Ch. 37: Neurons & Nervous Systems Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Ch. 37: Neurons & Nervous Systems

Invertebrate Nervous Organization Biology, 9th ed, Sylvia Mader Invertebrate Nervous Organization Chapter 39 Neurons & Nervous Systems Sponges Cellular level of organization; can respond to stimuli Cnidarians Tissue level of organization; can contract & extend body; move tentacles to capture prey 1. Hydras Nerve net composed of neurons in contact with one another & with contractile cells in body wall 2. Sea anemones & jellyfishes Two nerve nets: 1 fast & 1 slow

Evolution of the Nervous System Biology, 9th ed, Sylvia Mader Evolution of the Nervous System Chapter 39 Neurons & Nervous Systems

Invertebrate Nervous Organization Biology, 9th ed, Sylvia Mader Invertebrate Nervous Organization Chapter 39 Neurons & Nervous Systems Planarians (flatworms) Bilateral symmetry (left & right side symmetry) Ladderlike nervous system: 2 ventral nerve cords with transverse nerves that connect 2 nerve cords Cephalization - a concentration of ganglia and sensory receptors in the head Bilateral symmetry & cephalization are two important trends for the evolution of an active life

Evolution of the Nervous System Biology, 9th ed, Sylvia Mader Evolution of the Nervous System Chapter 39 Neurons & Nervous Systems

Invertebrate Nervous Organization Biology, 9th ed, Sylvia Mader Invertebrate Nervous Organization Chapter 39 Neurons & Nervous Systems Annelids, Arthropods and Molluscs Complex animals with true nervous systems Typical invertebrate nervous system: -Brain & ventral nerve cord -Ganglion in each segment of annelids & arthropods •Brain receives sensory information, controls activity of ganglia and nerves so muscle activity of entire animal is coordinated

Evolution of the Nervous System Biology, 9th ed, Sylvia Mader Evolution of the Nervous System Chapter 39 Neurons & Nervous Systems

Vertebrate Nervous Organization Biology, 9th ed, Sylvia Mader Vertebrate Nervous Organization Chapter 39 Neurons & Nervous Systems Central nervous system (CNS) Develops from an embryonic dorsal tubular nerve cord Cephalization and bilateral symmetry result in several types of paired sensory receptors that gather information about environment Vertebrates have thousands or billions more neurons than invertebrates

Vertebrate Nervous Organization Biology, 9th ed, Sylvia Mader Vertebrate Nervous Organization Chapter 39 Neurons & Nervous Systems Vertebrate brain is organized into three areas Hindbrain -Regulates organs below level of consciousness (heart & lung functions) -Also coordinates limb activity, posture & balance Midbrain -Optic lobes coordinate responses to visual input Forebrain -Receives sensory input from hind & midbrains. -Cerebrum of mammals associated with higher mental capabilities. Human cerebral cortex is especially large & complex.

Biology, 9th ed, Sylvia Mader Human Nervous System Chapter 39 Neurons & Nervous Systems Nervous system has three specific functions: Receives sensory input - Receptors respond to external & internal stimuli by generating nerve impulses that travel to CNS Performs integration -CNS sums up input it receives from entire body Generates motor output -Nerve impulses from CNS go to muscles & glands

Organization of the Human Nervous System Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Organization of the Human Nervous System

Biology, 9th ed, Sylvia Mader Human Nervous System Chapter 39 Neurons & Nervous Systems Central nervous system (CNS) Includes the brain and spinal cord Lies in the midline of the body Peripheral nervous system (PNS) Contains cranial nerves and spinal nerves that Gather info from sensors and conduct decisions to effectors Lies outside the CNS PNS consists of two parts: -Somatic nervous system -Autonomic nervous system - divided into sympathetic & parasympathetic systems

Biology, 9th ed, Sylvia Mader Nervous Tissue Chapter 39 Neurons & Nervous Systems Two principal types of cells: 1. Neurons (nerve cells) Functional units of the nervous system 2. Neuroglia •Provide support & nourishment to neurons

Biology, 9th ed, Sylvia Mader Nervous Tissue Chapter 39 Neurons & Nervous Systems Neurons consist of 3 major parts: Cell body contains nucleus & organelles Dendrites are short & highly branched; receive signals from sensory receptors or other neurons; transmit signals to cell body Axon conducts nerve impulse to other neurons or cells Axons can be bundled to form nerve fibers Covered by myelin sheath, white insulating layer

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader Nervous Tissue Chapter 39 Neurons & Nervous Systems Neuroglia greatly outnumber neurons in brain In CNS, there are several different types Myelin sheath formed from tightly spiraled neuroglia called oligodendrocytes. In PNS, Schwann cells create myelin sheath Gaps called nodes of Ranvier, appear between the Schwann cells.

