Artificial neurons activation function

The basic model of a single artificial neuron consists of a weighted summer and an activation (or transfer) function as shown in Figure 10.20. Figure 10.20 shows a neuron in the yth layer, where... 

A biological neuron can be active (excited) or inactive (not excited). Similarly, the artificial neurons can also have different activation status. Some neurons can be programmed to have only two states (active/inactive) as the biological ones, but others can take any value within a certain range. The final output or response of a neuron (let us call it a) is determined by its transfer function, f, which operates on the net signal (Netj) received by the neuron. Hence the overall output of a neuron can be summarised as ... 

Table 5.1 Activation functions currently employed in artificial neurons, where n represents the overall net input to the neuron and a denotes the result of the activation function...

A multilayer perceptron (MLP) is a feed-forward artificial neural network model that maps sets of input data onto a set of suitable outputs (Patterson 1998). A MLP consists of multiple layers of nodes in a directed graph, with each layer fully connected to the next one. Except for the input nodes, each node is a neuron (or processing element) with a nonlinear activation function. MLP employs a supervised learning techruque called backpropagation for training the network. MLP is a modification of the standard linear perceptron and can differentiate data that are not linearly separable. 

The 0 is a threshold value that a neuron must receive to be activated. Biologically, this happens inside the cell but the artificial neuron needs a dedicated function (equation). This threshold value (and its related mathematical function) is called bias. 

Practically, the most widespread network diagram (basically used in the models described in this book) is a multilayer perceptron (MLP) , composed of many processing units (artificial neurons), each of which performs weighted summation of output signals, passing to the output channel their nonlinear function called activation function. The multilayer perceptron has only unidirectional connections between neurons of adjacent layers (no feedback, no coimections between neurons of the same layer, no connections between neurons of layers simated further than the directly adjacent ones) (Fig. 3.6). 

The multilayer perceptron is thus an artificial neural network consisting of M layers. Each neuron is a simple processing unit it performs weighted summation of the inputs, calculates the excitation and generates the value specified by the activation function and the stimulation value in the output channel ... 

In conclusion, the chemical functionalization of synthetic materials with ECM molecules was an important step in the development of artificial implants with biological activities. Apart from these signals, which are always fixed to surfaces, peripheral nerve regeneration in vivo depends on a number of signals that are secreted from Schwann cells or the neurons themselves (Sect. 2.2). 

Activated CBi receptors can also change fhe function of several types of potassium channels. In oocytes and AtT20 cells artificially expressing the CBi receptor, stimulation of inwardly rectifying potassium channels was repeatedly observed (Henry and Chavkin 1995 Mackie et al. 1995 Garcia et al. 1998 McAllister et al. 1999). Potassium A currents in cultured hippocampal neurons are stimulated by cannabinoids (Deadwyler et al. 1995 Mu et al. 2000). The effects of cannabinoids on potassium M currents in hippocampal brain slices have also been studied M currents were inhibited, which means an enhancement of neuronal excitability (Schweitzer 2000). The potassium K current is inhibited by cannabinoids in cultured hippocampal neurons (Hampson et al. 2000). As in the case of calcium channels, anandamide can elicit a CBi receptor-independent effect on potassium... 

---