Chapter 17 Nervous System 1 The Nervous System Two Anatomical Divisions Central Nervous System (CNS) Brain and Spinal Cord Peripheral Nervous System (PNS) Two Types of Cells Neurons Transmit nerve impulses Neuroglia Support and nourish neurons 2 Three Parts of a Neuron Dendrites Extensions that receive signals from other neurons Cell Body Contains the nucleus and other organelles Axon Conducts nerve impulses away from the toward other neurons or effectors 3 Three Classes of Neurons 1. Sensory Neurons Takes messages to the CNS May have specialized endings ( receptors) 2. Interneurons Lies entirely within the CNS Receive input from neurons and other interneurons Summarize messages Communicate with motor neurons 3. Motor Neurons Carries messages from the CNS to an effector e.g. an organ, blood vessel, endrocrine gland, muscle etc 4 brain neurons receptors Motor neuron motor neurons other motor neurons skeletal muscles smooth muscle cardiac muscle glands Central nervous system Peripheral nervous system 5 direction of nucleus of Schwann cell (neuroglial cell) effector (muscle) 6 1
Sensory neuron direction of receptor (in skin) Interneuron 7 8 Motor neuron direction of nucleus of Schwann cell (neuroglial cell) effector Interneuron (muscle) terminal Sensory neuron direction of receptor (in skin) 9 Myelin Sheath Covers some s Nodes of Ranvier: gaps in the myelin sheath In the Peripheral Nervous System Acts as an insulator Important in nerve regeneration Myelin is formed by Schwann cells (neuroglia) In the Central Nervous System Myelin is formed by oligodendroglial cells Little nerve regeneration in the CNS 10 node of Ranvier a. myelin sheath Schwann cell b. 200 nm 11 CNS composed of two types of nervous tissue Gray matter contains cell bodies and short, nonmyelinated fibers Surface layer of brain Central part of spinal cord White matter contains myelinated s that run in tracts Deep in brain Surrounds gray matter in spinal cord 12 2
Resting Potential Axon is not conducting an impulse Inside of is negative compared to outside (-65mV) Resting potential is due to Unequal distribution of ions across membrane» More sodium outside than inside» More potassium inside than outside Resting potential is maintained by: Sodium-potassium pump Actively transports Na out and K in Membrane somewhat permeable to Na and K Tend to diffuse toward lesser concentration Polarity of membrane based on: More positive ions outside the membrane than inside Large, negatively charged organic ions inside 13 14 recording electrode Inside al membrane inside outside reference electrode outside K Na gated K gated Na Action Potential Rapid change in polarity across membrane as impulse occurs All or none phenomenon If threshold is reached, an action potential occurs Strong stimulus does not change strength of action potential changes frequency of firing a. Resting potential: more Na outside the and more K inside the causes polarization. 15 16 Action Potential Threshold Axonal membrane depolarized to threshold level Generates an action potential Requires two types of gated proteins» Sodium gated s» Potassium gated s Sodium Gates Open First Sodium flows down the concentration gradient into the Membrane potential changes from -65mV to 40mV Called depolarization because inside changes from negative to positive 17 18 3
Voltage (mv) 10/21/2011 direction of impulse Potassium Gates Open Second Potassium flows down its concentration gradient out of the Brings potential back to -65mV Called repolarization because it returns to original polarity open Na b. Action potential begins: depolarization occurs when Na gates open and Na moves inside the. 19 20 60 direction of impulse 40 20 Na moves to inside K moves to outside action potential 0 20 40 threshold 60 resting potential open K 0 1 2 3 4 5 6 Time (milliseconds) c. Action potential ends: repolarization occurs when K gates open and K moves outside the. 21 d. An action potential can be visualized if voltage changes are graphed over time. 22 Conduction of an Action Potential Nonmyelinated Axons Action potential travels down one small segment at a time Myelinated Axons Gated ion s concentrated in Nodes of Ranvier Action potential travels faster Jumps from node to node (saltatory ) 23 Synapse Every branches into many fine endings Each is tipped by an terminal Chemical synapse Neurons separated by synaptic cleft Presynaptic neuron sender Postsynaptic neuron receiver Neurotransmitter crosses synaptic cleft Released when action potential reaches terminal 24 4
Bone surrounds both the brain and the spinal cord Meninges are protective membranes that wrap around the brain and spinal cord Cerebral spinal fluid fills the spaces between the meninges Cushions and protects the CNS of the neuron terminals 25 26 The Spinal Cord Structure Extends from base of brain into vertebral canal Protected by vertebrae Intervertebral disks cushion and separate Cross-sectional anatomy Central gray matter» Shaped like letter H» Dorsal root- fibers entering gray matter» Ventral root-motor fibers leaving gray matter» Dorsal and ventral roots join as spinal nerve» Spinal nerves are part of the CNS Spinal Cord White matter Surrounds gray matter Ascending and descending tracts Ascending tract is located dorsally» Sends information to the brain Descending tract is located ventrally» Sends information from the brain to spinal nerves» Many tracts cross over to opposite side» Left side of brain controls right side of body and vice versa 27 28 spinal cord dorsal root dorsal root ganglion dorsal white matter gray matter central canal spinal nerve ventral root vertebra spinal nerve a. intervertebral disk b. ventral meninges 29 30 5
c. white central gray matter canal matter 31 Functions of Spinal Cord Communication between brain and body Center for many reflex arcs Sensory receptors generate an action potential Sensory neurons transmit impulse to the spinal cord Sensory neurons synapse with interneurons in the spinal cord Interneurons synapse with motor neurons Motor neurons carry impulse to effectors 32 The Peripheral Nervous System is composed of: Nerves (bundles of nerve fibers) Ganglia (collection of nerve cell bodies) nerve bundle of nerve fibers single nerve fiber Cranial and Spinal Nerves Cranial Nerves- 12 pairs Attached to the brain Some are purely, some motor, and some are mixed Largely concerned with head, neck, and face with the exception of the vagus nerve which extends to thorax and abdomen 33 34 spinal nerve dorsal root ventral root dorsal root ganglion Cranial and Spinal Nerves Spinal Nerves- 31 pairs Emerge from spinal cord between vertebrae All are mixed nerves Cell bodies of neurons are located in dorsal root ganglia Ventral roots contain s of motor neurons 35 b. 36 6
Divisions of the Peripheral Nervous System The Somatic System The Autonomic System Somatic System Serves the skin, skeletal muscles and tendons Includes nerves that carry information from receptors to the CNS and nerves that carry motor responses to muscles Some actions are due to reflexes Automatic responses to a stimulus Some actions are voluntary 37 38 A Somatic Reflex Arc pin dorsal root ganglion receptor (in skin) of neuron of neuron of interneuron dorsal horn Dorsal central canal white matter gray matter dorsal horn Autonomic System Regulates the activity of cardiac and smooth muscle and glands Function automatically and generally are involuntary Innervate all internal organs effector (muscle) of motor neuron ventral root of motor neuron ventral horn ventral horn Ventral 39 40 7