Sensory Neuron and Their Action

Sensory Neurons

The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons

Neurons can be classified according to the direction in which they conduct impulses or according to the number of processes they extend. Sensory neurons transmit nerve impulses toward the spinal cord and the brain. Motor neurons transmit nerve impulses from the brain and the spinal cord to the muscles and the glandular tissue. Multipolar neurons, bipolar neurons, and unipolar neurons are classified according to the number of processes they extend to the different kinds of neurons. Multipolar neurons have one axon and several dendrites, as do most of the neurons in the brain and the spinal cord. Bipolar neurons, which are less numerous than the other types, have one axon and only one dendrite. Unipolar neurons have one axon and no dendrites. All primary sensory afferents and some autonomic neurons are unipolar. All neurons have one axon, and most have one or more dendrites and have a slightly gray color when clustered, as in the nuclei of the brain and the spinal cord.

Classification of Neurons

1. Number of processes, or neurites

unipolar neuron (one process, usually an axon)

bipolar neuron (two processes, usually two dendrites or one axon, one dendrite)

multipolar neuron (many processes, usually only one axon)

2. Function

sensory neuron--input from sensory structure

motoneuron--output to muscles or glands

interneuron--inputs from other neurons, outputs to other neurons

3. Size and length of axon

projection neuron (Golgi type I)--large neurons with long axons

local neuron (Golgi type II)--small neurons with short axons

4. Shape

e.g. pyramidal = pyramid-shaped soma

stellate = star-shaped soma

granule = grain-like; small and round

many more exotic types;

e.g. chandelier cells have a dendritic trees that resemble chandeliers

5. Effect on target

excitatory neuron

inhibitory neuron

6. Identity of neurotransmitter (__-ergic)

e.g. cholinergic use acetylcholine, serotonergic use serotonin, etc

Organization of the Nervous System

The nervous system is divided into the

peripheral nervous system (PNS) and the

central nervous system (CNS) Link to discussion of the central nervous system.

The PNS consists of

sensory neurons running from stimulus receptors that inform the CNS of the stimuli

motor neurons running from the CNS to the muscles and glands - called effectors - that take action.

The CNS consists of the

spinal cord and the

brain

The peripheral nervous system is subdivided into the

sensory-somatic nervous system and the

autonomic nervous system

The Sensory-Somatic Nervous System

The sensory-somatic system consists of

12 pairs of cranial nerves and

31 pairs of spinal nerves.

Sensory neurons

These run from the various types of stimulus receptors, e.g.,

touch

odor

taste

sound

vision

to the central nervous system (CNS), the brain and spinal cord.

Touch

Light touch is detected by receptors in the skin. Many of these are found next to hair follicles so even if the skin is not touched directly, movement of the hair is detected.

In the mouse, light movement of a hair triggers a generator potential in mechanically-gated sodium channels in a neuron located next to the hair follicle. This potential opens voltage-gated sodium channels and if it reaches threshold, triggers an action potential in the neuron.

Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm2; the back of the hand fewer than 10 per cm2.

This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine (in a blindfolded subject) the minimum separation of the points that produces two separate touch sensations. The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back.

The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body. Link to illustrated discussion of the somatosensory cortex of the human brain.

Proprioception

Proprioception is our "body sense".

It enables us to unconsciously monitor the position of our body.

It depends on receptors in the muscles, tendons, and joints.

If you have ever tried to walk after one of your legs has "gone to sleep", you will have some appreciation of how difficult coordinated muscular activity would be without proprioception.

Two mechanoreceptors:

the Pacinian corpuscle and the muscle spindle

The Pacinian Corpuscle

Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron.

Because of its relatively large size, a single Pacinian corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency is applied to the corpuscle by the stylus. The electrical activity is detected by electrodes attached to the preparation.

Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.

Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron.

Adaptation

When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation.

Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing.

Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated.

The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception - such as spindle fibers - adapt slowly if at all.

The Sense of Smell

Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. The olfactory epithelium is made up of three kinds of cells:

sensory neurons each with a primary cilium

supporting cells between them

basal cells that divide regularly producing a fresh crop of sensory neurons to replace those that die (and providing an exception to the usual rule that neurons seldom are replaced).

The sequence of events

The cilia of the sensory neurons are immersed in a layer of mucus. Odorant molecules (molecules that we can smell) dissolve in the mucus and

bind to receptors on the cilia. These are "7-pass" transmembrane proteins.

