Describe the anatomy, physiology and classification of peripheral nerves
Peripheral nerves are made up of multiple axons grouped together to make fascicles which are held
together in a matrix of connective tissue called the epineurium
Sensory peripheral nerves respond to various stimuli (i.e. different forms of energy) and convert this
information into action potentials which are in turn transmitted to the CNS
o Modality (i.e. form) of a stimulus is represented by the type of receptor that detects it
o Intensity of a stimulus is encoded as the frequency of action potentials generated
o Duration of a stimulus is reflected in the duration of the action potentials
o Location of a stimulus is given by the local receptive field of the receptor that detects it
There are various different types of peripheral nerves with varying morphology to suit their particular
function of detection, e.g. photo-, mechano-, thermo-, noci- and chemoceptors
Each nerve can be classified in terms of its diameter and conduction velocity:
o Aα – largest diameter (13-20μm), fastest conduction (80-120ms-1) – found in proprioceptors
and motor neurons to skeletal muscle
o Aβ – found in cutaneous mechanoreceptors
o Aδ – found in noci- and thermoceptors
o B – found in preganglionic ANS neurons
o C – smallest diameter (0.2-1.5μm), slowest conduction (0.5-2ms-1); unmyelinated – found in
noci- and thermoceptors
Outline the principles of sensory transduction
Structurally, sensory nerve endings (sensory transducers) are either:
o Unencapsulated (free nerve endings) – e.g. noci- and thermoceptors
o Encapsulated by other (non-neural) specialised cells – e.g. mechanoreceptors
The process of sensory transduction can be broken down into the following steps:
o Detection – a particular stimulus is detected e.g. through specialised cilia, molecular
chemoreceptors etc.
o Response – a local receptor potential is generated through activation of ion channels
o Transmission – if the receptor potential is above the spike threshold then it triggers action
potentials which propagate along the neuronal axon
o Output – the action potentials reach the synaptic terminal which triggers a chemical output
Mechanoreceptors respond directly to physical stimuli through activation of ion channels
Chemoreceptors use second messengers to activate nearby ion channels
Sensory transducers can adapt to respond to sustained stimuli in different ways:
o Rapidly adapting (RA) receptors will induce receptor potentials at initiation and termination
of a stimulus only so that there is not a constant stream of sensation e.g. in some touch
o Slowly adapting (SA) receptors will induce a prolonged receptor potential for the duration of
the stimulus so that there is constant signal transmission e.g. in vision
Describe the structure and function of sensory receptors in the skin
There are a variety of receptors in the skin concerned with the detection of touch (including stretch,
vibration etc.) pain and temperature
Nociceptors are the most widely distributed receptors, present in the skin as free nerve endings that
terminate as ‘naked axons’ between superficial cells
o Mechanical (Aδ) nociceptors respond to sharp pain
o Polymodal (C) nociceptors respond to ‘slow pain’ (e.g. burning) as well as some thermal and
light touch stimuli
Thermoceptors are also free nerve endings and respond to coldness rapidly (Aδ) but warmth more
slowly (C)
Mechanoreceptors respond to various types of touch, stretch and vibration and as such have varying
o Merkel’s receptors (Aβ) are expanded nerve endings that make contact with Merkel cells in
the epidermis to sense steady skin indentation
o Meissner’s corpuscles (Aβ) are found in the dermal papillae particularly in the fingertips and
are sensitive to flutter; with Merkel’s receptors contribute to form and texture perception
o Pacinian corpuscles (Aβ) are the most numerous of the encapsulated receptors found deep in
the skin and respond primarily to vibration
o Ruffini corpuscles (Aβ) are also found deep in the skin and sense steady skin stretch