Unit 8 Seminar - The Nervous System

UNIT 8: THE NERVOUS SYSTEM NERVE SIGNALS MAINTAIN HOMEOSTASIS • Both the nervous system and the endocrine system control actions of the body and maintain homeostasis • Responses to changes in environment are made by electrochemical messages (relayed to and from the brain) or chemical messages (hormones – carried by the blood) • The nervous system is an elaborate communication system that contains more than 100 billion nerve cells in the brain alone • Memory, learning, and language are functions of the nervous system VERTEBRATE NERVOUS SYSTEM Two main divisions: 1. Central Nervous System (CNS) 2. Peripheral Nervous System (PNS) 1. CENTRAL NERVOUS SYSTEM • Consists of the nerves of the brain and spinal cord • Acts as a coordinating centre for incoming and outgoing information 2. PERIPHERAL NERVOUS SYSTEM • Consists of all other nerves that carry information in the form of electrical impulses between the organs of the body and the central nervous system • Further divided into Somatic and Autonomic nervous system DIVISIONS OF THE NERVOUS SYSTEM SOMATIC NERVOUS SYSTEM Part of the PNS Controls skeletal, muscle, bone and skin Under conscious control (lifting weights) Sensory somatic nerves relay information about the environment to the CNS • Motor somatic nerves initiate an appropriate response • • • • AUTONOMIC NERVOUS SYSTEM • Part of the PNS • Contains special motor neurons that control the internal organs of the body • Controls unconscious, life-sustaining activities (breathing, heart beat) • Two divisions: • Sympathetic: prepares the body for stress • Parasympathetic: returns the body to resting state SYMPATHETIC VS. PARASYMPATHETIC • Sympathetic nervous system sets off the “fight-orflight” reaction that prepares the body to deal with an immediate threat. When this system is stimulated, heart rate and breathing rate increase. The blood sugar is released from the liver to provide the energy required to deal with the perceived threat. • Parasympathetic nervous system has the opposite effect. When a threat has passed, the nerves of this system slow heart rate and breathing rate and reverse the effects of the sympathetic nervous system response SYMPATHETIC VS PARASYMPATHETIC Anatomy Sympathetic Function Controls organs in times of stress (fight or flight response) Primary neurotransmitter **Norepinephrine** Origin Parasympathetic Controls organs when body is at rest (restores body to normal balance) Acetylcholine Exit directly from the Exit from the thoracic brain or from either & lumbar vertebrae the cervical or caudal area area NERVOUS SYSTEM CELLS Two types of cells: 1. Glial Cells (neuroglial cells): nonconducting cells that support, nourish, and protect neurons : functional units of nervous system; conduct nerve impulses (action potentials) yellow structures = neurons purple & pink structures= glial cells NERVE • A nerve is made of a number individual neurons grouped together into a single structure • You can think of it like a cable-like bundle – a protective sheath surrounding many individual wires RECEPTORS • Specialized cells that receive stimuli from the (internal or external) environment and pass on the information to sensory neurons EXAMPLES OF RECEPTORS • Photoreceptors in your eyes respond to light • Chemoreceptors in your nose and on your tongue are sensitive to chemicals • Baroreceptors are pressure receptors in your skin that detect the fit of your clothes • Thermoreceptors on your skin respond to different temperatures NEURON STRUCTURE Myelin sheath Nodes of Ranvier All neurons contain dendrites, cell bodies and axons Dendrites: Projections of the cytoplasm Receive info (from sensory neurons or other nerve cells or receptors) and conducts nerve impulses toward the cell body Cell Body: • Contains the nucleus, ER, Golgi, ribosomes, lysosomes, mitochondria Axon: An extension of the cytoplasm Carries information and conducts nerve impulses away form the cell body (to other neurons or to effectors) Very thin – 100 axons could fit in a single hair strand MYELIN SHEATH • A white coat of fatty protein that covers many axons • Acts as insulation for the neuron allowing nerve impulses to travel more quickly than in unmyelinated axons • Prevents the loss of ions from the nerve cell • In the PNS, the myelin sheath is made of special glial cells called Schwann cells. In the CNS, it is formed by oligodendrocytes. NODES OF RANVIER • Areas between the sections of