Chapter 2 Nerve Cells and Nerve Impulses

1. Chapter 2Nerve Cells and Nerve Impulses

2. Neurons and How They Work:Link to video

3. Membrane: separates the inside of the cell from the outside environment comprised of two layers of lipids with proteins embedded Animal Cells

4. 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 Animal Cells

5. Neurons Approx. 100 billion in brain Receive and transmit info Behavior depends upon their communication Glia 10X the number of neurons Support neural communication The Human Nervous System—2 Kinds of Cells

6. 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

7. 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)

8. Neuroanatomy Handout #1: The Motor Neuron • Neurons are different from other cells of the body because they have distinctive shape and function

9. Neuroanatomy Handout #1: The Motor Neuron • The 4 major components of a motor neuron: • Soma/Cell body • Dendrites • Axon • Presynaptic terminals

10. 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

11. 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

12. 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

13. 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)

14. 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

15. 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

16. 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

17. 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. Other Cells of the Nervous System

18. 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

19. Spaniard Santiago Ramon y Cajal (1852-1934) was the first to demonstrate that neurons do not touch one another. With this understanding came new ideas about how neurons communicate. The Cells of the Nervous System

20. 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.

21. The Nerve Impulse • The resting potential (-70mV): state of the neuron prior to the sending of a nerve impulse • Electrical polarization: the difference in the electrical charge between two places

22. 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) Influencedby 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 -70mV resting potential

23. 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. Competing forces maintain a -70mV resting potential

24. 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

25. 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

26. The resting potential allows a neuron to respond quickly to a stimulus

27. The Action Potential • The resting potential (-70mV) remains stable until the neuron is stimulated. • If the neuron is stimulated to become more positive (i.e., excited), an action potential may occur.

28. 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

29. The threshold of excitement refers to stimulation beyond a certain level that results in a massive depolarization (action potential/nerve impulse/firing). -70mV can become +50mV The Action Potential

30. 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.

31. 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

32. 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

33. 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.

34. 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)

35. 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.

36. 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.

37. 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.