Biology 240 – Nervous System
SEVEN FUNCTIONAL CATEGORIES OF NERVE IMPULSES
Spinal and Cranial nerves contain fibers in seven functional categories: four afferent and three efferent.
4 AFFERENT CATEGORIES:
1. GENERAL SOMATIC AFFERENT (GSA) neurons convey impulses for cutaneous sensation (pain, temperature,
touch vibration, pressure, tickle, itch) from the skin and for proprioception (position from joints and muscles) via
spinal nerves and some cranial nerves.
2. GENERAL VISCERAL AFFERENT (GVA) neurons convey visceral information such as distention of organs and
chemical conditions within the body into the CNS via both cranial and spinal nerves.
3. SPECIAL SOMATIC AFFERENT (SSA) relay impulses for vision, hearing, equilibrium via cranial nerves, ONLY.
(Cranial Nerve II (Optic) for vision; Cranial Nerve VIII (Vestibulocochlear) for equilibrium and hearing)
4. SPECIAL VISCERAL AFFERENT (SVA) convey impulses for smell and taste via cranial nerves, ONLY.
(Cranial Nerve I (Olfactory) for smell; Cranial Nerves VII (Facial), IX (Glossopharyngeal), X (Vagus) for taste)
3 EFFERENT CATEGORIES:
1. GENERAL SOMATIC EFFERENT (GSE) neurons conduct impulses to the “voluntary effectors”, aka skeletal
muscles via spinal and some cranial nerves. (Note: Special Somatic Efferent (SSE) is the same as this component
for muscles of the head that were derived metamerically (that is, from metameres, or somites) in the same way that
all other general skeletal muscle in the body is derived. Skeletal muscle of the head that belongs to this category is
found in the Tongue and the Orbit, only. All other skeletal muscles of the head/neck was derived from branchial
arches and is innervated via SVE.)
2. GENERAL VISCERAL EFFERENT (GVE) neurons conduct impulses from the CNS to the “autonomic effectors”,
aka: smooth muscles, cardiac muscle, and glands via both cranial and spinal nerves.
3. SPECIAL VISCERAL EFFERENT (SVE) neurons conduct impulses from the CNS to skeletal muscles that control facial expression and position of the jaw, neck, larynx, and pharynx via cranial nerves, ONLY. (Note: Muscles
of Mastication and Muscles of Facial Expression are derived embryologically from branchial arches NOT from
metameres. As such, they develop with their own Cranial Nerve (V (Trigeminal) for Muscles of Mastication,
VII (Facial) for Muscles of Facial Expression; IX (Glossopharyngeal) for swallowing; XI (Spinal Accessory)
for swallowing, phonation, and certain neck muscles.
Clarifying Explanation of Nerve Components:
The CNS is composed of the brain and the spinal cord. The PNS is composed of twelve pairs of cranial nerves and
thirty-one pairs of spinal nerves. Each peripheral nerve is a collection of nerve fibers (axons and dendrites) which are
conductors for various types of afferent impulses (sensations) and efferent impulses (motor to effectors). The various
functional entities found in a peripheral nerve are expressed as nerve components or FUNCTIONAL CATEGORIES.
The four functional categories of impulses able to be conducted along any spinal nerve are:
GSA (General Somatic Afferent) along which fibers are transmitted sensation from the body to the spinal cord. Such
sensations may be exteroceptive, as pain, temperature, touch and pressure; or proprioceptive from muscles,
tendons, and joint capsules providing information on joint position, muscle/tendon tension, etc.
GVA (General Visceral Afferent) along which fibers are transmitted sensation from mucous membranes, glands,
blood vessels, and other smooth muscle components.
GSE (General Somatic Efferent) along which fibers are transmitted motor impulses to skeletal muscle effectors (of
metameric origin).
GVE (General Visceral Efferent) along which fibers are transmitted motor impulses to smooth muscle, cardiac muscle,
and glandular tissue.
Cranial nerves exhibit fundamentally the same components, but the type of sensation and certain of the muscle groups
resident in the head region are designated as “special”. Thus, the three additional functional categories of impulses able
to be conducted along any cranial nerve are:
SSA (Special Somatic Afferent) along which fibers are transmitted the impulses enabling vision, hearing and
equilibrium.
SVA (Special Visceral Afferent) along which fibers are transmitted the impulses enabling smell and taste.
SVE (Special Visceral Efferent) along which fibers are transmitted impulses to skeletal muscles which are embryologically related to the digestive tract and therefore derived from branchial arches (NOT metameres as are all other
skeletal muscles). These “special muscles” include muscles of mastication and muscles of facial expression along
with some others of the pharynx and larynx.
(NOTE: SSE (Special Somatic Efferent) components are composed of motor fibers to the skeletal muscles in the head
of metameric origin (like all other general skeletal muscles of the body). These include the skeletal muscles of
the TONGUE and ORBIT (ie, ocular muscles.) Thus this component, SSE, is essentially THE SAME as GSE
in spinal nerves; and since it is the same, it gets no separate designation of its own.)
While the special functional categories of impulses are carried only along cranial nerves, certain cranial nerves also
exhibit general visceral afferent and general visceral efferent categories in their distribution to the smooth muscle and
glandular structures of the head and body, and general somatic afferent components are also represented in the cranial
nerves.
There is nothing inherent in the structure of the nerve fiber that determines the functional category of impulse it will
convey; rather nerve fibers are simply conducting components of the neuron (nerve cell). Depending on whether the
fiber is a dendrite or an axon, it will convey the impulse in a particular direction, from a particular stimulus, from/to a
particular post-synaptic unit. The specificity which does exist in the intact functioning nerve fiber is a feature of its
origin and its termination. For example: A “pain” fiber conducts painful impulses because of its freely branched origin
in the epidermis of the skin and its ending in the spinal cord. A “touch” fiber conducts a touch stimulus because of its
specialized encapsulated origin in a dermal papilla and its ending in the spinal cord. A somatic “motor” fiber conducts
an impulse from its origin in the ventral grey horn along its axon which terminates in the motor end-plate of a specific
skeletal muscle.