Human Anatomy 4 th Edition Mc Kinley OLoughlin

Human Anatomy, 4 th Edition Mc. Kinley & O'Loughlin Chapter 14 Lecture Outline: Nervous Tissue 1

The Nervous System n n The nervous system is the body’s primary communication and control system. The nervous system can be divided according to structural and functional categories. 14 -2

Nervous System: Structural Organization Structural subdivisions of the nervous system: • Central nervous system (CNS) • brain and spinal cord • Peripheral nervous system (PNS) • cranial nerves (nerves that extend from the brain) • spinal nerves (nerves that extend from the spinal cord) • ganglia (clusters of neuron cell bodies located outside the CNS) 14 -3

Nervous System: Functional Organization Functional divisions of the nervous system: • Sensory division — receives sensory information (input) from receptors and transmits this information to the CNS. • Motor (or efferent) division — transmits motor impulses (output) from the CNS to muscles or glands. 14 -4

The sensory division is subdivided into two Sensory Division components: • Somatic sensory components are the general somatic senses—touch, pain, pressure, vibration, temperature, and proprioception. • Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS. The visceral senses primarily include temperature and stretch (of the organ wall). 14 -5

The motor division is subdivided into two components: • The somatic motor component (somatic nervous system; SNS) conducts nerve impulses from the CNS to skeletal muscles. • also known as the voluntary nervous system • The autonomic motor component (autonomic nervous system; ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands. • also known as the visceral motor system or involuntary nervous system 14 -6

Nerve Cells • Two distinct cell types form nervous tissue. - Neurons, which are excitable cells that initiate and transmit nerve impulses - Glial cells, which are nonexcitable cells that support and protect the neurons 14 -7

Characteristics of Neurons • Neurons have a high metabolic rate. • Neurons have extreme longevity. • Neurons typically are non-mitotic. 14 -8

Neuron Structure • Neurons come in all shapes and sizes, but all neurons share certain basic structural features. • A typical neuron has a cell body, dendrites, and axons. 14 -9

Neuron Structure – Cell Body • The cell body serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses. • Neurotransmitters like acetylcholine are synthesized in the cell body 14 -10

Neuron Structure – Dendrites • Dendrites tend to be shorter, smaller processes that branch off the cell body. • Some neurons have only one dendrite, while others have many. • Dendrites conduct nerve impulses toward the cell body; they receive input and then transfer it to the cell body for processing. • The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells. 14 -11

Neuron Structure – Axon • The larger, typically longer nerve cell process emanating from the cell body is the axon, sometimes called a nerve fiber. • Most neurons have only one axon. • The axon transmits a nerve impulse away from the cell body toward another cell. 14 -12

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a) Regions of neuron b) Structural components of a neuron 14

Classifications of Neurons • Neurons vary widely in morphology and location. • They can be classified according to either their structure or their function. • Neurons can be classified according to the number of processes extending from the cell body. • unipolar neuron has a single process • bipolar neurons have two processes • multipolar neurons have three or more processes 14 -15

Interneurons • Interneurons, or association neurons, lie entirely within the CNS and are multipolar. • They receive nerve impulses from many other neurons and carry out the integrative function of the nervous system. • Thus, interneurons facilitate communication between sensory and motor neurons. 14 -16

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Glial Cells • Sometimes referred to as neuroglia, occur within both the CNS and the PNS. • Glial cells are smaller and capable of mitosis. • Glial cells do not transmit nerve impulses. • Glial cells physically protect and help nourish neurons, and provide an organized, supporting framework for all the nervous tissue. • Glial cells far outnumber neurons. • Glial cells account for roughly half the volume of the nervous system. 14 -18

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Glial Cells of the CNS • Astrocytes exhibit a starlike shape due to projections from their surface. • Astrocytes are the most abundant glial cells in the CNS, and they constitute over 90% of the tissue in some areas of the brain. • Help form the blood-brain barrier (BBB) that strictly controls substances entering the nervous tissue in the brain from the bloodstream. • Regulate tissue fluid composition. 14 -22

Functions of Glial Cells • Forming a structural network. • Replacing damaged neurons. • Assisting neuronal development. 14 -23

