Sensory and Motor Mechanisms

● two basic mechanisms by which the nervous system produces graded contractions of whole muscles:

1. by varying the number of muscle fibers that contract

2. by varying the rate at which muscle fibers are stimulated

● a motor unit consists of a single motor neuron and all the muscle fibers it controls

○ number of muscle fibers in different motor units ranges from a few to hundreds

○ nervous system can regulate the strength of contraction in a muscle by determining how many motor units are activated and by selecting large or small motor units to activate

● when rate of muscle fiber stimulation is so high that muscle fiber cannot relax between stimuli, twitches fuse into one smooth, sustained contraction called tetanus

● several distinct types of skeletal muscle fibers that have adapted to a particular set of functions

○ oxidative rely mostly on aerobic respiration

■ have many mitochondria, a rich blood supply, and a large amount of an oxygen-storing protein called myoglobin

○ glycolytic fibers uses glycolysis as their primary source of ATP

■ larger diameter and less myoglobin

○ fast-twitch fibers enable brief, rapid, powerful contractions and develop tension two to three times faster than slow-twitch fibers

○ slow-twitch fibers are often found in muscles that maintain posture and can sustain long contractions

5 Basic Types of Sensory Receptors

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Changes in Synaptic Connections

Physical Interaction of Protein Filaments in Conjunction with Muscle Function

Parkinson's Disease

● in the sliding filament model, the filaments do not change in length when the sarcomere shortens; thin and thick filaments slide past each other, increasing overlap

○ relies on interaction of actin and myosin

○ myosin has a long “tail” region and a globular “head” region

○ the head can bind to ATP and hydrolyze into ADP and inorganic phosphate, converting myosin to a high-energy form that binds to actin, which then forms a cross-bridge and pulls the thin filament toward the center of the sarcomere

○ the cross-bridge is broken when a new molecule of ATP binds to the myosin head

○ muscle contraction requires repeated cycles of binding and release

Schizophrenia

Sensory Pathways

● cardiac muscle is found only in the heart

○ cardiac muscle is striated

○ have ion channels in plasma membrane that cause rhythmic depolarizations, triggering action potentials without input from the nervous system

○ plasma membranes of adjacent cardiac muscle cells interlock at specialized regions called intercalated disks, where gap junctions provide direct electrical coupling between cells

Memory and Learning

• Animals receive information through sensory receptors
• Sensory receptor formed by dendrites of an afferent neuron
• Sensory receptor formed by a cell that synapses with an afferent neuron

Transduction of stimuli by sensory receptors subject to 2 types of modification

● skeletal systems transform muscle contraction into locomotion

● different types of skeletal systems:

○ hydrostatic skeletons consist of fluid held under pressure in a closed body compartment

■ cnidarians, flatworms, nematodes, and annelids

● control form and movement by using muscles to change shape of fluid-filled compartments

● shape changes bring about peristalsis, a movement produced by rhythmic waves of muscle contractions passing from front to back

○ exoskeletons consist of a hard encasement deposited on an animal’s surface

■ shells of molluscs made of calcium carbonate

■ arthropods with a jointed exoskeleton called a cuticle consisting of chitin, polysaccharide similar to cellulose, that is embedded into a protein matrix that forms a composite material both flexible and strong

○ endoskeletons consist of a hardened internal skeleton buried within soft tissues

■ chordates have an endoskeleton consisting of cartilage, bone, or some combination

After embryonic development in which the overall structure of the nervous system has been established by the regulated gene expression and signal transduction, two processes encompass the remaining development of the nervous system

• Motor disorder, including muscle tremors, poor balance, a flexed posture, and a shuffling gait; facial muscles become rigid, limiting the ability of patients to vary their expressions.
• Like Alzheimer’s disease, Parkinson’s disease is a progressive brain illness and is more common with old age.
• Symptoms of Parkinson’s disease result from the death of neurons in the midbrain that normally release dopamine at synapses in the basal nuclei.
• Protein aggregates accumulate like in Alzheimer’s disease.
• At present, there is no definite cure for Parkinson’s disease; however, a potential cure is to implant dopamine-secreting neurons, either in the midbrain or basal nuclei.

● About 1% of the world’s population suffers from schizophrenia, a severe mental disturbance characterized by distorted perceptions of reality.

● Recent evidence indicates that schizophrenia impacts neuronal pathways that use dopamine as a neurotransmitter.

o The drug amphetamine, which stimulates dopamine release, can produce the same set of symptoms as schizophrenia.

● In addition, many of the drugs that alleviate the symptoms of schizophrenia block dopamine receptors.

● Schizophrenia may also alter glutamate signaling.

