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Central Nervous System

General Design of the Nervous System

The CNS contains more than 100 billion neurons. For different types of neurons, there may be only a few hundred

or as many as 200,000 such synaptic connections from input fibers.

Conversely, the output signal travels by way of a single axon leaving the neuron.

Then, this axon has many separate branches to other parts of the nervous system or peripheral body.

A special feature of most synapses is that the signal normally passes only in the forward direction (from the axon of a preceding neuron to dendrites on cell membranes of subsequent neurons).

This forces the signal to travel in required directions for performing specific nervous functions.

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Divisions of the Nervous System

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The Central nervous system

• Consists of:Brain. Spinal cord.The CNS:Receives input from sensory neurons. Directs activity of motor neurons. Association neurons (interneurons) maintain

homeostasis in the internal environmentenvironment

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The Spinal Cord

• The spinal cord has two main functions: 1. Common passageway for ascending and descending tracts.

Neurons in the white matter of the spinal cord transmit sensory signals from peripheral regions to the brain and motor signals from the brain to peripheral regions.

2. Center for reflexes. Neurons in the gray matter of the spinal cord integrate incoming sensory information and respond with motor impulses that control muscles (skeletal, smooth, or cardiac) or glands.

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The Spinal Cord cont’d….• The SC is an extension of

the brain stem that begins at the foramen magnum and continues down through the vertebral canal to the L1.

• It is held in position at its inferior end by the filum terminale, an extension of the pia mater that attaches to the coccyx.

• Along its length, the SC is held within the vertebral canal by denticulate ligaments, lateral extensions of the pia mater that attach to the dural sheath.

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The Spinal Cord cont’d….

• Cross section of the SC

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Spinal Cord Tracts

• The spinal cord white matter contains Ascending and Descending tracts

Ascending tracts emerge from the first order (1°) neuron located in the dorsal

root ganglion (DRG). transmit sensory information from the sensory receptors to

higher levels of the CNS. These ascending tracts are:

gracile and cuneate fasciculi occupying the dorsal column, and sometimes are named the dorsal funiculus.

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Spinal Cord Tracts cont’d…. These fibers carry information related to

TactileTwo point discrimination of simultaneously applied pressureVibration sense Position sense

Movement sense

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Spinal Cord Tracts cont’d….• In the ventral column (funiculus) there are four

prominent tracts: 1) the paleospinothalamic tract (or anterior spinothalamic

tract) carry pain, temperature, touch to the brain stem nuclei

and to the diencephalon2) the spinoolivary tract carries information from Golgi

tendon organs to the cerebellum 3) the spinoreticular tract –carries information to the RF4) the spino-tectal tract. Intersegmental nerve fibers

travelling for several segments and are located as a thin layer around the gray matter is known as fasciculus proprius, spinospinal or archispinothalamic tract.

carries pain information to the brain stem and diencephalon.

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Spinal Cord Tracts cont’d….

• In the lateral column (funiculus), the neospinothalamic tract (or lateral spinothalamic tract) is located more anteriorly and laterally, and carries pain, temperature and crude touch information from somatic and visceral structures.

• Nearby laterally, the dorsal and ventral spinocerebellar tracts carry unconscious proprioception information from muscles and joints of the lower extremities to the cerebellum.

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Spinal Cord Tracts cont’d….

• Descending tracts. The descending tracts originate from different cortical areas and from brain stem nuclei.

• The descending pathway carry information associated with maintenance of motor activities such as posture, balance, muscle tone, and visceral and somatic reflex activity.

• These include the lateral corticospinal tract and the rubrospinal tracts located in the lateral column (funiculus).

• These tracts carry information associated with voluntary

movement.

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Sensory Part of the Nervous System— Sensory Receptors

Most activities of the nervous system are initiated by sensory experience exciting sensory receptors.

This sensory experience can either cause

immediate reaction from the brain or the SC memory of the experience can be stored in the brain for

minutes, weeks, or years and determine bodily reactions at some future date.

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Sensory Part cont’d…..• As shown fig. (next slide) the somatic portion of the sensory system

transmitssensory information from the receptors of the entire body surface

and from some deep structures. This information enters the CNS through peripheral nerves and is

conducted immediately to multiple sensory areas in the spinal cord at all levels the reticular substance of the medulla, pons, and

mesencephalon of the brain the cerebellumthe thalamus areas of the cerebral cortex.

