The nervous system is a complex network that allows an organism to communicate with its environment. The network includes sensory components, which detect changes in environmental stimuli, and motor components, which generate movement, contraction of cardiac and smooth muscle, and glandular secretions. Integrative components of the nervous system receive, store, and process sensory information and then orchestrate the appropriate motor responses.
Sensory systems transmit information from the environment to the CNS via specialized sensory receptors and a series of first-, second-, third-, and fourth-order neurons in the CNS. Sensory receptors include mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, and nociceptors. A stimulus (e.g., light) is converted to electrical energy in the sensory receptors via transduction processes, which result in receptor potentials.
Somatosensory and pain systems process information about touch, position, pain, and temperature using the dorsal column and anterolateral systems.
The visual system detects and interprets light stimuli. Photoreceptors are rods and cones of the retina, which hyperpolarize in response to light. Photoreceptors synapse on bipolar cells and horizontal cells of the retina, where they produce either excitation or inhibition, depending on the type of receptor on the bipolar and horizontal cells. The output cells of the retina are ganglion cells, whose axons form the optic nerves. The optic nerves synapse in the lateral geniculate nucleus of the thalamus. Fibers from each nasal hemiretina cross at the optic chiasm and ascend contralaterally; fibers from each temporal hemiretina ascend ipsilaterally.
The auditory system involves transduction of sound waves. The mechanoreceptors are auditory hair cells located in the organ of Corti of the inner ear. Bending of cilia on the hair cells produces an oscillating receptor potential. Location of the hair cells along the basilar membrane encodes frequency.
The vestibular system is used to maintain equilibrium and balance. Vestibular hair cells are mechanoreceptors located in ampullae of semicircular canals and in otolith organs. The semicircular canals detect angular acceleration of the head, and the otolith organs detect linear acceleration.
The chemical senses are olfaction and gustation. Olfactory epithelium contains olfactory receptor cells, which are also primary afferent neurons. Axons from these neurons pass through the cribriform plate and synapse in glomeruli of the olfactory bulb. Taste receptors are found on taste buds, which are organized in papillae.
Muscle spindles are composed of intrafusal fibers and are arranged in parallel with extrafusal muscle fibers. Muscle spindles are stretch receptors, which detect changes in muscle length when extrafusal fibers contract or relax.
Spinal cord reflexes include the stretch reflex (monosynaptic), the Golgi tendon reflex (disynaptic), and the flexor-withdrawal reflex (multisynaptic).
Descending motor pathways from the cerebral cortex and brain stem are divided among the pyramidal tract and extrapyramidal tract. Pyramidal tracts pass through the medulla and synapse on lower motoneurons in the spinal cord. Extrapyramidal tracts include rubrospinal, pontine reticulospinal, medullary reticulospinal, lateral vestibulospinal, and tectospinal tracts.
The cerebellum regulates movement by controlling synergy. The cerebellar cortex includes a granular layer, a Purkinje cell layer, and a molecular layer. The output of the cerebellar cortex is via axons of Purkinje cells and is always inhibitory. Disorders of the cerebellum cause ataxia.
Basal ganglia are deep nuclei of the telencephalon, which are involved in planning and execution of smooth movements.
The motor cortex includes premotor and supplementary cortices, which are responsible for generating a motor plan. The primary motor cortex is responsible for execution of the motor plan.
Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are provided at the end of the book.
1 Cutting which of the following leads to total blindness in the right eye: optic chiasm, left optic tract, right optic tract, right optic nerve, left optic nerve?
2 A ballerina spins to the right. When she suddenly stops spinning, which way will her eyes move?
3 How many motoneurons are in a motor unit?
4 Which of the following reflexes comprise(s) only one synapse: knee-jerk reflex, Golgi tendon reflex, stretch reflex, the reflex involved when one removes a hand from a hot stove?
5 In which type of receptor, phasic or tonic, does the receptor potential fall below threshold, even as the stimulus continues?
6 Put these photoreception events in their correct order: release of neurotransmitter, decreased cyclic GMP, light, conversion of 11-cis rhodopsin to all-trans rhodopsin, transducin, hyperpolarization, closure of Na+ channels.
7 A hyperpolarizing receptor potential makes the membrane potential ______ (more or less) negative and ______ (increases or decreases) the likelihood of action potentials occurring.
8 Indicate whether each of the following is activated (increased), inhibited (decreased), or unchanged in the operation of the Golgi tendon reflex:
Golgi tendon organs
Ia afferent fibers
Ib afferent fibers
9 Which of the following is/are found in higher concentration in blood than in CSF: protein, osmolarity, Mg2+, glucose, Na+, K+?
10 If the head is rotated to the right, which horizontal semicircular canal (right or left) is activated during the initial rotation? When the head stops rotating, which canal (right or left) is activated?
11 Compared with the base, the apex of the basilar membrane is ______ (wider/narrower), is ______ (more compliant/less compliant), and responds to ______ (higher/lower) frequencies.
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