Integration and Control: Nervous Systems Chapter 9

Neurons Basic units of communication in nearly all nervous systems Monitor information in and around the body and issue commands for responsive actions

Three Classes of Neurons Sensory neurons – from stimuli to brain and spinal cord Interneurons - receive signal from sensory neurons and integrate them, send response signal to motor neurons Motor neurons - send signal from interneurons to effectors (muscle or gland cells) with response to stimuli

Neuroglia Make up more than half the volume of the vertebrate nervous system A variety of cells that metabolically assist, structurally support, and protect the neurons

Structure of a Neuron dendrites cell body TRIGGER ZONE INPUT ZONE CONDUCTING ZONE OUPUT ZONE axon axon endings Figure 34.2 Page 580

Resting Potential When a neuron is “at rest”, not being stimulated Charge difference across the plasma membrane of a neuron Fluid just inside cell is more negatively charged than fluid outside Potassium (K + ) - Higher inside than outside Sodium (Na + ) - Higher outside than inside Na+ channels are closed

Action Potential When a neuron receives a signal from a stimulus, an abrupt, temporary reversal in the polarity is generated –This is called an action potential –Na+ channels open in the area of the stimuli input, causing the inside becomes more positive Depending on the “size” of the stimuli, these reversals may only effect the local area or may reach the trigger zone and cause it to respond sending the signal down the axon to the output zone

Action potentials are all-or-nothing events. They only occur if the stimulus is strong enough to get the trigger zone to respond, otherwise the signal remains local. The action potential is self-propagating and moves away from the stimulation site to adjacent regions of the membrane (undiminished) to the output zone.

Propagation of Action Potentials An action potential in one part of an axon brings a neighboring region to threshold Action potential occurs in one patch of membrane after another until it reaches the output zone (end of the axon)

Action Potential Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K Figure 34.5d Page 583

Repolarization – return to rest Remember the action potential is only temporary. It only lasts for only about a millisecond, then the membrane polarity and concentrations return to resting potential. –When depolarization in one region is ended, the sodium gates close and potassium gates open. –The sodium-potassium pumps also become operational to fully restore the resting potential (concentrations of Na+ and K+) *sodium outside, potassium inside

Chemical Synapse Gap between the terminal ending of an axon and the input zone of another cell synaptic vesicle plasma membrane of axon ending of presynapic cell plasma membrane of postsynapic cell synaptic cleft membrane receptor Figure 34.7a Page 584

Synaptic Transmission Action potential in axon ending of presynaptic cell causes voltage-gated calcium channels in its membrane to open Flow of calcium into presynaptic cell causes release of neurotransmitter (signal molecules) into synaptic cleft

Synaptic Transmission Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell Binding of neurotransmitter to receptors opens ion channels in the membrane of postsynaptic cell Some neurotransmitters excite and some inhibit

Ion Gates Open ions neurotransmitter receptor for neurotransmitter gated channel protein Figure 34.7c Page 584

Many signals (excitatory and inhibitory) compete at the input zone. An excitatory postsynaptic potential (EPSP) is a summation of signals that brings the membrane closer to threshold (depolarizing effect). An inhibitory postsynaptic potential (IPSP) drives the membrane away from threshold by a hyperpolarizing effect. In synaptic integration, competing signals that reach the input zone are reinforced or dampened, sent on or suppressed.

Synaptic Integration Membrane potential (milliseconds) EPSP IPSP what action potential spiking would look like threshold resting membrane potential integrated potential Figure 34.9 Page 585

Myelin Sheath – on individual axons Many individual axons have a sheath series of Schwann cells (neuroglia) Figure 34.11a Page 586

Each section of the sheath is separated from adjacent ones by a node where the axon membrane (plentiful in gated sodium channels) is exposed. the sheath is an electric insulator and it enhances the propagation of the action potential The action potentials jump from node to node, which is fast and efficient. action potentials move about 120 meters per second in large sheathed axons

Nerve A bundle of axons enclosed within a connective tissue sheath Figure Page 586 axon myelin sheath nerve fascicle

