Ch. 7 Neurons: The Matter of the Mind

Transcription

1 Ch. 7 Neurons: The Matter of the Mind The nervous system: Controls, integrates and coordinates the body s activities Provides rapid and brief responses to stimuli Has 2 major divisions: 1. The central nervous system (CNS) = the brain and spinal cord 2. The peripheral nervous system (PNS) = nerves Is made up of nervous tissue, which includes: Neurons (nerve cells) are excitable; they can generate and transmit electrochemical signals that carry information Neuroglial (or glial) cells support and protect neurons

2 The anatomy of a typical neuron The red arrows ( ) indicate the direction of information (electrochemical signal or action potential or nerve impulse) flow Figure 7.2

3 Neurons vs. nerves Types of neurons: 1. Sensory (afferent) neurons carry info from sensory receptors toward the CNS 2. Motor (efferent) neurons carry info away from the CNS to an effector (= a muscle or gland) 3. Interneurons (association neurons) carry info within the CNS (between sensory and motor neurons) Nerves = bundles of neuron processes (axons and/or dendrites) in the PNS Nerves may be sensory, motor, or mixed (mixed nerves contain both sensory and motor neuron processes) CNS Figure 7.1

4 Some glial cells Some glial cells of the CNS: 1. Ependymal cells help produce and circulate cerebrospinal fluid (CSF) 2. Astrocytes connect neurons to blood vessels; help form the blood-brain barrier 3. Microglia = phagocytes that remove cell debris, wastes, and pathogens 4. Oligodendrocytes myelinate (= electrically insulate) axons in the CNS Some glial cells of the PNS 1. Schwann cells myelinate (= electrically insulate) axons in the PNS (see the next slide)

5 Schwann cells and the myelin sheath Functions: electrically insulates the axon; speeds up the conduction of the nerve impulse; protects the axon; aids in axon repair Figure 7.3

6 Ion flow across the neuron membrane The electrochemical signal (action potential or nerve impulse) depends on the flow of ions across the neuron membrane Ions may passively diffuse down their concentration gradients through open channels or be actively pumped against their concentration gradients K + K + Na + Na + Na+ Note: leak channels (which are always open) for Na + and K + are not shown here K + K + Na + Na + Na + Gated channels may open/close in response to electrical or chemical stimuli Na + -K + pump (requires ATP) Figure 7.4

7 Membrane potential (in general) = a voltage difference across a cell membrane due to a separation and unequal distribution of electrical charges The main charged particles involved: Na + and K + ions It s usually reported in millivolts (mv) A membrane potential is an electrochemical gradient: The ions have the potential to ( want to ) diffuse across the membrane due to concentration differences and electrical forces, but most of these ions are not able to diffuse across because of limited permeability (i.e., most of the channel proteins for these ions are closed) The inside of the membrane of most cells is more negative compared to the outside of the membrane

8 The membrane potential of a resting neuron Resting potential = the voltage difference across the cell membrane for an unstimulated ( resting ) neuron -70 mv for most neurons (the inside is more negative compared to the outside) Na + ions predominate outside, K + ions predominate (although not as strongly) inside, due to: K + leak channels being much more permeable than Na + leak channels (Note: leak channels are not shown in the figure below) The Na + -K + pump exchanges 3 Na + for every 2 K +, ejecting Na + as quickly as it enters Na + Na + Na+ Na + Na + Na + K + Na + Na+ Na + K + K + Na + Na + K + Na + Na + K + K + K + K + K + K + K + K + K + K + Na + Na + Na + Figure 7.4

9 The action potential (AP) (or nerve impulse) While most body cells have membrane potentials, only excitable cell types (such as neurons and muscle cells) are able to utilize membrane potentials to generate and transmit/ conduct moving electrochemical signals that can spread/ propagate like a wave along the membrane over long distances This electrochemical signal is called an action potential (AP) or nerve impulse Essentially, it s a sudden/rapid reversal of the charge of the membrane potential (followed just as rapidly by the restoration of the original resting potential) that travels along the membrane (which is the membrane of the axon in the case of a neuron) Once initiated, it is self-regenerating and does not diminish in intensity as it travels

10 (leak channels not shown) An AP in action polarized Gated Na + channels close and are inactivated; gated K + channels open Threshold is reached, and gated Na + channels open repolarization depolarization Na + flows inward Gated potassium channels close Figure 7.4

11 An AP (as measured on a voltmeter) All-or-nothing = either an AP happens all the way at full intensity if threshold is reached or it doesn t happen at all if threshold isn t reached Refractory period = the brief time during which a new AP cannot be generated (i.e., the neuron cannot be stimulated again) due to gated sodium channels being either open or inactivated until the resting potential is reestablished Depolarization Repolarization ^ always Refractory period Figure 7.5

12 Figure 7.4 The spread/propagation/ conduction of an action potential An AP initiated at one part of the axon membrane depolarizes adjacent areas of the membrane to the threshold level, causing nearby gated sodium channels to open and the AP to spread continuously down the membrane A wave of positivity enters the axon The refractory period keeps the AP moving in one direction only

13 The velocity of action potential propagation Is influenced by: 1. Axon size (diameter) Diameter velocity 2. The presence of electrical insulation (i.e., myelin) Myelin velocity See the next slide for saltatory conduction of an AP

14 Saltatory conduction of an AP Rather than opening and closing gated ion channels continuously along the entire length of the axon, it s quicker to do so only at the nodes of Ranvier (= the unmyelinated axon segments that are found between Schwann cells) Figure 7.3a

15 The synapse = the junction/connection between a neuron and another cell (typically another neuron, or a muscle or gland cell) Axon endings terminate in bulblike swellings called synaptic knobs, which: Store chemicals called neurotransmitter in synaptic vesicles Release neurotransmitter in response to an incoming AP Figure 7.6a

16 An electron micrograph of a synapse Figure 7.6b

17 Transmission at an excitatory synapse (Slide 1 of 3) Figure 7.7

18 Transmission at an excitatory synapse An example of a neurotransmitter that may have an excitatory effect is acetylcholine (ACh) (Slide 2 of 3) Figure 7.7

19 Transmission at an excitatory synapse Why is it excitatory? The receptors are gated Na + channels, (Slide 3 of 3) which when opened cause depolarization toward threshold If the receptors are gated K + channels, opening them would cause hyperpolarization away from threshold, and the synapse would be inhibitory Figure 7.7

20 Information processing by neurons A neuron may have up to 10,000 synapses with other neurons from different sources some of these are excitatory synapses, and some are inhibitory synapses The first part of the axon of the postsynaptic neuron sums up the input: If the net effect = threshold is reached, then it generates an AP If the net effect = threshold is not reached, then it doesn t generate an AP Figure 7.8