Page 335 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Central Conduction of Sensory Input Anterolateral spinothalamic pathway (! C;
violet). Afferent signals from nocisensors,
The posterior funiculus–lemniscus system thermosensors and the second part of pressure
(! C, green) is the principal route by which the and touch afferent neurons are already relayed
somatosensory cortex S1 (postcentral gyrus) (partly via interneurons) at various levels of
receives sensory input from skin sensors and the spinal cord. The secondary neurons cross to
propriosensors. Messages from the skin (su- the opposite side at the corresponding seg-
perficial sensibility) and locomotor system
ment of the spinal cord, form the lateral and
Central Nervous System and Senses afferent fibers project in tracts of the posterior hibit the flow of sensory input to the cortex at
(proprioceptive sensibility) reach the spinal
ventral spinothalamic tract in the anterolateral
funiculus, and project to the thalamus.
cord via the dorsal roots. Part of these primarily
Descending tracts (from the cortex) can in-
funiculus without synapses to the posterior
funicular nuclei of the caudal medulla oblon-
all relay stations (spinal cord, medulla oblon-
gata (nuclei cuneatus and gracilis). The tracts
gata, thalamus). The main function of these
tracts is to modify the receptive field and ad-
of the posterior funiculi exhibit a somatotopic
just stimulus thresholds. When impulses from
arrangement, i.e., the further cranial the origin
of the fibers the more lateral their location. At
different sources are conducted in a common
the medial lemniscus, the secondary afferent
afferent, they also help to suppress unimpor-
side (decussate) and continue to the post-
more important and interesting sensory mo-
erolateral ventral nucleus (PLVN) of the
dalities and stimuli (e.g., eavesdropping).
thalamus, where they are also somatotopically
12 somatosensory fibers cross to the contralateral tant sensory input and selectively process
Hemiplegia. (! D) Brown–Séquard syndrome occurs
arranged. The secondary afferent trigeminal
fibers (lemniscus trigeminalis) end in the post- due to hemisection of the spinal cord, resulting in
ipsilateral paralysis and loss of various functions
eromedial ventral nucleus (PMVN) of the below the lesion. The injured side exhibits motor pa-
thalamus. The tertiary afferent somatosensory ralysis (initially flaccid, later spastic) and loss of tactile
fibers end at the quaternary somatosensory sensation (e.g., impaired two-point discrimination,
neurons in the somatosensory cortex S1. The ! p. 314). An additional loss of pain and tempera-
main function of the posterior funiculus–lem- ture sensation occurs on the contralateral side (disso-
niscus pathway is to relay information about ciated paralysis).
tactile stimuli (pressure, touch, vibration) and Reticular activating system. (! E) The sensory
joint position and movement (proprioception) input described above as well as the input from
to the brain cortex via its predominantly the sensory organs are specific, whereas the
rapidly conducting fibers with a high degree of reticular activating system (RAS) is an un-
spatial and temporal resolution. specific system. The RAS is a complex process-
As in the motor cortex (! p. 325 B), each ing and integrating system of cells of the retic-
body part is assigned to a corresponding pro- ular formation of the brainstem. These cells re-
jection area in the somatosensory cortex S1 ceive sensory input from all sensory organs and
(! A) following a somatotopic arrangement ascending spinal cord pathways (e.g., eyes,
(! B). Three features of the organization of S1 ears, surface sensitivity, nociception), basal
are (1) that one hemisphere of the brain re- ganglia, etc. Cholinergic and adrenergic output
ceives the information from the contralateral from the RAS is conducted along descending
side of the body (tracts decussate in the medial pathways to the spinal cord and along ascend-
lemniscus; ! C); (2) that most neurons in S1 ing “unspecific” thalamic nuclei and “un-
receive afferent signals from tactile sensors in specific” thalamocortical tracts to almost all
the fingers and mouth (! p. 314); and (3) that cortical regions (! p. 333 A), the limbic system
the afferent signals are processed in columns and the hypothalamus. The ascending RAS or
of the cortex (! p. 333 A) that are activated by ARAS controls the state of consciousness and
specific types of stimuli (e.g., touch). the degree of wakefulness (arousal activity).
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