Motor Systems Lecture (Chapter 11) PDF

The Motor (Efferent) Division of the Nervous System: Autonomic and Somatic Motor Control Chapter 11  Efferent (Motor) Division Consists of TWO Subdivisions: 1) Somatic Motor subdivision: Innervates skeletal muscles of the body Responsible for voluntary movements...

The Motor (Efferent) Division of the Nervous System: Autonomic and Somatic Motor Control Chapter 11  Efferent (Motor) Division Consists of TWO Subdivisions: 1) Somatic Motor subdivision: Innervates skeletal muscles of the body Responsible for voluntary movements 2) Visceral Motor subdivision (a.k.a. Autonomic Nervous System, ANS): Regulates smooth muscle of digestive tract, blood vessels, ducts of glands, airways, urinary bladder, cardiac muscle, and secretion of salivary glands and some endocrine glands.  Features of the ANS Involuntary (you do not have conscious control of its output). Has two subdivisions: 1) Parasympathetic subdivision: “Rest and Digest” Prepares body for rest and digestion activities 2)Sympathetic subdivision: “Fight or flight” Prepares the body for energetic action However, these two subdivisions are always active, not just during times of rest or fighting. The dynamic balance in the output of these two subdivisions act to regulate homeostasis in the body. Most tissues, but not all, are under antagonistic regulation of the two ANS subdivision.  Autonomic Division: Homeostasis During “rest and digest” During “fight-or-flight” type type activities activities sympathetic parasympathetic output is output is higher than higher than sympathetic parasympathetic output. output.  Autonomic Control Centers Hypothalamus – Water balance, temperature, and hunger – Center for homeostasis integration Pons – Respiration, cardiac, and urinary bladder Medulla – Respiration, blood pressure Comparing the Sympathetic and Parasympathetic Subdivisions Autonomic pathways between CNS and Target cells consist of two neurons (i.e. they are di-synaptic). This is the case for both the sympathetic and parasympathetic subdivisions. axon axon Ganglion = cluster of cell bodies outside the CNS  Autonomic connections between the CNS and target tissue are di-synaptic. Preganglionic neuron: Postganglionic neuron: The preganglionic neuron synapses on the postganglionic neuron in the autonomic ganglion. The postganglionic neuron sends its axon out to synapse on the target tissue ( which may be an endocrine or exocrine gland cell, smooth muscle cell, or adipose cell). Comparing the Sympathetic and Parasympathetic Subdivisions Autonomic post-ganglionic axons form dispersed synapses on target cells. These dispersed synapses consist of varicosities (swellings along the axon) that release the neurotransmitter onto the surface of the target cell. The neurotransmitter receptors on the target cell are distributed across the surface of the target cell membrane, rather than at a specific synaptic location on the cell membrane, (as in a typical chemical synapse). Figure 11-8 The Sympathetic and Parasympathetic subdivisions differ in terms of: 1) Location of their pre- and post-ganglionic neurons. 2) The neurotransmitters used on target cells. 3) The receptors expressed by their target cells. Sympathetic versus Parasympathetic : Location of pre- and post-ganglionic cell bodies The cell bodies of the sympathetic preganglionic neurons are located in the thoracic and first two lumbar segments of the spinal cord. The cell bodies of the sympathetic post-ganglionic neurons are located in the sympathetic ganglion chain close to the spinal cord and three ganglia located along Aortic ganglia the aorta. The cell bodies of the parasympathetic preganglionic neurons are located in the pons, medulla and sacral segments of the spinal cord. The cell bodies of the parasympathetic post- ganglionic neurons are located close to or on the target tissue.  