Synapse number

Historically, varicosities have been thought to be the site of synaptic contact of dopamine axons in the striatum. The points of actual synaptic contact are so small that they are difficult to detect in a single thin electron microscopy (EM) sections serial sections are needed and synaptic specializations may only be present in one section. Studies using the serial EM have revealed that varicosities are not preferentially involved in synaptic contacts (Pickel et al., 1982 Freund et al., 1984 Groves et al., 1994). Counts of varicosities, therefore, do not necessarily reflect the dopamine synapse numbers it is necessary to use dopamine specific labels, such as antibodies to tyrosine hydroxylase (TH) or transporter-specific markers, such as 5-hydroxy-dopamine (5-OHDA) together with serial EM. 

What is renovating in your brain right now as you are learning about signaling of memory formation, a fraction of which you vdll perhaps retain in your memory for years to come The answer is neuronal plasticity, the key prelude to memory formation. It is a consequence of both, qualitative alteration in efficacy of synaptic transmission and quantitative alteration in synapse number due to synaptic growth. 

Is dendritic arborization and synapse number important for variability in cognitive ability (Anderson Donaldson 1995) Zagon has reported that postnatal administration of naltrexone, an opiate receptor antagonist, augments rat brain size and increases cortical neuron dendritic arborization (Hauser et al 1989, Zagon McLaughlin 1984). In MAM-treated rats, I showed that postnatal treatment with naltrexone could ameliorate the behavioural deficit for attention to novelty, habituation performance and response flexibility (see Fig. 5) with intermediate effects on time to perform a reasoning task. 

Barbosa AC et al (2008) MEF2C, a transcription factor that facilitates learning and memory by negative regulation of synapse numbers and function. Proc Natl Acad Sci USA 105(27) 9391-9396... 

Rett syndrome Decreased synapse number, reduced number of spines, and elevated LlNEl retrotransposon mobility [97]... 

Synaptic transmission is the transfer of biological information across synapses. Drugs that influence synaptic transmission play an eminent role in therapy, for two reasons. First, the nervous system controls all tissues. Second, with few exceptions synaptic transmission is chemical, operating by means of transmitter substances, and synapses therefore provide a large number of drug targets, such as the enzymes that synthesize the transmitter. However, the importance of... 

Although one neuron can receive hundreds of inputs releasing a number of different NTs, the correct and precise functioning of the nervous system presumably requires that a NT should only be able to act on appropriate receptors at the site of its release. This control is, of course, facilitated to some extent by having different NTs with specific receptors so that even if a NT did wander it could only work where it finds its receptors and was still present in sufficient concentration to meet their affinity requirements. Normally the majority of receptors are also restricted to the immediate synapse. 

MAO has been inhibited. As a result, transmitter accumulates in the cytoplasm and is exported into the synapse via the membrane-bound transporter. The ensuing (impulse-independent) sympathetic arousal can be disastrous, culminating in a hypertensive crisis and stroke. Although this process is a pharmacological curiosity and certainly contributed to the demise of MAOIs, it is possibly overrated (Tyrer 1979) it has been estimated that the number of deaths associated with the use of the MAOI, tranylcypromine, amounts to only 1 per 14000 patient years. However, this sequence of events echoes exactly the acute actions of methylenedioxymethamphetamine (MDMA, Ecstasy ) and undoubtedly accounts for some of the deaths attributed to this drug. 

Wilson K.C. and Raisman G. (1981). Estimation of numbers of vomeronasal synapses in the glomerular layer of the accessory olfactory bulb of the mouse at different ages. Brain Res 205, 245-254. 

As previously mentioned, a single action potential at a single synapse results in a graded potential only an EPSP or an IPSP. Therefore, generation of an action potential in the postsynaptic neuron requires the addition or summation of a sufficient number of excitatory inputs to depolarize this neuron to threshold. Two types of summation may occur ... 

A motor unit is defined as an alpha motor neuron and all of the skeletal muscle fibers it innervates. The number of muscle fibers innervated by an alpha motor neuron varies considerably, depending upon the function of the muscle. For example, the muscles of the eyes and hands have very small motor units. In other words, each alpha motor neuron associated with these muscles synapses with only a few muscle fibers. As a result, each of these muscles is innervated by a comparatively large number of alpha motor neurons. Densely innervated muscles are capable of carrying out more precise, complex motor activities. On the other hand, antigravity muscles have very large motor units. For example, the gastrocnemius muscle of the calf has about 2000 muscle fibers in each motor unit. Muscles with large motor units tend to be more powerful and more coarsely controlled. 

An interneuron together with a sensory afferent and motor efferent form a polysynaptic reflex (Figure 2.2) this comprises the initial stage of information input (sensory afferent), the processing/computing an appropriate response (interneurons) and the execution of a behavioural response (motor efferent). The simplest reflexes in the nervous system are monosynaptic reflexes, such as the familiar tendon (knee) jerk, these do not involve an interneuron. The sensory afferent activated by the mechano-receptor (the tap of the patellar hammer) forms a synapse with the motor efferent in the spinal cord, which then causes the skeletal muscle to contract and the crossed leg to jerk forward. With a synaptic delay of 1 millisecond (ms), the time between input and output increases with the number of synapses introduced into the circuit. As an... 

Lehre, K. P. and Danbolt, N. C. The number of glutamate transporter subtype molecules at glutamatergic synapses chemical and stereological quantification in young adult rat brain. /. Neurosci. 18 8751-8757,1998. 

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