Lesson 14 The Nervous System Introduction to Life Processes - SCI 102 1
Structures and Functions of Nerve Cells The nervous system has two principal cell types: Neurons (nerve cells) Glia The functions of a neuron are localized in separate parts of the cell A neuron has four distinct parts: dendrites, a cell body, an axon, and synaptic terminals These parts of the neuron perform four functions Neurons receive information from the environment Neurons process information and produce an electrical signal Neurons conduct signals over distances Neurons transmit signals to other neurons, or the cells of muscles or glands Introduction to Life Processes - SCI 102 2
Structures and Functions of Nerve Cells Dendrites respond to stimuli These structures receive information, usually in the form of chemicals called neurotransmitters The cell body processes signals from the dendrites These electrical signals are positive or negative If a signal is sufficiently positive, the neuron produces an electrical signal called an action potential Introduction to Life Processes - SCI 102 3
Structures and Functions of Nerve Cells The axon conducts action potentials over long distances Axons are typically bundled into nerves At synapses, signals are transmitted from one cell to another The synapse is the site where one neuron communicates with another cell A typical synapse consists of a synaptic terminal, a dendrite or cell body, and a small gap separating the two cells Introduction to Life Processes - SCI 102 4
Neurons Section 1: the dendrites, which receive information, signals from the environment Section 2: area of neurotransmitters production Path a nerve impulse travels within a single neuron Dendrite cell body axon synaptic terminal Introduction to Life Processes - SCI 102 5
Neurons Produce and Transmit Information Information within a single neuron is carried by electrical signals Most neurons have a negative resting potential, which is the difference in voltage across its membrane If the membrane potential reaches the threshold potential, the neuron will generate an action potential Introduction to Life Processes - SCI 102 6
Neurons Produce and Transmit Information Myelin speeds up the conduction of action potentials Many vertebrate axons have myelin covering them Between each myelin wrapping are nodes of exposed axon Action potentials jump from node to node, speeding up their transmission The thicker the axon, the faster the action potential will move When an axon is covered in fatty myelin, the action potential can travel even faster Introduction to Life Processes - SCI 102 7
Neurons Produce and Transmit Information Myelin sheath The myelin which protects an axon and helps speed up an action potential The myelin sheath enables nerve conduction to occur at a much more rapid rate than without it People with multiple sclerosis develop areas in which the myelin sheath on their neurons has disintegrated. This disease often results in paralysis and extreme weakness. When the myelin sheath disintegrates, impulses travel more slowly to and from the brain, contributing to the symptoms of multiple sclerosis Introduction to Life Processes - SCI 102 8
Neurons Produce and Transmit Information Creating action potential Electrical signals travel down the dendrites and converge on the neuron s cell body The cell body performs the processing information function The cell body adds up the electrical signals it receives from the dendrites Some signals are positive while some are negative When the sum of electrical signals is sufficiently positive, the neuron produces an action potential Action potential: a large, rapid electrical signal The action potential is carried down the axon to be transmitted through synaptic terminals to the dendrites of other nerve cells Caffeine lowers the threshold for an action potential That means the nerve needs less electrical signals to stimulate an action potential Introduction to Life Processes - SCI 102 9
Neurons Produce and Transmit Information Neurons use chemicals to communicate with one another at synapses The presynaptic neuron transmits a signal by releasing neurotransmitter molecules that are stored in vesicles in the presynaptic terminal into the synaptic cleft The postsynaptic neuron receives the signal because it has specialized receptor molecules that bind to the neurotransmitter and cause changes in the cell Introduction to Life Processes - SCI 102 10
Neurons Produce and Transmit Information Synapses produce inhibitory or excitatory postsynaptic potentials A postsynaptic potential is a small brief change in voltage of the neuron Inhibitory postsynaptic potentials (IPSPs) move the resting potential further from the threshold potential and make the cell less likely to generate an action potential Excitatory postsynaptic potentials (EPSPs) move the resting potential closer to the threshold potential and make the cell more likely to generate an action potential Introduction to Life Processes - SCI 102 11
Neurons Produce and Transmit Information Neurotransmitter action is usually brief Integration of postsynaptic potentials determines the activity of a neuron The dendrites and cell body of a neuron receive EPSPs and IPSPs from thousands of presynaptic neurons The voltages of all of these are added up by the postsynaptic neuron through a process called integration Introduction to Life Processes - SCI 102 12
