NEUROPHYSIOLOGY
THE SENSORY SYSTEMS
Dr. Jonathan H. Jaggar
Professor
Department of Physiology
STRUCTURE OF THE CENTRAL
NERVOUS SYSTEM
The CNS is comprised of the brain and spinal cord.
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BRAIN
3 major subdivisions of the brain:
1) Forebrain
2) Brainstem
3) Cerebellum
The forebrain is comprised of the cerebrum and the diencephalon.
The brainstem consists of the midbrain, pons, and medulla oblongata.
In addition there are 4 interconnected cavities called the cerebral ventricles that
contain cerebrospinal fluid.
Brainstem
All fibers that pass between spinal cord, forebrain and cerebellum go though
brainstem.
A large portion of the brainstem is a structure called the reticular formation, which is
absolutely essential for life. The reticular formation receives and integrates
information from all regions of the CNS.
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Cerebellum
Almost exclusively involved in coordinating movements and controlling posture and
balance. Receives information from muscles, joints, skin, eyes, ears, vicera and other
parts of the brain involved in the control of movement.
Forebrain
The cerebrum consists of left and right hemispheres. Each cerebral hemisphere is
divided into 4 lobes:
The 2 cerebral cortex hemispheres are connected by a massive bundle of nerve fibers
called the corpus callosum.
Cerebral hemispheres consist of the cerebral cortex and a number of cell clusters,
called the subcortical nuclei.
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Cells in cerebral cortex are organized into 6 layers. Cortical neurons are of 2 basic
types: pyramidal and non-pyramidal neurons.
In the cerebral cortex, basic information is processed into meaningful images, and fine
control of skeletal muscles occurs.
Nerve fibers enter the cortex from:
1) the diencephalon, particularly the thalamus.
2) other regions of the cortex.
3) the reticular formation of the brainstem.
The limbic system is an interconnected group of brain structures that are associated
with learning, emotion and behaviour.
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SPINAL CORD
Butterfly shaped gray matter composed of:
1) Interneurons
2) Cell bodies and dendrites of efferent (i.e. away from brain) neurons.
3) Fibers of afferent (i.e. towards brain) neurons.
4) Glial cells – physically and metabolically support neurons.
Gray matter surrounded by white matter, which consists of interneurons.
Afferent fibers enter on the dorsal side of spinal cord via dorsal roots. Axons of
efferent neurons exit the spinal cord on the ventral side, via the ventral roots. The
dorsal root ganglia, are small bumps that contain the cell bodies of the dorsal roots.
Dorsal and ventral roots combine to form a spinal nerve on each side of the spinal
cord. There are 31 pairs of spinal nerves:
•8 cervical: associated with the neck, shoulders, arms, hands.
•12 thoracic: chest and abdominal walls.
•5 lumbar: hip and legs.
•5 sacral: genitals and lower digestive tract.
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Afferent fibers enter on the dorsal side of spinal cord via dorsal roots. Axons of
efferent neurons exit the spinal cord on the ventral side, via the ventral roots. The
dorsal root ganglia, are small bumps that contain the cell bodies of the dorsal roots.
Dorsal and ventral roots combine to form a spinal nerve on each side of the spinal
cord. There are 31 pairs of spinal nerves:
•8 cervical: associated with the neck, shoulders, arms, hands.
•12 thoracic: chest and abdominal walls.
•5 lumbar: hip and legs.
•5 sacral: genitals and lower digestive tract.
Dorsal view,
spinal cord
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PERIPHERAL NERVOUS SYSTEM
12 pairs of Cranial Nerves
31 pairs of Spinal Nerves.
1) Afferent Division: carries information towards CNS.
i.e. sensory system
2) Efferent Division: carries information away from CNS.
i) Somatic nervous system.
Consists only of motor neurons.
Innervates skeletal muscle.
ii) Autonomic nervous system.
Innervates smooth and cardiac muscle, glands, GI.
