BIOL 3151: Principles of Animal Physiology

ANIMAL PHYSIOLOGY BIOL 3151: Principles of Animal Physiology Dr. Tyler Evans Email: tyler.evans@csueastbay.edu Phone: 510-885-3475 Office Hours: M8:30-11:30 or appointment Website: http://evanslabcsueb.weebly.com/

LAST LECTURE NEUROPHYSIOLOGY • each neuron has specialized regions that perform specific tasks: • these tasks are: reception, integration, conduction or transmission • e.g. VERTEBRATE MOTOR NEURON (Fig 4.2 pg. 145)

LAST LECTURE NEUROPHYSIOLOGY WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS? GRADED POTENTIALS: weak signals that occur in the soma and decrease in strength they get further away from the opened channel e.g. Graded potential created by ligand gated sodium channel • several properties of the neuron influence why a graded potential decreases as it travels: • leakage of ions across membrane • electrical resistance of cytoplasm • electrical properties of membrane textbook Fig 4.6 pg 151

LAST LECTURE NEUROPHYSIOLOGY WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS? GRADED POTENTIALS: weak signals that occur in the soma and decrease in strength they get further away from the opened channel e.g. Graded potential created by ligand gated sodium channel • more sodium outside of cells than inside. • when ligand-gated Na channels open, sodium enters cells and intracellular regions become more positively charged. • at +60mV there is no longer a gradient driving sodium inward. textbook Fig 4.4 pg 148

LAST LECTURE NEUROPHYSIOLOGY WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS? 2. ACTION POTENTIALS: are stronger signals used to transmit information over longer distances without degrading textbook Fig 4.10 pg 155

LAST LECTURE NEUROPHYSIOLOGY WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS? 2. ACTION POTENTIALS: are stronger signals used to transmit information over longer distances without degrading • the three phases of an action potential are driven by the opening and closing of ion channels textbook Fig 4.10 pg 155 • Na+ channels open during depolarization and Na+enters cell • K+ channels open during repolarization and K+ exits cells, making interior more negatively charged than exterior • K+channels close slowly which causes the hyperpolarization response • membrane returns to resting potential.

NEUROPHYSIOLOGY TRANSMISSION ACROSS THE SYNAPSE • once the action potential reaches the AXON TERMINAL, the neuron must transmit the signal across the SYNPASE to the target cell • the cell that transmits the signal is called the PRE-SYNAPTIC CELL and the cell that receives the signal is referred to as the POST-SYNAPTIC CELL • the space between the pre-synaptic and post-synaptic cell is the SYNAPTIC CLEFT. • collectively, these three components make up the SYNPASE textbook Fig 4.2 pg 145

NEUROPHYSIOLOGY TRANSMISSION ACROSS THE SYNAPSE • in the example we have been describing so far, the vertebrate motor neuron, the synapse forms at a NEUROMUSCULAR JUNCTION

NEUROPHYSIOLOGY TRANSMISSION ATTHE NEUROMUSCULAR JUNCTION • when an action potential reaches the axon terminal of the neuromuscular junction it triggers calcium (Ca+2) channels to open • the concentration of Ca+2 inside the neuron is much lower than outside, so Ca+2moves into the neuron along its concentration gradient • this increase in internal Ca+2concentration triggers the release of SYNAPTIC VESICLES, synaptic vesicles contain neurotransmitters, which are then released across the synapse Textbook Fig 4.16 pg 162

NEUROPHYSIOLOGY TRANSMISSION ATTHE NEUROMUSCULAR JUNCTION • the main neurotransmitter released at vertebrate neuromuscular junctions is ACETYLCHOLINE • acetylcholine is released from synaptic vesicles and binds to specific cell surface receptors in the membranes of post-synaptic cells • acetylcholine binds to receptors to induce muscle contraction • the enzyme ACETYLCHOLINESTERASE removes acetylcholine from its receptor to terminate the signal. Textbook Fig 4.17 pg 163

NEUROPHYSIOLOGY PATHOLOGIES THE NEUROMUSCULAR JUNCTION • strength of contraction is determined by two factors: • 1. amount of neurotransmitter released • 2. number of receptors on target cells • if the amount of neurotransmitter or density of receptors is high a strong muscle contraction will result. In contrast, a weak muscle contraction will result when amount of neurotransmitter or density of receptors is low • disease called MYASTHENIA GRAVIS occurs when muscles contain a reduced number of acetylcholine receptors • experience muscle weakness and muscle fatigue • weakened eye muscles can cause a drooping eyelid or PTOSIS, a common symptom

NEUROPHYSIOLOGY Diversity in Neurophysiology • although all neurons have the same basic components, each of these components has been modified by evolution to better perform specific tasks • all neurons have DENDRITES, a CELL BODY (SOMA) and an AXON, but details of each structure are variable EXAMPLES OF NEURON DIVERSITY textbook Fig 4.18 pg 166

NEUROPHYSIOLOGY Diversity in Neurophysiology Brain, Sensory or Muscle?

