Nervous System, Sensory Mechanisms and Motor Mechanisms AP Bio Chapters 39, 40 & 41

Organization of the Nervous System Central Nervous System (CNS) – brain and spinal cord Peripheral Nervous System (PNS) – nerves outside of CNS – cranial nerves connect brain w/ upper body – spinal nerves Connect spine w/ regions of the body below the head

Peripheral Nervous System (PNS) PNS – sense stimuli inside and outside the body, helps to control internal environment Sensory/Afferent Division – send impulses to the CNS Motor/Efferent Division – send impulses away from the CNS – effectors: control voluntary and involuntary muscles

Autonomic Nervous System Involuntary: smooth and cardiac muscle – Sympathetic increases energy consumption and prepare for action Fight or Flight response – Parasympathetic Enhance activity to gain and conserve energy

Neuron cell bodydendritesaxon (hillock) myelin sheathSchwann cellsnodes of Ranvier synaptic terminalsynapseterminal branches

Neuron Structure Cell body: – performs cellular functions Dendrites: – Receive information Axon (hillcock): – which impulse starts when threshold is exceeded myelin sheath: – lipid layer surrounding nerve cell Nodes of Ranvier: – Holes in myelin where Na can move into the cell Schwann Cell: – produce myelin in the PNS synaptic terminal: – End of axon Synapse: – communication junction between 2 nerve cells, where neurotransmitters move

Functional Organization of Neurons 3 Classes of Neurons sensory neurons convey impulse from sensory receptors to CNS interneurons integrate sensory input and motor output motor neurons convey impulses from CNS to effector cells Function as a reflex arc: Sensory neuron, to interneuron, to motor neuron

The Knee-jerk Reflex 1. Tap patellar tendon 2. Sensory receptors sense stretch in quadriceps 3. Sensory neurons convey info. to spinal cord 4. Synapses with motor neuron in spinal cord 5. Motor neuron conveys signal to quadriceps 6. Synapse with inter- neuron in spinal cord 7. Interneurons inhibit other motor neurons (hamstring) 8. Prevents the hamstring from contracting (no resistance to quads contracting).

The Nature of Neural Signals Membrane Potential the difference in voltage across the plasma membrane + arises from differences in ionic composition (Na + /K + pump) - normal: positive outside; negative inside (-70mV) 48_06RestingPotential_A.swf

Action Potential Excitable Cells cells that have the ability to change their membrane potentials neurons and muscle cells Resting potential (unexcited) Change from resting potential can result in active electrical impulse Gated ion channels open or close in response to stimuli Hyperpolarization increase in the electrical gradient opens K+ channel; increase outflow of K+; more negative, no impulse Depolarization reduction in the electrical gradient opens Na+ channel increase inflow of Na + ; less negative, can cause nerve impulse action potential: a brief reversal of membrane polarity

Graded Potentials and the Action Potential in a Neuron

Propagation of the Action Potential Membrane becomes depolarized by reacting to a stimulus. – Must cross a threshold of -55 mV – Action potential – all or none response – Increase frequency = increased stimulus Na+ rushes into the axon causing the charge reversal from – to + inside the axon K+ leaves the axon after the action potential is finished – Refractory period – another action potential can not occur yet – Resting potential is restored

Action Potential 48_10ActionPotential_A.swf 48_10ActionPotential_A.swf

Saltatory Conduction speeds the propagation of action potential + nodes of Ranvier: gaps between myelinated regions - action potentials “jump” from node to node

Conversion of Signal: Electrical to Chemical Depolarization causes influx of Ca2+ Release of synaptic vesicle contents Neurotransmitter re- leased into cleft Molecules bind to receptors Opens ion channels 48_15Synapse_A.swf

Diversity of Nervous Systems

The Brain Structures Cerebrum – Cerebral cortex – outer portion of the cerebrum – gray matter – Cerebral hemispheres – left and right sides – Corpus callosum – allows communication between the left and right hemispheres Brainstem – Medulla oblongata – controls autonomic and homeostatic functions – Pons – regulates breathing – Midbrain – large scale body movements - walking Cerebellum – coordination, learning, and decision making Diencephalon – Thalamus – input center for sensory info and output center for motor info – Hypothalamus – homeostasis – links to the endocrine system

Cerebral Hemispheres Left Language Math Logical operations Visual and auditory details Right Pattern recognition Face recognition Spatial relations Nonverbal thinking Emotional processing Understanding and reacting to stress Music

Cerebral Cortex Frontal lobe – Speech Temporal lobe – Smell – Hearing Occipital lobe – Vision Parietal lobe – Speech – Taste – Reading

Muscle Contraction Skeletal Muscle two kinds fast-twitch (white meat) Tend to go anaerobic slow-twitch (dark meat) myoglobin-rich “twitch” contraction of protein filaments causes muscles to shorten - thin (actin) and thick (myosin) bands - interleaved with each other myosin grabs actin and pulls - sliding filament theory of muscle contraction

Muscle Contraction Sliding Filament Theory relaxed muscle + length of each sarcomere is greater - Z-line to Z-line Contracting Muscle + actin/myosin slide past each other - shortening the sarcomere Contracted Muscle (maximum) + actin filaments overlap each other - sarcomere is very short

Myosin & Actin Interactions

Regulation of Muscle Contraction ACh synaptic terminal, diffuse across cleft & bond to muscle cell receptors Action potential moves down PM along T-tubule Action Potential triggers Ca 2+ release from sarcoplasmic reticulum Ca2+ bond to troponin in thin filament Myosin cross bridges alternative attach to actin pulling thin filament toward sarcomere Cystolic Ca 2+ removed Tropomyosin blocks myosin binding sites, contraction ends and muscle relaxes