Autonomic can be further subdivided into:
a) Sympathetic
b) Parasympathetic
c) Enteric
Major differences in Sympathetic/Parasympathetic
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Autonomic Nervous System
Sympathetic and Parasympathetic usually have opposing effects:
e.g. in heart
Sympathetic
Parasympathetic
Sinoatrial Node
↑ heart rate
↓ heart rate
Atria
↑ contractility
↓ contractility
Atrioventricular Node
↑ conduction velocity
↓ conduction velocity
Ventricles
↑ contractility
↓ contractility
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Cranial Nerves
NAME
FIBERS
COMMENTS
I. Olfactory
Afferent
Neuroepithelium
II. Optic
Afferent
Eye receptors
III. Oculomotor
Efferent
Eye muscles
IV. Trochlear
Efferent
Afferent
Eye muscles
Eye muscles
V. Trigeminal
Efferent
Afferent
Chewing muscles
Skin and skeletal muscles
VI. Abducens
Efferent
Afferent
Eye muscles
Muscle
VII. Facial
Efferent
Afferent
Skeletal muscles/ salivary glands
Taste buds
VIII. Vestibulocochlear
Afferent
Ear receptors
IX. Glossopharyngeal
Efferent
Afferent
Swallowing/salivary
Taste buds
X. Vagus
Efferent
Afferent
Thorax/abdomen muscle and glands
Receptors in thorax/abdomen
XI. Accessory
Efferent
Neck skeletal muscles
XII. Hypoglossal
Efferent
Tongue skeletal muscles
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Enteric Nervous System
Localized to gastrointestinal tract.
Consists of: 1) Myenteric plexus 2) Submucous plexus
Many interactions between these 2 nerve networks.
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SENSORY SYSTEM - GENERAL PRINCIPLES
The sensory system is part of the nervous system. It consists of sensory receptors
that receive stimuli, the neural pathways that transmit that information to the brain,
and the parts of the brain that process the information received.
Definitions:
Sensory information: conscious or unconscious.
Sensation: conscious detection of sensory information.
Perception: an understanding of sensory information that results from neural
processing.
Afferent Neuron: carries information towards CNS.
Efferent Neuron: carries information away from CNS.
1) Receptors
Respond to changes in environment.
Two forms: i) On peripheral end of an afferent neuron.
ii) Located on a separate cell that is adjacent to an afferent neuron.
Receptors are activated by stimuli. Sensory information is transformed into
an electrical response via a process known as signal transduction. Sensory
receptors respond to specific stimuli, but can be activated by other stimuli if
the stimulus is sufficiently high. The type of energy to which a receptor
responds with most sensitivity is known as its adequate stimulus.
The Receptor Potential
Definition: A graded change in membrane potential that is induced by a stimulus that
alters the activity of ion channels in a specialized receptor membrane. The localized
steady depolarization induces subsequent action potential generation in the attached
axon at the first node of Ranvier.
When the receptor membrane is on a separate cell, activation of the receptor induces
release of neurotransmitter that binds to specific sites on the afferent neuron and
induces a graded membrane potential change.
After initial stimulation, receptors may undergo adaptation, which is a decrease in the
rate of firing of action potentials. The degree of adaptation varies widely between
receptor types. The significance of adaptation will be discussed later.
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2) Neural Pathways in Sensory Systems
Sensory Unit: A single afferent neuron and all receptor endings.
Receptive Field: Portion of the body that, when stimulated, activates an afferent
neuron. Receptive fields of different afferent neurons overlap, so that stimulation
activates several sensory units.
Sensory unit
and receptive field
Ascending Pathways
Afferent sensory neurons synapse on interneurons, termed “second-order” neurons, in
the spinal cord or brain. In turn, these neurons synapse on “third-order” neurons, etc.,
etc., until the action potential reaches the cerebral cortex. Specific ascending
pathways carry single types of stimuli (e.g. from thermoreceptors) to the brainstem
and thalamus, before going to the cerebral cortex. Almost all specific pathways cross
over to the opposite side of the central nervous system to that from which the stimulus
came (i.e. contralateral).
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Primary sensory areas
cerebral cortex
Examples of specific ascending pathways and primary receiving areas:
•Somatic receptors => somatosensory cortex in parietal lobe of the brain
•Eyes => visual cortex in occipital lobe
•Ears => auditory cortex in temporal lobe
•Taste buds => cortical area adjacent to somatosensory cortex
•Olfactory => terminate in limbic system rather than going to thalamus
Processing of this information does not end here but continues to the association areas
of the cerebral cortex.
Non-specific ascending pathways
•Activated by sensory units of different types, i.e. signal general information
•Neurons receiving input from multiple non-specific pathways are termed “polymodal
neurons”.
