• A sheep's brain is part of its central nervous system.sheep brain & dissection tools image by PHOTOFLY from Fotolia.com
  
  According to "Biology," neurons are cells that transmit information in the form of electrical and chemical signals, and as such are the most important cells in the nervous system. A neuron has a long cable-like projection called an axon that transmits signals to its target, and slender branching projections called dendrites through which incoming signals are relayed to the cell body. The junction between an axon of one neuron and the dendrite of another is called a synapse.
  
  

Chemical Synapses

• Most synapses are chemical synapses, notes "Biology." At a chemical synapse, the electrical signal traveling down the axon causes the release of chemicals called neurotransmitters by the presynaptic neuron. The neurotransmitter diffuses across the gap between the two neurons, a gap called a synaptic cleft. On the other side of the cleft, the neurotransmitter binds to proteins called ligand-gated ion channels in the membrane of the post-synaptic cell; these ion channels open in response to permit charged ions like chlorine or calcium ions to cross the membrane of the post-synaptic cell and induce a response.
  
  

Electrical Synapses

• Unlike chemical synapses, electrical synapses permit current to flow directly from one cell to another. "Biology" explains that neurons at electrical synapses are typically connected by gap junctions, proteins in the membrane of both cells that form a channel through which ions can flow from one cell to the other. Electrical synapses, unlike chemical synapses, cannot modify the information they transmit, so electrical synapses are most common in cases where the organism needs to generate a rapid and unvarying response. Squid and lobsters, for example, use electrical synapses in certain pathways responsible for a rapid escape response they use to flee predators.
  
  

Excitatory Synapses

• According to Kimball's Biology Pages, excitatory synapses are a type of chemical synapse that depolarizes the membrane of the postsynaptic cell; in other words, it makes the postsynaptic cell more likely to fire. Excitatory synapses often use neurotransmitters like glutamate, although as noted in "Biology," some neurotransmitters are used both by excitatory and inhibitory synapses -- acetylcholine and norepinephrine being two prominent examples.
  
  

Inhibitory Synapse

• As described in Kimball's Biology Pages, an inhibitory synapse is the opposite of an excitatory synapse. Neurotransmitter release at an inhibitory synapse hyperpolarizes the membrane of the postsynaptic cell; that is, it makes the postsynaptic neuron less likely to fire. When a neuron receives both excitatory and inhibitory inputs at different synapses at the same time, the net effect is cumulative, so the inhibitory inputs can cancel the effect of the excitatory inputs. "Biology" lists glycine and GABA as common neurotransmitters at inhibitory synapses.