AP Biology I. Nervous System Notes 1. General information: passage of information occurs in two ways: Nerves - process and send information fast (eg. stepping on a tack) Hormones - process and send information slowly (eg. growth hormone etc) 2. Major components of the nervous system: Two major divisions The central nervous system (CNS) - made up of the spinal cord and brain The peripheral nervous system (PNS) - made up of the cranial and spinal nerves 3. Neurons a) Definition - neurons are the cells which make up the nervous system. They consist of an axon, 2 or more dendrites and a cell body containing a nucleus. b.) Types of Neurons i. Sensory neurons - send information from the sense organs (outside) to the C.N.S. They have long dendrites and short axons. The diagram above shows a picture of a sensory neuron. Be able to label sensory receptors, myelin sheath, Schwann cells, Node of Ranvier, axon, dendrite, cell body and nucleus in the diagram of a sensory neuron above ii. Motor neurons - send information from the C.N.S to the muscles. Causing the muscles to move. They have short dendrites and long axons. The diagram below show a diagram of a motor neuron
Be able to label effector, myelin sheath, Schwann cells, node of ranvier, axon, dendrite, cell body and nucleus in the diagram of the motor neuron above iii. Interneurons - connect different neurons together, send information between neurons. They have short dendrites and short axons. 4. Myelin sheath - is a sheath that surrounds the axon. It is like a bunch of sausages joined together. The sausages are the Schwann cells and the spaces between the sausages the nodes of ranvier. The myelin sheath increases the speed of the nerve impulse because the nerve impulse jumps from one Node of Ranvier to the next (the node is the depression between the Schwann cells that make up the myelin sheath. The myelin sheath also insulates axon preventing the nerve impulse from "shorting" out. 5. Reflex arc - Be able to label the kinds of neurons and their parts, in the two diagrams of reflex arcs in the diagrams below and be able to describe how a reflex arc works
The correct order of a reflex arc is: sensory receptor --> sensory neuron-->spinal cord (CNS) --> motor neuron --> effector (muscle or gland) 5. The Nerve Impulse A. Passage of a nerve impulse within a Neuron i. Resting potential - polarized membrane -higher concentration of potassium ions (K+) inside the axoplasm (axoplasm = cytoplasm of the axon) than outside and a higher concentration of sodium ions (Na+) outside the axoplasm than inside. -membrane of the axon impermeable to sodium ions (Na+) (Sodium gates are closed) -membrane permeable to potassium ions (K+), -potassium ions (K+) diffuse from a high concentration inside the axon to a lower concentration outside the axon according to the law of diffusion. (they are slightly more permeable that Na+ ions) -sodium ions (Na+) diffuse from a high concentration outside the axon to a lower concentration inside the axon according to the law of diffusion. - sodium/potassium pump located in the membrane of the axon, pumps potassium ion (K+ ions) from the outside of the axoplasm to the inside and sodium ions (Na+) from the inside of the axon to the outside - The axoplasm is negatively charged because of large negatively charged ions located there. - the area outside the axoplasm is positively charged because of the excess (Na+) and (K+) ions. - (-60)mV membrane potential difference b. Threshold stimulus - in order for a nerve to depolarize it much reach a threshold level of stimulus, the level at which an action potential is generated ii. Action potential a. Upswing - depolarized membrane -membrane is permeable to Sodium ions (Na+) (Sodium gates are open) -sodium ions move into the axoplasm making the axoplasm more positive. -Outside the axoplasm becomes more negative as the sodium ions move into the axoplasm. -Inside the the axoplasm becomes more positive -membrane potential changes from -60mV to +40 mv b. Downswing - repolarization
-Sodium gates close, potassium gates open -potassium ions diffuse from inside the axoplasm to outside. The movement of (K+) ions to the outside causes the polarity to change back to positive and the loss of (K+) ions on the inside causes the polarity to change back to negative. -membrane potential changes from +40mV to -60 mv c. Recovery phase -Even though the polarity and membrane potential (voltage) of resting potential has been reestablished, The sodium and potassium ions are in the wrong places (eg. the sodium on the inside and the potassium on the outside). The Sodium/Potassium pumps re-establishes the resting potential configuration of sodium ions on the outside and potassium ions on the inside during this time. d. Refractory period As soon as the action potential has move on, the axon undergoes a refractory period. At this