myelin cells • Nerve impulses jump from one node to another, increasing the speed of nerve impulses NEUROLEMMA • Thin outer membrane that surrounds a myelinated axon only in the PNS • Helps promote the regeneration of damaged axons • When you get a paper cut, neurons are severed • Feeling gradually returns to your fingers as the severed neurons are rejoined 2W 3W 3M Several Months • Nerves within the brain that contain myelin are called white matter (because of their whitish appearance) • Grey matter is grey because it contains mostly cell bodies, dendrites and unmyelinated neurons. • These are found within the brain and spinal cord MULTIPLE SCLEROSIS (MS) • Multiple sclerosis is caused by destruction of the myelin sheath • The myelin sheath hardens and produces scarlike tissue that prevents normal transmission of information • Symptoms include double vision, speech difficulty, jerky movements, partial paralysis TYPES OF NEURONS 1. Sensory (afferent) neurons 2. Interneurons 3. Motor (efferent) neurons TYPES OF NEURONS: 1. SENSORY NEURONS • Also known as afferent neurons • Sense and relay information from receptor cells and then pass it on to an interneuron • Located in clusters called ganglia outside of the spinal cord TYPES OF NEURONS: 2. INTERNEURONS • Link neurons within the body • Found predominantly in the brain and the spinal cord • They interpret sensory information and connect neurons to outgoing motor neurons TYPES OF NEURONS: 3. MOTOR NEURONS • Also known as efferent neurons • Relay information from the CNS to the effectors • Effectors include muscles, organs, and glands. They produce responses to the stimuli. SIMPLE REFLEX In this simple handwithdrawal reflex, information is carried from skin receptors along a sensory neuron to the spinal cord. From here it is passed via the interneurons to motor neurons that lead to muscles in the hand and arm which elicit the stimulus (hand movement). Because this reflex involves only the spinal cord, the hand jerks away from the candle flame, even before information about the event has reached the brain, causing the experience of pain. This is known as a reflex. It is involuntary and often happens unconsciously. REFLEX ARC • The simplest nerve pathway is the reflex arc • Most reflexes occur without brain coordination • Reflex arcs contain five essential components: 1. Receptor 2. Sensory neuron 3. Interneuron (in spinal cord) 4. Motor neuron 5. Effector  An excitatory neuron causes muscle contractions  An inhibitory neuron causes muscle relaxation in the opposing muscle  This allows for the quick removal of the body part from the harmful stimulus All of this happens in less than 1 second, even before the information has been sent to your brain! The sensation of pain will become noticeable, and you will probably let out a yell! Imagine if your reflexes did not work this way! You would be badly burned if you had to wait for the sensation of pain to react. https://www.youtube.com/watch?v=wLrhYzdbbpE&t=1s How do neurons communicate and relay information? ACTION POTENTIALS ACTION POTENTIAL • Neurons use electrical signals to communicate with other neurons, muscles, and glands. • These signals, called nerve impulses are electrochemical messages created by the movement of ions through the nerve cell membrane • Through experimentation, it was found that every time a nerve is excited, there was a rapid change in electrical potential difference (electric potential) SOME IMPORTANT TERMS • Resting potential: the voltage difference (mV) across a nerve cell membrane during the resting stage (when it is not relaying information). The resting potential is normally -70 mV. The area outside the neuron is more positive than the inside. • Action potential: the voltage difference (mV) across a nerve cell membrane when the nerve is excited. When excited, the potential on the inside of the membrane can register at +40mV. HOW DO NERVE CELLS BECOME CHARGED? • Nerve cells have a rich supply of positive and negative ions inside and outside the cell • However, the negative ions are not really involved in creating the charged membrane – they are large and cannot easily cross the membrane • The membrane becomes charged because of an unequal concentration of positive ions across the membrane (possibly through protein channel/ion gates) PRE-EXCITATION • Before an action potential is initiated, there is a high concentration of Na+ outside of the neuron and a high concentration of K + inside the neuron • Negative ions (Cl-) will want to move out of the axon (where it is more positive) but they cannot get through the membrane, so they will accumulate along the inside of the membrane AT RESTING POTENTIAL PRE-EXCITATION / RESTING POTENTIAL • The membrane is now a polarized membrane – charged by the unequal distribution of ions (positive outside and negative inside, -70 mV) • The membrane has the potential to do work – expressed in mV EXCITATION 1. When the nerve cell becomes excited (due to a stimulus), the membrane becomes more permeable to sodium than potassium. That is, Na+ gates open and K + gates close. 