2 Animal Cells Membrane: separates the inside of the cell from the outside environmentcomprised of two layers of lipids with proteins embedded
3 Animal CellsNucleus refers to the structure that contains the chromosomesMitochondria perform metabolic activities and provide energy that the cell requires.Ribosomes: Sites at which the cell synthesizes new protein moleculesEndoplasmic reticulum: Transports newly synthesized proteins
4 The Human Nervous System—2 Kinds of Cells NeuronsApprox. 100 billion in brainReceive and transmit infoBehavior depends upon their communicationGlia10X the number of neuronsSupport neural communication
5 Neuroanatomy Handout #1: The Motor Neuron A motor neuronhas its soma in the spinal cordreceives excitation from other neuronsconducts impulses along its axon to a muscle or glandis the largest of the nerve cells
6 Neuroanatomy Handout #1: The Motor Neuron Neurons are similar to other cells of the bodyAll neurons have a cell body (soma, A):responsible for the metabolic work of the neuronsurrounded by cell membrane (A1)Containing a nucleus (A2), mitochondria (A3), ribosomes (A4), endoplasmic reticulum (A5)
7 Neuroanatomy Handout #1: The Motor Neuron Neurons are different from other cells of the body because they have distinctive shape and function
8 Neuroanatomy Handout #1: The Motor Neuron The 4 major components of a motor neuron:Soma/Cell bodyDendritesAxonPresynaptic terminals
9 Neuroanatomy Handout #1: The Motor Neuron Dendrites (B)- branching fibers responsible for receiving information from other neuronsDendritic spines (B1) further branch out and increase the surface area of the dendrite
10 Neuroanatomy Handout #1: The Motor Neuron Axon (C) - thin fiber responsible for sending impulses to other neurons, glands, or musclesSome 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
11 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
12 Neuroanatomy Handout #1: The Motor Neuron Axons from other neurons (F) converge on receiving neuronSynapse: gap between neuronsPostsynaptic neuron (G) and dendrite (G1)
13 Sensory and Motor Neurons A motor neuron receives excitation from other neurons and conducts impulses along its axon to a muscle or glandIt carries information from the brain to the perimeter of the body
14 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
15 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.
16 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.
17 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.
18 The Cells of the Nervous System Spaniard Santiago Ramon y Cajal ( ) was the first to demonstrate that neurons do not touch one another.With this understanding came new ideas about how neurons communicate.
19 The Nerve ImpulseA 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.
20 The Nerve ImpulseThe resting potential: state of the neuron prior to the sending of a nerve impulseElectrical gradient: a difference in the electrical charge inside and outside of the cellAt rest, the membrane is slightly negative with respect to the outside (approximately -70 millivolts)
21 Competing forces maintain a -70mV resting potential Concentration gradient: The difference in the distribution of ions between the inside and the outside of the membraneSodium (Na+) more abundant outside cell than inside (10:1)Potassium (K+) more abundant inside cell than outside (20:1)Negatively charged proteins inside cell
22 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 closedK+ channels are partially closed allowing the slow passage of potassium
23 Cellular mechanisms of the resting potential The sodium-potassium pump puts 3 Na+ ions out of the cell while drawing in 2 K+ ions.helps restore and maintain resting potentialElectrical and concentration gradients attract sodium ions into the cell.Electrical gradient pulls potassium ions into the cellAnimation of Sodium Potassium Pump
24 The resting potential allows a neuron to respond quickly to a stimulus
26 The Action PotentialThe resting potential (-70mV) remains stable until the neuron is stimulated.Electrical polarization: the difference in the electrical charge between two places
27 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).
28 The Action PotentialAn 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
29 The Nerve ImpulseVoltage-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.
30 The Nerve Impulse Scorpion venom attacks the nervous system by keeping sodium channels open andclosing potassium channelsLocal anesthetic drugs block sodium channels and therefore prevent action potentials from occurring.Example: NovocainGeneral anesthetics open potassium channels wider than usual
31 The Nerve ImpulseThe 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.
32 The Nerve ImpulseA 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 potentialThe relative refractory period: the second part, when it takes a stronger than usual stimulus to trigger an action potential.
33 The Nerve ImpulseIn 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.
34 The Nerve ImpulseThe 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.
35 The Nerve ImpulseSaltatory conduction: the “jumping” of the action potential from node to node.Provides rapid conduction of impulsesConserves energy for the cellMultiple sclerosis: disease in which myelin sheath is destroyed; associated with poor muscle coordinationAnimation of propagation of the action potential along unmyelinated and myelinated axons
36 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.