10 THE NERVOUS SYSTEM: SENSORY SYSTEMS
Perception: the conscious interpretation of the world based on the sensory systems, memory,
and other neural processes.
General Principles of Sensory Physiology
The afferent division of the PNS transmits information from the periphery to the CNS. The
information is detected by sensory receptors (detect stimuli in the external environment).
Visceral receptors (detect stimuli within the body) transmit information to the CNS by visceral
The somatosensory system is necessary for perception of sensations associated with receptors in
the skin (somesthetic sensations) and for proprioception (the perception of the position of the
limbs and the body). The proprioception depends on specific proprioceptors in muscles and
joints and on more generalized receptors in the skin.
Special senses are necessary for senses of vision, hearing, balance/equilibrium, taste, and small.
Sensory receptors: specialized neuronal structures that detect a specific form of energy in the
internal or external environment.
Modality: the energy form of a stimulus (i.e. light waves, sound waves, pressure, etc.).
The law of specific nerve energies states that a given sensory receptor is specific for a particular
modality (i.e. photoreceptors detect light).
The modality to which a receptor responds best is called the adequate stimulus. A sufficiently
strong “inadequate” stimulus will induce the perception of the adequate stimulus.
o The function of sensory receptors is transduction: the conversion of one form of energy
o Sensory transduction: receptors convert energy of a sensory stimulus into changes in
membrane potential (receptor potentials/generator potentials).
o Receptor potentials resemble postsynaptic potentials since they are graded potentials,
but are triggered by sensory stimulus not neurotransmitters.
o The greater the strength of the stimulus, the greater the change in membrane potential.
o A sensory receptor can be a specialized structure at the peripheral end of an afferent
neuron. When depolarized to threshold, an action potential is generated in the afferent
neuron and propagated to the CNS.
o A sensory receptor can be a separate cell that communicates through a chemical synapse
with an associated afferent neuron. Changes in the receptor ‘s membrane potential
causes the release of neurotransmitters. If the afferent neuron is depolarized to
threshold, action potential is generated.
o Receptor adaptation: a decrease over time in the magnitude of the receptor potential in
the presence of a constant stimulus.
o Slowly adapting or tonic receptors show little adaptation and, therefore, can function in
signaling the intensity of a prolonged stimulus (i.e. muscle stretch receptors – detect
muscle length, proprioceptors – detect position of body in 3D)
o Rapidly adapting or phasic receptors adapt quickly, and thus function best in detecting
changes in stimulus intensity. Some can show a second, smaller response upon the
termination of a stimulus, “off response” (i.e. olfactory receptors – detects odors,
Pacinian corpusles – detect vibration).
o Labeled lines: the specific neural pathways that transmit information pertaining to a
particular modality (each modality follows its own label line).
o Activation of a specific pathway causes perception of the associated modality, regardless
of the actual stimulus that activated it.
o Sensory unit: a single afferent neuron and all the receptors associated with it. 10 THE NERVOUS SYSTEM: SENSORY SYSTEMS
o Receptive field: the area over which an adequate stimulus can produce a response. It
corresponds to the region containing receptors for that afferent neuron.
o The afferent neuron transmits information from the periphery to the CNS is called first-
order neuron (diverges and converges).
o Some of the associated interneurons transmit information to the thalamus, the major
relay nucleus for sensory input (second-order neurons).
o In the thalamus, these second-order neurons form synapses with third-order neurons
that transmit info to the cerebral cortex, where sensory perception occurs.
CODING FOR STIMULUS TYPE
o Stimulus type is coded by the receptor and pathway activated when the stimulus is
applied (i.e. light waves to photoreceptors to the visual cortex).
o Often the brain must integrate information from different sensory systems
CODING STIMULUS INTENSITY
o Stimulus intensity is coded by the frequency of action potentials (frequency coding) and
the number of receptors activated (population coding).
o Stronger stimuli produce a high frequency of action potentials.
o In population coding, a stronger stimulus activates, or recruits, a greater number of
o Receptor potentials that are generated at the individual receptors sum and produce a
greater frequency of action potentials in that neuron.
o A stimulus may also recruit receptors associated with different afferent neurons, where
more afferent neurons transmit signals to the CNS.
o In either case, a greater frequency is transmitted, indicating that the stimulus is strong.
The Somatosensory System
Somesthetic sensations of stimuli associated with the surface of the body require
mechanoreceptors to detect pressure, force, or vibration. Thermoreceptors detect temperature.
Nociceptors detect tissue-damaging stimuli.
Most somatosensory receptors in the skin are specialized structures at nerve endings. Those
without identifiable specialized structures are called free nerve endings.
