Chapter 48 : Nervous Systems

Megan Otto

F Block

AP Bio

48.1 Nervous Systems Consist of Circuits of Neurons and Supporting Cells

• Key Terms: • 1. Neuron: bundles of fiber like extensions• 2. Cephalization: cluster of neurons in the brain near the front end in animals with elongated,

bilaterally symmetrical bodies• 3. Ganglion: cluster of nerve cell bodies in a centralized nervous system• 4. Central Nervous System (CNS): simple nervous system with small brain and longitudinal

nerve cords• 5. Peripheral Nervous System (PNS): sensory and motor neurons that connect to the CNS• 6. Sensory Neurons: transmit info from sensors that detect external stimuli and internal

conditions• 7. Interneurons: association neurons, form synapses, integrate sensory input and motor

output• 8. Motor Neurons: nerve cells that transmit signals from brain or spinal cord to muscles or

glands• 9. Effector Cells: muscle/gland cells that perform body’s responses to stimuli• 10. Axon: extension that transmites signals to other cells• 11.Glia: supporting cells that are essential for the structural integrity of nervous system • 12. Synapse: site of communication between synaptic terminal and another cell

Organization of Nervous Systems

• Animal groups differentiate by how their neurons are organized into circuits

• Symmetrical body is organized around gastro vascular cavity, some arranged in nerve nets like a hydra

• In vertebrates, for example a salamander,

the CNS consists of a brain, spinal cord that

runs along the dorsal side of the body; nerves

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Information Processing

• Three stages are 1. Sensory input 2. Integration and 3. Motor output

• For example, the knee-jerk reflex involves all three steps• Tapping of tendon connecting to quadriceps-->• sensors detect stretch in muscle--> • sensory neurons convey info to spinal cord-->• sensory and motor neurons communicate, motor neurons convey signals to

quadriceps, causing it to contract and jerk the leg forward-->• sensory neurons from the quadriceps

communicate with interneurons in the spinal cord-->• the interneurons inhibit motor neurons

that supply the hamstring muscle,

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Structure of Typical Neuron

• The presynaptic cell is the neuron transmitting and the postsynaptic cell is the receiving cell by means of chemical messengers called neurotransmitters.

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In the PNS, glia called Schwann cells that wrap themselves around axons (orange). Forming layers ofMyelin sheath (yellow). Gaps between adjacent Schwann cells are called nodes of Ranvier. Nucleus of neuron (green). (purple) Dendrites are highly branched extensions that receive signals from other neurons.

48.2 Ion Pumps and Ion Channels Maintain the Resting Potential of a Neuron

• Key terms:• 1. Membrane potential: charge difference between a cells cytoplasm

and extracellular fluid, due to distribution of ions• 2. Resting potential: membrane potential of a neuron that isn’t

transmitting signals depends on ionic gradients that exist across plasma membrane

• 3. Equilibrium potential: the magnitude of cell’s membrane voltage at equilibrium; calculated using Nernst equation

• 4. Gated-ion channels:open or close in response to three kinds of stimuli; a)stretch-gated ion channels-cells that sense strech and open when the membane is mechanically deformed B)ligand-gated ion channels-found at synapses, open/close when a specific chemical like a neurotransmitter binds to channel C)voltage-gated channels-found in axons, open/close when the membrane potential changes

48.3 Action Potentials are the Signals Conducted by Axons

• Key terms:• 1. Hyperpolarization: an increase in the magnitude of the membrane potential(inside

of membrane becomes more negative)• 2. Depolarization: a reduction in the magnitude of the membrane potential(inside of

membrane becomes less negative)• 3. Graded potentials:change in membrane potential, magnitude of hyperpolarization

or depolarization varies with strenght of stimulus• 3. Threshold: depolarizations are graded only up to certain membrane voltage• 4. Action potential: stimulus strong enough to a produce a depolarization that

reaches the threshold• 5. Refractory period- “down time” following an action potential when a second action

potential can’t be initiated• 6. Salutatory conduction-rapid transmission of a nerve impulse along an axon,

resulting from the action potential jumping from one node of Ranvier to another, slipping the myelin-sheathed regions of the membrane

Course of Depolarization

• Resting State: activation gates closed

• Depolarization: stimulus opens activation gate, influx through channels depolarizes membrane; if reaches threshold-->action potential!

