Sensory system - introduction

introduction

    The nervous system along with endocrine system provide most of control functions of the body 
nervous system control the rapid activities of the body, such as muscular contractions
 Endocrine system in contrast regulates principally the metabolic functions of body
Organization of the nervous system

1- Historical organization:

NS is comprised of billion of cells which are major two types:

 A- neurons: are the basic units of nervous tissues, specialized for transmission of AP.

 Human NS functions roughly with about 10 million sensory neurons, 50 billion central neurons & ½ million motor neurons.

 B- Glial cells (neuroglia= supporting cells):

 Are the matrix of CNS & maintain its structure.

They are 10-50 times the No of neurons.

 They are not directly involved in the transfer of information but they do play a key role in the overall functioning of the NS.

 They are of 3 types:

 1/ microglial cells: are the macrophages (scavenger) of CNS which phagocytose tissue debris, thus helping the process of tissue repair

11/ oligodendrites: are responsible for production & maintenance of myelin sheath around axons in CNS

 111/ astrocytes: have extensive branches that surround the cerebral capillaries forming a blood brain barrier.

  Function also to maintain a constant K environment around neuron this is necessary for the normal functioning of the nerve cell

 2- functional & anatomical organization:

NS is divided into:

 A. central nervous system CNS: include the brain (lying within skull) & spinal cord (lying within vertebral column).

It contain centers for the integration of the input signals & for the production of output signals.

 B. peripheral NS: consist of nerves which enter & leave the CNS. Is subdivided into:

afferent (sensory) division: carry impulses from the periphery to CNS
efferent (motor) divisions: carry impulses from CNS to effector organs eg muscles & glands. includes somatic NS (controlling skeletal muscle contraction) + & autonomic NS ( controlling smooth & cardiac muscles & glands)
Somatic division is functionally considered the motor nervous system
The reflex arc

 Most neural activities in the body are in form of reflex arc a change in external or internal environment receptors sensitive to various stimuli (eg light, sound, touch, blood pressure) are stimulated send nerve impulses along afferent fibers to CNS where they are processed & integrated before appearing in the efferent fibers to produce appropriate responses in the effectors organs (eg muscle contraction, glandular secretion)

The pathway the impulse follow from receptor to effector is called reflex arc & action that result is called reflex action

 Reflex arc vary on complexity simple at the level of SC & get more complex at the level of brain stem but most complex RA are in cereberal cortex

The sensory system

 sensory system provide us with information about our environment- external or internal

Sensory system is  consist of sensory receptors which receive stimuli from external or internal environment, the neural pathways that conduct information from receptors to the brain & those parts of the brain that deal primarily with processing the information.   

Information processed by the sensory system may or may not lead to conscious awareness of the stimulus. If the information does reach consciousness, it can be called sensation. A person understanding of the sensations meaning is called perception 

Sensory receptors

Definitions:

         Sensory receptors: are specialized structures or modified nerve ending present at the peripheral termination of afferent nerve fibers

Sensory unit:  is a single sensory afferent with all its receptor endings

Receptive field of a neuron: is the region which contains all the receptor endings of a sensory neuron & from which a stimulus produces a response in that particular afferent neuron

Functions of receptors:

 1- detectors: they detect change in the internal or external environment (adequate stimuli) & inform CNS about different sensations

2- transducers: they transform any form of energy (chemical, mechanical, thermal etc) into action potentials that lead to generation of nerve impulses in the sensory nerve.

 Classification of receptors:

 1- Mechanoreceptors: detect mechanical deformity in the receptors or cells adjacent to them as

  A-  touch & pressure receptors: in skin & deep  tissues.

 B- proprioceptors in tendons, muscles & joints: they are concerned with information about the position of the body in space, stretch & tension

 C- Vestibular receptors: equilibrium

D- cochlear receptors: sound

 E- baroreceptors: blood pressure

2- Thermoreceptor: detect thermal form of energy

 A- cold receptors

B- warm receptors

 3- Pain receptors (nociceptors): detect physical or chemical damage

4- photo receptors (electromagnetic receptors):  detect light: rods & cones

 5-chemoreceptors: detect chemical change:

A- taste & smell

 B- blood gases

 C- osmo & glucoreceptors in the hypothalamus

Structures & characters of cutaneous receptors:

1- free nerve endings: present everywhere in the body. They can detect pain, temp, crude touch & tickle & itch sensation

 2- encapsulated nerve ending: Rapidly adapting receptors

1/ Meissner's corpuscles: they are present in large no in lips & finger tips & are suitable for fine touch sensation. They also respond to low frequency vibrations up to 80cycle/sec 

11/ Paccinian corpuscles: are present in subcut- tissues, deep tissues & joint capsules. They respond to deep pressure, position & to high frequency vibration up to 500cycles/sec 

 3- Expanded nerve endings: slowly adapting receptors:

Merkel’s discs: they are present in large no in lips & fingertips. Being slowly adapting they are suitable for detection of sustained touch & pressure.

Ruffinis  end organs: they are present in deep tissues & joint capsules & ligaments. They detect deep pressure & position sensation

Spray type endings: they resemble Golgi tendon organs & are present in joints & tendons. They are suitable for sense of position

 4- hair follicles (hair end organs): in hairy skin. It detect mainly movement of the object on the surface of the skin

Properties of receptors: specificty, excitability, adaptation

1- specificity: each type of receptor is highly sensitive to one specific type of stimulus (or energy) which is called its adequate stimulus & normally nonresponsive to the normal intensities of other forms of energy (unless they are stronger than normal range) eg

light is the adequate stimulus for the rods & cones of the eyes but they do not respond to heat or cold.