MECHANORECEPTORS IN THE SKIN
o Some receptors are found in the superficial layers of the skin close to the epidermis (i.e.
o Other receptors are located deeper, in the dermis (i.e. hair follicle receptors).
o Meissner’s corpuscles are found only in glabrous skin (hairless skin), whereas hair
follicle receptors are found only in hairy skin.
o Slowly adapting mechanoreceptors respond to pressure (a sustained stimulus), whereas
rapidly adapting receptors respond best to vibration (a constantly changing stimulus).
o The size of the receptive fields for the mechanoreceptors varies greatly (smaller
receptive fields better tactile acuity).
THERMORECEPTORS IN THE SKIN
o Respond to temperature of the receptor endings and the surrounding tissue.
o Warm receptors: respond to temperatures between 30 C and 45 C; the frequency of
action potentials increases as skin temperature increases to 45 C.
o Cold receptors: respond to 35 C-20 C; the frequency of action potentials peaks at a skin
temperature of 25 C. They also respond to temperatures greater than 45 C (hot
stimulus). The perception of cold at these hot temps is referred to as paradoxical cold.
o Free nerve endings that contain temperature-sensitive ion channels (transient receptor
potential, TRP channels) that open/close to thermal stimuli for thermal transduction
and pain transduction. 10 THE NERVOUS SYSTEM: SENSORY SYSTEMS
o 4 types of heat-activated TRP channels (TRPV1-4) and 2 types of cold activated channels
(TRPM8 and TRPA1) exist.
o Both warm and cold receptors are rapidly responding and they respond best when the
o Both types of thermoreceptors have tonic activity during the background (resting)
temperature. When temp decreases, the frequency of action potentials in axons
associated with cold receptors increases sharply and then gradually declines.
o Axons associated with warm receptors show the opposite response; the frequency of
action potentials decreases sharply when the temp drops and increases rapidly when
the temp returns to the resting state.
NOCICEPTORS IN THE SKIN
o Nociceptors are the sensory receptors responsible for the transduction of noxious
stimuli that we perceive as pain.
o Free nerve endings that respond to tissue-damaging (or potentially damaging) stimuli.
o Mechanical nociceptors; respond to intense mechanical stimuli.
o Thermal nociceptors: respond to intense heat (greater than 44 C)
o Polymodal nociceptors: respond to a variety of stimuli, including intense mechanical
stimuli, intense heat/cold, and chemicals released from damaged tissue (i.e. histamine,
bradykinin, and prostaglandins).
THE SOMATOSESORY CORTEX
Perception of somatic sensations from all parts of the body begins in the primary somatosensory
cortex. Sensory information from neighboring areas of the body generally projects to
neighboring areas of the cortex. Size of the cortex area devoted to somatic sensations is
proportional to the sensitivity of the body region.
The cerebral cortex has columnar organization. Vertical columns are organized according to
Dorsal column-mediated lemniscal pathway and the spinothalamic tract. Both transmit different
types of sensory information to the thalamus, and then to the primary somatosensory cortex.
In both cases, the pathways enter the spinal cord on one side and cross to the other side before
reaching the thalamus (info from the right side of the body is perceived in the left somatosensory
THE DORSAL COLUMN-MEDIAL LEMNISCAL PATHWAY
o It transmits information from mechanoreceptors and proprioceptors to the thalamus. It
crosses to the other side of the CNS in the medulla oblongata.
o 1 order neurons originate in the periphery and enter the dorsal horn. Collaterals from
the main axon may terminate in the spinal cord, communicating with interneurons (i.e.
o The main branch ascends from the spinal cord to the ipsilateral (same side of stimulus)
brainstem in the dorsal columns.
o The 1 order neurons terminate in the dorsal column nuclei, which are located in the
medulla, where they form synapses with 2 order neurons.
o The 2 ndorder neurons cross over to the contralateral side of the medulla in a tract called
medial lemnisus, and ascend to the thalamus.
o In the thalamus, the 2 order neurons synapse with 3 order neurons, which transmit
information from the thalamus to the somatosensory cortex.
THE SPINOTHALAMIC TRACT
o Transmit information from the thermoreceptors and nociceptors to the thalamus. It
crosses to the other side of the CNS within the spinal cord before it reaches the brain.
o 1 order neurons originate in the periphery at either thermoreceptors or nociceptors
and enter the dorsal horn. 10 THE NERVOUS SYSTEM: SENSORY SYSTEMS
o 1 order neurons may ascend or descend for a short distance along Lissauer’s tract, but
form synapses with 2 ndorder neurons in the dorsal horn.
o The 2 order neurons cross over to the contralateral side of the spinal cord, ascend in
the anterolateral quadrant of the spinal cord through the brainstem and then terminate
in the thalamus.
o In the thalamus, 2ndorder neurons from synapses with 3 order neurons that ascend to
the somatosensory cortex.
