) Peripheral Nervous System (PNS) Group of nerve cell bodies - ganglion Group of parallel axons - nerve, ramus, or root III. Describe the differences between neurons and glia. Neurons (i.e., nerve cell) use electrical signals to receive, conduct, and propagate information. They operate on a fast time scale (i.e., milliseconds). Glial cells are primarily supportive with little or no electrical processing. They shuttle nutrients and wastes between neurons and the blood, and they maintain the electrochemical environment, provide structural support, guide growth of axons during development, and provide myelin coating for some axons. CNS has astrocytes and oligodendrocytes. PNS has Schwann cells. Oligodendrocytes and Schwann cells can myelinate axons. IV. Classification of Neurons: By processes: Identify and describe the three main types of neurons (multipolar, pseudounipolar, and bipolar), and include differences. (Fig. 1-4) Multipolar - multiple processes from the soma. Usually multiple dendrites and an axon. Examples include Purkinje cell in cerebellum and pyramidal cell in cerebral cortex. Bipolar - two processes from soma. An example in the retina is the retinal bipolar cell. Unipolar & Pseudounipolar - single process from the soma. A pseudounipolar neuron has one process that branches into two processes. Example includes the posterior/dorsal root ganglion cell, which has a central and a peripheral branch. By Target: projection vs. local circuit (Fig 22-4) projection neuron = neuron that projects out of a structure to another structure interneuron = local circuit neuron that projects to an area inside a structure By direction of conduction: afferent vs. efferent (Fig 1-4) afferent = in, a neuron or fiber that projects into a structure (e.g., DRG neuron projecting into the CNS) efferent = out, a neuron or fiber that projects out of a structure (e.g., 1° motor neuron projecting out of the CNS) Note: this term is always relative to a particular structure!!! By Shape: (Fig 22-4 blow up) Refers to shape of soma and/or dendrites (e.g., pyramidal, basket, stellate, and Chandelier cell) By Transmitter: Refers to the type of transmitter that the neuron releases at synapses (e.g., glutamate, GABA, Acetylcholine, Norepinephrine...) Thus these neurons can be referred to as glutamatergic, GABAergic, cholinergic and noradrenergic (norepinephrin is the same as noradrenaline). Neurotransmitter = a chemical released by a presynaptic neuron that activates receptors on the postsynaptic membrane, and usually causes a change in the membrane potential of the cell (i.e., depolarization or hyperpolarization). By Effect: excitatory vs. inhibitory Refers to whether the neurotransmitter released from the terminal excites or inhibits next neuron(s). Neurons with transmitters that depolarize other neurons are generally excitatory; neurons with transmitters that hyperpolarize other neurons are usually inhibitory. However, remember that the effect depends on both the neurotransmitter and the post-synaptic receptors. Some neurotransmitters can be excitatory or inhibitory based on the receptor they interact with. V. Electrical Properties of Neurons: Be able to write a sentence that describes what each of the following terms means, and describe their general role in neural circuits. (Fig 7-1 and integration schematic). Resting membrane potential - the negative voltage across the cell membrane, usually between -60 mV and -80 mV. This is generally due to a high permeability to potassium ions. Depolarization and hyperpolarization - a change in voltage that makes the membrane potential less negative (i.e., depolarizing or excitatory) or more negative (i.e., hyperpolarizing or inhibitory). Graded potential - a depolarization or hyperpolarization whose amplitude is a continuous function of the amplitude of the input. Decreases in amplitude as it moves away from the site of induction - good for short distances only! Receptor potential - a graded potential (depolarization or hyperpolarization) generated in a receptor cell in response to a sensory stimulus. Action potential - a relatively brief change in membrane potential caused by the fast opening and closing of voltage-activated ion channels in the membrane. A depolarization to "threshold” causes an all-or-none "spike” that propagates in a wave along an axon to transmit electrical information over long distances in the nervous system. Postsynaptic potential (excitatory versus inhibitory) - a graded potential caused by a neurotransmitter that is either depolarizing (i.e., excitatory) or hyperpolarizing (i.e., inhibitory). If an excitatory postsynaptic potential (EPSP) is large enough to depolarize a neuron to threshold, the neuron will fire an action potential. Inhibitory postsynaptic potentials (IPSPs) can prevent action potentials by hyperpolarizing the membrane potential away from threshold. Integration Schematic VI. Axonal Conduction: Describe the principles of conduction velocity and the frequency of action potentials. Axon diameter - Describe the relationship between axon diameter and conduction velocity. Axons with a larger diameter have a faster conduction velocity. The lower internal resistance associated with the larger diameter enhances the spread of the local currents responsible for propagation of the action potential. Myelin – Explain the structure of a myelinated axon and describe how myelin affects conduction velocity. (Fig 1-23 & 1-29) Some axons are mostly "coated” by a sheath of myelin. This allows the action potential to "jump” from one spot without myelin to the next spot without myelin, which increases the speed of conduction. Frequency - the frequency and pattern of the action potentials are the "information” conducted along axons. Because action potentials are not graded in amplitude, but rather are all-or-none, they represent the intensity of a signal by their frequency. If the sum of all inputs just barely brings the neuron to threshold, then this neuron will likely fire only one action potential - indicative of a low intensity signal. If the sum of all inputs depolarizes the neuron well above the threshold potential, then this neuron will likely fire several action potentials in a row - indicative of a high intensity signal. NOTE: Graded potentials (both receptor potentials and synaptic potentials) do not propagate very far. As you increase the distance from the site of initiation, these potentials become smaller and smaller - they are completely gone within just a few mm's. Action potentials are actively propagated over long distances with NO decrease in amplitude. This is critical for sending electrical signals long distances (e.g., some motor neurons in the spinal cord must send electrical signals all the way to muscles in the toe - that can be more than a meter in a tall individual!). Graded potentials: useful for information processing. If the SUM of all the synaptic potentials (both excitatory and inhibitory) received by a neuron causes the trigger region to depolarize to threshold, then that neuron will fire an action potential down its axon and signal the downstream cells. If it does not reach threshold, then no signal is transmitted by that neuron. Action potentials: useful for transmitting an electrical signal over long distances.