Sensory1_0708.pdf

Sensory system 1

Contents

1. Receptor2. Receptor potentials• Characteristics• Its role in generation of an action potential3. Characteristics of the stimuli e.g.

localisation, intensity

Sensory system

• Consists of:• Sensory receptor cells that receive stimuli

from external or internal environment• Neural pathways that conduct information to

the spinal cord and the brain• Parts of the brain that deal with processing of

the information

Sensory system

• Sensory information – Information processed by the sensory system

• Sensation – if the information reaches consciousness

• Perception - a person’s understanding of the sensation’s meaning

Sensory receptors• Peripheral ends of afferent neurons

(detectors and transducers)• structures that detect changes in external & internal

environmentmodified neurons or epithelial cells that have evolvedto respond to stimuli (eye, ear, nose)

• The energy that activates a sensory receptor – stimulus

• The process by which a stimulus is transformed into an electrical response –stimulus transduction.

• The type of energy to which a particular receptor responds – adequate stimulus

Sensory receptors

• Several classes of receptors• Mechanoreceptors e.g. touch, BP and

muscle tension• Thermoreceptors• Chemoreceptors e.g. smell and taste• Nociceptors

Senses

CONSCIOUS SENSES• Vision• Hearing• Smell• Taste• Touch-pressure• Warmth• Cold• Pain

Information that does not reach consciousness (unconscious senses)

• Muscle length• Muscle tension• Arterial blood pressure• Central venous pressure• Inflation of lungs• Arterial PaO2 and PaCO2

• Osmotic pressure in the plasma

Cutaneous sense organs

• Touch-pressure, cold, warmth and pain• Variety sensory endings1) Naked nerve endings2) Expanded tips on sensory nerve terminals

- Merkel’s disks and Ruffini’s endings3) Encapsulated endings

- Meissner’s corpuscles, Pacinian corpuscles and Krause’s end-bulbs

Cutaneous sense organs

• Meissner’s and pacinian corpuscles – rapidly adapting touch receptors

• Merkel’s disks & Ruffini endings are slowly adapting touch receptors

• Nerve endings around hair follicles – touch• Movement of hair follicle- tactile sensation• Each given endings are physiologically

specific- responds to 1 kind of cutaneoussensation

Pacinian corpuscle

Pacinian corpuscle

• Relatively large and can easily be isolated

• Straight unmyelinated ending of sensory fibre

• Concentric lamellas of connective tissue• The first node of Ranvier is inside

Receptor potentials• Transduction process involves opening and

closing of ion channels• Mechanical distortion opening of Na+

channels• Number of channels α intensity of

stimulus• Ion fluxes across the receptor membrane – a

change in the membrane potential • The graded potential – a receptor potential• The local current flows a short distance to a

region where the membrane has voltage gated ion channels – action potentials

Receptor potentials

• Its magnitude decreases with distance • Its magnitude determines the distance it will

travel and its duration• An increase in the receptor potential – an

increase in the frequency of action potential (limited by neuron’s refractory period)

• Does not determine the ACTION POTENTIAL (AP)’s magnitude (All or none law)

An afferent neuron and recording electrodes at 1 and 2. The receptor potential arises at 1 and action potentials (APs) arises at 2. APs are not generated at the lowest stimulus intensity.

Receptor potentials

Factors that control the magnitude of receptor potential-

• Stimulus strength• Rate of change of stimulus strength• Temporal summation of successive

potentials• Adaptation

Adaptation

• When a maintained stimulus of constant strength is applied, the frequency of action potentials declines

• Adapt rapidly-phasic receptors• Adapt very slowly and incompletely• Carotid sinus, muscle spindles, sense

organs that detect cold and lung inflation-tonic receptors

Coding of sensory information

• Conversion of receptor potentials into a pattern of action potentials that conveys relevant sensory information

• Coding enables higher centres to discriminate• Modality• Locality• Intensity• Duration of different sensory stimuli

Modality

• A specific stimulus for each receptor –adequate stimulus

• Law of specific nerve energy – when the nerve pathway is stimulated the sensation evoked depends on the site of brain that the pathway activates

• A specific pathway for each modality of sensation

ADEQUATE STIMULUS (LIGHT STIMULUS)

INADEQUATE STIMULUS (HEAT STIMULUS)

RECEPTOR (CONES

AND RODS)

RECEPTOR

A SPECIFIC SENSATION (VISION)

NO RESPONSE

THE CONCEPT OF ADEQUATE STIMULUS

Localisation

• Law of projection- the sensation is projected to the locality of the receptor

A sensory unit (SU) - a single afferent neuron with all its receptor endingsReceptive field – a portion of the body that when stimulated produces activity in the particular afferent neuron

A stimulus point falls within overlapping receptive fields of 3 afferent neurons. Note the difference in the number of APs frequency due to the difference in the receptor distribution.

A high AP frequency in B

(B > A; B > C) – provide accurate localization

Firing frequency by B -intensity

LOCATION OF THE STIMULUS

SU

The density of nerve endings around A is greater than around B. The frequency of APs in response to stimulus A is greater than the response to stimulus B.

LOCATION OF THE STIMULUS

Lateral Inhibition

• Information from afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus’s centre.

• The number of APs from lateral neurons will further decreases

• Enhances the contrast between relevant & irrelevant information- to increase effectiveness and focusing sensory processing mechanisms on important messages (e.g. retinal processing).

LOCATION OF THE STIMULUS

Afferent pathways showing lateral inhibition. 3 sensory units have overlapping receptive field. The central fiber (B) is firing at the highest frequency and inhibits the lateral neurons (A & C) via the inhibitory interneuron

Intensity

• Increase in the number of action potentials generated per second (frequency)

• Increase in the number of sensory units activated

Stimulus duration

• affected by adaptation• Rapidly adapting – touch and

movement• Slowly adapting - pressure

Stimulus on

AP response

Stimulus off

Stimulus duration

• affected by adaptation• Rapidly adapting – touch and

movement• Slowly adapting - pressure

Stimulus on

AP response

Stimulus off

Descending pathways may influence sensory information by (a) directly or (b) indirectly inhibiting the central terminals of afferent neuron. Arrows indicate the direction of APs.