Binding of the odorant activates a G protein coupled to the receptor on its cytoplasmic side.

This activates adenylyl cyclase, an enzyme embedded in the plasma membrane of the cilia.

Adenylyl cyclase catalyzes the conversion of ATP to the "second messenger" cyclic AMP (cAMP) in the cytosol. Odorant receptors represent one family of many types of G-protein-coupled receptors (GPCRs). Some other examples of GPCRs:

receptors of peptide hormones

taste receptors

the light receptor rhodopsin

GABAB receptors at certain synapses in the brain

cAMP opens up ligand-gated sodium channels for the facilitated diffusion of Na+ into the cell

The influx of Na+ reduces the potential across the plasma membrane.

If this depolarization reaches threshold, it

generates an action potential.

The action potential is conducted back along the olfactory nerve to the brain.

The brain evaluates this and other olfactory signals reaching it as a particular odor.

The Sense of Taste

Taste is the ability to respond to dissolved molecules and ions called tastants.

Humans detect taste with taste receptor cells. These are clustered in taste buds. Each taste bud has a pore that opens out to the surface of the tongue enabling molecules and ions taken into the mouth to reach the receptor cells inside.

There are five primary taste sensations:

salty

sour

sweet

bitter

umami

Properties of the taste system.

A single taste bud contains 50–100 taste cells representing all 5 taste sensations (so the classic textbook pictures showing separate taste areas on the tongue are wrong).

Each taste cell has receptors on its apical surface. These are transmembrane proteins which

admit the ions that give rise to the sensations of salty and sour;

bind to the molecules that give rise to the sensations of sweet, bitter, and umami.

A single taste cell seems to be restricted to expressing only a single type of receptor (except for bitter receptors).

Taste receptor cells are connected, through an ATP-releasing synapse, to a sensory neuron leading back to the brain.

However, a single sensory neuron can be connected to several taste cells in each of several different taste buds.

The sensation of taste — like all sensations — resides in the brain

Salty

At least one of the receptors for salty substances (e.g., table salt, NaCl), is an ion channel that allows sodium ions (Na+) to enter directly into the cell. This depolarizes it allowing calcium ions (Ca2+) to enter [Link] triggering the release of ATP at the synapse to the attached sensory neuron and generating an action potential in it.

In lab animals, and perhaps in humans, the hormone aldosterone increases the number of these salt receptors. This makes good biological sense:

The main function of aldosterone is to maintain normal sodium levels in the body.

An increased sensitivity to sodium in its food would help an animal suffering from sodium deficiency (often a problem for ungulates, like cattle and deer).

Sour

Sour receptors are transmembrane ion channels that admit the protons (H+) liberated by sour substances (acids) into the cell.

Sweet

Sweet substances (like table sugar — sucrose) bind to G-protein-coupled receptors (GPCRs) at the cell surface.

Each receptor contains 2 subunits designated T1R2 and T1R3 and is

coupled to G proteins.

The complex of G proteins has been named gustducin because of its similarity in structure and action to the transducin that plays such an essential role in rod vision.

Activation of gustducin triggers a cascade of intracellular reactions:

activation of adenylyl cyclase

formation of cyclic AMP (cAMP)

the closing of K+ channels that leads to depolarization of the cell.

The mechanism is similar to that used by our odor receptors [View].

The hormone leptin inhibits sweet cells by opening their K+ channels. This hyperpolarizes the cell making the generation of action potentials more difficult.

Bitter

The binding of substances with a bitter taste, e.g., quinine, phenyl thiocarbamide .also takes place on G-protein-coupled receptors that are coupled to gustducin.

In this case, however, cyclic AMP acts to release calcium ions from the endoplasmic reticulum . which triggers the release of neurotransmitter at the synapse to the sensory neuron.

Humans have genes encoding 25 different bitter receptors ("T2Rs"). However, each taste cell responsive to bitter expresses many of these genes. (This is in sharp contrast to the system in olfaction where a single odor-detecting cell expresses only a single type of odor receptor.)

Despite this — and still unexplained — a single taste cell seems to respond to certain bitter-tasting molecules in preference to others.

The sensation of taste — like all sensations — resides in the brain. Transgenic mice that

express T2Rs in cells that normally express T1Rs (sweet) respond to bitter substances as though they were sweet;

express a receptor for a tasteless substance in cells that normally express T2Rs (bitter) are repelled by the tasteless compound.

