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The Nervous System Chapter 44

Nervous System Organization All animals must be able to respond to environmental stimuli Sensory receptors – detect stimulus Motor effectors – respond to it Nervous system links the two Consists of neurons and supporting cells

Nervous System Organization Vertebrates have three types of neurons Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)

Nervous System Organization Central nervous system (CNS ) Brain and spinal cord Peripheral nervous system (PNS) Sensory and motor neurons Somatic NS stimulates skeletal muscles Autonomic NS stimulates smooth and cardiac muscles, as well as glands Sympathetic and parasympathetic NS Counterbalance each other

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

Nervous System Organization Neurons have the same basic structure Cell body Enlarged part containing nucleus Dendrites Short, cytoplasmic extensions that receive stimuli Axon Single, long extension that conducts impulses away from cell body

Nervous System Organization

Nervous System Organization Neuroglia Support neurons both structurally and functionally Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons In the CNS, myelinated axons form white matter Dendrites/cell bodies form gray matter In the PNS, myelinated axons are bundled to form nerves

Nervous System Organization

Nerve Impulse Transmission A potential difference exists across every cell’s plasma membrane Negative pole – cytoplasmic side Positive pole – extracellular fluid side When a neuron is not being stimulated, it maintains a resting potential Ranges from –40 to –90 millivolts (mV) Average about –70 mV

Nerve Impulse Transmission The inside of the cell is more negatively charged than the outside Sodium–potassium pump Brings two K+ into cell for every three Na+ it pumps out Ion leakage channels Allow more K+ to diffuse out than Na+ to diffuse in

Nerve Impulse Transmission Two major forces act on ions in establishing the resting membrane potential Electrical potential produced by unequal distribution of charges Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other

Nerve Impulse Transmission Sodium–potassium pump creates significant concentration gradient Concentration of K+ is much higher inside the cell Membrane not permeable to negative ions Leads to buildup of positive charges outside and negative charges inside cell Attractive force to bring K+ back inside cell Equilibrium potential – balance between diffusional force and electrical force

Nerve Impulse Transmission

Nerve Impulse Transmission Uniqueness of neurons compared with other cells is not the production and maintenance of the resting membrane potential Rather the sudden temporary disruptions to the resting membrane potential that occur in response to stimuli 2 types of changes Graded potentials Action potentials

Nerve Impulse Transmission Graded potentials Small transient changes in membrane potential due to activation of gated ion channels Each gated channel is selective Most are closed in the normal resting cell

Nerve Impulse Transmission Chemically-gated or ligand-gated channels Ligands are hormones or neurotransmitters Induce opening and cause changes in cell membrane permeability

Nerve Impulse Transmission Depolarization makes the membrane potential more positive Hyperpolarization makes it more negative These small changes result in graded potentials Size depends on either the strength of the stimulus or the amount of ligand available to bind with their receptors Can reinforce or negate each other Summation is the ability of graded potentials to combine

Nerve Impulse Transmission

Nerve Impulse Transmission Action potentials Result when depolarization reaches the threshold potential (–55 mV) Depolarizations bring a neuron closer to the threshold Hyperpolarizations move the neuron further from the threshold Caused by voltage-gated ion channels Voltage-gated Na+ channels Voltage-gated K+ channels

Nerve Impulse Transmission Voltage-gated Na+ channels Activation gate and inactivation gate At rest, activation gate closed, inactivation gate open Transient influx of Na+ causes the membrane to depolarize Voltage-gated K+ channels Single activation gate that is closed in the resting state K+ channel opens slowly Efflux of K+ repolarizes the membrane

Nerve Impulse Transmission The action potential has three phases Rising, falling, and undershoot Action potentials are always separate, all-or-none events with the same amplitude Do not add up or interfere with each other Intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

Nerve Impulse Transmission Propagation of action potentials Each action potential, in its rising phase, reflects a reversal in membrane polarity Positive charges due to influx of Na+ can depolarize the adjacent region to threshold And so the next region produces its own action potential Meanwhile, the previous region repolarizes back to the resting membrane potential Signal does not go back toward cell body

Nerve Impulse Transmission Two ways to increase velocity of conduction Axon has a large diameter Less resistance to current flow Found primarily in invertebrates Axon is myelinated Action potential is only produced at the nodes of Ranvier Impulse jumps from node to node Saltatory conduction

Nerve Impulse Transmission

Synapses Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells Presynaptic cell transmits action potential Postsynaptic cell receives it Two basic types: electrical and chemical

Electrical synapses Chemical synapses Involve direct cytoplasmic connections between the two cells formed by gap junctions Relatively rare in vertebrates Chemical synapses Have a synaptic cleft between the two cells End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

Synapses Chemical synapses Action potential triggers influx of Ca2+ Synaptic vesicles fuse with cell membrane Neurotransmitter is released by exocytosis Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins Produces graded potentials in the postsynaptic membrane Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

Synapses

Neurotransmitters Acetylcholine (ACh) Crosses the synapse between a motor neuron and a muscle fiber Neuromuscular junction

Neurotransmitters Acetylcholine (ACh) Binds to receptor in the postsynaptic membrane Causes ligand-gated ion channels to open Produces a depolarization called an excitatory postsynaptic potential (EPSP) Stimulates muscle contraction Acetylcholinesterase (AChE) degrades ACh Causes muscle relaxation

Neurotransmitters Amino acids Glutamate Major excitatory neurotransmitter in the vertebrate CNS Glycine and GABA (g-aminobutyric acid) are inhibitory neurotransmitters Open ligand-gated channels for Cl– Produce a hyperpolarization called an inhibitory postsynaptic potential (IPSP)