Sympathetic versus Parasympathetic: Neurotransmitters used Parasympathetic Pre- Sympathetic ganglionic neuron Pre-ganglionic neuron Sympathetic Post- Parasympathetic ganglionic neuron Produces fast Post-ganglionic synaptic response neuron Produces slow synaptic response Figure 11-7 Focus on the Sympathetic Subdivision: Examples of some actions Pupil dilation to allow more light to enter the eye increasing visual acuity. Promotes mucus secretion from the salivary glands to help increase cleaning of inhaled air. Increases rate and depth of breathing to increase gas exchange. Bronchiole dilation reducing resistance to airflow in the lungs. Speeds Heart Rate and increases the force of heart contraction increasing volume of blood pumped per beat. Blood vessel constriction of arteries supplying blood to the digestive tract. Promotes Lipolysis (Fat breakdown) to release stored energy. Sympathetic Target Tissues use Adrenergic Receptors Adrenergic receptors bind norepinephrine (and some also bind epinephrine released from the adrenal medulla). Adrenergic receptors are linked to G-proteins inside the target cell producing slow synaptic responses. Two general types of adrenergic receptors (a and b): 1) Alpha receptors (α): (two subtypes) - a1 receptors are found on most, but not all, target tissues of the sympathetic subdivision of ANS. Will cause smooth muscle contraction Respond most strongly to binding of norepinephrine. Will also bind epinephrine, but response is weak. - a2 receptors are found only on the smooth muscle of the digestive tract and secretory cells of the pancreas. Will cause digestive tract smooth muscle relaxation and decrease pancreatic secretions. Respond most strongly norepinephrine. Will also bind epinephrine, but response is weak. Sympathetic Target Tissues use Adrenergic Receptors 2) Beta Receptors (β): (three subtypes) – β 1 receptors are found on cardiac pacemaker cells, heart muscle cells, and smooth muscle of kidney arterioles. Will cause increase in heart rate, force of heart contraction, and increase secretion of renin (more later). Respond equally well to binding of norepinephrine and epinephrine. – β 2 receptors are found on smooth muscle of arterioles, bronchioles, digestive tract, and urinary bladder. Activated by epinephrine released from adrenal medulla. – Will causes relaxation of smooth muscles that they’re expressed on. – Will also respond to norepinephrine, but response is weak. However, are not innervated by sympathetic post-ganglionic neurons so don’t receive norepinephrine. Sympathetic Target Tissues use Adrenergic Receptors β 3 receptors are mostly found on adipose tissues (fat) cells. Activation will stimulate lipolysis (breakdown of stored fats). Respond most strongly to norepinephrine, but will also respond to epinephrine but response is weak. Medulla of the Adrenal Gland Focus on Parasympathetic Subdivision: Examples of some actions Constricts pupils decreasing visual acuity Stimulates salivation to preparing the oral cavity for food. Slows heart rate Constricts bronchioles increasing resistance to airflow in the lungs. Stimulates digestion by stimulating increased release of digestive acid and enzymes, and increased stomach and intestinal motility. Stimulates insulin secretion from the pancreas preparing the body to receive increased glucose and amino acids absorbed during a meal.  Focus on Parasympathetic Subdivision Neurotransmitter used by both the pre- and post- ganglionic neurons is Acetylcholine The parasympathetic receptors are called “cholinergic receptors” – Two types: Nicotinic receptors and muscarinic receptors. - Nicotinic receptors are expressed on post- ganglionic neurons of this subdivision (also expressed on sympathetic post-ganglionic neurons). - Activation produces fast synaptic response. - Muscarinic receptors are expressed on the target cells of the parasympathetic subdivision. – At least five subtypes of muscarinic receptors have been identified. – All are G protein-coupled producing slow synaptic responses in the target cell.  Summary of Sympathetic versus Parasympathetic Subdivisions In terms of Parasympathetic Sympathetic Location of pre-ganglionic Located in pons, medulla, Located in thoracic & neurons in the CNS & segments 2-4 of sacral lumbar segments of spinal spinal cord cord Location of autonomic Located on or very near Located in sympathetic ganglia target tissue chain of ganglia. Close to vertebral column, and along abdominal aorta. Receptor Types Post-ganglionic neurons Post-ganglionic neurons express nicotinic receptors express nicotinic receptors Target Cells express Target cell receptors muscarinic receptors express adrenergic receptors Neurotransmitter used Both pre & post-ganglionic Pre-ganglionic neurons neurons use Acetylcholine use ACh. Post-ganglionic as their neurotransmitter neurons use norepinephrine.  