Communication Lines – normal flow Stimulus (input) Receptors (sensory neurons) Integrators (interneurons) motor neurons Effectors (muscles, glands) Response (output) Figure 34.1 Page 579

Reflexes Automatic movements made in response to stimuli In the simplest reflex arcs, sensory neurons synapse directly on motor neurons Most reflexes involve an interneuron

Invertebrate Nervous Systems All animals except sponges have some sort of nervous system Nerve cells are oriented relative to one another in signal-conducting and information-processing highways

Nerve Net - Cnidaria Diffuse mesh of nerve cells that take part in simple reflex pathways Nerve cells interact with sensory and contractile cells The nerve net (a loose mesh of nerve cells associated with epithelial tissue) in the cnidarians reflects their radially symmetrical bodies. The nerve net is composed of reflex pathways that result in simple, stereotyped movements. information flow is not focused and there is not precise reaction.

Bilateral Nervous Systems Flatworm -ladder like Annelids -ventral nerve cord and rudimentary brain Arthropods Crayfish & Grasshopper -double ventral nerve cord, anterior brain and acute senses Fig a Page 589

Functional Regions Expansion and modification of the dorsal nerve cord produced functionally distinct regions FOREBRAIN MIDBRAIN HINDBRAIN (start of spinal cord) Figure 34.15a Page 590

Vertebrate Brains olfactory lobe (part of forebrain) forebrain midbrain hindbrain fish (shark) reptile (alligator) mammal (horse) forebrain midbrain hindbrain olfactory lobe Figure 34.15b Page 590

Vertebrate Nervous System Central nervous system (CNS) –Brain and Spinal cord –The communication lines within the brain and spinal cord are called tracts –The white matter contain axons with glistening myelin sheaths and specialize in rapid transmission of impulses. –Gray matter consists of unmyelinated axons, dendrites, nerve cell bodies, and neuroglia cells, that protect and support neurons. Peripheral nervous system –Nerves that thread through the body

Peripheral Nervous System Somatic nerves –Motor functions –voluntary –(Shown in green) Autonomic nerves –Visceral functions –involuntary –(Shown in red) Figure Page 591

Two Types of Autonomic Nerves The autonomic system is broken down into the sympathetic and parasympathetic systems. Parasympathetic nerves tend to slow down body activity when the body is not under stress. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also call on the hormone epinephrine to increase the “fight-flight” response.

Both Systems Are Usually Active Most organs are continually receiving both sympathetic and parasympathetic stimulation For example, sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down Which dominates depends on situation

Function of the Spinal Cord Expressway for signals between brain and peripheral nerves Sensory and motor neurons make direct reflex connections in the spinal cord Spinal reflexes do not involve the brain

Structure of the Spinal Cord spinal cord ganglion nerve vertebra meninges (protective coverings) Figure 34.19a Page 593

Development of the Brain Brain develops from a hollow neural tube Forebrain, midbrain, and hindbrain form from three successive regions of tube Brain stem is tissue that evolved first and develops first in all three regions

Divisions of Brain DivisionMain Parts Forebrain Midbrain Hindbrain Cerebrum Olfactory lobes Thalamus Hypothalamus Limbic system Pituitary gland Pineal gland Tectum Pons Cerebellum Medulla oblongata anterior end of the spiral cord Figure Page 594

Cerebrospinal Fluid Surrounds the spinal cord Fills ventricles within the brain Blood-brain barrier controls which solutes enter the cerebrospinal fluid because of tight junctions Figure Page 595

Reticular Formation Mesh of interneurons extends from top of spinal cord, through brain stem, and into higher integrating centers of cerebral cortex (throughout brain)

Chapter 10 – Sensory Receptors 6 types of sensory receptors Mechanoreceptors – detects physical stimuli (mechanical energy) such as touch, pressure, vibrations Thermoreceptors – detects heat or cold Pain Receptors (nociceptors) – detect tissue damage Chemoreceptors – detect chemical energy of specific substances in fluid around them –ex. taste (gustatory) and smell (olfactory) Osmoreceptors – detect changes in water volume in fluid around them Photoreceptors – detect visible and ultraviolet light