• sensory receptors respond to stimuli by undergoing sensory transduction, which is the conversion of a stimulus into a change in membrane potential
• change in membrane potential itself is known as a receptor potential
• in most receptors, sensory transduction is caused by changes in the rate at which channels conduct positive ions across the plasma membrane
• change may generate one or more action potentials which travel along the axon of an afferent neuron to reach the interneuron networks of the central nervous system
• the interneurons integrate the action potentials and the brain forms an appropriate response to the stimulus
• perception (colors, smells, sounds, and tastes) are formed by the brain and do not exist outside it

● The formation of memories is an example of neural plasticity.

● Information in short-term memory is accessed via temporary links formed in the hippocampus. (Does that mean Spickelmier possesses an abundance of these temporary links? I mean, he did spell my last name wrong and I knew the man for four years, but it’s ok he’s still loved...)

● When memories are made more long-term the links in the hippocampus are replaced by more permanent connections within the cerebral cortex itself; consolidation of memory supposedly takes place during sleep.

Pain Receptors (Nociceptors)

● physical interaction of protein filaments is required for muscle function

● vertebrate skeletal muscle moves bones and body, and is characterized by a hierarchy of smaller units

○ most skeletal muscles consists of a bundle of long fibers running parallel to the length of the muscle

○ also called striated muscle because the regular arrangement of the filaments creates a pattern of light and dark bands

■ each repeating unit is a sarcomere, the basic contractile unit of the muscle

■ inside the fiber are bundles of smaller myofibrils, which are arranged longitudinally

● composed of two types of filaments:

1. thin filaments consist of two strands of actin and two strands of a regulatory protein coiled around one another

2. thick filaments are staggered arrays of myosin molecules

Mechanoreceptors

● smooth muscles are found mainly in the walls of hollow organs, such as blood vessels and organs of the digestive tract

○ lack striation because their actin and myosin filaments are not regularly arrayed along length of cell

○ thick filaments are scattered throughout cytoplasm and thin filaments attached to structures called dense bodies

○ contract and relax more slowly than striated muscle

○ have no troponin complex or T tubules

Alzheimer's Disease

● calcium ions and proteins bound to actin play crucial role in both muscle cell contraction and relaxation

○ tropomyosin (a regulatory protein) and troponin complex (set of additional regulatory proteins) are bound to the actin strands of thin filaments

○ when calcium concentration rises in the cytosol, the thin and thick filaments slide past each other, and the muscle fiber contracts; when it falls and the binding sites are covered, the contraction stops

○ arrival of an action potential at the synaptic terminal of a motor neuron causes release of the neurotransmitter acetylcholine

■ binding to receptors on muscle fiber leads to depolarization and triggers an action potential

■ action potential spreads deep into muscle fiber interior, following infoldings of the plasma membrane, called transverse (T) tubules

■ T tubules make contact with the sarcoplasmic reticulum (SR), a specialized endoplasmic reticulum, and triggers changes in the SR and opens Ca2+ channels

■ calcium ions flow through these open channels into the cytosol and bind to the troponin complex and initiates contraction of the muscle fiber

○ several diseases cause paralysis by interfering with the excitation of skeletal muscle fibers by motor neurons

1. pain receptors (nociceptors) detect stimuli that reflect such noxious (or harmful) conditions

● each type or receptor produces specific response (ex. action potentials from eyes travel along the optic nerve to visual cortex, where they are interpreted by brain as differences in pattern, color, and intensity of light

○ two ways in which intensity and extent of stimulus is registered

1. frequency (number per unit of time) of action potentials traveling along each axon of an afferent pathway; the stronger the stimulus, more frequently afferent neurons fire action potentials

2. number of afferent neurons that stimulus activates to generate action potentials in pathway; more sensory receptors activated, more axons carry information to brain

Sensory and Motor Mechanisms

Nervous System Disorders

Neural Plasticity

Mechanoreceptors detect physical deformation caused by forms of mechanical energy such as pressure, touch, stretch, motion, and sound

a. typically consist of ion channels linked to structures that extend outside the cell

b. bending or stretching of external structure generates tension that alters the permeability of ion channels, altering the membrane potential and resulting in depolarization or hyperpolarization

● responsible for hearing and equilibrium detect moving fluid or settling particles

● statocysts are mechanoreceptors located in organs that help invertebrates sense gravity and maintain equilibrium

○ in a typical statocyst, a layer of ciliated receptor cells surrounds a chamber that contains statoliths, which are grains of sand or other dense granules

○ when statoliths settle to the low point in the chamber, they stimulate mechanoreceptors within the location

Chemoreceptors

• A mental deterioration, or dementia, characterized by confusion and memory loss and its incidence is age related, rise from about 10% at age 65 to about 35% at age 85.
• Leads to the death of neurons in many areas of the brain; as a result, there is often extreme shrinkage of brain tissue.