Pain receptors are not stimulated by a blunt objects touching the skin but they discharge as soon as the blunt object is pushed with enough force to damage tissues

The sensation perceived as a result of stimulation of a receptor is called the modality of sensation. Thus cold, warmth, touch & pain are different modalities of sensation

2- excitability: different types of receptors are excited in different ways. Some are stimulated by mechanical deformation, others by application or release of chemical & others by changes of temperature.

The receptors transforms theses various stimuli into a local electrical change called generator or receptor potential (non propagated partial depolarization).

Successive receptor potentials can be summated & when threshold values is reached AP is triggered. Then propagated along the sensory nerve.

 The response to receptor stimulation is not a single AP but a volley of APs the frequency of which is directly related to the intensity of the applied stimulus

3- Adaptation: when a stimulus of constant strength is continuously applied to a receptor the frequency of APs in the afferent nerve fiber  gradually declines.

 According to the rate of adaptation receptors are classified into:

1- slowly adapting receptors (tonic receptors):

continuous stimulation The receptors continue to transmit impulses to the brain at relatively a constant rate during (at least for many minutes or hours)  keep the brain continuously informed about the status of the body & its relation to surroundings. Also  they protect & warn the body against any dangerous change in the internal environment. Example of tonic receptors are:

Muscle spindle for information about posture

 Joint receptors for information about the position of different parts of the body

Baroreceptor for information about arterial BP

 2- rapidly adapting (phasic receptors):

 Although they show rapid decline in the magnitude of the receptor potential (thus decrease in AP along its nerve) WITH maintained stimulation  they discharge strongly while a change in the intensity of the stimulus is taking place. As touch & pressure receptors

sudden pressure applied to the tissue excites this receptor for a few milliseconds, and then its excitation is over even though the pressure continues. But later, it transmits a signal again when the pressure is released

They are therefore is useful in transmitting information about events which undergo rapid changes but are of no use in transmitting information about constant events

 Pain receptors do not adapt at all  this is useful because pain serves as protective mechanism for the body

3- moderately adapting: occupy position between the two groups eg temperature, smell & taste 

Coding of sensory information

Definition: mean the ability of CNS to recognize the modality (type), the locality (site) & the intensity (degree) of sensation.

All stimuli produce APs which are all alike. The question is how can CNS convert a receptor stimulus into a recognizable sensation.

 a/ Modality of sensation: modality discrimination is make possible by the following:

1-adequate stimulus: there is specific stimulus for each receptor. each receptor is specialized to receive a particular type of energy (stimulus)

2- Specific pathway exists for each modality of sensation ie pain pathway, touch pathway etc. stimulation of the pathway at any point with any form of energy evokes its specific sensation

 3- a receptor or its nerve pathway when stimulated gives rise to one type of sensation regardless the method of stimulation this is called Mullers law of specific nerve energy. Example stimulation of pain receptor or the pathway leading from it will cause the subject to perceive pain whether the stimulus was heat, electric shock or crushing of the tissues.

4- each nerve fiber in the pathway end at specific area in the CNS. The modality perceived is determined by that area ie a subject perceived touch or temperature because their respective pathways end at specific touch or temperature area in the brain

  b/ locality of the stimulus:

 Locality discrimination is explained by law of projection.

Conscious perception of a particular sensation is always projected to the site of the receptor, irrespective of where the sensory pathway (leading from the receptor)is stimulated

There is a separate representation area for each part of the body in the cereberal cortex (point to point representation). When the impulse reach the specific area in the cortex, it project the stimulus to its original site  

This explain the phantom limb phenomenon which occurs after amputation  squeezing the nerve in the stump causes the patient to report pain in the little toe or ankle (lost parts)  as the impulses reaches the sensory cortex the sensation projected to the site of the receptor previously removed with the amputated part  give rise to the false interpretation

 c/ intensity of sensation:

The intensity of stimulus is coded to the brain in two ways:

  Frequency of action potential

 &/Or Number of receptor stimulated:

Somatic sensation

Objectives:

 Definition of somatic sensation

  Classification of somatic sensation

  Transmission of somatic sensation

 1/ somatic sensory afferents

 2/ somatosensory pathway

  Dorsal column pathway

Ventrolateral pathway

 The somatic sensation are those sensations carried from the various parts of the body by somatic nerves

Classification of somatic sensation:

1- mechanoceptive sensation: stimulated by mechanical displacement of body tissues

A- tactile sensation:

1/ touch

 A- crude touch

 B- fine touch

 Tactile localization

Texture

11/ pressure: crude & fine

111/ stereognosis

1V/   vibration sense

     V/    itch & tickling

B/ Position senses:

 1/ static position sensation

11/ kinetic position sensation

 2- thermoceptive sensation:

 Cold       Warm

  3- pain sensation

Classification & characters of somatic sensory nerves:

1- according to their diameter they classified into 4 types from the largest to the least thickness1,11,111,1V

2- according to their velocity of conduction they are classified into 3 types from fastest to lowest A (Aα, Aβ, Aδ), B & C

 Each type of sensation has its own importance thus some sensations must be carried to the brain v fast (eg proprioceptive carried by Aα fiber) while others may be carried rather slowly (slow pain carried by C fibers) .