Fig. 41.1 Divisions of the nervous system
Direction of information flow divides nerve fibers into two types: afferent (sensory) fibers, which transmit impulses toward the central nervous system (CNS), and efferent (motor) fibers, which transmit impulses away. The nervous system may also be divided into a somatic and an autonomic part. The somatic nervous system mediates interaction with the environment, whereas the autonomic (visceral) nervous system coordinates the function of the internal organs.
Fig. 41.2 Organization of the gray matter
Left oblique anterosuperior view. The gray matter of the spinal cord is divided into three columns (horns). Afferent (blue) and efferent (red) neurons within these columns are clustered according to function.
Fig. 41.3 Muscle innervation
Indicator muscles are innervated by motor neurons in the anterior horn of one spinal cord segment. Most muscles (multisegmental muscles) receive innervation from a motor column, a vertical arrangement of motor nuclei spanning several segments.
Fig. 41.4 Reflexes
Muscular function at the unconscious (reflex) level is controlled by the gray matter of the spinal cord.
Fig. 41.5 Sensory and motor systems
The sensory system (see p. 614) and motor system (see p. 615) are so functionally interrelated they may be described as one (sensorimotor system).
Sensory & Motor Pathways
Fig. 41.6 Sensory pathways (ascending tracts)
Fig. 41.7 Motor pathways (descending tracts)
Sensory Systems (I)
Fig. 41.8 Visual system: Overview
Fig. 41.9 Visual pathways
90% of optic nerve fibers terminate in the lateral geniculate body on neurons that project to the striate area (visual cortex). This forms the geniculate pathway, responsible for conscious visual perception. The remaining 10% travel along the medial root of the optic tract, forming the non-geniculate pathway. This pathway plays an important role in the unconscious regulation of vision-related processes and reflexes.
Fig. 41.10 Reflexes of the visual system
The reflexes of the visual system are mediated by the optic (afferent) and oculomotor (efferent) nerves.
Sensory Systems (II)
Fig. 41.11 Balance
Human balance is regulated by the visual, proprioceptive, and vestibular systems. All three systems send afferent fibers to the vestibular nuclei, which then distribute them to the spinal cord (motor support), cerebellum (fine motor function), and brainstem (oculomotor function). Proprioception (“position sense”) is the perception of limb position in space. Note: Efferents to the thalamus and cortex control spatial sense; efferents to the hypothalamus regulate vomiting in response to vertigo.
Fig. 41.12 Oculomotor nuclei
The oculomotor nuclei receive efferent fibers from both the vestibular and visual systems. Conjugate eye movement requires the activity of multiple extraocular muscles and their corresponding nerves. The oculomotor nuclei are therefore coordinated at a supranuclear level by premotor nuclei (purple).
Fig. 41.13 Vestibular system and nuclei
The receptors of the vestibular system are located in the membranous labyrinth. The maculae of the utricle and saccule respond to linear acceleration, whereas the semicircular duct organs in the ampullary crests respond to angular (rotational) acceleration.
Fig. 41.14 Auditory system (hearing)
See p. 480 for the vestibulocochlear nerve (CN VIII).
Fig. 41.15 Gustatory system (taste)
When specialized epithelial cells (secondary sensory cells with no axon) in the tongue are chemically stimulated, the cell bases release glutamate, stimulating the peripheral processes of afferent cranial nerves VII, IX, and X. Note: Spicy foods may also stimulate trigeminal fibers (not shown).
Sensory Systems (III)
Fig. 41.16 Olfactory system (smell)
The olfactory system is the only sensory system not relayed in the thalamus before reaching the cortex (the prepiriform area is considered the primary olfactory cortex). The olfactory system is linked to other brain areas and can therefore evoke complex emotional and behavioral responses (mediated by the hypothalamus, thalamus, and limbic system): noxious smells induce nausea; appetizing smells evoke salivation.
The limbic system, which exchanges and integrates information between the telencephalon, diencephalon, and mesencephalon, regulates drive and affective behavior. It plays a crucial role in memory and learning.
Fig. 41.17 Limbic system nuclei
This neuronal circuit (Papez circuit) establishes a connection between information stored at the conscious and unconscious level.
Fig. 41.18 Limbic regulation of the peripheral autonomic nervous system
The limbic system receives afferent feedback signals from its target organs. See p. 623 for the autonomic nervous system.