time the sodium gates are unable to open. This ensures that the action potential cannot move backwards and always moves down an axon to the axon branches B. Transmission of a Nerve Impulse Between two Different Neurons (across the Synapatic cleft) -presynaptic means anything before the synapse and postsynaptic means anything after the synapse. Therefore the cell transmitting the nerve impulse is called the presynaptic cell and the cell receiving the information is called the postsynaptic cell. -nerve impulses reaching the presynaptic ending cause calcium ions to interact with contractile proteins. These proteins pull the synaptic vesicles (containing neurotransmitters) to the inner surface of the presynaptic cell membrane. -the synaptic vesicles merge with the presynaptic cell membrane (like in exocytosis) and neurotransmitters are released into the synaptic cleft (the small space between the neur -the neurotransmitters diffuse across the cleft and fit into receptor sites on the postsynaptic cell membrane in a lock and key manner. (Inhibitor substances stop the impulse because they can fit into the receptor sites and block the normal neurotransmitter.) -this generates an action potential in the postsynaptic membrane and the nerve impulse continues on -after their release the neurotransmitters are quickly broken down by enzymes such as cholinesterase or reabsorbed by the presynaptic ending. The diagram below represents a synaptic knob (terminal knob) of an axon. Be able to identify parts A ( the synaptic vesicles) and B (the postsynaptic membrane), substance X (neurotransmitters) and the other parts of the synapse
This is a close up view of the vesicles that contain the neurotransmitters and the receptor sites that the neurotransmitters fit into 7. Differences between the Sympathetic and Parasympathetic systems The autonomic nervous system, part of the PNS is made up of two divisions: the sympathetic and parasympathetic systems. Characteristic Sympathetic Parasympathetic When functioning? emergencies normal/everyday Digestive system inhibits/slows down promotes Pupil dilates constricts Heartbeat accelerates retards Breathing rate increases retards Neurotransmitter norepinephrine acetylcholine 8. Parts of the brain a. Medulla oblongata - Contains the centres for heartbeat, breathing and blood pressure. It also
contains the reflex centers for vomiting, coughing, sneezing, hiccuping and swallowing b. Thalamus - It is the relay centre for sensory impulses travelling upwards from other parts of the cord and the brain to the cerebrum. It receives all sensory impulses (except for smell) and channels them to appropriate regions of the cerebrum c. Cerebrum - The area responsible for consciousness. d. Cerebellum - It functions in muscle co-ordination, integrating impulses received from the higher centres to ensure that all the skeletal muscles work together to produce a smooth and graceful motions. it is also responsible for maintaining muscle tone, and transmitting impulses that maintain posture. e. Hypothalamus - Is concerned with homeostasis or the constancy of the internal environment. Contains the centers for hunger, thirst, body temperature, water balance and blood pressure. It controls the pituitary gland and links the nervous and endocrine glands. f. Corpus callosum - is a connection between the two cerebral hemispheres. This allows the two hemisphere to share information. Severing the Corpus callosum can cause severe epileptic seizures. Be able to labeled the parts of the Brain below
III. Nerve subjective questions 1. Describe the events that occur during resting potential. 2. Describe, in detail, what occurs during the passage of a nerve impulse between 2 neurons. 3. Explain why a nerve impulse can only travel in one direction. 4. Explain how a chemical could disrupt the transmission of a nerve impulse 5. Describe the chemical and physical events that occur during action potential 6. Define reflex. Describe the sequence of events during a reflex action 7. Describe the physical and chemical events happening to the nerve membrane and what is happening inside and outside the nerve axon during the following: a. resting potential b. action potential 8. Describe the changes that would occur in the: membrane surrounding the axon; distribution and kinds of ions inside and outside the membrane and; polarity inside and outside the axon, during the following: a. An action potential is generated and travels down an axon b. After the action potential has passed and resting potential is being re-established 9. Create a diagram of a neuron and describe their functions 10. List 4 differences between the Central and Peripheral nervous systems. 11. List 4 difference between the sympathetic and parasympathetic nervous systems 12. Create or find a diagram of the brain and label the parts as well as describe their functions