More On Cholinergic Receptors Acetylcholine (ACh) uses 2 types of receptors: 1) Nicotinic receptors: Called “nicotinic” because nicotine is an agonist for these receptors (i.e. They can also be activated by nicotine). – Found on dendrites and cell bodies of post-ganglionic neurons of both subdivisions of the ANS & on skeletal muscle fibers (more later). – Consists of four protein subunits that combine to form a chemically gated ion channel in the membrane of the target cell. – Has two receptor sites for ACh as part of the protein subunits that make up the ion channel, both sites must bind ACh for the channel to open. Channel allows both Na+ and K+ to move across the cell membrane producing a fast synaptic response. At the resting membrane potential more Na+ than moves in than K+ moves out => depolarization in the target cell!  More On ACh Receptors 2) Muscarinic Receptors: are called “muscarinic” because muscarine is an agonist for these receptors. – Found on cell membrane of target cells of parasympathetic post-ganglionic neurons, pacemaker cells of the heart, and other target cells of the parasympathetic subdivision. – Consists of a single protein subunit that spans the cell membrane. – All muscarinic receptors are linked to G-proteins. – Activated by binding one molecule of ACh, which activates a second messenger system, thus producing a slow synaptic response in the target cell.  Somatic Motor Division Function: Provides voluntary control of body movement. Neuron innervating a skeletal muscle fiber is called a somatic motor neuron. The cell body of a somatic motor neuron is located in CNS. – These neurons send their axons out to directly synapse on target cells, which are always skeletal muscle fibers. Ganglion: Cluster of neuronal cell bodies in PNS Nucleus: Cluster of neuronal cell bodies in the CNS  Somatic Motor Division In the somatic motor subdivision the connection between the CNS and the target (skeletal muscle fiber) is monosynatptic (as opposed to di- Cell body & synaptic as in the ANS). axon of a somatic So, the somatic motor neuron sends it motor axon out to synapse directly on the neuron target (skeletal muscle fiber). These axons may be many feet in length. The somatic motor axon may branch to synapse with multiple muscle fibers in a muscle. The axon terminals of a somatic motor axon forms a chemical synapse on a muscle fiber called a Neuromuscular junction  Structure of the Neuromuscular Junction Somatic motor axon Somatic muscle fiber Structure of the Neuromuscular Junction Somatic motor axon terminal Muscle fiber Figure 11-12 (3 of 3)  Events at the Neuromuscular Junction leading to muscle fiber contraction. (1) AP 1. Action potential comes down axon and depolarizes the axon terminal. 2. Depolarization of the axon terminal triggers opening of voltage gated Ca2+ channels in membrane of axon terminal. (3) 3. Ca2+ enters axon terminal and triggers binding of synaptic vesicles to docking proteins on the presynaptic membrane. (2) 4. ACh is released into the synaptic cleft, (4) diffuses across and binds to nicotinic receptors on motor end plate. 5. (see next slide).  Events at the Neuromuscular Junction leading to muscle fiber contraction 5. Binding of two molecules of ACh to the nicotinic receptor triggers the opening of the Na+/K+ channels in the motor end plate allowing Na+ influx/K+ efflux.. out Motor end plate in *At the resting membrane potential of the muscle fiber the driving force on Na+ into the cell is greater than the driving force on K+ out of the cell, so result is a depolarization that leads to muscle fiber contraction.  Summary of the Motor Division Motor Division consists of: Somatic Motor Division ANS ANS – Has central role in homeostasis – CNS control centers in the hypothalamus. – Divided into sympathetic and parasympathetic subdivisions – Consists of di-synaptic pathways between CNS and target tissues: – pre-ganglionic motor neuron with its cell body in the CNS, axon synapses on postganglionic motor neuron – Postganglionic motor neuron with its cell body in an autonomic ganglion, axon synapses on target tissue – Regulates gland secretion, smooth and cardiac muscle activity, and adipose cell fat storage. – Antagonistic regulation of most, but not all, target tissues.  Summary of the Motor Division Somatic Motor Division. – Somatic motor neurons control skeletal muscle contraction. – Somatic motor neuron cell body located in CNS. – Single long myelinated axon from somatic motor neuron to skeletal muscle fiber. (monosynaptic connection between CNS and muscle fiber) – Release of ACh at Neuromuscular junction leads to muscle fiber contraction.