The Nervous System Chapter 48 and Section 49.2 Biology – Campbell Reece

Neurons Neurons – nerve cells that transfer information within the body Communication by neurons consists of long-distance electrical signals and short-distance chemical signals Signals are interpreted by groups of neurons organized into a brain or simple clusters called ganglia

Information Processing Three stages: sensory input, integration, motor output Central nervous system (CNS) – includes the brain and spinal cord; carries out integration Peripheral nervous system (PNS) – neurons that carry information into and out of the CNS Nerve – a bundle of neurons of the PNS

Information Processing

Types of Neurons Sensory neurons – transmit information from eyes and other sensors that detect external stimuli or internal conditions Neurons in the brain (or ganglia) integrate (analyze and interpret) the sensory input Interneurons – connects sensory and motor neurons in the central nervous system Motor neurons – transmit signals to muscle cells, causing them to contract

Neuron Structure Cell body – contains most of a neuron’s organelles, including the nucleus Dendrites – numerous highly branched extensions The cell body and dendrites receive signals from other cells Axon – a single extension that transmits signals to other cells

Neuron Structure Synapse – a junction between an axon of a neuron and an adjacent cell Neurotransmitter – a chemical messenger that passes information from the transmitting neuron to the receiving cell Glial cells or glia – supporting cells that nourish neurons, insulate the axons and regulate the extracellular fluid around neurons

Neuron Structure

Resting Potential The inside of the cell is negatively charged relative to the outside Between -60 and -80 mV (millivolts) In mammalian neurons, the K + concentration is highest inside the cell, while the Na + concentration is highest outside the cell

Resting Potential

Action Potentials Gated ion channels – ion channels that open or close in response to stimuli The opening or closing of gated ion channels alters the membrane’s permeability to certain ions, which alters the membrane potential Opening K+ channels makes the inside of the membrane more negative (hyperpolarization) Opening Na+ channels makes the inside of the membrane less negative (depolarization)

Action Potentials Action potential – a massive change in membrane voltage Occurs when depolarization increases the membrane voltage above the threshold (-55 mV) Voltage-gated sodium channels open increasing the flow of Na + into the neuron, which triggers more channels to open (positive feedback) All-or-none response to stimuli

Action Potentials

Steps of an Action Potential 1.The gated Na + and K + channels are closed. 2.A stimulus opens some Na channels. Na + inflow depolarizes the membrane. If depolarization reaches the threshold, it triggers an action potential. 3.Depolarization opens most Na channels, while K channels remain closed. Na + influx makes the inside positive compared to outside.

Steps of an Action Potential 4.Most Na channels become inactivated, blocking Na + inflow. Most K channels open, permitting K + outflow, which makes the inside negative again. 5.Na channels close, but K channels are still open. As these close, the membrane returns to its resting state.

Myelin Sheath Insulation that surrounds vertebrate axons Produced by two types of glia – oligodendrocytes and Schwann cells Increases conduction speed compared to neurons of comparable diameter

Synapses The presynaptic neuron synthesizes neurotransmitters and packages them in synaptic vesicles When the action potential reaches the terminal, Ca 2+ diffuse into the terminal causing the synaptic vesicles to fuse with the membrane, releasing the neurotransmitter

Synapses The neurotransmitter crosses the synaptic cleft, binds to and activates a specific receptor Can be inhibitory (causes hyperpolarization) or excitatory (causes depolarization)

Synapses

The Cerebrum Controls skeletal muscle contraction Center for learning, emotion, memory, and perception Cerebral cortex is vital for perception, voluntary movement and learning The left side receives information from and coordinates the movement of the right side and vice versa Corpus callosum enables the two cerebral cortices to communicate

The Cerebellum Coordinates movement and balance Helps in learning and remembering motor skills Receives sensory information about the positions of the joints and lengths of the muscles as well as input from the auditory and visual systems Hand-eye coordination

Cerebrum and Cerebellum

The Diencephalon Thalamus – main input center for sensory information going to the cerebrum Sorts and sends it to the appropriate cerebral center Hypothalamus – contains the body’s thermostat Controls the pituitary gland Regulates hunger and thirst, sexual and mating behaviors, flight-or-flight response

The Brainstem Midbrain – receives and integrates several types of sensory information and sends it to specific regions of the forebrain Pons and medulla transfer information between the PNS and the midbrain and forebrain Help coordinate large-scale body movements Medulla also controls several automatic, homeostatic functions such as breathing, heart and blood vessel functions, swallowing, and digestion

Diencephalon and Brainstem

Four Lobes