Chapter 12 lecture notes PDF

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Summary

Lectures will provide an overview of functional anatomy at all levels, from the systems to the tissues. The goal is to provide a mechanistic understanding of the different structures in our body as a foundation for human-health oriented studies....

Description

Lecture notes 1 Nervous system – Ch. 12 • 2 types of Nervous tissue cells = Neurons + Neuroglia (supportive cells) Nervous System (NS): 1. Regulates body activities by rapid response using nervous impulses 2. Responsible for our perception, behavior and memories 3. Responsible for all voluntary movements (somatic NS) and some involuntary move – Cerebral cortex = where emotions occur 3 Basic Functions of the NS: (1) Sensory function → Sense changes in external/internal environment through senso • Sensory (afferent) neurons serve this function (2) Integrative function→ Analyze incoming sensory info, store some aspects, & make decisions regarding appropriate behaviors • Association or interneurons serve this function

(3) Motor function → Respond to stimuli by initiating action • Motor (efferent) neurons serve this function – the primary function of the NS is to provide rapid communication within the body to maintain homeostasis – this function underlies behaviors, thinking, Nervous System Divisions (1) Central nervous system (CNS) = brain and spinal cord • spinal cord connects to the brain through the foramen magnum of the skull & is encircled by the bones of the vertebral column • processes incoming sensory info • source of thoughts, emotions, & memories • most signals that stimulate muscles to contract & glands to secrete originate here (2) Peripheral nervous system (PNS) = nerves, ganglia, enteric plexuses, sensory receptors; connects CNS to muscles, glands & all sensory receptors • Nerve =bundle of hundreds or thousands of axons, each serving a specific region of the body. • cranial & spinal nerves contain both sensory & motor fibers • 12 pairs of cranial nerves emerge from the base of the brain through foramina of the skull. • Thirty-one pairs of spinal nerves emerge from the spinal cord • Ganglia (swelling or knot) = small masses of nervous tissue, containing primarily cell bodies of neurons; • located outside brain & spinal cord • closely associated with cranial/spinal nerves • Enteric plexuses = extensive networks of neurons located in the walls of organs of the GI tract ernal or external environment rs in the nose

Subdivisions of PNS Somatic (voluntary) nervous system (SNS) (1) Sensory neurons = convey information from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell to the CNS (2) Motor neurons = conduct impulses from the CNS to skeletal muscles only • Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

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neurons from cutaneous and special sensory receptors to the CNS

Autonomic (involuntary) nervous systems (ANS) (1) Sensory neurons = convey information from autonomic sensory receptors in visceral organs (stomach & lungs) to the CNS (2) Motor neurons = conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. • Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. • The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. ◦ sympathetic division (speeds up heart rate)

◦ parasympathetic division (slow down heart rate) Enteric (involuntary) nervous system (ENS) = consists of neurons that extend the length of the GI tract (1) Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. (2) Enteric motor neurons govern contraction of GI tract smooth muscle to propel food through the GI tract, secretions of the GI tract organs such as acid from the stomach, & activity of GI tract endocrine cells, which secrete hormones • Many neurons of the enteric plexuses function independently of the CNS and ANS.

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Sensory neurons of the ENS monitor chemical changes within the GI tract and stretching of its walls Motor neurons of the ENS govern contraction of GI tract organs, and activity of the GI tract endocrine cells

Histology of the Nervous System Neurons vs. Neuroglia Neurons: • Electrically excitable = the ability to respond to a stimulus & convert it into an action potential • Stimulus = any change in the enviroment strong enough to initiate an AP • Action Potential (nerve impulse) = electrical signal that propogates along the surfece of the neuron membrane

• Very specialized – nucleus, cytoplasm, cell membrane, mitochondria, ER, ribosomes, microtubules… • Form complex processing networks within brain & spinal cord • Highly specialized cells that make intricate connections with other cells • Function in sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions • cannot undergo mitotic divisions due to their specialization Parts of a Neuron: (1) Dendrites = where signals are received/inputted

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Plasma membrane of dendrites contain the most receptor sites for binding chemical messengers from other cells Short, tapering, highly branched