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader Types of Neurons Chapter 39 Neurons & Nervous Systems Motor (Efferent) Neurons Accept nerve impulses from the CNS Transmit them to muscles or glands. Said to “innervate” these structures. Sensory Neurons Accept impulses from sensory receptors Transmit them to the CNS Interneurons Convey nerve impulses between various parts of the CNS

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Motor Interneuron Sensory

Biology, 9th ed, Sylvia Mader Nerve Impulses Chapter 39 Neurons & Nervous Systems Nerve Impulses These are waves of electrical activity Involve movement of unequally distributed ions on either side of an axon’s membrane. Can be measured with tiny electrodes inserted into axons. A voltage difference can be measured between the inside & outside of the axon. This is called a membrane potential

Nerve Impulses: Resting Potential Biology, 9th ed, Sylvia Mader Nerve Impulses: Resting Potential Chapter 39 Neurons & Nervous Systems Resting Potential The membrane potential when the axon is not conducting an impulse The inside of a neuron is more negative than the outside. Resting potential = -65 mV Due to following factors: Higher concentration of Na+ outside the axon Higher concentration of K+ inside axon Unequal distribution is partly due to activity of sodium-potassium pump Also due to large, negatively charged proteins in cytoplasm of axon

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Resting Potential of the Axonal Membrane Uneven distribution of Na+ and K+ create membrane potential of -65 mV More K+ More Na+

Biology, 9th ed, Sylvia Mader Action Potential Chapter 39 Neurons & Nervous Systems In order for a nerve impulse to begin an action potential needs to be generated. An action potential is a rapid change in polarity across a portion of the axon. It will be generated only by a stimulus larger than the threshold value. •This is the minimum change in polarity across the axon membrane required to generate an action potential. Stimuli cause gated ion channels to open in the axon membrane. There are two types of gated channels: •Sodium gates: allow Na+ ions to enter axon •Potassium gates: allow K+ ions to leave axon

Biology, 9th ed, Sylvia Mader Action Potential Chapter 39 Neurons & Nervous Systems Once a stimulus is detected by the axonal membrane, depolarization will begin. •Sodium ion gates open causing Na+ to flow into axon •This causes the inside of the neuron to become positively charged (depolarized) •If threshold depolarization occurs, many more sodium channels will open & the action potential begins. •The membrane potential rapidly swings up to +40 mV

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Depolarization of the Axonal Membrane Na+ ions move inward; membrane potential becomes positive

Biology, 9th ed, Sylvia Mader Action Potential Chapter 39 Neurons & Nervous Systems The reversal in polarity causes sodium channels to close & the potassium channels to open. •K+ ions now move from the inside of the axon to the outside •As K+ ions leave, the membrane potential changes from +40 mV back to -65 mV. •Repolarization has occurred. •The entire action potential takes only two milliseconds

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Repolarization of the Axonal Membrane K+ ions rush outside & membrane potential returns to -65 mV

Biology, 9th ed, Sylvia Mader Action Potential Chapter 39 Neurons & Nervous Systems As soon as an impulse has occurred at a particular spot on an axon, that region undergoes a short refractory period. •During this time Na+ gates cannot open yet. •Thus, that locale is unable to conduct an impulse. •This ensures that the impulse can only move in one direction.

