- Slides: 37
Chapter 2 Nerve Cells and Nerve Impulses
Neurons and How They Work: Link to video
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 – 10 X 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 (A 1) – Containing a nucleus (A 2), mitochondria (A 3), ribosomes (A 4), endoplasmic reticulum (A 5)
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 (B 1) 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 (C 2). • Axon hillock (C 1) – 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 (G 1)
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 (-70 m. V): state of the neuron prior to the sending of a nerve impulse • Electrical polarization: the difference in the electrical charge between two places
Competing forces maintain a -70 m. V resting potential Electrical gradient: a difference in the electrical charge inside and outside of the cell – Influenced by the distribution of negatively and positively charged ions, including Na+ (sodium) and K+ (potassium) – Influenced by negatively charged proteins inside cell – Opposites attract: ions with positive charges are attracted to negative environments and ions with negative charges are attracted to positive environments
Competing forces maintain a -70 m. V resting potential 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) – In the absence of competing forces, particles will move from areas of higher concentration to areas of lower concentration.
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 potassium
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 potential • Electrical and concentration gradients attract sodium ions into the cell. • Electrical gradient pulls potassium ions into the cell Link to animation of Sodium Potassium Pump
The resting potential allows a neuron to respond quickly to a stimulus
The Action Potential • The resting potential (-70 m. V) remains stable until the neuron is stimulated. • If the neuron is stimulated to become more positive (i. e. , excited), an action potential may occur.
Electrical Stimulation of a Resting Neuron • Hyperpolarization: increasing the difference (polarization) between the electrical charge of two places – For a neuron, this means it becomes even more negative internally (less likely to fire) • Depolarization refers to decreasing the polarization towards zero – This makes a neuron less negative internally and more likely to fire
The Action Potential • The threshold of excitement refers to stimulation beyond a certain level that results in a massive depolarization (action potential/nerve impulse/firing). • -70 m. V can become +50 m. V
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 • 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. – Link to animation of propagation of the action potential
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 Link to video: How myelin sheath speeds up neural transmission (5 min)
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 • 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.