(2) Cell Body (aka soma) = contains a nucleus surrounded by cytoplasm that contains typical cellular organelles • Lysosomes, rough ER, Golgi complex, mitochondria, ribosomes (site of protein synthesis)

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Cytoskeleton includes neurofibrils (bundles of filaments that provide shape & structure) & microtubules (assist in moving materials b/w the cell body and axon)

(3) Axon = propagates nerve impulses toward another neuron, muscle fiber, or a gland cell • long, thin, cylindrical projection that joins to the cell body at a cone-shaped elevation called the Axon Hillock • no rough ER → no protein synthesis • Axon Hillock = connects cell body and axon • Initial Segment = part of axon closest to the axon hillock– where first action potential is generated • Nerve impulses arise at the junction of the axon hillock & the initial segment, an area called the trigger zone • Axolemma = membrane of the axon • Synapse = site of communication b/w 2 neurons or b/w a neuron and an effector cell • Synaptic end bulb = some axon terminals swell into a bulb-shaped structure • contain many tiny membrane-enclosed sacs called synaptic vesicles that store a chemical called a neurotransmitter • Neurotransmitter = molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell – could be connected to another muscle • Myelin sheath – around axon • Schwann cell – part of PNS

(b) Bipolar Neurons = 1 dendrite, 1 axon – from cell body you have 2 extensions – one becomes axon and other becomes dendrites – few neurons have this – found in the retina of the eye, inner ear, olfactory area of the brain – photoreceptors (c) Unipolar = 1 dendrite, 1 axon fused together to form a continuous process that emerges from the cell body – most sensory neurons – sense of touch, pressure, pain, thermal stimuli – cell bodies located in the ganglia of spinal and cranial nerves – makes dorsal ganglion – dendrites are more specialized

Functional Classification of Neurons: Direction of Action Potential with respect to the CNS (1) Sensory/afferent neurons – entering spine from the back… – Unipolar neurons – receive signal from sense (skin) and then send signal to the CNS (to the spine) – stimulus activates a sensory receptor → sensory neuron forms an AP in its axon – AP is conveyed into the CNS through cranial or spinal nerves (2) Interneurons/association neurons • located within CNS b/w sensory & motor neurons

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integrate incoming sensory info from sensory neurons and then elicit a motor response by activating the appropriate motor neurons Multipolar neurons

(3) Motor/efferent neurons • convey AP away from the CNS to effectors (muscles & glands) in the periphery (PNS) through cranial or spinal nerves

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most are multipolar in structure exit → efferent exiting spine from ventral/front go to muscle/glands to do action

Nuroglia Cells • Not electrically excitable • Make up about half the volume of the nervous system • Smaller in size • Support, nourish and protect neurons • Can multiply and divide – this is how cancers happen • 6 kinds total (4 in CNS, 2 in PNS) • 4 in CNS = astrocytes, oligodendrocytes, microglia, ependymal • 2 in PNS = Schwann cells & satellite cells Neuroglia of the CNS:

(1) Astrocytes = supports the shape, strength of nervous tissue • Provides connection bw blood vessels and the neurons • Maintains blood-brain barrier • Fibrous Astrocytes = many long unbranched processes found in white matter • Protoplasmic Astrocytes = many short branched processes found in gray matter (2) Oligodendrocytes form/maintain the myelin sheath around CNS axons (3) Microglial cell = function as phagocytes; remove cellular debris formed & phagocytize microbes & nervous damaged tissue (4) Ependymal cells + line the ventricles of the brain & central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects & nourishes the brain & spinal cord) • produce/assist in circulation of cerebral spinal fluid • cells that send extension → Oligodendricytes (same function as Shwaan cell) – form myelin sheath

Neuroglia of the PNS: Composed of Schwann cell and Satellite cells Myelination of Neurons: • Myelin sheath → produced by Schwann cells & oligodendro

Electrical Signals in Neurons:

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Objectives: ◦ Describe the cellular features that permit communication o ◦ Compare the types of ion channels

◦ Describe graded potential, action potential and resting pot

◦ List the events that generate an action potential Communication between Neurons: • Excitable cells communicate with each other via action potentials or graded potentials

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Action potentials (AP) allow communication over short and long distances whereas graded potentials (GP) allow communication over short distances only

