Nervous System All animals must respond to environmental stimuli Sensory receptors – detect stimulus Motor effectors – respond to stimulus The nervous system links the two Consists of neurons (nerve cells) and supporting cells (neuroglia)

Nervous System Vertebrates have three types of neurons (nerve cell) Sensory – carry impulses from sensory receptors to the central nervous system (CNS) Motor – carry impulses from the CNS to effectors (muscles and glands) Interneurons (association neurons) – located in the brain and spinal cord; provide more complex reflexes and higher associative functions such as learning and memory; “integrators” Sensory communicate information about the external and internal environment to the CNS Approx 12 billion neurons in human body, most in brain and spinal cord Sensory – receive stimuli from external enviro Motor – transmit impulses from brain and spinal cord to the muscle or gland that will produce the stimulus Interneurons – connect sensory and motor neurons and carry stimuli in brain and spinal cord; more than 90% of neurons in the body are association neurons

Nervous System The central nervous system (CNS) consists of the brain and the spinal cord The peripheral nervous system (PNS) consists of sensory and motor neurons; a network of nerves extending into different parts of the body; carries sensory input to the CNS and motor output away from the CNS

Nervous System: Neurons The basic structure of the neuron (consists of): Cell body – an enlarged region containing nucleus Dendrite – short cytoplasmic extensions extending from the cell body; receive stimuli Axon – single, long extension that conducts impulses away from the cell body The axons controlling the muscles in a person’s feet can be more than 1 meter (3 ft) long!; axons from the skull to the pelvis in a giraffe are 3 meters (9 ft) long! Motor and associaton neurons possess a profusion of highly branched dendrites which enables them to receive information from many different sources simultanously Only one axon per neuron

Neuron

Nervous System: Neuroglia Neurons are supported structurally and functionally by supporting cells called neuroglia 1/10th size of a neuron, but 10x more abundant Schwann cells and Oligodendrocytes – produce the myelin sheaths in the PNS and the CNS, respectively Myelin sheaths surround and insulate the axon of many types of neurons; “myelinated” axons Each Schwann cell tightly wraps around the axon numerous times to form a multi-layered insulation In the CNS, myelinated axons form the white matter and unmyelinated dendrites form the gray matter; in the PNS, myelinated axons are bundled together like wires in a cable to form nerves

Nervous System: Neuroglia Other neuroglia provide neurons with nutrients, remove wastes Small gaps known as nodes of Ranvier interrupt the myelin sheath at intervals of 1-2μm; uninsulated, capable of generating electrical activity (sites of action potential) Ion transfer occurs here, action pot jumps from one node to another rather than propogating smoothly

Conduction of the nerve impulse Upon stimulation of a nerve cell, electrical changes spread or propagate from one part of the cell to another Neuron function depends on a changeable permeability to ions; an electrical difference exists across the plasma membrane Membrane potential – voltage measured across a membrane due to differences in electrical charge; inside of cell is negative relative to outside The nerve impulse is an electrochemical event that occurs in the neuron The minus sign indicates that the inside of the cell is negative with respect to the outside

Conduction of the nerve impulse When a neuron is not being stimulated, it maintains a resting potential; -70mV (average; ranges from -40 to -90; “polarized”) The inside of the cell is negatively charged relative to the outside Polarization is established by maintaining an excess of Na+ ions on the outside, and an excess of K+ ions on the inside Most animal cells have a low concentration of Na+ and a high K+ relative to their surroundings Negative charge maintained by the active transport of Na ions out of the cytoplasm Sodium, potassium pump – helps establish and maintain conc gradients resulting in high K and low Na inside the cell, and high Na and low K outside of the cell Ion leakage channels – in the cell membrane are more numerous for K than for N; form pores through the membrane; more numerous for K than for Na allowing more K to diffuse out of cell than for Na to diffuse into cell

Conduction of the nerve impulse A certain amount of Na+ and K+ ions are always leaking across the membrane through leakage channels, but Na+/K+ pumps actively restore the ions to their appropriate sides The Na+/K+ pump: brings in 2 K+ for every 3 Na+ pumped out Ion leakage channels: allow more K+ to diffuse out than Na+ to diffuse in

Conduction of the nerve impulse Other ions, such as large, negatively-charged proteins and amino acids, reside within the cell It is these large, negatively-charged ions that contribute to the overall negative charge on the inside of the cell membrane relative to the outside Negative pole: Cytoplasm (inside cell) Positive pole: Extracellular (outside cell)

Remember: Cells contain relatively high [K+] inside the cell, but low [Na+] Na/K pump – transfers 3 Na outside of the cell and simultaneously transports 2 Ks inside the cell; requires ATP maintains cell membrane potential

Conduction of the nerve impulse A nerve impulse is generated when the difference in electrical charge disappears Occurs when a stimulus contacts the tip of a dendrite and increases the permeability of the cell membrane to Na+ ions Na+ ions rush into the cystoplasm, and the difference in electrical charge across the membrane disappears (depolarized) Remember, the concentration of Na+ inside the cell is low relative to its surroundings

Conduction of the nerve impulse Some stimuli open K+ channels As a result, K+ leaves the cell (remember: high [K+] inside the cell) Membrane potential becomes more negative (more negative inside the cell) “hyperpolarization” Some stimuli open Na+ channels Causes Na+ to enter the cell Membrane potential becomes less positive “depolarization”