• Rising: open activation gates of Na+, K+ activation gates closed, Na+ influx makes inside of membrane positive

• Falling: Na+ activation gates close, blocking Na+ influx, K+ activation gates open permitting K+ efflux, inside of cell negative

• Undershoot: both Na+ gates closed, as K+ channels close, inactivation gates open on Na+ channels, turn to resting state

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48.4 Neurons Communicate With Other Cells at Synapses

• Key terms:• 1. Synaptic vesicle: a membrane sac containing neurotransmitter molecules at the tip of the

presynaptic cell• 2. Synaptic cleft: a narrow gap that separates the presynaptic neuron from the postsynaptic cell• 3. EPSPs: an electrical charge, depolarization, in the membrane of a postsynaptic neuron caused

by the binding of an excitatory neurotransmitter from a presynaptic to a postsynaptic neuron to generate an action potential

• 4. IPSPs: an electrical charge, hyperpolarizaton in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic neuron to generate action potential

• 5. Temporal summation: membrane potential of the postsynaptic cell in a chemical synapse is determined by the combined effect of EPSPs or IPSPs produced in rapid succession

• 6. Axon hillock: neuron’s integrating center, represents the summed effect of all EPSPs and IPSPs• 7. Acetylcholine: in vertebrate CNS, can be inhibitory or excitatory depending on receptor; released

by motor neuron it binds to ligand-gated channels on skeletal muscle cell producing EPSP through direct synaptic transmission

• 8. Biogenic amines: neurotransmitters dervided from amino acids; included hormones like epinephrine and norpinephrine

• 9. Neuropeptides: short chains of amino acids, substance p- mediates perception of pain;

endorpins-natural analgesics, decreasing pain perception

Chemical Synapse

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48.5 The Vertebrate Nervous System is Regionally Specialized

• Key terms: • 1. Central canal: the narrow cavity in the center of the spinal cord that’s continuous with

fluid-filled ventricles of the brain• 2. Ventricle: space in the vertebrate brain, filled with cerebrospinal fluid• 3. Cerebrospinal fluid: blood derived fluid that surounds, protects against infection,

nourishes and cushions the brain and spinal cord, supplying nutrients and hormones• 4. Medulla oblongata: controls homeostatic functions like breathing, heart and blood vessel

activity, swallowing, vommiting and digestion• 5. Pons: regulate breathing in the medulla• 6. Arousal: state of awareness of visual world; opposite is sleep, external stimuli received

but you’re unconscious of them• 7. Reticulur formation:diffuse network of neurons• 8. Serotonin: neurotransmitter of sleep-producing centers• 9. Melatonin: occurs at night during sleep cycle, dietary supplement treating sleep

disturbances• 10. Cerebellum: important for coordination and error checking during motor, perceptual,

and cognitive functions involved in learning and remembering motor skills• 11. Suprachiasmatic nuclei (SCN): biological clock in mammals

PNS

• PNS system is divided into two functional components

• A)somatic nervous system:carries signals to and from skeletal muscles, in response to external stimuli, controlled by reflexes B)autonomic nervous system: regulates enternal environment by controlling smooth, cardiac muscles, and organs of digestive, cardiovascular, excretory, and endocrine systems- 3 divisions 1. Sympathetic 2. Parasympathetic 3. enteric

Sympathetic and Parasympathetic Divisions

• Sympathetic: corresponds to arousal and energy generatoion (“fight or flight” response)

• Parasympathetic: oppositie responses that promote calming and a return to self-maintenance functions

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Embryonic Brain Regions

• Forebrain:a) telencephalon:cerebrum (cerebral hemispheres;includes cerebral cortex, white matter, basal nucelei b)diencephalon:thalamus, hypothalamus, epithalamus

• Midbrain:a)mesencephalon:midbrain (part of brainstem)

• Hindbrain:a) metencephalon:pons(part of brainstem, cerebellum b)myelencephalon: medulla oblongata (part of brainstem)

48.6 The Cerebral Cortex Controls Voluntary Movement and Cognitive Functions

• Key terms:• 1. Lateralization: competing functions

segregate and displace each other in the cortex of the left and right cerebral hemispheres

• 2. Limbic systems: ring of structures around the brainstem, emotions

Human Cerebral Cortex

• Each side of cerebral cortex is divided into four lobes, with own specialized functions. (frontal=association and speech, parietal=taste and reading, occipital=vision,temporal=smell and hearing)

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Primary Motor and Primary somatosensory cortices

• Cortical surface area devoted to each body part is represented by the relative size of that part.

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48.7 CNS Injuries and disease are the focus of much research

• Key terms:• 1. Growth cone: responsive region at leading edge of growing

axon• 2.Schizophrenia: mental disturbance characterized by psychotic

episodes in which patients lose the ability to estblish reality• 3. Bipolar Disorder: swings of moods from high to low• 4. Alzheimer’s: mental deterioration, confusion and memory loss• 5. Parkinson’s: motor disorder characterized by difficulty in

initiating movements, slowness, ridgity