Nervous Systems Three Main Functions: 1. Sensory Input 2. Integration

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Presentation transcript:

Nervous Systems Three Main Functions: 1. Sensory Input 2. Integration 3. Motor Output

Figure 48.1 Overview of a vertebrate nervous system

Two Main Parts of Nervous Systems Central nervous system (CNS) brain and spinal cord integration  Peripheral nervous system (PNS) network of nerves extending into different parts of the body carries sensory input to the CNS and motor output away from the CNS

Two Cell Types in Nervous Systems Neurons Cells that conduct the nerve impulses Supporting Cells - Neuroglia

Neuron

Figure 48.2x Neurons

Three Major Types of Nerve Cells Sensory neurons communicate info about the external or internal environment to the CNS Interneurons integrate sensory input and motor output makes synapses only with other neurons Motor neurons convey impulses from the CNS to effector cells

Supporting Cells - Neuroglia provide neurons with nutrients, remove wastes Two important types in vertebrates Oligodendrocytes – myelin sheath in CNS Schwann cells -myelin sheath in PNS

Myelin Sheath Formation

Conduction of the Nerve Impulse Membrane Potential Voltage measured across a membrane due to differences in electrical charge Inside of cell is negative wrt outside Resting potential of neuron = -70 mV

Figure 48.6 Measuring membrane potentials

Sodium-Potassium Pump 1. Carrier in membrane binds intracellular sodium. 2. ATP phosphorylates protein with bound sodium. 3. Phosphorylation causes conformational change in protein, reducing its affinity for Na+. The Na+ then diffuses out. 4. This conformation has higher affinity for K+. Extracellular K+ binds to exposed sites. 5. Binding of potassium causes dephosphorylation of protein. Extracellular Intracellular ATP ADP Pi P + K+ Na+ 6. Dephosphorylation of protein triggers change to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats.

Excitable Cells Neurons & muscle cells Have gated ion channels that allow cell to change its membrane potential in response to stimuli

Gated Ion Channels Some stimuli open K+ channels K+ leaves cell Membrane potential more negative hyperpolarization Some stimuli open Na+ channels Na+ enters cell Membrane potential less negative depolarization

Gated Ion Channels Strength of stimuli determines how many ion channels open = graded response

Nerve Impulse Transmission

Action Potentials Occur once a threshold of depolarization is reached -50 to –55 mV All or none response (not graded) Magnitude of action potential is independent of strength of depolarizing stimuli Hyperpolarization makes them less likely

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

Refractory Period During undershoot the membrane is less likely to depolarize Keeps the action potential moving in one direction

Propagation of Action Potential Action potential are very localized events DO NOT travel down membrane Are generated anew in a sequence along the neuron

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

Saltatory Conduction

Transfer of Nerve Impulse to Next Cell Synapse the gap between the synaptic terminals of an axon and a target cell

Transfer of Nerve Impulse to Next Cell Electrical synapses Gap junctions allow ion currents to continue Chemical synapses More common Electrical impulses must be changed to a chemical signal that crosses the synapse

Synapses

Neurotransmitters

Summation of postsynaptic potentials

Effects of Cocaine

Addiction Transporter protein Dopamine Cocaine Receptor Receptor protein Drug molecule Synapse 1. Reuptake of neuro- transmitter by transporter at a normal synapse. 2. Drug molecules block transporter and cause overstimulation of the postsynaptic membrane. 3. Neuron adjusts to overstimulation by decreasing the number of receptors. 4. Decreased number of receptors make the synapse less sensitive when the drug is removed. Neurotransmitter Transporter protein

Diversity of Nervous Systems Human Cerebrum Cerebellum Spinal cord Cervical nerves Thoracic Lumbar Femoral nerve Sciatic Tibial Sacral Nerve net Nerve cords Associative neurons Brain Giant axon Mollusk Echinoderm Central nervous system Peripheral Ventral nerve cords Radial ribs Cnidarian Flatworm Earthworm Arthropod

Parasympathetic nervous CNS Brain and Spinal Cord Sensory Pathways Motor Pathways Sensory neurons registering external stimuli Sensory neurons registering external stimuli PNS Somatic nervous system (voluntary) Autonomic nervous system (involuntary) Sympathetic nervous system "fight or flight" Parasympathetic nervous system "rest and repose" central nervous system (CNS) peripheral nervous system (PNS)

Parasympathetic Constrict Secrete saliva Dilate Stop secretion Dilate bronchioles Speed up heartbeat Increase secretion Empty colon Increase motility Empty bladder Slow down heartbeat Constrict bronchioles Sympathetic ganglion chain Stomach Secrete adrenaline Decrease secretion Decrease motility Retain colon contents Delay emptying Adrenal gland Bladder Small intestine Large intestine Spinal cord Parasympathetic

Vertebrate Central Nervous System Spinal Cord Receives info from skin & muscles Sends out motor commands for movement & response Simple integration -- reflexes Brain More complex integration Homeostasis, perception, movement, emotion, learning

Spinal cord Stretch receptor (muscle spindle) Nerve fiber Sensory Quadriceps muscle (effector) Spinal cord Dorsal root ganglion Gray matter White Monosynaptic synapse Sensory neuro Nerve fiber Stretch receptor (muscle spindle) Skeletal Stimulus Response Motor neuron

The knee-jerk reflex