Thermoreceptors

2. when stimuli remains constant, the effect of the stimulus is reduced

● sensory adaptations reduce the frequency of action potentials generated in afferent neurons

○ increases sensitivity of receptor systems to changes in environmental stimuli

● The capacity of the nervous system to be “remodeled,” in response to its own activity, is called neural plasticity; this concept is extremely similar to the concept of price elasticity in Microeconomics, which measures the response of consumers to the change of prices of products.

● When the activity of a synapse correlates with that of other synapses, changes may occur that reinforce that synaptic connection.

o However, the opposite is also true; if the synapse does NOT correlate with other synapses, it could potentially weaken the synaptic connection.

1. amplification is the strengthening of a sensory signal during transduction

● often requires signal transduction pathways involving second messengers

○ amplify signal strength through formation of many product molecules by a single enzyme molecule

2nd developmental process is known as synapse elimination, which shapes the nervous system

• A developing neuron forms numerous synapses more than required for its proper function
• The activity of that neuron then stabilizes some synapses and destabilizes others

1st process involves a sort of competition between neurons for survival

• Neurons compete for growth-supporting factors, which are produced in limited quantities by tissues that direct neuron growth
• Cells that fail to reach the proper locations fail to receive these factors and undergo an inevitable death

Chemoreceptors include general receptors (those that transmit information about total solute concentration) and specific receptors (those that respond to individual kinds of molecules)

● both perceptions of gustation (taste) and olfaction (smell) depend on chemoreceptors

● in terrestrial animals, taste is the detection of chemicals called tastants present in a solution and smell is the detection of odorants carried through the air

○ no distinction between taste and smell in aquatic animals

● humans and other mammals recognize five types of tastants (sweet, sour, salty, bitter, and umami (delicious)

○ receptor cell are modified epithelial cells organized into taste buds scattered in areas of tongue and mouth

● in olfaction, sensory cells are neurons

○ line the upper portion of nasal cavity and send impulses along axons directly to olfactory bulb of the brain

○ receptive ends of the cells contain cilia that extend into layer of mucus coating nasal cavity

○ when an odorant diffuses in this region, it binds to a specific GPCR protein called an odorant receptor (OR) on the plasma membrane of the olfactory cilia, triggering signal transduction leading to the production of cyclic AMP

■ cyclic AMP opens channels in the plasma membrane permeable to both Na+ and Ca2+

■ the flow of these ions into the receptor cell leads to depolarization of the membrane and generates action potentials

Thermoreceptors detect heat and cold

● located in the skin and the anterior hypothalamus

● send information to the body’s thermostat in posterior hypothalamus

○ mammals have different thermoreceptors that are specific for a particular temperature range

Electromagnetic Receptors

Electromagnetic receptors detect various forms of electromagnetic energy (visible light, electricity, and magnetism)

● visual receptors in diverse animals depend on light-absorbing pigments

● light detectors contain photoreceptors, cells that contain light-absorbing pigment molecules

● most invertebrates have some kind of light-detecting organ

○ insects and crustaceans have compound eyes, which consist of several thousand light detectors called ommatidia (facets of the eye)

■ effective at detecting movement

○ vertebrates have single-lens eyes with a pupil (a small opening where light enters) and an iris that contracts or expands to let in more or less light

■ behind the pupil, a single lens focuses light on a layer of photoreceptors

○ light passes through cornea -> through aqueous fluid -> through lens -> through vitreous body -> images form on retina (photoreceptor cells) -> signals bipolar cells -> signals ganglion cells -> optic nerve transmits signals to thalamus -> integration by visual areas of the cerebral cortex

● sensory organs for hearing and equilibrium are closely associated

● in hearing, the ear reduces mechanical stimulus (pressure waves) into nerve impulses that the brain perceives as sound

○ hair cells are sensory receptors with hair-like projections on the cell surface that detect motion

■ used to hear sounds in our environment

○ sound waves enter external auditory canal -> tympanic membrane vibrates -> vibrations amplified -> oval window vibrates -> vibrations are conducted through fluid -> basilar membrane vibrates -> hair cells in the organ of Corti are stimulated -> cochlear nerve transmits impulses to brain

● chambers called the utricle and saccule allow us to perceive position with respect to gravity or linear movement

○ contains a sheet of hair cells that project into a gelatinous material, which has otoliths (small calcium carbonate particles) embedded within

ACT I: Nervous System

ACT II: Sensory and Motor Mechanisms