Complete Action Potential Graph Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Complete Action Potential Graph Refractory period

Propagation of Action Potentials Biology, 9th ed, Sylvia Mader Propagation of Action Potentials Chapter 39 Neurons & Nervous Systems In unmyelinated fibers, an action potential at one locale stimulates adjacent ions channels to open & continue the action potential along the axon. In myelinated fibers, an action potential at one node causes an action potential at the next node. This is called Saltatory Conduction. It is much faster than in unmyelinated fibers: -100 m/sec versus 1.0 m/sec The strength of the nerve impulse does NOT decrease with distance traveled along an axon

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Unmyelinated neurons

Biology, 9th ed, Sylvia Mader All-or-None Response Chapter 39 Neurons & Nervous Systems Conduction of a nerve impulse is an all-or-nothing event Either a fiber conducts a nerve impulse or it does not. -If the threshold value is met an action potential will be generated. -If the threshold value is not met no action potential will occur •Increased intensity of stimulus above threshold does NOT alter the intensity or speed of impulse produced. -Intensity of message is determined by how many nerve impulses are generated per second.

Biology, 9th ed, Sylvia Mader A Synapse Chapter 39 Neurons & Nervous Systems A synapse is a region where axon terminals of one neuron nearly touch the dendrites or cell bodies of another neuron. •At a synapse, the membrane of the first neuron is called the presynaptic membrane. •The membrane of the second neuron is called the postsynaptic membrane. The small gap between neurons is the synaptic cleft Electrical nerve impulses cannot cross a synaptic cleft. How do impulses move between neurons?

Transmission Across a Synapse Biology, 9th ed, Sylvia Mader Transmission Across a Synapse Chapter 39 Neurons & Nervous Systems Transmission across a synapse is carried out by molecules called neurotransmitters •These are stored in synaptic vesicles inside of axon terminal. When a nerve impulse reaches an axon terminal: • Gated channels for calcium ions, in the axon terminal membrane, suddenly open -Ca2+ enters the terminal Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane Neurotransmitter molecules are released into the synaptic cleft -They diffuse across cleft to postsynaptic membrane & bind with specific receptor proteins.

Synapse Structure and Function Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Synapse Structure and Function

Synapse Structure and Function Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Synapse Structure and Function

Biology, 9th ed, Sylvia Mader Chapter 39 Neurons & Nervous Systems Synapse Function & Games Click on link to get to a good internet activity

Biology, 9th ed, Sylvia Mader Synaptic Integration Chapter 39 Neurons & Nervous Systems A single neuron is on the receiving end of Many excitatory signals - those that drive neurons closer to an action potential. Have a depolarizing effect on neuron. Many inhibitory signals - those that drive a neuron further from an action potential. Have a hyperpolarizing effect on neuron. Neurons integrate these incoming signals. •Integration is the summing up of excitatory & inhibitory signals from different synapses or from a rapid firing rate from one synapse.

Biology, 9th ed, Sylvia Mader Synapse Integration Chapter 39 Neurons & Nervous Systems

CNS: Brain and Spinal Cord Biology, 9th ed, Sylvia Mader CNS: Brain and Spinal Cord Chapter 39 Neurons & Nervous Systems Spinal cord and brain are wrapped in three protective membranes, meninges Spaces between meninges are filled with cerebrospinal fluid which cushions & protects the CNS Fluid is continuous with that of central canal of spinal cord and the ventricles of the brain Meningitis is a bacterial or viral inflammation of the meninges which can be life-threatening.

Biology, 9th ed, Sylvia Mader Spinal Cord Chapter 39 Neurons & Nervous Systems The spinal cord has two main functions: Center for many reflex actions Means of communication between the brain and spinal nerves The spinal cord is composed of central gray matter and peripheral white matter Cell bodies and short unmyelinated fibers give the gray matter its color Myelinated long fibers of interneurons running in tracts give white matter its color

Biology, 9th ed, Sylvia Mader Chapter 39 The Spinal Cord Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader The Brain Chapter 39 Neurons & Nervous Systems Cerebrum is the largest portion of the brain in humans Communicates with, and coordinates the activities of, the other parts of the brain Carries out higher thought processes required for learning, memory, language & speech Longitudinal fissure divides into left and right cerebral hemispheres -Right = artistic & musical ability, emotions, spatial relationships -Left = mathematics, language, analytical reasoning

Biology, 9th ed, Sylvia Mader The Human Brain Chapter 39 Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader Cerebral Cortex Chapter 39 Neurons & Nervous Systems A thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres It contains: •motor areas - organizes commands to muscles •sensory areas - where sensory information is processed •association areas - where integration occurs