Production of an AP or a GP depends up channels – voltage does not diminish with distance in APs

– wherever you have synapse from 1 ne – trigger zone is where it becomes actio 4. Axon of interneuron forms an action potent 5. Interneuron is activated and perception occ 6. Another graded and action potential is generated in upper motor neuron – motor neurons exit from the ventral/ front 7. Neurotransmitter released and generate another action potential 8. Neurotransmitter stimulates muscle fibers that control finger movements – synapse here is called Neuromuscular Junction graded → dendrites and cell body potential → axon

Resting Membrane Potential • Plasma membrane of excitable cells exhibit a membrane potential, an electrical potential difference across the membrane • This voltage is called resting membrane potential

• This works like a battery • In cell instead of electron current we have ion current – outside is more positive – inside is more negative Ion Channels: • Electrochemical gradient = concentration and electrical difference of two sides of a membrane – Na+ outside & K+ inside – more Na+ than K+ – inside cell has phosphate and protein that are more negative • Ion channels allow movements of ions down their electrochemical gradient • Gate is a part of channel protein that can seal the channel shut or move aside to open it 4 types of channels in nervous system: 1. leak channel 2. ligand-gated channel 3. mechanically gated channel 4. voltage gated channel Passive Transport: Oxygen

Some ions

Glucose/AA

(1) Leakage channels randomly alternate between ope • K+ channels are more numerous than Na+ channels • K+ channels are leakier than Na+ channels (2) Ligand-gated channels respond to chemical stimul • Ligand can be an ion, hormone or a neurotransmitter • They are located on dendrites of some sensory neuro

(3) Mechanically-gated channels respond to mechanical vibration (sound waves) or pressure stimuli (touch, pressure, tissue stretching) • The force distort the channel and open the gate • Located on dendrites of some sensory neurons

(4) Voltage-gated channels respond to direct changes in m • They participate in generation and conduction of action p

Resting Membrane Potential: Voltage Difference • The greater the difference in charge across the membrane, the larger the m • The cytosol is elec How to Measure a • In neuron resting m • A cell that has mem

Resting Membrane Potential: The membrane of a non-conducting neuron is positive outside and negative inside. This is determined by: (1) Unequal distribution of ions across the plasma membrane & the selective permeability of the neuron’s membrane to K+>Na+ (2) Most anions cannot leave the cell because they are attached to non-diffusible molecules (ATP & protein) (3) Na+/ K+ pumps (small contributio Factors Contributing to Resting Membrane Potential: why negative inside the cell 3 reasons are above

Graded Potentials: • Small deviations in resting membrae potentials • Hyperpolarization (makes it more negative) • Depolarization (makes it more positive) • A graded potential occurs in response to the opening/closing of a mechanicallygated or ligand-gated ion channel • These two channels are present on cell body and/or dendrites of neurons • Hence graded potential occur only in the dendrites and cell body of a neuron Generation of Graded Potential:

Graded Potentials: Various Amplitudes • The amplitude of a graded potential depends on the stimulus strength • How many channel have opened and how long they remained open → -55mV is threshold → if less than -55mV than no AP

Decremental Conduction: • flow of ion current upon generation of graded potential is localized • It spreads to adjacent region of stimulus source for a short distance (few mm) • It gradually dies out as the charges are lost because of leak channels • This mode of travel is called decremental conduction Graded Potentials: Summation: • Graded potentials can be added together to become larger in amplitude Graded Potentials: • They have diff names depending on the type & location of a stimulus • Postsynaptic potential (neurotransmitter) • Sensory Potential (sensory receptor) • Generator Potential (sensory neuron) Action Potential (impulse): • An action potential is a sequence of rapidly occurr and eventually restore it to the resting stage (repola • Following repolarization there may be an after-hy • Two voltage gated channels are involved Action Potentials: • Voltage-gated Na+ channels open first • Voltage-gated K+ channel opens after • Different neurons may have different thresholds* Action Potentials: the Status of Na+ & K+ Voltag ****know potassium has only 1 gate

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Action Potentials: Stimulus Strength Action Potentials can only occur if the membrane potential reaches the threshold

