NEUROPHYSIOLOGY

THE SENSORY SYSTEMS

Dr. Jonathan H. Jaggar

Professor

Department of Physiology

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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.

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.

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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.

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.

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).

Sensory unit

and receptive field

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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.

Primary sensory areas

cerebral cortex

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

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.

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.

(a) Small receptive field

(b) Large receptive field

Stimulus location/two neurons

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

action-

potential

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