14Fig Somatosensory Axis of the Nervous System

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Motor Part of the NS -Effectors

Eventual role of the NS is to control the various bodily activities. This is achieved by controlling contraction of appropriate skeletal muscles throughout the body contraction of smooth muscle in the internal organs secretion of active chemical substances by both exocrine and

endocrine glands in many parts of the body.

Figure on the slide 17 shows the “skeletal” motor nerve axis of NS controlling collectively called motor functions of the NS, the muscles and glands are called effectors.

Operating parallel to this axis is another system, called the

autonomic nervous system, for controlling smooth muscles, glands, and other internal organs.

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Motor Part of the NS cont’d…..Note in Figure that the skeletal muscles can be controlled by the : spinal cord Reticular substance of the medulla, pons, and mesencephalon basal ganglia Cerebellum motor cortex Each of these areas plays its own specific role the lower regions are concerned primarily with automatic,

instantaneous muscle responses to sensory stimuli the higher regions with deliberate complex muscle movements

controlled by the thought processes of the brain.

17Fig Skeletal Motor Nerve Axis of the Nervous System

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Major Levels of CNS Function The human nervous system has inherited special

functional capabilities from each stage of human evolutionary development.

There are three major levels of the CNS having specific functional characteristics:

the spinal cord level the lower brain or subcortical levelthe higher brain or cortical level.

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Major Levels of CNS Function Cont’d….

Spinal Cord Level

We often think of the spinal cord as being only a

conduit for signals from the periphery of the body to

the brain, or in the opposite direction from the brain

back to the body. This is far from the truth.• Neuronal circuits in the cord can cause:

walking movementsreflexes that withdraw portions of the body reflexes that stiffen the legs to support the body against

gravity reflexes that control local blood vessels, gastrointestinal

movements, including defecation reflex or urinary excretion.

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Major Levels of CNS Function Cont’d….Lower Brain or Subcortical Level

Many, if not most, of what we call subconscious activities of the body are controlled in the lower areas of the brain:

medulla ponsmesencephalonHypothalamusthalamuscerebellumBasal ganglia

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Major Levels of CNS Function Cont’d…

For instance, subconscious control of arterial

pressure and respiration is achieved mainly in the medulla and pons

Control of equilibrium is a combined

function of the portions of the cerebellum

and the reticular substance of the medulla,

pons, mesencephalon.

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Major Levels of CNS Function Cont’d…

• Higher Brain or Cortical Level • After the preceding account of the many NS

functions that occur at the cord and lower brain levels, one may ask, what is left for the cerebral cortex to do?

The answer to this is complex, but it begins withthe fact that the cerebral cortex is an extremely largememory storehouse and responsible for many other

intellectual functions. The cortex never functions alone but always inassociation with lower centers of the NS.Without the cerebral cortex, the functions of thelower brain centers are often imprecise.

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Higher Brain or Cortical Level cont’d..The vast storehouse of cortical information usually converts these

functions to determinative and precise operations.

The cerebral cortex is essential for most of our thought processes,

but it cannot function by itself.

It is the lower brain centers, not the cortex, that initiate wakefulness

in the cerebral cortex, thus opening its bank of memories to the

thinking machinery of the brain.

But it is the cortex that opens a world of stored information for use

by the mind.

Thus, each portion of the NS performs specific functions.

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Central Nervous System SynapsesObviously information is transmitted in the NS mainly in the form

of nerve action potentials

In addition, each impulse may be:blocked in its transmission from one neuron to the next, changed from a single impulse into repetitive impulses,integrated with impulses from other neurons to cause

highly intricate patterns of impulses in successive neurons.

All these functions can be classified as synaptic functions of

neurons.

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CNS Synapses Cont’d…

Types of Synapses

There are two major types of synapses: the chemical synapse and the electrical synapse.

Almost all the synapses used for signal transmission in the CNS

of the human being are chemical synapses.

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CNS Synapses cont’d……More than 40 important transmitter substances havebeen discovered thus far. Some of the best known are:

Acetylcholine Norepinephrine Epinephrine Histamine Gamma aminobutyric acid (GABA) Glycine Serotonin, and Glutamate.

