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Download Neuroanatomy Handout #1: The Motor Neuron
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Chapter 2 Nerve Cells and Nerve Impulses Animal Cells • Membrane: – separates the inside of the cell from the outside environment – comprised of two layers of lipids with proteins embedded Animal Cells • Nucleus refers to the structure that contains the chromosomes • Mitochondria perform metabolic activities and provide energy that the cell requires. • Ribosomes: Sites at which the cell synthesizes new protein molecules • Endoplasmic reticulum: Transports newly synthesized proteins The Human Nervous System—2 Kinds of Cells • • Neurons – Approx. 100 billion in brain – Receive and transmit info – Behavior depends upon their communication Glia – 10X the number of neurons – Support neural communication Neuroanatomy Handout #1: The Motor Neuron • A motor neuron – has its soma in the spinal cord – receives excitation from other neurons – conducts impulses along its axon to a muscle or gland – is the largest of the nerve cells Neuroanatomy Handout #1: The Motor Neuron • Neurons are similar to other cells of the body • All neurons have a cell body (soma, A): – responsible for the metabolic work of the neuron – surrounded by cell membrane (A1) – Containing a nucleus (A2), mitochondria (A3), ribosomes (A4), endoplasmic reticulum (A5) Neuroanatomy Handout #1: The Motor Neuron • Neurons are different from other cells of the body because they have distinctive shape and function Neuroanatomy Handout #1: The Motor Neuron • The 4 major components of a motor neuron: – Soma/Cell body – Dendrites – Axon – Presynaptic terminals Neuroanatomy Handout #1: The Motor Neuron • Dendrites (B)- branching fibers responsible for receiving information from other neurons • Dendritic spines (B1) further branch out and increase the surface area of the dendrite Neuroanatomy Handout #1: The Motor Neuron • Axon (C) - thin fiber responsible for sending impulses to other neurons, glands, or muscles • Some neurons are covered with an insulating material called the myelin sheath (D) with interruptions in the sheath known as nodes of Ranvier (C2). • Axon hillock (C1) – bulge in the cell body where axon begins Neuroanatomy Handout #1: The Motor Neuron • Presynaptic terminals (E) refer to the end points of an axon responsible for releasing chemicals (neurotransmitters) to communicate with other neurons Neuroanatomy Handout #1: The Motor Neuron • Axons from other neurons (F) converge on receiving neuron • Synapse: gap between neurons • Postsynaptic neuron (G) and dendrite (G1) Sensory and Motor Neurons • A motor neuron receives excitation from other neurons and conducts impulses along its axon to a muscle or gland – It carries information from the brain to the perimeter of the body Sensory and Motor Neurons • A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation (touch, temperature, odor etc.) – It carries information from the perimeter of the body to the brain Other Cells of the Nervous System • Terms used to describe the neuron include the following: – Afferent axon - refers to bringing information into a structure. – Efferent axon - refers to carrying information away from a structure. – Interneurons or Intrinsic neurons are those whose dendrites and axons are completely contained within a structure. Other Cells of the Nervous System • Glia are the other major component of the nervous system and include the following: – Astrocytes help synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon. – Microglia - remove waste material and other microorganisms that could prove harmful to the neuron. The Cells of the Nervous System – Oligodendrocytes & Schwann cells- build the myelin sheath that surrounds the axon of some neurons. – Radial glia- guide the migration of neurons and the growth of their axons and dendrites during embryonic development. The Cells of the Nervous System • Spaniard Santiago Ramon y Cajal (18521934) was the first to demonstrate that neurons do not touch one another. • With this understanding came new ideas about how neurons communicate. The Nerve Impulse • A nerve impulse is the electrical message that is transmitted down the axon of a neuron. • The impulse is regenerated at points along the axon. • The speed of nerve impulses ranges from approximately 1 m/s to 100 m/s. The Nerve Impulse • The resting potential: state of the neuron prior to the sending of a nerve impulse • Electrical gradient: a difference in the electrical charge inside and outside of the cell – At rest, the membrane is slightly negative with respect to the outside (approximately -70 millivolts) Competing forces maintain a -70mV resting potential 1. Concentration gradient: The difference in the distribution of ions between the inside and the outside of the membrane – Sodium (Na+) more abundant outside cell than inside (10:1) – Potassium (K+) more abundant inside cell than outside (20:1) 2. Negatively charged proteins inside cell Cellular mechanisms of the resting potential • Selective permeability of the membrane allows some molecules (e.g. water, oxygen) to pass more freely than others. • Charged ions, like sodium (Na+), potassium (K+), calcium (Ca++) and chloride (Cl-) pass through channels in the membrane. • When the membrane is at rest: – Na+ channels are closed – K+ channels are partially closed allowing the slow passage of sodium Cellular mechanisms of the resting potential • The sodium-potassium pump puts three sodium ions out of the cell while drawing in two potassium ions. – helps to maintain the electrical gradient • The electrical gradient and the concentration gradient work to pull sodium ions into the cell. • The electrical gradient tends to pull potassium ions into the cells. The resting potential allows a neuron to respond quickly to a stimulus The Action Potential • The resting potential (-70mV) remains stable until the neuron is stimulated. • Electrical polarization: the difference in the electrical charge between two places Two things can happen to a resting neuron: • Hyperpolarization: increasing the difference (polarization) between the electrical charge of two places (less likely to fire) • Depolarization refers to decreasing the polarization towards zero (more likely to fire) • The threshold of excitement refers any stimulation beyond a certain level that results in a massive depolarization (action potential). The Action Potential • An action potential is a rapid depolarization of the neuron. • Stimulation of the neuron past the threshold of excitation triggers a nerve impulse, action potential, or “firing” • -70mV can become +50mV The Nerve Impulse • Voltage-activated channels are membrane channels whose permeability depends upon the voltage difference across the membrane. – Sodium channels are voltage activated channels. • When sodium channels are opened, positively charged sodium ions rush in and a subsequent nerve impulse occurs. The Nerve Impulse • Scorpion venom attacks the nervous system by keeping sodium channels open and closing potassium channels • Local anesthetic drugs block sodium channels and therefore prevent action potentials from occurring. – Example: Novocain • General anesthetics open potassium channels wider than usual The Nerve Impulse • The all-or-none law states that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it. – Action potentials are equal in intensity and speed within a given neuron. The Nerve Impulse • A refractory period happens after an action potential occurs, during which time the neuron resists another action potential. • The absolute refractory period: the first part, when membrane cannot produce an action potential • The relative refractory period: the second part, when it takes a stronger than usual stimulus to trigger an action potential. The Nerve Impulse • In a motor neuron, the action potential begins at the axon hillock (a swelling where the axon exits the soma). • Propagation of the action potential is the term used to describe the transmission of the action potential down the axon. The Nerve Impulse • The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier. • At each node of Ranvier, the action potential is regenerated by a chain of positively charged ions pushed along by the previous segment. The Nerve Impulse • Saltatory conduction: the “jumping” of the action potential from node to node. – Provides rapid conduction of impulses – Conserves energy for the cell • Multiple sclerosis: disease in which myelin sheath is destroyed; associated with poor muscle coordination The Nerve Impulse • Not all neurons have lengthy axons. • Local neurons have short axons, exchange information with only close neighbors, and do not produce action potentials. • When stimulated, local neurons produce graded potentials which are membrane potentials that vary in magnitude and do not follow the all-or-none law,. • A local neuron depolarizes or hyperpolarizes in proportion to the stimulation.