P.R. Burgess

Patterning in the regeneration of type I cutaneous receptors

1. Type I sensory fibres in cat hairy skin innervate structures characterized by twenty to fifty specialized epithelial (Merkel) cells aggregated in a small dome‐shaped elevation. Only one fibre enters each dome and it branches repeatedly to supply at least one terminal to each Merkel cell. After the nerve is cut, the Merkel cells and the dome ultimately disappear.

Inhibition of regeneration: the ultrastructure of reactive astrocytes and abortive axon terminals in the transition zone of the dorsal root

Publisher Summary This chapter presents a study in which regenerating sensory axons have been shown to form abortive terminals in the transition zone of the dorsal roots of the spinal cord after a lesion to the nerve at a distant site. The axons vigorously regenerated in the peripheral nerve, penetrated the basal lamina, and then abruptly ceased to grow as soon as they entered central nervous tissue and met processes of reactive astrocytes. These abortive terminals contained little cytoskeletal material and an abundance of vesicles, endoplasmic reticulum, and abnormal organelles. Membrane specializations indicative of cell–cell interaction were observed. Mechanisms whereby the astrocytic processes might inhibit axonal outgrowth are discussed in the chapter. Astrocytes have long been known to play a major role in the reactive processes that occur following an injury to the central nervous system. One particularly advantageous model for studying the interaction between astrocytes and regenerating sensory axons is provided by the transition zone between peripheral and central portions of the nervous system in the dorsal root of the spinal cord.

Ascending collaterals of cutaneous neurons in the fasciculus gracilis of the cat.

Primary sensory neurons with myelinated axons in the sural nerve of the cat were found to be divisible into 3 systems on the basis of the length of their central collaterals in the dorsal columns. The short system consists of neurons that ascend only a segment or two in the fasciculus gracilis above their level of entry into the spinal cord. It is composed of all neurons with peripheral conduction velocities in the Adelta range and thus includes both D hair and nociceptive neurons. Approximately 35% of the Aalpha neurons join the intermediate system and ascend 4-12 segments before leaving the forsal columns. This system is composed of all sural type I neurons, as well as about 40% of the G2 hair, 40% of the intermediate field, and 50% of the F2 field neurons in the nerve. Those nociceptive neurons conducting at Aalpha velocities also contribute to the intermediate system. The remaining G2 hair, intermediate field, and F2 field neurons, together with almost all the sural type II, G1 hair, intermediate hair and F1 field neurons, join the long system and ascend to the nucleus gracilis. Fibers in the intermediate system showed a relatively abrupt decrease in conduction velocity usually of 50% or more (median 71%) a few millimeters rostral to their entry into the spinal cord. Members of the long system also decreased in conduction velocity at this point, but the magnitude of the changes was typically less than 50% of the peripheral velocity (median 36%). In addition, the ascending collaterals of the long system underwent a second reduction in conduction velocity near the cervical enlargement.

Joint angle signaling by muscle spindle receptors

Nerve impulses were recorded from sensory fibers supplying the tibialis anterior and soleus muscles of anesthetized cats as the ankle joint was moved from one end of the flexion-extension axis to the other and back again in steps of 6-7 degrees. The rate of movement from one position to the next was 40 deg/s and each position was held for 16-18 s. Plots were made of receptor discharge frequency as a function of ankle joint angle during joint movement (dynamic input-output (I-O) functions) as well as 2 and 15 s after movement terminated (2 and 15 s static I-O functions). Only receptors with a sustained (5s) static response within the physiological range were studied. A total of 229 tibialis anterior receptors met this criterion, of which 11 were identified as tendon organs. One hundred and five soleus receptors were studied, of which 6 were tendon organs. Thus tendon organ activity accounted for only a small part of the muscle afferent signal under passive conditions. The spindle receptors in soleus and tibialis anterior divided the ankle flexion-extension range about equally between them, those in soleus signaling over the flexion half of the range and those in tibialis anterior over the extension half. At angles where the receptors in a particular muscle did not signal joint angle, the tendon of the muscle was observed to be slack. Thus the total muscle afferent discharge in a relaxed animal is high at one end of the range, declines progressively as the ankle is displaced to an intermediate position, and then increases again as the joint moves toward the opposite end of the range. The spindle receptors within an individual muscle were recruited rather early as the muscle came under tension so that over most of a muscles signaling range joint angle could have been coded by changes in receptor discharge frequency but not by which spindle receptors were active. To evaluate the information signaled by individual muscle spindle receptors, the following measurements were made from plots of impulse frequency vs joint angle: dynamic response, defined as the frequency difference between the dynamic and 2 s static I-O functions during muscle lengthening; adaptation, defined as the frequency difference between the 2 and 15 s static I-O functions during muscle lengthening.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid displacements of the skin lack clear positional information.

Rapid skin indentations can produce strong tap-like sensations that contain little information about skin indentation depth. Although slower stimuli produce weaker sensations, subjects can more accurately identify how the position of the skin surface changes with respect to the deeper tissues. This dissociation between intensity and depth information suggests that intensity and depth are served by different neural circuits.