NEUROPHYSIOLOGY Diversity in Neurophysiology 1. SENSORY NEURONS dendrites • sensory neurons are found in animal senses: sight, hearing, touch, taste, smell • at one end of the neuron is a receptor that is associated with that particular sense • for example, olfactory receptors involved in smell are activated by airborne chemicals • at the other end are lots of dendrites that allow sensory neuron to connect to the brain for processing receptors

NEUORPHYSIOLOGY Diversity in Neurophysiology 2. BRAIN NEURONS • this type of neuron is called an INTERNEURON • have large numbers of dendrites on both ends to maximize connections with other neurons • often lack an obvious axon because are only transmitting signals over short distances between other neurons densely packed in the brain

NEUROPHYSIOLOGY Diversity in Neurophysiology 3. VERTEBRATE MOTOR NEURONS • have long axons covered in MYELIN SHEATH that allows signals to travel long distances • one end branches into NEUROMUSCULAR JUNCTIONS • other end has lots of dendrites for connecting muscle to brain or spinal cord

NEUROPHYSIOLOGY Diversity in Neurophysiology • diversity in neural signaling can also be achieved by varying cells associated with each neuron. These accessory cells are called GLIAL CELLS • in vertebrates, there are five types of glial cells: SCHWANN CELLS: form MYELIN SHEATHS and are associated with neurons with long axons. Increase the conduction speed and prevent the decay of action potentials. OLIGODENDROCYTES: form myelin sheaths in the central nervous system. ASTROCYTES: found in central nervous system and have a number of functions including transport of nutrients and neuron development. MICROGLIA: are the smallest glial cells and are involved in neuron maintenance (e.g. removes debris and dead cells) EPENDYMAL CELLS: found in fluid-filled cavities of central nervous system. They are often CILIATED (tiny-hairs) and circulate spinal fluid.

NEUROPHYSIOLOGY Diversity in Neurophysiology • SCHWANN CELLS: form MYELIN SHEATHS and are associated with neurons with long axons. • myelin increases the conduction speed and prevent the decay of action potentials. • Schwann cells wrap around an axon many times to form the myelin sheath Fig 4.14 pg. 160

NEUROPHYSIOLOGY Diversity in Neurophysiology • invertebrates lack a true myelin sheath, but are instead wrapped in membranes of glial cells called GLIOCYTES THE EVOLUTION OF MYELIN SHEATHS textbook Box 4.2 pg 170 • certain invertebrate neurons are wrapped in multiple layers of cell membrane similar in appearance to vertebrate myelin sheaths • includes nerve fibers in the ventral nerve cords of shrimp, crabs and earthworms.

NEUROPHYSIOLOGY THE EVOLUTION OF MYELIN SHEATHS • protein complexes called SEPTATE JUNCTIONS hold cells in place as they wrap around invertebrate axons. • this structure suggests invertebrate wrappings play a similar role to vertebrate myelin sheaths, but evolved independently INVERTEBRATE VERTEBRATE

NEUROPHYSIOLOGY THE EVOLUTION OF MYELIN SHEATHS Invertebrate vs. Vertebrate Axon Wrappings • the layers of membrane in the invertebrate wrappings are not as closely stacked as vertebrate layers of myelin sheath • protein composition of myelin sheath is different • proteins critically important forming myelin sheath are largely missing from invertebrate axon wrappings • wrappings likely evolved separately by CONVERGENT EVOLUTION e.g. wings of birds, reptiles and mammals are all used for flying, but evolved independently and thus have different structures textbook Box 4.2 pg 170

NEUROPHYSIOLOGY Diversity in Neurophysiology • neurons also differ in the speed at which signals are transmitted • axons conduct action potentials at different speeds: some quickly, some slowly • speed of action potentials are influenced by two variables: • 1. PRESENCE OF MYELIN • 2. AXON DIAMETER textbook Table 4.3 pg. 172

NEUROPHYSIOLOGY Diversity in Neurophysiology • OHM’S LAW describes the speed of an action potential traveling down an axon • speed or CURRENT (I) of the signal depends on two variables: voltage and resistance V R I I = = V or R voltage current resistance • essentially, the strength of the signal along an axon (current) is greatest when voltage (input energy) is high and resistance is low

NEUROPHYSIOLOGY Diversity in Neurophysiology • resistance (the force opposing conduction) is applied by different components of the cell each component is has a different resistance value, which will reduce the strength of the signal over space textbook Fig 4.20 pg. 173

NEUROPHYSIOLOGY Diversity in Neurophysiology • MYELIN SHEATH prevents the signal from traveling through these areas of high resistance • as a result, current or conduction speed increases

NEUROPHYSIOLOGY Diversity in Neurophysiology • the same can be said for large diameter axons • less of the axon surface area is exposed to the membrane and cytoplasm that slows down the signal, so resistance is low • as a result, current or conduction speed increases

LECTURE SUMMARY • once the action potential reaches the AXON TERMINAL, the neuron must transmit the signal across the SYNPASE to the target cell • in muscles involves the flow of calcium and the neurotransmitter ACETYLCHOLINE • all neurons have DENDRITES, a CELL BODY (SOMA) and an AXON, but details of each structure are variable • interneurons in the brain have very short axons and many dendrites • sensory neurons have a receptor on one end and dendrites on the other • motor neurons have neuromuscular junctions • SCHWANN CELLSform MYELIN SHEATHS that prevent the decay of action potentials when traveling down the axon • OHM’S LAW describes the speed of an action potential traveling down an axon • signals travel fastest down myelinated and large diameter axons because resistance is lowered

BIOL 3151: Principles of Animal Physiology