Neurotransmitters Biogenic amines Epinephrine (adrenaline) and norepinephrine are responsible for the “fight or flight” response Dopamine is used in some areas of the brain that control body movements Serotonin is involved in the regulation of sleep

Neurotransmitters Neuropeptides Substance P is released from sensory neurons activated by painful stimuli Intensity of pain perception depends on enkephalins and endorphins Nitric oxide (NO) A gas – produced as needed from arginine Causes smooth muscle relaxation

Synaptic Integration Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body Small EPSPs add together to bring the membrane potential closer to the threshold IPSPs subtract from the depolarizing effect of EPSPs Deter the membrane potential from reaching threshold

Synaptic Integration

Synaptic Integration There are two ways that the membrane can reach the threshold voltage Spatial summation Many different dendrites produce EPSPs Temporal summation One dendrite produces repeated EPSPs

Drug Addiction Habituation Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it Cell decreases the number of receptors because there is an abundance of neurotransmitters In long-term drug use, means that more of the drug is needed to obtain the same effect

Drug Addiction Cocaine Affects neurons in the brain’s “pleasure pathways” (limbic system) Binds dopamine transporters and prevents the reuptake of dopamine Dopamine survives longer in the synapse and fires pleasure pathways more and more

Drug Addiction Nicotine Binds directly to a specific receptor on postsynaptic neurons of the brain Binds to a receptor for acetylcholine

The Central Nervous System Sponges are only major phylum without nerves Cnidarians have the simplest nervous system Neurons linked to each other in a nerve net No associative activity Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity Two nerve cords run down the body Permit complex muscle control All of the subsequent evolutionary changes in nervous systems can be viewed as a series of elaborations on the characteristics already present in flatworms

Vertebrate Brains All vertebrate brains have three basic divisions: Hindbrain Midbrain Forebrain In fishes, Hindbrain – largest portion Midbrain – processes visual information Forebrain – processes olfactory information

Vertebrate Brains

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

Vertebrate Brains Cerebrum The increase in brain size in mammals reflects the great enlargement of the cerebrum Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum Each hemisphere receives sensory input from the opposite side Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes

Cerebrum

Cerebrum Cerebral cortex Outer layer of the cerebrum Contains about 10% of all neurons in brain Highly convoluted surface Increases threefold the surface area of the human brain Divided into three regions, each with a specific function

Cerebrum Cerebral cortex Primary motor cortex – movement control Primary somatosensory cortex – sensory control Association cortex – higher mental functions Basal ganglia Aggregates of neuron cell bodies – gray matter Participate in the control of body movements

Each of these regions of the cerebral cortex is associated with a different region of the body

Other Brain Structures Thalamus Integrates visual, auditory, and somatosensory information Hypothalamus Integrates visceral activities Controls pituitary gland Limbic system Hypothalamus, hippocampus, and amygdala Responsible for emotional responses

Complex Functions of the Brain Sleep and arousal One section of reticular formation is the reticular-activating system Controls consciousness and alertness Brain state can be monitored by means of an electroencephalogram (EEG) Records electrical activity

Complex Functions of the Brain Language Left hemisphere is “dominant” hemisphere Different regions control various language activities Adept at sequential reasoning Right hemisphere is adept at spatial reasoning Primarily involved in musical ability Nondominant hemisphere is also important for the consolidation of memories of nonverbal experiences

Complex Functions of the Brain Memory Appears dispersed across the brain Short-term memory is stored in the form of transient neural excitations Long-term memory appears to involve structural changes in neural connections Two parts of the temporal lobes, the hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory

Complex Functions of the Brain Alzheimers disease Condition where memory and thought become dysfunctional Two causes have been proposed Nerve cells are killed from the outside in External protein: b-amyloid Nerve cells are killed from the inside out Internal proteins: tau ()

Spinal Cord Cable of neurons extending from the brain down through the backbone Enclosed and protected by the vertebral column and the meninges

Spinal Cord 2 zones Inner zone is gray matter Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia Outer zone is white matter Contains cables of sensory axons in the dorsal columns and motor axons in the ventral columns

Spinal Cord It serves as the body’s “information highway” Relays messages between the body and the brain It also functions in reflexes The knee-jerk reflex is monosynaptic However, most reflexes in vertebrates involve a single interneuron

Knee-jerk reflex is monosynaptic

Most reflexes in vertebrates involve a single interneuron

The Peripheral Nervous System Consists of nerves and ganglia Nerves are bundles of axons bound by connective tissue Ganglia are aggregates of neuron cell bodies Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells

The Peripheral Nervous System Sensory neurons Axons enter the dorsal surface of the spinal cord and form dorsal root of spinal nerve Cell bodies are grouped outside the spinal cord in dorsal root ganglia Motor neurons Axons leave from the ventral surface and form ventral root of spinal nerve Cell bodies are located in the spinal cord

The Somatic Nervous System Somatic motor neurons stimulate the skeletal muscles to contract In response to conscious command or reflex actions Antagonist of the muscle is inhibited by hyperpolarization (IPSPs) of spinal motor neurons

The Autonomic Nervous System Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata In both, efferent motor pathway has 2 neurons Preganglionic neuron – exits the CNS and synapses at an autonomic ganglion Postganglionic neuron – exits the ganglion and regulates visceral effectors Smooth or cardiac muscle or glands

The Autonomic Nervous System Sympathetic division Preganglionic neurons originate in the thoracic and lumbar regions of spinal cord Most axons synapse in two parallel chains of ganglia right outside the spinal cord Parasympathetic division Preganglionic neurons originate in the brain and sacral regions of spinal cord Axons terminate in ganglia near or even within internal organs