•Non-specific pathways terminate in areas of brainstem, thalamus, and cerebral
cortex that are non discriminative, but control alertness.
Specific/nonspecific
sensory pathways
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3) Association Cortex and Perceptual Processing
The areas of the association cortex lie outside the primary cortical sensory or motor
areas. These areas are not considered to be part of the sensory pathways, but their
function is to process and analyze sensory information. Therefore, the association
cortex is important for determining perception.
These factors can affect perception
•Sensory receptor adaptation
•Personal experience, emotion, personality, social background
•Afferent sensory processing and signal discrimination. Occurs at the level of
receptor and higher centers.
•No feasible sensation – lack receptor for certain energy forms.
•Faulty perception from damaged neural networks. e.g. phantom limb.
•Pharmacological modulation. e.g. drug induced hallucinations
Areas of association cortex
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4) Primary Sensory Coding
4 aspects of a stimulus are coded: stimulus type, intensity, location, and duration.
a) Stimulus Type
Modalities are broken down into Submodalities. e.g Taste/sweet. All receptors of a
single afferent neuron respond preferentially to the same stimulus type. Overlap of
different sensory neuron receptive fields allow a single stimulus to provide multiple
sensations because multiple neurons that respond to different modalities will be
stimulated.
b) Stimulus Intensity
Stimulus intensity is translated in 3 primary ways:
•Action potential firing frequency. Action potentials do not vary in amplitude, they
are all-or-none. Thus, increased stimulus intensity at a single receptor is translated
into sensory information by increasing the frequency of evoked action potentials.
•Multiple receptor stimulation. An increased stimulus will most likely activate
receptors on other branches of the same afferent neuron. Action potentials evoked by
these receptors will add to the train of action potentials in the main fiber.
•Recruitment. A stronger stimulus will usually affect a larger tissue area and stimulate
receptors on other afferent neurons.
Action potentials/afferent fiber
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c) Stimulus Location
Touch: location on body.
Smell, vision, hearing: origin of stimulus.
Stimulus location is coded by the site of the stimulated receptor.
Several factors affect the acuity (or precision) of stimulus location:
i) The amount of convergence in the ascending pathway.
> convergence = < acuity.
ii) Size of the receptive field covered by the afferent neuron.
> receptive field = < acuity.
Stimulus location/two neurons
(a) Small receptive field
(b) Large receptive field
iii) Receptor density. Neurons produce more action potentials if a stimulus occurs in
center of receptive field, due to increased receptor density. However, this is not a
precise mechanism because an increase in the number of action potentials could also
mean a more intense stimulus was applied.
Two stimulus points
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iv) Receptive field overlap. Stimuli will usually activate receptors in the receptive
field of more than 1 neuron. Thus, differential firing frequency of multiple neurons
will allow coding of stimulus location.
A
stimulus
point
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v) Lateral Inhibition. This is the most important mechanism to determine stimulus
localization. Focuses sensory processing on important messages, allowing signal
discrimination by inhibiting information from afferent neurons whose receptors are at
the edge of a stimulus. Lateral inhibition can occur at all levels in the sensory
pathway, but it usually occurs at the early stages. It occurs most commonly in
pathways that require accurate localization, e.g. skin.
Afferent pathways/lateral inhibition
Effect on
actionpotential
frequency
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d) Stimulus Duration
The action potential frequency that occurs at the beginning of a stimulus indicates
intensity. After the initial response the action potential frequency depends on the
receptor type:
Rapidly adapting receptors: after initial burst, fires slowly or stops firing. Important
for signaling rapid changes in stimuli. Some of these receptors only fire 1 action
potential at the initial stimulus onset (“on-response”). Some receptors respond at the
beginning and at the end of the stimulus.
Slowly adapting receptors: Maintain firing near to initial frequency throughout
stimulus duration. Signal slow changes in stimuli, or prolonged events.
Rapidly/slowly adapting receptors
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Central Control of Afferent Information
In addition to information filtering that occurs in the ascending pathways, information
is also controlled from the higher centers in the brain by descending pathways. In
particular, the reticular formation and the cerebral cortex control afferent information
via descending pathways.
Inhibition from descending pathways can occur:
1) directly, by synapses onto the axon terminals of the primary afferent neuron
(termed “presynaptic inhibition”).
2) indirectly, via interneurons.
Descending pathways
control sensory information
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