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Structure and Function of the Nervous System -------------------------------------------- \ OUTLINE **23** ### The Structure of Neurons   TAKE-HOME MESSAGES - Neurons and glial cells make up the nervous system. - Neurons are the cells that tra...

Structure and Function of the Nervous System -------------------------------------------- \ OUTLINE **23** ### The Structure of Neurons   TAKE-HOME MESSAGES - Neurons and glial cells make up the nervous system. - Neurons are the cells that transmit information throughout the nervous system. Most neurons consist of a cell soma (body), axon, and dendrites. - Neurons communicate with other neurons and cells at specialized structures called synapses, where chemical and electrical signals can be conveyed between neurons. ### Neuronal Signaling ##### The Membrane Potential Membrane ##### The Action Potential 20 0 --20 --40 --60 --80 0 30 20 10 0 a. An idealized neuron with myelinated axon and axon terminals. Voltage-gated ion channels located in the spike-triggering zone at the axon hillock, and along the extent to the axon, open and close rapidly, changing their conductance to specific ions (e.g., Na^+^), alerting the membrane potential and resulting in the action potential **(inset)**. **(b)** Relative time course of changes in membrane voltage during an action potential, and the underlying causative changes in membrane conductance to Na^+^ (*g*Na) and K^+^ (*g*K). The initial depolarizing phase of the action potential (red line) is mediated by increased Na^+^ conduc- TAKE-HOME MESSAGES - The presynaptic cell is located before the synapse with respect to information flow; the postsynaptic cell is located after the synapse with respect to information flow. Nearly all neurons are both pre- and postsynaptic, since they both receive and transmit information. - The resting membrane potential is the difference in the voltage across the neuronal membrane during rest (i.e., not during any phase of the action potential). - The electrical gradient results from the asymmetrical distribution of ions across the membrane. The electrical - Ion channels are formed by transmembrane proteins that create passageways through which ions can flow. - Ion channels can be either passive (always open) or gated (open only in the presence of electrical, chemical, or physical stimuli). - Passive current conduction is called electrotonic conduc- tion or decremental conduction. A depolarizing current makes the inside of the cell more positive and therefore more likely to generate an action potential; a hyperpolar- izing current makes the inside of the cell less positive and therefore less likely to generate an action potential. - Action potentials are an all-or-none phenomena: The am- plitude of the action potential does not depend on the size of the triggering depolarization, as long as that depolariza- tion reaches threshold for initiating the action potential. - Voltage-gated channels are of prime importance in generating an action potential because they open and close according to the membrane potential. - Myelin allows for the rapid transmission of action potentials down an axon. - Nodes of Ranvier are the spaces between sheaths of myelin where voltage-gated Na^+^ and K^+^ channels are located and action potentials occur. ### Synaptic Transmission ##### Chemical Transmission ##### Neurotransmitters - It is synthesized by and localized within the presynaptic neuron, and stored in the presynaptic terminal before release. - It is released by the presynaptic neuron when action potentials depolarize the terminal (mediated primarily by Ca2+). - The postsynaptic neuron contains receptors specific for the neurotransmitter. - When artificially applied to a postsynaptic cell, the 1. *Tachykinins* (brain-gut peptides). This group includes substance P, which affects vasoconstriction and is a spinal neurotransmitter involved in pain. 2. *Neurohypophyseal hormones.* Oxytocin and vaso- pressin are in this group. The former is involved in 3. *Hypothalamic releasing hormones.* This group includes corticotropin-releasing hormone, involved 4. *Opioid peptides.* This group is so named for its simi- lariγ to opiate drugs, permitting the neuropeptide to bind to opiate receptors. It includes the endorphins and enkephalins. 5. *Other neuropeptides*. This group includes peptides that do not fit neatly into another category. ##### Inactivation of Neurotransmitters after Release ##### Electrical Transmission TAKE-HOME MESSAGES - Synapses are the locations where one neuron can transfer information to another neuron or specialized non-neuronal cell. They are found on dendrites and at axon terminals but can also be found on the neuronal cell body. - Chemical transmission results in the release of neurotransmitters from the presynaptic neuron and the binding of those neurotransmitters on the postsynaptic neuron, which in turn causes excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs), depending on the properties of the postsynaptic receptor. - Classes of neurotransmitters include amino acids, biogenic amines, and neuropeptides. - Neurotransmitters must be removed from the receptor after binding. This removal can be accomplished by a. active reuptake back into the presynaptic terminal, b. enzymatic breakdown of the transmitter in the synaptic cleft, or (c) diffusion of the neurotransmitter away from the region of the synapse. - Electrical synapses are different than chemical synapses as they operate by passing current directly from one neuron (presynaptic) to another neuron (postsynaptic) via specialized channels in gap junctions that connect the cytoplasm of one cell directly to the other. ### The Role of Glial Cells Astrocytes are recognized for their supporting roles, so to speak, but recent evidence suggests that they have TAKE-HOME MESSAGES - An astrocyte is a type of glial cell that helps form the blood--brain barrier. - Astrocytes have an active role in modulating neural activity. - Glial cells aid in the speed of information transfer by forming myelin around the axons of the neurons. - An oligodendrocyte is a type of glial cell that