Processing Information and Controlling Behavior Most behaviors are controlled by pathways composed of four elements Sensory neurons: respond to a stimulus, either internal or external to the body Interneurons: receive signals from many different sources and often activate motor neurons Motor neurons: receive messages from sensory neurons and interneurons and activate muscles or glands Effectors, such as muscles and glands: perform the response directed by the nervous system Introduction to Life Processes - SCI 102 13
Processing Information and Controlling Behavior The four elements carry out the basic operations required of any nervous system Nervous systems must determine the type of stimulus Nervous systems must determine and signal the intensity of a stimulus Nervous systems must integrate information from many sources Nervous systems must initiate and direct appropriate responses Introduction to Life Processes - SCI 102 14
Processing Information and Controlling Behavior The nature of a stimulus is encoded by specialization of sensory neurons and their connections to specific parts of the brain Sensory neurons respond to different stimuli Information gathered by the senses is converted to action potentials, which are sent to the brain All nervous systems interpret what a stimulus is by monitoring which neurons are firing action potentials Introduction to Life Processes - SCI 102 15
Processing Information and Controlling Behavior The intensity of a stimulus is encoded by the frequency of action potentials Intensity can also be coded by the number of similar neurons that fire at the same time The nervous system processes information from many sources The brain receives sensory stimuli from many different sources Introduction to Life Processes - SCI 102 16
Processing Information and Controlling Behavior The nervous system produces outputs to muscles and glands The nervous system stimulates activity in muscles and glands, which produce behaviors Behaviors are controlled by networks of neurons in the nervous system Simple behaviors such as the reflex may be controlled by activity in as few as two or three neurons Complex behaviors are organized by interconnected neural pathways Introduction to Life Processes - SCI 102 17
Nervous System Organization All animals have two basic types of nervous systems Radially symmetrical animals have a diffuse network of neurons called a nerve net Bilaterally symmetric animals have a centralized nervous system in which cell bodies tend to be clustered in the head region and along the central axis of the animal Introduction to Life Processes - SCI 102 18
Structures and Functions of the Human Nervous System The nervous system of all mammals is divided into the peripheral nervous system (PNS) and the central nervous system (CNS) The PNS links the CNS with the rest of the body The PNS consists of three major categories of axons Axons of sensory neurons that carry sensory information from all parts of the body Sensory neurons carry this information to sensory organs (eyes, ears, nose, etc.) Axons of motor neurons that control the skeletal muscles, regulating voluntary movements Axons of motor neurons that control involuntary movements Introduction to Life Processes - SCI 102 19
Structures and Functions of the Human Nervous System Motor neurons make up the somatic and autonomic nervous systems The somatic nervous system controls voluntary movement Most neurons of this system synapse with skeletal muscles The autonomic nervous system controls involuntary actions The sympathetic division of the autonomic nervous system prepares the body for fight or flight The parasympathetic division of the autonomic nervous system dominates during rest and digest Introduction to Life Processes - SCI 102 20
Structures and Functions of the Human Nervous System Important Neurotransmitters Epinephrine and norepinephrine Part of the sympathetic nervous system Activate target organs of the sympathetic nervous system Also known as adrenaline and noradrenaline When you experience something very scary, the automatic response of the body is to produce epinephrine. This may cause you to feel your heart racing, lungs pumping, blood rushing. Introduction to Life Processes - SCI 102 21
Structures and Functions of the Human Nervous System Endorphins Important Neurotransmitters Found in many areas of the brain and spinal cord Tasks: block the transfer of pain sensations and alter mood Serotonin Found in the midbrain, pons, and medulla Influences mood and sleep Glutamate Found in many areas of the brain and spinal cord Is the major excitatory neurotransmitter in the CNS Introduction to Life Processes - SCI 102 22
Structures and Functions of the Human Nervous System The CNS consists of the spinal cord and brain First line of defense around the CNS: the skull and backbone Second line of defense around the CNS: A triple layer of connective tissue, called the meninges Cerebrospinal fluid between the meninges aids in cushioning the CNS The blood brain barrier, a wall of brain capillaries that are much less permeable than capillaries in the rest of the body, forms the third line of defense Tasks of the CNS: Receive and process information Initiates action Introduction to Life Processes - SCI 102 23
