diff --git a/brion/synapse.cpp b/brion/synapse.cpp
index 5549697..d563cd8 100644
--- a/brion/synapse.cpp
+++ b/brion/synapse.cpp
@@ -71,7 +71,8 @@ class SynapseFile : public boost::noncopyable
 
         _numAttributes = dataset.dims[1];
         if( _numAttributes != SYNAPSE_ALL &&
-            _numAttributes != SYNAPSE_POSITION_ALL )
+            _numAttributes != SYNAPSE_POSITION_ALL &&
+            _numAttributes != 1 /* nrn_extra */)
         {
             LBTHROW( std::runtime_error( source + " not a valid synapse file"));
         }
@@ -179,6 +180,9 @@ class SynapseFile : public boost::noncopyable
             return read< SYNAPSE_ALL >( gid, attributes );
         case SYNAPSE_POSITION_ALL:
             return read< SYNAPSE_POSITION_ALL >( gid, attributes );
+        case 1:
+            // nrn_extra
+            return read< 1 >( gid, 1 );
         default:
             LBERROR << "Synapse file " << _file.getFileName()
                     << " has unknown number of attributes: " << _numAttributes
diff --git a/brion/synapse.h b/brion/synapse.h
index 4f26133..7f826ce 100644
--- a/brion/synapse.h
+++ b/brion/synapse.h
@@ -31,6 +31,22 @@ namespace detail { class Synapse; }
 
 /** Read access to a Synapse file.
  *
+ * There are three types of synapse files that can be read by this class:
+ * - nrn[_efferent].h5: These files store the synaptic model parameters,
+ *   connectivity information and morphological location of synapses. They
+ *   are organized as sets of tables with one table per neuron and one row
+ *   per affernt (or efferent) synapse.
+ * - nrn_positions_[_efferent].h5: These files store precomputed spatial
+ *   locations of synapses on the dendrite/axon longitudinal axes and on the
+ *   cell membranes.
+ * - nrn_extra.h5: This file stores the index of each synapse in the
+ *   list of afferent synapses of a post-synaptic neuron before any pruning
+ *   or filtering is carried out. This indices together with post-synaptic
+ *   neuron GIDs can be used as synapse GIDs.
+ *
+ * There exists a forth file type, nrn_summary.h5, with the aggregated
+ * connectivity per cell. SynapseSummary is the clase used to parse it.
+ *
  * Following RAII, this class is ready to use after the creation and will ensure
  * release of resources upon destruction. Threadsafety is guaranteed for all
  * provided methods.
diff --git a/doc/Changelog.md b/doc/Changelog.md
index d47a1dd..75afb66 100644
--- a/doc/Changelog.md
+++ b/doc/Changelog.md
@@ -5,6 +5,8 @@ Changelog {#Changelog}
 
 * [38](https://github.com/BlueBrain/Brion/pull/38):
   Fix crash while reading more than `ulimit -Sn` (1024 default) morphologies
+* [37](https://github.com/BlueBrain/Brion/pull/37):
+  Added support for synapse nrn_extra.h5 files.
 * [31](https://github.com/BlueBrain/Brion/pull/31):
   Fix crash while reading more than `ulimit -Sn` (1024 default) NEST gdf files
 * [30](https://github.com/BlueBrain/Brion/pull/30):