Conduction of the nerve impulse When the strength of stimuli determines how many ion channels will open; graded response Caused by the acvtivation of a gated ion channels which behave like a door that can open or close, unlike ion leakage channels that are always open Each gated channel is selective, opening only to allow diffusion of one type of ion Normally closed in a resting cell allow cell to change its membrane potential in response to stimuli

Graded Potentials Depends on stimulus, 1 weak, 2 stronger, 3 inhibitory (produces hyperpolarization); resulting change will be the sum of all 3 (if they occur together)

Action Potentials Permeability changes are measurable as depolarizations or hyperpolarizations of the membrane potential Depolarization – makes membrane potential less negative (more positive) Hyperpolarization – makes membrane potential more negative Ex: -70mV  -65mV = Depolarization -70mV  -75mV = Hyperpolarization These are graded potentials b/c their size depends on either the strength of the stimulus or the amount of ligand available to bind with their receptors

Action Potentials Action potentials are rapid, reversals in voltage across the plasma membrane of axons Once a threshold of depolarization is reached (-50 to -55 mV), an action potential will occur An ‘all or nothing’ response, not graded Magnitude of the action potential is independent of strength of depolarizing stimuli Action potentials are the signals by which neurons communicate and spread messages Action potentials result when depolarization reaches a threshold Also in muscle cells

Action Potentials An action potential is caused by a different class of ion channels, voltage-gated ion channels These channels open and close in response to changes in membrane potential; only open at certain membrane potentials; flow of ions controlled by these channels creates the action potential only open at certain membrane potentials (theres a threshold, above it – open, below it – closed)

Action Potentials Voltage-gated channels are very specific; each ion has its own channel Voltage-gated Na+ channels Voltage-gated K+ channels

Action Potentials When the threshold voltage is reached, Na+ channels open rapidly Influx of Na+ causes the membrane to depolarize K+ channels open slowly, eventually repolarizing the membrane Action potential consists of three phases: Rising, falling, and undershoot

Action Potentials – the Spoiler At threshold, membrane is depolarized enough that Na+ voltage-gated channels open; Na+ moves into interior of cell, becoming less negative; rapid depolarization ( 45mV), then stops Stops because channels will close after a specific amount of time has elapsed

Action Potentials II – the Spoiler K+ voltage-gated channels will also open, before membrane potential reaches zero; K+ moves out of cell, making cell become more negative, returns cell to resting Na+/K+ pump is also activated, moving 3 Na+ out for every 2 K+ in, makes cell more negative Returns cell to rest (~-70mV) Na+ is the primary ion responsible for changing cell membrane potential

Action Potentials III – the Spoiler Excess K+ diffuse out before K+ channel closes, or over-activity of the Na+/K+ pump; results in undershoot (hyperpolarization) Entire process occurs very rapidly: 2-4ms from start to finish

Action Potentials Each action potential, in its rising, reflects a reversal in membrane polarity Positive charges due to Na+ influx can depolarize adjacent region to threshold, causing the next region to produce an action potential of its own The previous region then repolarizes back to its resting membrane potential

3. Top curve 2. Rising Phase 1. Resting Phase 4. Falling Phase Membrane potential (mV) 2. Rising Phase Stimulus causes above threshold voltage +50 1 2 3 –70 Time (ms) 1. Resting Phase Equilibrium between diffusion of K+ out of cell and voltage pulling K+ into cell Voltage-gated potassium channel Potassium channel sodium channel Potassium channel gate closes Sodium channel activation gate closes. Inactivation gate opens. activation gate opens 3. Top curve Maximum voltage reached gate opens 4. Falling Phase gate open Undershoot occurs as excess potassium diffuses out before potassium channel closes Equilibrium restored Na+ channel inactivation gate closes K+ Na+ closed At rest, voltage-gated ion channels are closed, but there is some leakage of K. In response to stimulus, the cell begins to depolarize, and once a threshold is met, an action potential is produced; rapid depolarization occurs (bringing in Na+ into cell) because voltage-gated Na+ channels open, allowing Na+ to diffuse into the axon. At the top of the spike, Na channels close, and K+ channe;s (that were previously closed) open. Repolarization occurs because of the diffusion of K outside of the cell. And undershoot occurs before the membrane returns to its resting potential

Action Potentials Action potentials are localized events They DO NOT travel down the membrane They are generated anew in a sequence along the neuron as they propogate along axon During undershoot, the membrane is less likely to depolarize This keeps the action potential moving in one direction

Cell membrane Cytoplasm resting repolarized depolarized + + + + + + + + + – – – – – – – – – – – + + + + + + + + + – – – – – – – – – – – – – – + + + + + + + + + – – + + – – + + + + + – – + + – – – – – – – – – – + + – – + + + + + – – + + + + + + – – – + + – – – – + + + – – – – + + + – – – – + + – – – + + + + + + + + + + + – – – – – – – – – + + – – + + + + + + + Na+ K+

Saltatory Conduction Two ways to increase velocity of conduction: Increase diameter of axon; reduces resistance to current flow; found primarily in invertebrates Axon is myelinated; impulse jumps from node to node (Nodes of Ranvier – the only site of action potentials) = saltatory conduction one action potential still serves as stimulus for the next one, but the impulse (depolarization at one end) spreads quickly beneath the insulating myelin to trigger the opening of voltage-gated ion channels at the next node Squid have a “giant” axon, allows for very rapid escape response Saltare means to jump

Saltatory Conduction http://www.flickr.com/photos/photoklick/2829645922/

Saltatory Conduction