THE PAIN RESPONSE
o The activation of nociceptors leads not only to pain but also for other body responses: 1.
Autonomic responses (i.e. increase in blood pressure and heart rate, increase in blood
epinephrine levels, increases in blood glucose, dilation of the pupils, sweating). 2.
Emotional responses (i.e. fear or anxiety). 3. A reflexive withdrawal from the stimulus.
o Fast pain: perceived as sharp pricking sensation that can be easily localized. Lightly
myelinated A delta fibers.
o Slow pain: perceived as a poorly localized, full aching sensation. Unmyelinated C fibers.
o The primary afferents form synapses with 2 ndorder neurons in the dorsal horn.
o Communication between the 1 and 2 ndneurons involves substance P. It is released
from primary afferent neurons and binds to receptors on 2 order neurons.
o The 2 ndorder neurons ascend to the thalamus via spinothalamic tract, the pathway
involved in perception and discrimination of pain.
o Nociceptors activate ascending pathways for interpreting components of pain. It ascends
to the reticular formation of the brainstem, the hypothalamus, and the limbic system.
o The viscera are subject to tissue damage, and nociceptors detect this damage.
o Referred pain: activation of nociceptors in the viscera that produces pain.
o Occurs because 2 order neurons that receive input from visceral afferents also receive
input from somatic afferents. The 2dorder neurons are activated by somatic afferents,
and thus the brain has learned that signals from these neurons are somatic in origin.
ANATOMY OF THE EYE
The eye can be divided into 3 concentric layers.
The outer layer consists of sclera and cornea.
o Sclera: a tough connective tissue that makes up the white of the eye.
o Cornea: in the front of the eye, the sclera gives way to the cornea, a transparent
structure that allows light to enter.
The middle layer consists of choroid, ciliary body, and iris.
o Choroid: beneath the sclera and contains blood vessels that nourish the inner layer of
o Ciliary body: contains ciliary muscles, which are attached to the lens by strands of
connective tissue (zonular fibers).
o Lens: focuses the light on the retina, where the visual information is transduced. Iris:
consists of two layers of pigmented smooth muscle (in the front of the lens).
o Pupil: a hole in the iris that allows light to enter the posterior part of the eye (regulated
by the iris).
The innermost layer is the retina.
o The retina has neural tissue and contains photoreceptors (rods for dim light, and cones
for bright light).
o Functions in phototransduction, the conversion of light energy to electrical energy. 10 THE NERVOUS SYSTEM: SENSORY SYSTEMS
o Retinal-pigmented epithelium: located outside the retina and attached to the choroid.
Has a high concentration of black pigment melanin, which absorbs the light that strikes
the back of the eye, preventing it from reflecting back across the retina.
o Fovea: the central region of the retina, where light from the center of the visual field
strikes. Area with greatest visual acuity.
o Optic disk: the portion of the retina where the optic nerve and blood vessels supplying
the eye pass through the retina. No photoreceptors here, therefore is the blind spot (a
region where light striking the retina can’t be transduced into neural impulses).
The lens and the ciliary body separate the eye into 2 fluid filled chambers.
o Anterior segment: in front of the lens and ciliary body; divided into an anterior chamber
between the cornea and iris and a posterior chamber between the iris and lens.
o The anterior segment contains aqueous humor (clear, watery fluid), which supplies
nutrients to the cornea and lens.
The cornea and lens are transparent so light can pass through. If relied on blood for nutrients,
the blood vessels would obstruct the light.
Behind the lens and the ciliary body are posterior segment (vitreous chamber) containing a
firmer jellylike material (vitreous humor), which maintains the spherical structure of the eye.
If an object is to be seen clearly, the light reflected from any given point on the object must
converge at a single point on the retina.
When viewing something far away, the light waves enter the eye almost parallel to each other, so
little refractive power is needed to focus the light on the retina.
Light waves from close-up objects diverge as they enter the eye, sot he greater refractive power
of a rounder lens is necessary to overcome this divergence and focus the light on the retina.
The ciliary muscle controls the shape of the lens; tension is applied to the zonular fibers, which is
attached to the ciliary muscles to the lens.
For far objects, ciliary muscles relax (increase diameter) and tighten the zonular fibers (pulls the
lens into a flatten shape).For close objects, the ciliary muscles contracts, reducing the diameter of