Same

All or none

Refractory Period • Refractory Period = the period of time after an action potential begins during which an excitable cell can not generate another AP to a stimulus • Absolute Refractory Period = coincide with inactivated Na+ channel (repolarization phase) • Relative Refractory Period = larger stimulus can initiate it; coicincide with K+ channel open & Na+ channel in resting state (end of repolarization phase)

Refractory Period Importance: • Larger-diameter axon has shorter refractory period (0.4 msec), which allows 1000 impulse per second. • Smaller-diameter axon have absolute refractory period of 4msecwhich allows 250 impulse per second • in normal condition the maximum frequency of impulse range from 10-1000 per second

Propogation of AP: • Action potential in contrast to graded potential is not decremental and it does not die out • Instead it spread along the membrane in one direction and this mode of conduction is called propagation • This is because of positive feedback of Na+ voltage-gated channels • Action potential regenerates over and over from trigger zone to axon terminals (like dominos) Clinical Connection: • Some neurotoxins block action potential by clogging Na+ voltage-gated channel. • Local anesthetics such as procaine (Novocaine), also block Na+ voltage-gated channel and prevent propagation of sensory signals from pain receptors to the CNS. Continuous vs. Saltatory Conduction: • Continuous conduction = step by step polarization and depolarization of each adjacent segment of plasma membrane • Saltatory (leaping) conduction = faster mode of conduction in myelinated axons because of uneven distribution of voltage-gated channel • Action potential leap from one node of Ranvier to next one along the myelinated axolemma • This mode is faster and more energy-efficient

Factors that Affect Propagation Speed: • Axon diameter • Amount of myelination

• Location of synapses: a. Axodenritic b. Axosomatic c. Axoaxonic Synapses Between Neurons:

postsynaptic = receiving the signal vs presynaptic

Synapses are critical part of the NS: 1. They provide homeostasis 2. They allow info to be filtered and integrated 3. During learning the structure and function of particular synapsis change 4. They are important in some diseases and the therapeutic drugs target Signal Transmission at Synapses Electrical vs Chemical Synapses (1) Electrical Synapse ◦ Gap junctions connect cells and allow the transfer of info to synchronize the activity of a group of cells (2) Chemical Synapse ◦ One-way transfer of ingo from a presynaptic neuron to a postsynaptic neuron’s Electrical Signal • At an electrical synapase, impulses conduct directly between the plasma membranes of adjacent neurons through gap junctions • • •

Chem • • • • Signa

Steps of Action in Chemical Synapse 1. Nerve impulse arrives at a synaptic end bulb of a presynaptic axon. 2. The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels 3. An increase in the concentration of Ca2+ inside the presynaptic neuron triggers exocytosis of the synaptic vesicles which contain neurotransmitter molecules 4. The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron's plasma membrane. 5. This binding opens the ligand-gated channel and allows particular ions to flow across the membrane. 6. As ions flow through the opened channels, the voltage across the membrane changes. This could results in depolarization or hyperpolarization. 7. When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron. Postsynaptic Graded Potentials: • Excitatory postsynaptic potentials (EPSP) ◦ A depolarizing postsynpatic potential • Inhibitory postsynaptic potentials (IPSP) ◦ A hyperpolarizing postsynaptic potential • A postsynaptic neuron can receive many signals at once • Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable Structure of Neurotransmitter Receptors: • Neurotranmsitters at chemical synapses cause either an excitatry or inhibatory graded potential • Neurotransmitter receptors have 2 structures: 1. Ionotropic receptors • neuotransmitter binding site and on the ion channel are components of the same protein 2. Metabotropic receptors • neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structures Ionotropic receptor generating ESPS: • Binding of Ach to the ionotropic acetylcholine (ACh) three most plentiful cations and an excitatory postsyna Ionotropic receptor generating ISPS

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Binding of GABA to the ionotropic gamma-aminobutyric acid (GABA) receptor causes the Cl− channel to open, allowing a larger number of chloride ions to diffuse inward and an inhibitory postsynaptic potential (IPSP) to be generated.

Metabotropic receptor generating ISPS •

Binding of Ach to the metabotropic acetylcholine (AC allowing a larger number of potassium ions to diffuse o

Removal of Neurotransmitter: • Neurotransm...