Myelination • Neurolemmocytes also called Schwann cells, are associated with PNS axons and are responsible for myelinating PNS axons. • Myelination is the process by which part of an axon is wrapped with a myelin sheath, a protective fatty coating that gives it glossy-white appearance. • The myelin sheath supports, protects, and insulates an axon. 14 -24

Myelination • No change in voltage can occur across the membrane in the insulated portion of an axon. • In the PNS, myelin sheaths form from neurolemmocytes. • In the CNS, they form from oligodendrocytes. 14 -25

• In the CNS, they form from oligodendrocytes. • In the PNS, myelin sheaths form from neurolemmocytes. 26

Myelinated vs. Unmyelinated Axons • In a myelinated axon, the nerve impulse “jumps” from neurofibril node to neurofibril node and is known as saltatory conduction. • In an unmyelinated axon, the nerve impulse must travel the entire length of the axon, a process called continuous conduction. • A myelinated axon produces a faster nerve impulse. 14 -27

Myelinated vs. Unmyelinated Axons • In an unmyelinated axon, a nerve impulse takes longer to reach the end of the axon. • A myelinated axon also requires less energy (ATP) than does an unmyelinated axon. • Using continuous conduction, unmyelinated axons conduct nerve impulses from pain stimuli. 14 -28

Regeneration of PNS Axons • PNS axons are vulnerable to cuts, crushing injuries, and other trauma. • A damaged axon can regenerate, however, if at least some neurilemma remains. • PNS axon regeneration depends upon three factors. • the amount of damage • neurolemmocyte secretion of nerve growth factors to stimulate outgrowth of severed axons • the distance between the site of the damaged axon and the effector organ • https: //www. youtube. com/watch? v=Ua. Ku. Y 1 WYJc. A 14 -29

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Structure of a Nerve • A nerve is a cable-like bundle of parallel axons. • Like a muscle, a nerve has three successive connective tissue wrappings. - endoneurium - a delicate layer of loose connective tissue - perineurium - a cellular and fibrous connective tissue layer that wraps groups of axons into bundles called fascicles - epineurium - a superficial connective tissue covering • This thick layer of dense irregular fibrous connective tissue encloses the entire nerve, providing both support and protection 14 -32

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Nerves • Nerves are a component of the peripheral nervous system. • Sensory (afferent) nerves convey sensory information to the CNS. • Motor (efferent) nerves convey motor impulses from the CNS to the muscles and glands. • Axons terminate as they contact other neurons, muscle cells, or gland cells. • An axon transmits a nerve impulse at a specialized junction with another neuron called synapse. 14 -34

Synapses • Presynaptic neurons transmit nerve impulses along their axonal membranes toward a synapse. • Postsynaptic neurons conduct nerve impulses through their dendritic and cell body membranes away from the synapse. • Axons may establish synaptic contacts with any portion of the surface of another neuron, except those regions that are myelinated. • Link http: //highered. mheducation. co m/sites/0072495855/student_vi ew 0/chapter 14/animation__the _nerve_impulse. html 14 -35

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Electrical Synapses • Electrical synapses are not very common in mammals. • In humans, these synapses occur primarily between smooth muscle cells where quick, uniform innervation is essential. • Electrical synapses are also located in cardiac muscle. 14 -37

Chemical Synapses • The most numerous type of synapse is the chemical synapse. • It facilitates most of the interactions between neurons and all communications between neurons and effectors. • At these junctions, the presynaptic membrane releases a signaling molecule called a neurotransmitter, such as acetylcholine (ACh). • Other types of neurons use other neurotransmitters. • Link • http: //highered. mheducation. com/ sites/0072495855/student_view 0/c hapter 14/animation__chemical_sy napse__quiz_1_. html 14 -38

Neurotransmitters • Are released only from the plasma membrane of the presynaptic cell. • It then binds to receptor proteins found only on the plasma membrane of the postsynaptic cell. • A unidirectional flow of information and communication takes place. • Two factors influence the rate of conduction of the impulse: the axon’s diameter and the presence (or absence) of a myelin sheath. 14 -39