The Lobes of a Cerebral Hemisphere Biology, 9th ed, Sylvia Mader The Lobes of a Cerebral Hemisphere Chapter 39 Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader Diencephalon Chapter 39 Neurons & Nervous Systems Consists of hypothalamus and thalamus Hypothalamus is an integration center - helps maintain homeostasis by regulating: hunger, sleep, thirst, body temp, water balance - controls the pituitary gland & is thus a link between nervous & endocrine systems Thalamus receives all sensory input except smell - integrates sensory information & sends to cerebrum

Biology, 9th ed, Sylvia Mader Cerebellum Chapter 39 Neurons & Nervous Systems Largest part of hindbrain. Receives sensory input from the eyes, ears, joints, and muscles about position of body parts Sends motor impulses out the brain stem to the skeletal muscles Thus, it helps to maintain posture & balance Ensures smooth, coordinated muscle movements

Biology, 9th ed, Sylvia Mader Brain Stem Chapter 39 Neurons & Nervous Systems Contains the midbrain, the pons, and the medulla oblangata Midbrain Acts as a relay station for tracts passing between The cerebrum, and the spinal cord or cerebellum Pons Helps regulate breathing and head movements Medulla oblongata Contains reflex centers for heartbeat, breathing, & blood pressure, vomiting, coughing, sneezing, hiccuping, and swallowing

Biology, 9th ed, Sylvia Mader Limbic System Chapter 39 Neurons & Nervous Systems Complex network of tracts and “nuclei” Integrates higher mental functions and primitive emotions into a united whole Accounts for why certain activities are deemed as pleasurable: eating, sex, etc.

Biology, 9th ed, Sylvia Mader The Limbic System Chapter 39 Neurons & Nervous Systems

Peripheral Nervous System Biology, 9th ed, Sylvia Mader Peripheral Nervous System Chapter 39 Neurons & Nervous Systems The peripheral nervous system lies outside the CNS and contains nerves, bundles of axons. Two types of nerves: •Cranial nerves -12 pairs attached to brain. -Concerned with head, neck & facial region •Spinal nerves -31 pairs; one from each vertebrae -Dorsal root contains axons of sensory neurons -Ventral root contains axons of motor neurons

Cranial and Spinal Nerves Biology, 9th ed, Sylvia Mader Cranial and Spinal Nerves Chapter 39 Neurons & Nervous Systems

Biology, 9th ed, Sylvia Mader The Reflex Arc Chapter 39 Neurons & Nervous Systems Spinal Reflex Involves only the spinal cord Sensory receptors activated (touch a sharp pin) Nerve impulses move along sensory axons toward spinal cord •Sensory neurons enter cord dorsally & pass signals to interneurons •Interneurons synapse with motor neurons •Motor neuron axons leave cord ventrally & carry impulse to an effector -Effectors bring about a response (muscle contracts)

A Reflex Arc Showing the Path of a Spinal Reflex Biology, 9th ed, Sylvia Mader A Reflex Arc Showing the Path of a Spinal Reflex Chapter 39 Neurons & Nervous Systems

Peripheral Nervous System Biology, 9th ed, Sylvia Mader Peripheral Nervous System Chapter 39 Neurons & Nervous Systems Autonomic system Controls the smooth muscles, cardiac muscles, and glands Usually unaware of its actions Divided into two divisions Sympathetic division Parasympathetic division

Sympathetic and Parasympathetic Divisions Biology, 9th ed, Sylvia Mader Sympathetic and Parasympathetic Divisions Chapter 39 Neurons & Nervous Systems Sympathetic division Especially important during fight or flight responses during emergencies Accelerates heartbeat and dilates bronchi Inhibits digestive tract Neurotransmitter is norepinephrine (NE) Parasympathetic division Promotes all internal responses associated with a relaxed state Promotes digestion and retards heartbeat Neurotransmitter is acetylcholine (ACh)

Autonomic System Structure and Function Biology, 9th ed, Sylvia Mader Autonomic System Structure and Function Chapter 39 Neurons & Nervous Systems