So it is the activation of hard-wired neurons that determines the sensation of taste, not the molecules nor the receptors themselves.

Umami

Umami is the response to salts of glutamic acid — like monosodium glutamate (MSG) a flavor enhancer used in many processed foods and in many Asian dishes. Processed meats and cheeses (proteins) also contain glutamate.

The binding of amino acids, including glutamic acid, takes place on G-protein-coupled receptors that are coupled to heterodimers of the protein subunits T1R1 and T1R3.

Another umami receptor (at least in the rat's tongue) is a modified version of the glutamate receptors found at excitatory synapses in the brain. [More]

Hearing

The sense of hearing is the ability to detect the mechanical vibrations we call sound.

Sound waves

pass down the auditory canal of the outer ear

strike the eardrum (tympanic membrane) causing it to vibrate

these vibrations are transmitted across the middle ear by three tiny, linked bones, the ossicles:

hammer (malleus)

anvil (incus)

stirrup (stapes)

The ossicles also magnify the amplitude of the vibrations.

The middle ear is filled with air and is connected to the outside air by the eustachian tube, which opens into the nasopharynx. Opening of the tube — during swallowing or yawning — equalizes the air pressure on either side of the eardrum.

Allergies or a head cold may inflame the walls of the eustachian tubes making them less easily opened. Rapid changes in pressure at such times — such as descending in an aircraft or during a SCUBA dive, may be quite painful because of the unequal pressure against the eardrums.

The Human Eye

The human eye is wrapped in three layers of tissue:

the sclerotic coat

This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The cornea

admits light to the interior of the eye and

bends the light rays to that they can be brought to a focus.

The surface of the cornea is kept moist and dust-free by secretions from the tear glands.

the choroid coat

This middle layer is deeply pigmented with melanin. It reduces reflection of stray light within the eye. The choroid coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "color". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability (as does "stopping down" the iris diaphragm of a camera).

the retina The retina is the inner layer of the eye. It contains the light receptors, the rods and cones (and thus serves as the "film" of the eye). The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain. (Note: the rods and cones are not at the surface of the retina but lie underneath the layer of interneurons.)

Sensory neurons are PARtial to pain

Recent insights into the receptors expressed by sensory neurons are providing the beginnings of a biological basis for designing therapies to block nociceptive information before it reaches the spinal cord. This approach could potentially avoid some of the side effects in the central nervous system caused by currently

Sensory or Afferent Nerve Fibres

Sensory or afferent nerve fibres conduct messages from the peripheral tissues to the spinal cord. They finally join the grey matter of the spinal cord. They produce the sensation of touch, pain or reflex actions (involuntary movements..

Sensory Nerves or the Receptor Nerves

They are made up of only sensory neurons. For example, the cranial nerves that conduct impulses from the organs to the central nervous system.

Neuropathy may be associated with varying combinations of weakness, autonomic changes and sensory changes. Loss of muscle bulk or fasciculations, a particular fine twitching of muscle may be seen. Sensory symptoms encompass loss of sensation and "positive" phenomena including pain. Symptoms depend on the type of nerves affected; motor, sensory, autonomic, and where the nerves are located in the body. One or more types of nerves may be affected. Common symptoms associated with damage to the motor nerve are muscle weakness, cramps, and spasms. Loss of balance and coordination may also occur. Damage to the sensory nerve can produce tingling, numbness, and pain. Pain associated with this nerve is described in various ways such as the following: sensation of wearing an invisible "glove" or "sock", burning, freezing, or electric-like, extreme sensitivity to touch. The autonomic nerve damage causes problems with involuntary functions leading to symptoms such as abnormal blood pressure and heart rate, reduced ability to perspire, constipation, bladder dysfunction (e.g., incontinence), and sexual dysfunction

The somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch is considered one of the five traditional senses, the impression of touch is formed from several modalities; In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved.

The system reacts to diverse stimuli using different receptors: thermoreceptors, mechanoreceptors and chemoreceptors. Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.

At its simplest, the system works when a sensory neuron is triggered by a specific stimulus such as heat; this neuron passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location. The mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image.

In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons.

The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber.

In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory.

For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in vertebrates is obtained only by inhibition of the motor neuron itself. Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

Sensory Neuron - receptor neurons that receive messages from the external environment and sends it TO the brain for processing.