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

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 PiPi P + K+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. PiPi PiPi PiPi Sodium-Potassium Pump

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

Membrane potential (mV) 2. Rising Phase Stimulus causes above threshold voltage –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 Voltage-gated sodium channel Potassium channel gate closes Sodium channel activation gate closes. Inactivation gate opens. Sodium channel activation gate opens 3. Top curve Maximum voltage reached Potassium gate opens 4. Falling Phase Potassium gate open Undershoot occurs as excess potassium diffuses out before potassium channel closes Equilibrium restored Na + channel inactivation gate closes K+K+ Na + Na + channel inactivation gate 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+K+ K+K+ K+K+ K+K+ K+K+ K+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

Effects of Cocaine

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 Transporter protein Dopamine Cocaine Receptor protein

Integration of multiple synaptic inputs

Summation of postsynaptic potentials

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

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

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 ParasympatheticSympathetic

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

Vertebrate Central Nervous System White matter –Internal part of brain & external part of spinal cord –Myelinated axons Gray matter –Cell bodies of neurons

Figure 48.16x Spinal cord

Vertebrate Central Nervous System Cerebrospinal Fluid –Fills central canal of spinal cord and ventricles of brain –Shock absorption

Functions of Spinal Cord Carrying information to and from the brain Integration of simple responses –Reflexes Unconscious programmed response to stimuli

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

The knee-jerk reflex

Evolution of Vertebrate Brain Evolved from a set of three bulges at the anterior end of spinal cord –Forebrain (cerebrum) –Midbrain (optic lobe) –Hindbrain (cerebellum & medulla oblongata) Regions have been further subdivided structurally and functionally

Vertebrate Brains Olfactory bulb CerebrumThalamus Optic tectum Cerebellum Spinal cord Medulla oblongata Pituitary Hypothalamus Optic chiasm Forebrain (Prosencephalon) Midbrain (Mesencephalon) Hindbrain (Rhombencephalon)

Vertebrate Brains The relative sizes of different brain regions have changed as vertebrates evolved -Forebrain became the dominant feature