forms myelin in the central nervous system. - A Schwann cell is a type of glial cell that forms myelin in the peripheral nervous system. - As part of the immune response of the nervous system, microglial cells are phagocytic cells that engulf damaged cells. ### The Bigger Picture ### Overview of Nervous System Structure ##### The Autonomic Nervous System ##### The Central Nervous System Parasympathetic branch Sympathetic branch Constricts pupil Inhibits tear glands Increases salivation Slows heart and pancreas (erection) TAKE-HOME MESSAGES - The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of all nerves and neurons outside of the central nervous system. - The autonomic nervous system is involved in controlling the action of smooth muscles, the heart, and various glands. - The sympathetic system uses the neurotransmitter norepinephrine. This system increases heart rate, diverts blood from the digestive tract to the somatic musculature, and prepares the body for fight-or-flight responses by stimulating the adrenal glands. - The parasympathetic system uses acetylcholine as a neurotransmitter. It is responsible for decreasing heart rate and stimulating digestion. - Groups of neurons are called ganglia. - The cerebral cortex is a continuous sheet of layered neurons in each hemisphere. - The axons of cortical neurons and subcortical ganglia travel together in white matter tracts that interconnect neurons in different parts of the brain and spinal cord. - The corpus callosum is the main fiber tract that connects the two hemispheres of the brain. ##### The Spinal Cord ### A Guided Tour of the Brain Coronal section tricle ##### The Brainstem: Medulla, Pons, Cerebellum, and Midbrain Cervical roots  - Many neurochemical systems have nuclei in the brain- stem that project widely to the cerebral cortex, limbic system, thalamus, and hypothalamus. - The cerebellum integrates information about the body and motor commands and modifies motor outflow to effect smooth, coordinated movements. ##### The Diencephalon: Thalamus and Hypothalamus TAKE-HOME MESSAGES - The spinal cord conducts the final motor signals to the muscles, and it relays sensory information from the body's peripheral receptors to the brain. - The brainstem's neurons carry out many sensory and motor processes, including visuomotor, auditory, and ves- tibular functions as well as sensation and motor control of the face, mouth, throat, respiratory system, and heart. - The brainstem houses fibers that pass from the cortex to the spinal cord and cerebellum, and sensory fibers that run from spinal levels to the thalamus and then to the cortex. Anterior nucleus Dorsal medial cortex TAKE-HOME MESSAGES - The thalamus is the relay station for almost all sensory information. - The hypothalamus is important for the autonomic nervous system and endocrine system. It controls functions necessary for the maintenance of homeostasis. It is also involved in control of the pituitary gland. - The pituitary gland releases hormones into the bloodstream where they can circulate to influence other tissues and organs (e.g., gonads). ##### The Telencephalon: Limbic System, Basal Ganglia, and Cerebral Cortex ### The Cerebral Cortex [ ] human cerebral cortex is about 2,200 to 2,400 cm2, but because of extensive folding, about two thirds of this area TAKE-HOME MESSAGES - The limbic system includes subcortical and cortical struc- tures that are interconnected and play a role in emotion. - The basal ganglia are involved in a variety of crucial brain functions, including action selection, action gating, reward-based learning, motor preparation, timing, task switching, and more. Central sulcus Right hemisphere Left hemisphere Central sulcus Parietal lobe ##### Dividing the Cortex Anatomically notch ##### Dividing the Cortex Cytoarchitectonically **a**  **b** A diagram of a brain Description automatically generated ##### Functional Divisions of the Cortex Central sulcus S1 S2 Hip Knee Ankle Toes Tongue Tongue Swallowing \ TAKE-HOME MESSAGES - Gyri are the protruding areas seen on the surface of the cortex; sulci, or fissures, are the enfolded regions of cortex. - Brodmann divided the brain into distinct regions based on the underlying cytoarchitectonics. - The lobes of the brain include the frontal, parietal, temporal, and occipital lobes. - The frontal lobe is for planning, cognitive control, and execution of movements. The parietal lobe receives sensory input about touch, pain, temperature, and limb position, and it is involved in coding space and coordinating actions. - The temporal lobe contains auditory, visual, and multi- modal processing areas. The occipital lobe processes visual information. The limbic lobe (not really a lobe) is involved in emotional processing, learning, and memory. - Topography is the principle that the anatomical organiza- tion of the body is reflected in the cortical representa- tion of the body, both in the sensory cortex and motor cortex. - Association cortices are those regions of cortex out- side the sensory specific and motor cortical regions. Association cortex receives and integrates input from multiple sensory modalities. - *Decreased long distance brain connectiv- ity with increasing size.* The number of neurons that an average neuron connects to actually does not change with increas- ing brain size. By maintaining absolute connectivity, not proportional connectivity, large brains became less interconnected. No need to worry about this, because evo- lution came up with two clever solutions. - *Minimizing connection lengths.* Short con- nections keep processing localized, with the result that less space is needed fro the shorter axons, less energy is required, and signaling is faster over shorter distances. - *Not all connections are minimized, but some very long connections between distant sites are retained.* Primate brains in general, and human brains in particular, have developed what is known as "small-world architecture," which is common to many complex sys- tems, including human social relations. This type of organizational structure combines many short fast local connections with a few long distance ones to communicate