Unformatted text preview: Molecular Cell Biology 32 Professor Terry Machen
09/21/10 Lecture 8
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These notes are copyrighted by the University of California and are for your personal use only. ANNOUNCEMENTS The exam is a week from today. It will be in this classroom and will begin at 9:30. Today’s schedule is: Somatic motor pathway Ch. 8 Motor cortex. Motor nerve excites skeletal muscle contraction by Ach (acetylcholine) and EPSP coordination: basal ganglia, thalamus and cerebellum reflexes. Autonomic Motor pathway CH. 9. Medulla oblongata+hypothalamus. Sympathetic and parasympathetic nerves excite or inhibit glands, heart, smooth muscle. Anatomy. Neurotransmitters:ACH and norepinephrine. LECTURE The left cortex controls the right side of the body and vice versa. When you want to move your fingers there, is a decision in your brain that sends a burst of action potentials down the spine. There are two different pathways that these signals can be brought down. Some go down the middle part of the brain, down to the brain stem, and to the medulla. Another path crosses over to the other side of the spinal cord and makes a connection to a second nerve which innervates the muscle causing the muscle to contract. The nerve and all the muscles that are innervated are known as a motor unit. One nerve can give rise to two muscles. Each nerve can innervate multiple muscles. Nerves that move from the brain to the spine are known as Corticospinal tracts. These tracts can either go straight down or they can cross over to the other side. Anterior corticospinal tract goes down the spinal cord farther than the lateral corticospinal tract before it crosses over. Student What happens if a lateral corticospinal tract is damaged? The crossover of all the nerves is not shown in the picture. There are many nerves that connect the brain to the muscle. Ganglia are bundles of nerves in the brain. Basal nuclei regulate the activity of the nerves indirectly by signaling to the motor cortex in the brain. Defects in the basal nuclei make it difficult to coordinate movement. This is what happens in Huntington’s and Parkinson’s disease. For some reason a genetic defect affects the genes that code DO NOT COPY Sharing or copying these notes is illegal and could end note taking for this course. MCB 32 ASUC Lecture Notes Online: Approved by the UC Board of Regents 9/21/10 for the basal nuclei. Cerebellum, ganglia and basal nuclei are all involved in motor coordination. The thalamus, basal nuclei and cerebellum are connected in a complicated feedback network that allows you to make precise movements. Inhibition of the lower motor neurons is important in contracting your muscles. You need both inhibition and contraction in order to control your muscle movements precisely. For example, when you need to contract your bicep, you would need to inhibit the contraction of your triceps. The pathway starts in the cerebral cortex (precentral gyrus, nerve #1).The nerve then passes through and/or interacts with nerves in basal ganglia, thalamus and cerebellum which regulate coordination. After reaching the spinal cord, the nerve can cross over (from left brain right side muscle). The motor nerve from the spinal cord then leads to the muscle (nerve #2). Nerve #2 + all muscles innervated = motor unit. Knee jerk reflex: You wouldn’t want both muscles to contract simultaneously. The way your body prevents this is that it sends an action potential through the sensory neuron that sends in action potential to the synapse which connects to an interneuron which releases an inhibitory signal to the motor neuron to relax the other corresponding muscles. These interneurons convert the excitatory signals from the sensory nerve to an inhibitory signal sent to the motor nerve. (This is the second outer loop shown in the picture). Student: How does it sense the length of the tendon? There is a monitor that senses the length of the tendon. When you hit the tendon it stretches and sends the signal to your spine. Summary: The muscle spindle senses muscle length. A stretch caused by hitting the tendon of the quadriceps triggers the muscle spindle. If the stimulus reaches threshold then the afferent nerves (via dorsal root) send an action potential to the spinal cord. This signal from the sensory nerve triggers the motor nerve to the quadriceps muscle where there is a release of excitatory neurotransmitters (Ach), EPP, and action potential in muscles. The muscles will then contract. The synapse with inhibitory interneuron to motor neuron to the hamstring (antagonist) muscle releases inhibitory neurotransmitter into motor nerve. This is to prevent both muscles in your leg from contracting. There are IPSPs and no action potentials in this motor nerve. The inhibitory signals cause the hamstring muscle quiescent to relax. Somatic (motor - only skeletal muscle) is