2. Na + moves into the cell following a concentration gradient (diffusion) and charge attraction. The positive charge moving into the neuron reduces the potential difference of the membrane. This charge reversal is called depolarization. When the potential difference is above zero, Na + movement will now only be driven by diffusion. 3. When the membrane potential reaches +40mV, the sodium gates will close and the K+ gates will open. 4. K+ will now exit the neuron by diffusion and charge attraction. • As the positive K+ leave the neuron, the potential difference across the membrane will start to decrease – repolarization. This helps restore the original polarity of the nerve membrane. 5. The K+ gates will close when the resting potential (of -70mV) is reached again. • However, the Na+ and K+ ions are on the “wrong” side of the cell membrane. 6. Refractory Period: a resting period; during this time, the neuron cannot start another action potential • The ions will move back to their original positions through the Na/K pump (fuelled by ATP) which restores the resting membrane condition by transporting 3 Na+ out of the neuron and 2 K+ into the neuron • The time required for the nerve cell to become fully repolarized is called the “refractory period” and lasts 1 to 10 milliseconds. • Action Potentials are an “All-or-None Response” ALL OR NONE PRINCIPLE • The depolarization and repolarization events described above are called an action potential. • During an action potential, the depolarization spreads all the way to the axon terminal where the axon joins another cell. The action potential is "all or nothing." A slight depolarization will not cause the gates to open but if the depolarization is greater than a threshold value, the gates will open. Threshold levels vary for different neurons. • The intensity of an action potential does not diminish as it spreads along an axon. ACTION POTENTIAL GRAPH • Resting membrane potential is normally -70mV • When the nerve is excited, the potential on the inside of the membrane will change to +40mV • This is described as an action potential (does not last for more than a few milliseconds) +40 Refractory Period (Na+ and K+ channels cannot open during this time) +40 NEURONS COMMUNICATE VIA SYNAPSES • As the nerve impulses travel along the axon of a neuron, away from the dendrite, it reaches the far end, called the synaptic terminal. • Neurons are not directly connected, but have a small gap between them called the synaptic cleft which allows them to transmit the signal to another neuron or effectors (i.e. muscle or gland cell) • The connection between two neurons, or a neuron and an effector, is called a synapse. • In a synapse, small vesicles containing chemicals called neurotransmitters are located in the end plates of neurons • Once the nerve impulses reach the end plate, neurotransmitters are released • The neurotransmitters are released from the presynaptic neuron and diffuse across the synaptic cleft, creating a depolarization of the dendrites of the post-synaptic neuron • The neurotransmitters in the synaptic cleft then act on the dendrites of the postsynaptic neuron and the nerve impulse is carried along the axon • This allows the signal to be passed on between separate neurons • Therefore, the electrochemical signal can be broken down as:  Electrical: along the neuron/axon  Chemical: in the synapse NEURONS COMMUNICATE VIA NEUROTRANSMITTERS Neurotransmitters have either excitatory or inhibitory effects on the post-synaptic membrane • Acetylcholine: excitatory neurotransmitter that acts on the post-synaptic neurons to open the Na+ channels, causing depolarization • Cholinesterase: from the post-synaptic membrane, destroys acetylcholine and thus sodium channels close again and repolarization happens. https://www.youtube.com/watch?v=BbUcWbtVjT4&t=7s