Structures and Functions of the Human Nervous System The spinal cord controls many reflexes and conducts information to and from the brain The spinal cord has axons of both sensory neurons and motor neurons Gray matter: The center of the spinal cord Consists mostly of cell bodies of neurons White matter: The outer part of the spinal cord Consists mostly of the myelin-coated axons of the neurons Introduction to Life Processes - SCI 102 24
Structures and Functions of the Human Nervous System The spinal cord controls many reflexes and conducts information to and from the brain The neurons that control many reflexes reside in the spinal cord and PNS A reflex is an involuntary movement of a body part in response to a stimulus The pain-withdrawal reflex is a good example Some complex actions are coordinated within the spinal cord Neurons needed for a variety of complex actions, such as walking and running, are found in the spinal cord Introduction to Life Processes - SCI 102 25
Structures and Functions of the Human Nervous System The brain consists of many parts that perform specific functions All vertebrate brains consist of three main parts: The hindbrain The midbrain The forebrain In nonmammalian vertebrates, the three divisions are prominent In mammals, the brain regions are significantly modified The human brain has a variety of structures Introduction to Life Processes - SCI 102 26
Structures and Functions of the Human Nervous System The hindbrain consists of the medulla, pons, and cerebellum The medulla controls several automatic functions The pons plays a role in the transition from sleep to wakefulness, as well as between the stages of sleep The cerebellum is crucial in coordinating movements of the body The midbrain contains clusters of neurons that contribute to movement, arousal, and emotion The midbrain is quite small in humans The reticular formation is an important relay and filtering station that extends from the medulla into the lower regions of the forebrain Introduction to Life Processes - SCI 102 27
Structures and Functions of the Human Nervous System The forebrain includes the thalamus, hypothalamus, and cerebrum The thalamus channels sensory information from all parts of the body to the limbic system and cerebral cortex The hypothalamus, through its hormone production and neural connections, helps maintain homeostasis The cerebrum consists of two cerebral hemispheres Each hemisphere is composed of an outer cerebral cortex Structures in the Interior of the cerebrum The amygdala produces sensations of pleasure, fear, or sexual arousal The hippocampus is critical for the formation of long-term memory The basal ganglia are important in the overall control of movement Introduction to Life Processes - SCI 102 28
Structures and Functions of the Human Nervous System The forebrain includes the thalamus, hypothalamus, and cerebrum The limbic system is a group of structures that produce our most basic emotions, drives, and behaviors The cerebral cortex The cerebral cortex is folded into convolutions The cerebral cortex is divided into two hemispheres connected by a large band of axons, called the corpus callosum Each hemisphere is divided into four anatomical regions: the (1) frontal, (2) parietal, (3) occipital, and (4) temporal lobes, which all have specific functions Introduction to Life Processes - SCI 102 29
Structures and Functions of the Human Nervous System Cerebral cortex Thin outer layer of each cerebral hemisphere Neurons in the cortex: Receive and process sensory information Sound, sight, smell, taste, touch Direct voluntary movements Create memories Allow us to be creative Envision the future Is made up of four parts called lobes Occipital lobe: visual association area Parietal lobe: sensory association area, primary sensory area Frontal lobe: premotor area, area of higher intelligence functions, speech motor area, primary motor area Temporal lobe, primary auditory area, memory, language comprehension Introduction to Life Processes - SCI 102 30
Structures and Functions of the Human Nervous System How do neuroscientists learn about the functions of brain regions? The functions of different parts of the brain were discovered by examining the behaviors and abilities of people who suffered brain injuries, such as the case of Phineas Gage Introduction to Life Processes - SCI 102 31
Structures and Functions of the Human Nervous System The left and right sides of the brain are specialized for different functions Studies in which the corpus callosum has been cut have shown the differing functions of each hemisphere Axons from the eyes go to opposite sides of the brain Further studies have confirmed the different functions of each hemisphere Example: It is possible that brain damage from a stroke or other injury could leave a person with the ability to comprehend written language but not spoken language (assume the person's hearing is fine) because different aspects of language comprehension are controlled by different parts of the brain Introduction to Life Processes - SCI 102 32
Structures and Functions of the Human Nervous System Learning and memory involve biochemical and structural changes in specific parts of the brain Learning has two phases Short-term memory Long-term memory The frontal and parietal lobes and some of the basal ganglia deep in the cerebrum are the primary sites of short-term memory Short-term memory probably requires repeated activity, whereas long-term memory involves structural changes Introduction to Life Processes - SCI 102 33