Lecture 3 Flashcards
Cellular Neuroanatomy
Neurons and synapses, glial cells, neural tissue
The nervous system is composed of cells
Most neurons are interneurons (communicate between two other neurons). Cell bodies are about 100 micrometers in diameter. Synapses are at the scale of nanometers. Cells of the nervous system: neurons - foundation for communication within nervous system, glial cells - provide support for neurons and participate in information processing.
Nerve cells (neurons)
Foundation for most simple and complex of all behavior. - All neurons working in same manner for the most part, higher-level processing mediated by same process as low-level things. About 100 billion neurons and 100 trillion synapses. They are enormously varied in size and form. Neuron doctrine: neurons are distinct entities, information travels in one direction from dendrite to axon, information is transmitted from cell to cell across tiny gaps called synapses.
Anatomy of a neuron
Four zones. Input zone - receives information from other cells through dendrites (can be hundreds to thousands of these, receive neurotransmitters, dendritic spines). Integration zone - cell body or soma: regions where inputs are combined and transformed, organelles, typically covered in receptors that are more impactful than those on the dendrites. Final integration happens at the axon hillock (making decision to fire or not). Conduction zone - single axon or nerve fiber conducts output information away from the cell body as an electrical impulse. Output zone - axon terminals at the end of the axon communicate activity to other cells (neurons or target muscles or glands). Axon hillock - integrates input into all or none electrical signals that travel down axon and innervate target cells. Dendritic spine - small protrusion from dendrite that receives input from a single axon or synapse. Cover the dendrites, each spine is a synapse, changing brain/learning is a remodeling of the dendritic spines. Ratio, responsivity, sensitivity, number, and organization of these spines. Use it or lose it model, strenghten synapses or they go away (not holding on to things you don’t need).
Sizes and shapes
Axons are only a few micrometers long in many neurons but may reach longer than a meter in neurons that connect the spinal cord to the body. Classified into multipolar (most common), bipolar (common in sensory systems, one dendrite), unipolar (common in touch sensation, myelinated dendrite and axon.
The synpase
Axon terminals typically form synapses on the cell body or dendrites of a postsynaptic neuron. On dendrites, synapses may form on dendritic spines or dendrite shaft. Communication within a neuron along the axon is electrical. Communication between neurons is chemical (neurotransmitters, can be electrical, but it’s rare). Each neuron can be pre or post synaptic. Synaptic vesicles in axon terminals where neurotransmitters are stored. Synapse: cellular location where information transmitted from one neuron to another via neurotransmission. Some have up to 100,000 synaptic contacts although a more common number is 5-10 thousand. Synapses have three components: presynaptic membrane on axon terminal of presynaptic neuron, postsynaptic membrane on dendrite or cell body of postsynaptic neuron. Synaptic cleft - a gap that separates the membrane. Neurotransmitters released in response to electrical activity in the axon. Typically a neuron will fire one neurotransmitters (up to 3 maybe, but it’s not like they all have the same ones), different firing rates may correspond to which one is released. Can track neurons corresponding to neurotransmitters, can track where in the brain they’re released, if released in different areas, can do different functions. Neurotransmitter receptors in postsynaptic membrane are specialized proteins that react to neurotransmitters. Neurotransmitters can either increase (depolarize) or decrease (polarize) the likelihood of the postsynaptic neuron firing. Inhibitory or excitatory, GABA is inhibitory. Synaptic remodeling underlies much of neuroplasticity. Changing amount of receptors (cocaine downregulates receptors on dopamine neurons), learning modulates glutamate receptors, pharmacology based on this.
Glial Cells
Neuronal cells that provide structural, nutritional, and other types of support to the brain (do some modulatory activity). Directly affect neuronal processes by providing neurons with raw materials and chemical signals that alter neuronal structure and excitability. Neurons don’t divide in standard mitosis, glial cells do (get tumors in glial cells, not neurons).
Astrocytes
Monitor neural activity and regulate adjacent capillaries to regulate blood flow to meet metabolic demands. Most common, biggest, some neuronal communication, tentacles that go out and modulate activity.
Oligodendrocytes/Schwan
Myelin sheath - fatty wrapping around axon, oligodendrocytes in CNS (wrap self around axon), Schwan cells in PNS. Facilitates efficient communication. Facilitates speed and conduction of electrical impulses down the axon. Nodes of Ranvier - gaps in myelin sheath that further facilitate speed of action potential. MS is deterioration of myelin in in nervous system. DTI models microstructural integrity of myelin sheath.
Microglial cells
CNS immune cells. Break down debris in the brain and target pathogens. Important following brain damage. The “trimming” of unused dendrites by microglia is central to learning and brain development. Implicated in overall general brain health and psychiatric disorders such as schizophrenia and variants of depression. Abnormal microglia processes may overprune the prefrontal cortex - schizophrenia.
Neural tissue
Two “colors.” Gray matter: contains more cell bodies and dendrites which lack myelin, prominent in cortex and subcortical nuclei. White matter: consists mostly of axons with white matter tracts. Cortex is only 3-4mm thick. Nervous system is a connectome - brain is a communication network.
Within the CNS and PNS
Nucleus - collection of neurons. Cortex - “bark,” outer surface which contains layers of neurons (e.g. cerebral cortex, cerebrum, 2-4 mm thick). Tract: a bundle of axons (white matter). PNS: ganglia - collection of neurons. Nerves - bundles of axons.
Neuroplasticity
Synaptic remodeling. Alteration in number and configurations of neurons, dendrites, dendritic spines, synapses, receptors, neurotransmission, etc.
The Nervous System
CNS - brain (control center for entire nervous system) and spinal cord (connects brain and PNS and enables spinal reflexes). Encased in protective structure. PNS: all other parts of the nervous system, found outside the skull and spinal cord. Branches of PNS : somatic (nerves that interconnect brain and major muscles and sensory systems in the body, conduit for incoming sensory input and outgoing commands from brain to skeletal muscles, moving and feeling in space) and autonomic (nerves that connect to viscera, internal organs, regulates internal bodily environment, innervate internal organs/viscera). Sympathetic (fight or flight, controls body when it’s aroused), parasympathetic (rest and digest, controls body during its normal rest state).
ANS
Brain’s main system for controlling/regulating organs of the body. Two branches, sympathetic and parasympathetic, it’s a ratio between the two of activity. Para helps body repair and recuperate. Para and sympa often in opposition. Two opposing neuronal tracts. Stomach inhibited with stress, resources not allocated to digestion.