Time trends for injuries and illness, and their relation to performance in the National Basketball Association

OBJECTIVES To survey injury/illness in the National Basketball Association over a 25-year period and examine the relationship of injury/illness to team performance. DESIGN A retrospective correlational design. METHODS Trends were examined in reported numbers of players injured/ill during a season and games missed due to injury/illness from seasons ending in 1986 through 2005. This period was compared to years 2006-2010, when NBA teams were allowed to increase the total number of players on the team from 12 to 15. RESULTS There was a highly significant trend (p<0.0001) of increasing numbers of players injured/ill and games missed from 1986 through 2005. After the team expansion in 2006, these rates fell abruptly by 13% and 39% respectively (both p<0.0001 compared to the previous 5-year period). We also found a significant inverse association between games missed due to injury/illness and percent games won (r=-0.29, p<0.0001). CONCLUSIONS Results demonstrate an increased rate of injury in the National Basketball Association up until the expansion of team size in 2006. Following 2006, team expansion was positively associated with decreased injury/illness rates. The latter finding suggests the importance of maintaining a healthy roster with respect to winning outcomes.

Signaling of ankle joint position by receptors in different muscles.

Plots were made of multiunit activity versus ankle joint position for receptors in each of the 12 muscles crossing the cat ankle joint, except peroneus tertius, by recording from populations of afferent fibers in muscle nerves. The discharge was measured 15 or 30 sec after terminating the movements that altered the position of the joint. These recordings were dominated by large-spike activity that would be expected to originate mainly from primary spindle endings. Seven of the 12 muscles also cross other joints. Their responses at a given ankle joint position were so altered by changes in the position of the knee or toe joints that they could not reliably signal the position of the ankle joint. As judged from multiunit recording, receptors in each of the five muscles specific to the ankle joint were influenced by more than one axis of ankle joint displacement. Single-unit recording from dorsal root filaments was used to determine whether primary or secondary spindle receptors in soleus and tibialis anterior could selectively signal one axis of ankle joint rotation. Individual soleus receptors were tested both on the flexion-extension axis and with a combined adduction-eversion movement. For 38 of the 70 soleus receptors examined (54%), firm adduction-eversion produced a level of activity greater than that caused by 10 degrees of flexion, and for 77% the level of activity was greater than that caused by 5 degrees of flexion. For 168 of the 184 tibialis anterior receptors studied (91%), firm abduction-inversion produced a level of activity greater than that caused by 10 degrees of extension. Thus few receptors were found that responded exclusively to one axis of rotation. One way in which the position of the ankle joint could be specified in the face of multiaxial receptor activity is by examining the receptor discharge from more than one muscle. A suggestion for how the nervous system might do this is given in the discussion.

Classification of muscle spindle receptors.

The conduction velocities of muscle spindle afferent fibers have a bimodal distribution, and classifications of spindle receptors based on afferent fiber diameter have therefore divided these receptors into two groups, the well known primary and secondary endings. However, measures of spindle function that are likely to be important for kinesthetic sensibility such as dynamic response, adaptation and linear directionality (hysteresis) are distributed rather uniformly. Therefore, from this functional perspective it might be argued that muscle spindle receptors should not be subdivided at all. On the other hand, different receptors demonstrate these properties to varying degrees, and there are simple, linear correlations among log (dynamic response), log (adaptation), linear directionality and conduction velocity. Thus, the receptors can be divided into as many as 5-10 different subpopulations that differ significantly in one or more of these properties.

General Properties of Mechanoreceptors that Signal the Position of the Integument, Teeth, Tactile Hairs and Joints

Publisher Summary Receptors that signal the position of some part of an animals body are usually organized in an opponent fashion. The activity of one population of receptors increases as the body part deviates progressively from some intermediate (rest) position toward one extreme of the range and the activity of another (the opponent) population of receptors decreases. The converse pattern of activity occurs when the body part deviates from the rest position in the opposite direction. This is the method whereby receptors signal the positions of joints, teeth, vibrissae, and the skin in mammals. Joint position in arthropods is also signalled in this fashion. In a few cases, one of the opponent receptor populations may be absent. Rarely has it been seen that individual receptors respond over only a limited portion of the range at intermediate positions. It is apparent, therefore, that position signalling with opponent populations is widely used by animals that are distantly related phylogenetically.

The Association Between Knee Extensor Force Steadiness, Force Accuracy, and Mobility in Older Adults Who Have Fallen.

Background:Older adults often experience limited mobility, lower extremity muscle weakness, and increased fall risk. Furthermore, when older adults perform tasks that require control of submaximal force, impairments in their ability to maintain steady and accurate force output have been reported. Such problems may be related to deteriorating levels of mobility, particularly in older adults who have fallen. Purpose:The purpose of this study was to determine whether an association exists between muscle force steadiness (MFS) or muscle force accuracy (MFA) of the knee extensors and mobility in older adults who have fallen. Methods:Twenty older adults ( = 77.5 ± 7 years, 5 males and 15 females) with 2 or more comorbid conditions and who experienced a fall in the past year underwent assessment of maximal voluntary isometric contraction of the knee extensors. A submaximal target force of 50% of their maximal voluntary isometric contraction was used to determine concentric and eccentric (ECC) steadiness (the fluctuations in force production) and accuracy (the average distance of the mean force from the target force) measures. Mobility was indicated by the 6-minute walk test, the Timed Up and Go, stair ascent, and stair descent tests. Correlation analysis was used to assess the relationship between measures of muscle force control and mobility. Results:The correlations between muscle force steadiness and mobility were not significant (P > .05) for either contraction type. However, MFA during ECC contractions only was correlated significantly with all measures of mobility—6 minute walk test (r = −0.48; P = .03), Timed Up and Go (r = 0.68; P = .01), stair ascent (r = 0.60; P = .01), and stair descent (r = 0.75; P < .01). Conclusion:The identification of the relationship between ECC MFA and mobility in older adults who have fallen is novel. Although the correlations are not causal, these relationships suggest that inaccurate force output during ECC contractions of the knee extensors is linked to impaired mobility.