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Nervous System Objectives 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Explain the general functions (3 Functions) of the nervous system. Identify the two main parts of the Nervous System Describe the structure of a neuron and the function of each major part. Distinguish between sensory neurons, motor neurons, and interneurons. Summarize the electrical and chemical conditions of resting potential. Outline the electrical and chemical changes during an action potential. Explain the role of neurotransmitters in transmitting a signal across a synapse. Describe the role of sensory and motor divisions of the peripheral nervous system. Distinguish between the autonomic and somatic nervous systems. Summarize a spinal reflex. Introduction The nervous system controls and coordinates all essential functions of the human body. ► The Nervous System has 3 main functions ► 1. SENSORY FUNCTION – gather information and transmit to brain and spinal cord 2. INTEGRATIVE FUNCTION - Processes the information to determine the best response. 3. MOTOR FUNCTION - Sends information to muscles, glands, and organs (effectors) so they can respond by muscular contraction or glandular secretions. Divisions of the Nervous System ► Central Nervous System Consists of the Spinal Cord and Brain Spinal cord carries messages to the brain where they are analyzed and interpreted Response messages are sent from the brain to the spinal cord and the rest of the body Peripheral Nervous System Consists of the neurons not found in the brain and spinal cord Divided into 2 divisions: Sensory and Motor Afferent Neurons – Collect info and send it toward the CNS Efferent Neurons – Transmit info away from the CNS The Vertebrate Nervous System LE 48-3 Sensory input Sensor Integration Motor output Effector Peripheral nervous system (PNS) Central nervous system (CNS) LE 48-4 Quadriceps muscle Cell body of sensory neuron in dorsal root ganglion Gray matter White matter Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron Neuron Structure Schwann Cell ► ► ► Neurons are the basic functional units of the nervous system All consist of a cell body, dendrites, and one axon Messages take the form of electrical signals known as impulses – Neurons carry impulses in one direction Types of Neurons ► Sensory Neurons Carry messages from sense organs to the CNS Receptors detect stimuli from external or internal changes and send info to CNS by way of Afferent neurons Motor Neurons Carry impulses from the CNS to muscles and glands Interneurons Connect Sensory and Motor Neurons Found entirely in the CNS Anatomy of a Neuron ► ► Neuron Video Cell Body Contains nucleus and most of the cytoplasm Site of most of the metabolic activity including ATP production and protein synthesis ► Dendrites Short branch extensions spreading out from the cell body. Receive impulses (action potentials) and carry them toward the cell body ► Axon Carries action potential away from the cell body Each neuron only has one axon Axon ends with a series of swellings called axon terminals Anatomy cont. ► Myelin Sheath Lipid layer that insulates the axon Speeds up the transmission of action potentials Multiple Sclerosis Video ► Schwann Cells In peripheral nervous system Produce the myelin that surrounds the axon ► Nodes of Ranvier Gaps in the myelin sheath along the axon Synapses Point of contact where impulses are passed from one neuron to another Supporting Cells (Glia) ► Glia are essential for structural integrity of the nervous system and for functioning of neurons ► Types of glia: astrocytes, radial glia, oligodendrocytes, and Schwann cells ► In the CNS, astrocytes provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters Nerve Impulses ► Resting potential --70 mV potential In a non-conducting neuron Result of an imbalance of charged particles (ions) between the extracellular and the intracellular fluids – positive charge on the outer surface and negative on inner surface Remember the Na/K pump? Well, if not… Mechanism for Resting potential 1.Na/K pump pumps 3 Na+ ions to the outside while bringing in 2 K+ ions 2.Cell membrane is more permeable to K+ than to Na+ so that K+ diffuses out faster than Na+ diffuses in 3.Cell membrane is basically impermeable to large negatively charged anions present inside the neuron, therefore fewer negative particles move out than positive Resting Potential ION CONCENTRATION Action Potential ► ► Nerve impulses carry info from one point of the body to another by progression along the neuron membrane of an abrupt change in resting potential. This traveling disturbance is called an action potential. ACTION POTENTIAL ANIMATION LE 48-10 CYTOSOL EXTRACELLULAR FLUID [Na+] 15 mM [Na+] 150 mM [K+] 150 mM [K+] 5 mM [Cl–] [Cl–] 120 mM 10 mM [A–] 100 mM Plasma membrane LE 48-11 Inner chamber –92 mV Outer chamber 150 mM KCl Inner chamber 15 mM NaCl 5 mM KCl +62 mV Outer chamber 150 mM NaCl Cl– K+ Potassium channel Cl– Na+ Sodium channel Artificial membrane Membrane selectively permeable to K+ Membrane selectively permeable to Na+ Steps of Action Potential 1. A stimulus (chemicalelectrical-mechanical) is sufficient to alter the resting membrane potential 2. The membrane’s permeability to Na+ increases at the point of stimulation 3. Na+ moves into the cell rapidly; the membrane becomes locally depolarized 3 2. 1. 4. Na+ continues to move inward; inside of cell becomes positively charged relative to outside (reverse polarization or depolarization) 5. Reverse polarization at original site acts as a stimulus to adjacent region of membrane 4 3 5 2. 