AP BIOLOGY ANIMAL FORM AND FUNCTION Nervous System

 The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body.

Central and Peripheral Nervous System  The Central Nervous System consists of the brain, spinal cord and retina  The Peripheral Nervous System consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system

Neurons  The basic structural unit of the nervous system is a nerve cell, or neuron. It consists of the following parts:  The cell body-main body of the neuron  The dendrite-short, branched extensions that bring impulses to the cell  The axon-long extensions that leave from a neuron and carry impulses to the target cell.

Nerve Impulse  A nerve impulse begins at the tips of the dendrite branches, passes through the dendrites to the cell body, then through the axon and finally terminates at branches of the axons.

Sensory neurons (Afferent Neurons)  Sensory neurons receive the initial stimulus.  For example, sensory neurons embedded in the retina of the eye are stimulated by light  Sensory neurons in the hand are stimulated by touch.

Motor Neurons (Efferent neurons)  Motor neurons stimulate effectors (target cells) that produce some kind of response.  For example, motor neurons may stimulate muscles, sweat glands (to cool the body) or cells in the stomach (to secrete gastrin in response to the smell of food).

Association neurons (Interneurons)  Association neurons (interneurons) are located in the spinal cord or brain and receive impulses from sensory neurons or send impulses to motor neurons. Interneurons function to make synaptic connections with other neurons.

Transmission of a nerve signal—due to chemical changes across the membrane  The membrane of an un-stimulated neuron is polarized (there is a difference in charge between the inside and outside of the cell)  Polarization is established when the inside is negative with respect to the outside—due to an excess of Na+ outside and an excess of K+ on the inside. The net negative charge on the inside is primarily due to the presence of large, negatively charged proteins and nucleic acids residing inside the cell.

Following the events in transmitting a nerve impulse:  1. Resting potential—the resting potential describes the un- stimulated, polarized state of a neuron.  2. Action potential—in response to a stimulus, gated ion channels in the membrane suddenly open and permit the Na+ on the outside to rush in. As this happens, the charge on the membrane is depolarized (become more + on the inside).

Following the events in transmitting a nerve impulse  If the stimulus is strong enough, more Na+ gates open, increasing the inflow of Na+ even more, causing an action potential, or complete depolarization.  This, in turn, stimulates neighboring Na+ gates, further down the neuron to open.  The action potential travels down the length of the neuron as opened Na+ gates stimulate neighboring Na+ gates to open.

Following the events in transmitting a nerve impulse  3. Repolarization—In response to the inflow of Na+, another kind of gated channel opens, this time allowing the K+ on the inside to rush out of the cell.  The movement of K+ out of the cell causes repolarization by restoring the original membrane polarization (a condition where it is once again more negative inside the cell)  Unlike the resting potential at the beginning, however, this time there are more Na+ on the inside and more K+ on the outside! Soon after the K+ gated channels open, the Na+ gates close.

Following the events in transmitting a nerve impulse  4. Hyperpolarization—By the time the K+ gated channels close again, more K+ have moved out of the cell than is actually necessary to establish the original polarized potential. Thus, the membrane becomes hyperpolarized.

Following the events in transmitting a nerve impulse  5. Refractory period. With the passage of the action potential, the cell membrane is polarized, but the Na+ and the K+ are on the wrong sides of the membrane.  During this refractory period, the neuron will not respond to a new stimulus.  To establish the original distribution of these ions, the Na+ and K+ are returned to their resting potential by Na+/K+ pumps in the cell membrane.

Sodium-Potassium Pumps  A Sodium-Potassium Pump is an active transport protein in the membrane of nerve cells. It actively transports Na+ and K+ ions against their concentration gradients to restore the original polarized state of the nerve cell. w- hill.com/sites/ /student_view0/c hapter2/animation__h ow_the_sodium_pota ssium_pump_works.h tml

Transmission of a nerve signal  mpulses.htm mpulses.htm

Nerve Impulse Animations  hill.com/sites/ /student_view0/chapte r14/animation__the_nerve_impulse.html hill.com/sites/ /student_view0/chapte r14/animation__the_nerve_impulse.html  ce/63.swf ce/63.swf

Synapses (aka: Synaptic Cleft)  A synapse is the gap that separates adjacent neurons.  Transmission of an impulse across a synapse may be electrical or chemical.  In electrical synapses, the action potential travels along the membranes of gap junctions, small tubes of cytoplasm that connect adjacent cells.

Synapses (aka synaptic cleft)  In most animals, the synapse between two neurons are traversed by chemicals in the following steps:  1. Calcium (Ca2+) gates open. When an action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open and allows Ca2+ to enter the cell  2. Synaptic vesicles release transmitters. The influx of Ca2+ into the terminal end of the axon causes synaptic vesicles to merge with the membrane of the next nerve cell. This releases molecules of a chemical called neurotransmitter into the synaptic cleft.

Synapses  3. Neurotransmitters released by the first cell bind to proteins on the surface of the second nerve cell.  4. The second cell’s membrane is either excited or inhibited.  5. The neurotransmitter is degraded and recycled. hill.com/sites/ /student_vie w0/chapter14/animation__transmission _across_a_synapse.html

Neurotransmitters  Common neurotransmitters:  1. Acetycholine—commonly secreted at a neuromuscular junction, the gaps between motor neurons and muscle cells, where it stimulates muscles to contact.  2. Epinephrin—derived from amino acids and are mostly secreted between neurons of the central nervous system  3. Gamma aminobutyric acid (GABA) is usually an inhibitory neurotransmitter

Peripheral Nervous System  Peripheral Nervous System (All nerves going to and from the Central Nervous System) Consists of:  Somatic Nervous System Autonomic Nervous System  All nerves carrying sensoryRegulates bodily functions and motor informationAutomatic Voluntary ParasympatheticSympatheticNervous System Maintains basicActivates body to bodily functionsdeal with stress

Parasympathetic Vs. Sympathetic Fight or flight Response Normal everyday life, when not excited, just sitting at home watching tv (unless it’s a really scary show!)