The loudness of a sound is encoded in the frequency of action potentials in the afferent nerve. An adult has more neurons than an infant. Activation of a synapse just prior to an action potential in the postsynaptic cell strengthens that synapse. The hippocampus is needed for memory. Grey matter contains cell bodies of neurons.
White matter contains axonal processes plus glial cells.
A special fluid bathes the brain and spinal cord and also provides cushioning for the brain. Peripheral tissues of the body and head receive nerves from the spinal cord and not directly from the brain. The dorsal half of the spinal cord contains the sensory nerves, the ventral half the motoneurons. Nerve cells can reach one meter in length. Glial cells help wrap axons to enhance propagation of electrical signals.