1. Action Potential cont. 6. At the point originally stimulated, the membrane’s permeability to sodium decreases, and its permeability to K+ increases. 7. K+ rapidly moves outward, again making the outside of the membrane positive in relation to the inside (repolarization) 8. Na+/K+ pumps transport Na+ back out of, and K+ back into the cell. The cycle repeats itself, traveling in this manner along the neuron membrane. Threshold Stimulus An action potential is produced in response to a threshold stimulus. Resting potential is about -70mV If the stimulus raises the potential to -55mV, a threshold potential has been reached Complete depolarization and repolarization occur and an action potential is generated Impulses cont. ► An impulse is self-propagating – once it starts it continues, and moves in one direction Impulses cont. Synapse and Synaptic Transmission ► 1. 2. 3. 4. 5. 6. AP reaches the axon terminal Influx of Ca2+ causes synaptic vesicles with neurotransmitter to fuse with presynaptic membrane Neurotransmitter is released by exocytosis into synaptic cleft Neurotransmitter binds to receptors in postsynaptic membrane Permeability of postsynaptic membrane is altered initiating on impulse on the second neuron Neurotransmitter is removed from the synapse (reuptake) Refractory Period ► A period when a neuron is unable to conduct an impulse Na+/K+ pumps are restoring the resting potential SYNAPSE FUNCTION VIDEO LE 48-17 Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Na+ K+ Presynaptic membrane Neurotransmitter Postsynaptic membrane Ligandgated ion channel Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ Synaptic cleft Ligand-gated ion channels Neurotransmitters ► Ach - acetylcholine ► Norepinephrine ► Epinephrine ► Dopamine ► Serotonin ► GABA ► Derived acids from amino Neuroglia ► Make up more than half the volume of the vertebrate nervous system ►A variety of cells that metabolically assist, structurally support, and protect the neurons Fig. 34-14, p.583 Peripheral Nervous System ► ► All nerves not in the spinal cord and brain Sensory Division (Afferent) Transmits impulses from the sense organs (e.g. ears and eyes) to the CNS ► Motor Division (Efferent) Transmits impulses from the CNS to the muscles and glands Two divisions: Somatic and Autonomic Somatic System ► Regulates activities that are under conscious control (ex: skeletal muscle movement) ► Many nerves are part of reflexes and can act automatically Patellar reflex – By passes the brain completely Reflexes ► Automatic movements made in response to stimuli ► In the simplest reflex arcs, sensory neurons synapse directly on motor neurons ► Most reflexes involve an interneuron Stretch Reflex STIMULUS Biceps stretches. sensory neuron motor neuron Response Biceps contracts. Figure 34.16 Page 585 Central and Peripheral Nervous Systems ► Central nervous system (CNS) Brain Spinal cord ► Peripheral nervous system Nerves that thread through the body Peripheral Nervous System ► Somatic external nerves- Motor functions (Shown in green) ► Autonomic internal nerves - Visceral functions (Shown in red) Two Types of Autonomic Nerves ► Sympathetic ► Parasympathetic ► Most organs receive input from both ► Usually have opposite effects on organ Sympathetic Nerves ► Originate in the thoracic and lumbar regions of the spinal cord ► Ganglia are near the spinal cord ► Promote responses that prepare the body for stress or physical activity (fight-or-flight response) Parasympathetic Nerves ► Originate in the brain and the sacral region of the spinal cord ► Ganglia are in walls of organs ► Promote housekeeping responses such as digestion “Rest & Digest” Both Systems Are Usually Active ► Most organs are continually receiving both sympathetic and parasympathetic stimulation ► For example, sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down ► Which dominates depends on situation Function of the Spinal Cord ► Expressway for signals between brain and peripheral nerves ► Sensory and motor neurons make direct reflex connections in the spinal cord ► Spinal reflexes do not involve the brain Structure of the Spinal Cord spinal cord ganglion nerve vertebra meninges (protective coverings) Figure 34.18 Page 587 Anatomy of the Brain Cerebrospinal Fluid ► Surrounds the spinal cord ► Fills ventricles within the brain ► Blood-brain barrier controls which solutes enter the cerebrospinal fluid Figure 34.20 Page 588 The Brainstem ► “Lower Brain” ► Medulla oblongata – controls automatic, homeostatis functions (breathing, hear and blood vessels, swallowing, vomiting, digestions ► Pons – breathing regulation & other homeostatic functions ► Midbrain – recipt and integeration of sensory info Cerebellum ► Coordination, error checking in motor, perceptual & cognitive functions (learning, consciousness) ► Sensory info about joint position and length of muscle – how you “know” where your parts are ► Auditory & visual information Cerebrum ► Right and Left Cerebral Hemispheres which are responsible for opposite sides of body ► Gray matter outside; white matter inside ► CORPUS CALLOSUM – connection between two halves Sensory Receptors Convert the energy of a stimulus into action potentials Mechanoreceptors Chemoreceptors Thermoreceptors Osmoreceptors Pain receptors Photoreceptors Assessing a Stimulus ► Action potentials don’t vary in amplitude ► Brain tells nature of stimulus by: Particular pathway that carries the signal Frequency of action potentials along an axon Number of axons recruited Somatic Sensations ► Touch ► Pressure ► Temperature ► Pain ► Motion ► Position