Ch 13 The Nervous System (PDF)

The Nervous System Ch 13 Intro Along with the endocrine system, the Nervous System controls and adjusts the activities of the other systems Both require chemical communication with target organs or systems Differ in the time of onset/duration of control Endocrine system has slow onset and long-lasti...

The Nervous System Ch 13 Intro Along with the endocrine system, the Nervous System controls and adjusts the activities of the other systems Both require chemical communication with target organs or systems Differ in the time of onset/duration of control Endocrine system has slow onset and long-lasting effects (minutes-days) Nervous system has fast onset (milli-seconds) and short-term effects (seconds) Overview of the Nervous System The nervous system consists of all nervous tissues in the body. Like the previous systems has 2 divisions: Central –Brain and spinal cord Processes and coordinates sensory input The location of all higher order thinking and function Peripheral –The peripheral nerves Provides sensory information to the CNS Carries out the motor commands sent down by the CNS Both contain somatic and visceral components The afferent division carries information From somatic sensory receptors that monitor and send info about muscles, joints and skin And from visceral sensory receptors that monitor other internal organs from systems such as cardiovascular and respiratory The efferent division is divided into the somatic and autonomic nervous systems: The somatic Nervous System controls skeletal muscle contractions These actions may be voluntary or involuntary The Autonomic Nervous System regulates smooth & cardiac muscle and glandular activity Completely involuntary Cellular Organization The nervous system contains only 2 types of cell: 1. Neurons Nerve cells that are responsible for the transfer and processing of information in the nervous system 2. Neuroglia Support and protect the neurons Neuroglia of the CNS There are 4 types of neural glial cells: 1. 2. 3. 4. Astrocytes Oligodendrocytes Microglia Ependymal Cells Astrocytes The largest and most numerous of the neuroglial cells: Function to: 1. 2. 3. 4. 5. Control the interstitial environment Maintain the Blood-Brain barrier Form a 3D framework for the CNS Repair damaged nervous tissue Guide neuronal development (embryonically) Oligodendrocytes The processes of oligodendrocytes contact the axons or cell bodies of the neurons and tie clusters of axons together. Thus, they improve conduction of the axon by creating a myelin sheath Wrapped areas are called internodes Myelin sheath gaps are referred to as Nodes of Ranvier Microglia Microglia migrate into the CNS is it develops and remain within nervous tissues These act as the phagocytic cells of the CNS Engulf debris and waste produced by CNS and Phagocytize viruses, microorganisms, & tumor cells Make up approximately 5% of CNS cells Numbers greatly increase when infections/injury present Ependymal Cells The ependyma lines the ventricles of the brain and central canal of the spinal cord. Passageways filled with cerebrospinal fluid This surrounds the CNS: Providing a protective cushion Transporting dissolved gasses, waste and nutrients In adults the apical surface of ependymal cells of the 4th ventricle and spinal cord are lined with cilia or microvilli Those of the 3rd ventricle lack villi Cells of the PNS Neuron cell bodies of the PNS are clustered into groups called ganglia Axons of the PNS group together to form the peripheral nerves The neuroglia of the PNS is composed of only 2 cell types: Satellite Cells Schwann Cells Schwann Cells The neurolemmal (cell membrane) of a Schwann cell extends and wraps itself around part of the axon of a single nerve cell Multiple Schwann cells line a single nerve axon Unlike oligodendrocytes that encircle multiple axons at once Gaps called Nodes of Ranvier are still present between cells The Neuron The neuron is the functional unit of the nervous system, It transmits information from one part of the nervous system to another through electrical impulses: The typical neuron is designed as: Classification of Neurons Neurons are classified into 4 structural groups: 1. 2. 3. 4. anaxomic : small & found only in the CNS bipolar: Cell body lies in between the dendrite and axon pseudounipolar : Cell body lies outside the plane of the dendrite and axon multipolar: The most common neuron in the CNS Functionally neurons are classified into 3 groups: 1. sensory: From the afferent division of the PNS Deliver information about the internal and external environment to the CNS 2. Motor: Form the efferent division of the CNS Stimulate the activity of the peripheral motor or organ systems Divided into Somatic (body) and Visceral (organ) Motor 3. Interneurons: Located in between the sensory and motor neurons Found within the brain and spinal cord Analyze the sensory input and organize the motor outputs Classified as excitatory or inhibitory based on post synaptic membranes Nerve Impulses Excitability is the ability of the plasma membrane to conduct electrical impulses An action potential is the change in membrane potential after the membrane stimulation reaches “threshold” All cells possess a standing membrane potential When stimulus is applied a temporary change in the permeability of the axolemella This changes the overall membrane potential of the membrane The development of an action potential will cause propagation of that signal along the axon Once initiated the speed of transmission depends on the length of travel, the diameter of the neuron, and the level of myelination A myelinated neuron transmits at about 40x’s the speed of an identical unmyelinated axon Synaptic Communication A synapse is the site of cellular/neuronal communication Synapses may be chemical ( vesicular) or electrical In a chemical synapse the neurotransmitter is released at the presynaptic membrane of an axon and bonds to the receptor protein of the postsynaptic membrane Neuromuscular junctions are chemical synapses that rely on acetylcholine The general sequence of excitation (vesicular transport): Arrival of the action potential at the axon terminal causes the release of neurotransmitter Neurotransmitter diffuses across the synaptic cleft , binding to receptor proteins This binding changes the membrane permeability of the postsynaptic membrane The result may be excitatory ( generating new action potentials) or Inhibitory ending or the propagation of action potentials If excitation occurs the action potential is propagated in a new axon ( neuronal connection) or a motor unit is activated (muscular connection) The effects of a single action potential are short lived as the neurotransmitter is reabsorbed or broken down by enzymes almost immediately upon its release At an electrical synapse, the presynaptic and postsynaptic membranes are bound tightly together Because of this the 2 cells function as if they had one membrane communicating junctions allow for the passage of the electrical signals between cells Unlike chemical synapses, electrical synapses can covey information on both directions Neuronal Organization Anatomical Organization of the Nervous System