voluntary. You must think about the movement; it is specific to the skeletal muscle which is innervated by only one motor nerve. There is only a single neurotransmitter with a single responsecontract. Examples: hamstring, quadriceps, biceps etc. F igur e 1 M uscle “ tone” . This is a simple r eflex. If you hit the tendon in your knee it will shorten, causing your leg to have a reflex. If the tendon is stretched the sensory nerves sends a signal to the interneuron. An action potential from the sensory fiber in your knee sends an action potential through a synapse (in spine) to nerve #2. The signal is passed to a motor nerve which makes a response. DO NOT 2 COPY Sharing or copying these notes is illegal and could end note taking for this course. MCB 32 ASUC Lecture Notes Online: Approved by the UC Board of Regents 9/21/10 Autonomic (motor - glands, heart, sm. muscle) is involuntary and non-specific. The target cells can be innervated by two different autonomic nerves. There are two neurotransmitters with varied responses. Examples: heart, pulmonary smooth muscle, blood vessel smooth muscle, intestinal smooth muscle, eye pupil, stomach. Intestine secretions, kidney functions, adrenal gland are all innervated by the autonomic smooth muscle. The sympathetic nervous system has the same action except for the length of the pre and post synaptic nerves. Also, the synapse releases norepinephrine instead of acetylcholine when it reaches the visceral effector organ. Symp. Parasymp. ANATOMY When the sympathetic nervous system sends a signal to the adrenal gland, it causes the gland to release the hormone adrenaline which amplifies the response by causing the chemical to circulate the blood and travel throughout your body. Some smooth muscles are innervated by both sympathetic and parasympathetic. Norepinephrine causes contraction and acetylcholine causes relaxation. This is a ying and yang relationship. These are the two parts of the autonomic nervous system. The sympathetic nervous system is activated during stress. When you eat and your body wants to rest and digest, it activates the parasympathetic nerves. In every case, the first nerve of the PS has a long presynaptic nerve and a short postsynaptic nerve. In the sympathetic nervous system, the presynaptic nerve is short and close to the spine. The vagus nerve innervates multiple organs that you don’t have to know about. The adrenal gland releases adrenaline which is released into the blood and amplifies the sympathetic response. Now let’s discuss the parasympathetic and sympathetic transmitters. They have different synapses and second massagers. The first nerve from the brain sends an action potential down the spinal cord to the synapse which releases acetylcholine whose action depends on the organ. This effect is in the visceral effector organs. The receptors for norepinehprine (NE) are adrenergic receptors. When treating high blood pressure you will want to inhibit these receptors. DO NOT 3 COPY Sharing or copying these notes is illegal and could end note taking for this course. MCB 32 ASUC Lecture Notes Online: Approved by the UC Board of Regents 9/21/10 The Ach receptors act on cholinergic receptors also known as muscarinic receptors. Nicotinic receptor is on muscles and responds to nicotine. Acetylcholine released from postganglionic neuron attaches to muscarinc receptors and acts by second massagers, not by action potentials. We will learn more about this later. The release of norepinephrine is the same but the receptors are different and trigger different responses. For acetylcholine, it affects the heart by causing it to slow. It acts on the blood vessels and the lung airways, causing them to constrict. The pupils will become smaller and the GI tract will increase in activity. The overall effect of acetylcholine is a REST and DIGEST response. Norepinephrine causes responses that depend on the receptor type (alpha or beta). These responses include an increase in heart rate and strength of contraction. The blood vessels will contract. The lung airways will relax allowing you to get more oxygen. Overall this is a FIGHT or FLIGHT response, which will also cause of pupils to dilate. The GI tract will also slow activity. 1. Drug blocking nicotinic acetylcholine receptors would: A) Block skeletal muscle contraction? YES – e.g., curare isolated from SA frog skin B) Block smooth muscle contraction? NO – different receptor 2. Drug blocking muscarinic acetylcholine receptors would: A) Block skeletal muscle contraction? NO – nicotinic receptors involved B) Slow or stop heart contractions? NO –Muscarinic Ach receptors slow the heart, so blocking this receptor speeds the heart. End of lecture. Notes prepared by Victor Hoang. Edited by Y.Alhlou. DO NOT 4 COPY Sharing or copying these notes is illegal and could end note taking for this course. ...
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