Electrical synapses, in contrast, are characterized by direct open fluid channels that conduct electricity from one cell to the next.

This conduction is via the gap junctions that allow free movement of ions from the interior of one cell to the interior of the next.

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Sensory ReceptorsThe are five basic types of sensory receptors: Mechanoreceptors-which detect mechanical compression or

stretching of the receptor or of tissues adjacent to the receptor Thermoreceptors, which detect changes in temperature, some

receptors detecting cold and others warmth

Nociceptors (pain receptors), which detect damage occurring in the tissues

Electromagnetic receptors, which detect light on the retina of the eye

Chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration etc.

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Sensory Receptors cont’d…Differential Sensitivity of Receptors

The first question that must be answered is, how do two types of sensory receptors detect different types of sensory stimuli?

The answer is, by “differential sensitivities.” That is, each type of receptor is highly sensitive to one type of stimulus

Thus, the rods and cones of the eyes are highly responsive to light but are almost completely non-responsive to other stimuli

The osmoreceptors of the supraoptic nuclei in the hypothalamus detect minute changes in the osmolality of the body fluids but have never been known to respond to sound.

Pain receptors in the skin are almost never stimulated by usual touch or pressure stimuli but do become highly active the moment tactile stimuli become severe enough to damage the tissues.

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Sensory Receptors cont’d…Modality of Sensation— The “Labelled Line”

Principle

Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation.

Yet despite the fact that we experience these different modalities of sensation, nerve fibers transmit only impulses.

Therefore, how is it that different nerve fibers transmit different modalities of sensation?

The answer is that each nerve tract terminates at a specific point in the CNS, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the NS to which the fiber leads.

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Sensory Receptors cont’d… For instance, if a pain fiber is stimulated, the person perceives

pain regardless of what type of stimulus excites the fiber.

The stimulus can be electricity, overheating of the fiber, crushing of the fiber, or stimulation of the pain nerve ending by damage to the tissue cells. In all these instances, the person perceives pain.

Likewise, if a touch fiber is stimulated by electrical excitation of a touch receptor or in any other way, the person perceives touch because touch fibers lead to specific touch areas in the brain.

Similarly, fibers from the retina of the eye terminate in the vision areas of the brain, fibers from the ear terminate in the auditory areas of the brain, and temperature fibers terminate in the temperature areas.

• The fact that every sensory information is conducted to definitive areas in the brain is called The “Labelled Line” Principle

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CLASSIFICATION OF SOMATIC SENSES

The somatic senses can be classified into three physiologic types:

the mechanoreceptive somatic senses, which include both tactile and position sensations that are stimulated by mechanical displacement of some tissue of the body

the thermoreceptive senses, which detect heat and cold

the pain sense (nociceptive), which is activated by any factor that damages the tissues.

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Other Classifications of Somatic Sensations Somatic sensations are also often grouped

together in other classes, as follows.Exteroreceptive sensations are those from the surface of

the body.

Proprioceptive sensations are those having to do with the physical state of the body, including

position sensations tendon and muscle sensationspressure sensations from the bottom of the feetthe sensation of equilibrium (which is often

considered a “special” sensation rather than a somatic sensation).

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Other Classifications cont’d…

Visceral sensations are those from the viscera of the body; in using this term, one usually refers specifically to sensations from the internal organs.

Deep sensations are those that come from deep tissues, such as from fasciae, muscles, and bone. These include mainly

o deep pressure, o paino vibration

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Tactile Receptors.There are at least six entirely different types of tactile receptors,

but many more similar to these also exist. 1. Free nerve endings, found everywhere in the skin and in many

other tissues, can detect: touch and pressure

2. Touch receptor called Meissner’s corpuscles, elongated encapsulated nerve endings of a large (type Aß) myelinated sensory nerve fiber

They are rapidly adapting receptors sensitive to movement of objects over the surface of the skin

as well as to low frequency vibration.

3. Merkel’s discs, responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin.

These receptors are slowly adapting type

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Tactile Receptors cont’d….4. Hair end-organ, slight movement of any hair on the body stimulates a

nerve fiber entwining its base. Like Meissner’s corpuscles, detects mainly

movement of objects on the surface of the body initial contact with the body.

5. Ruffini’s end-organs, which are Multi-branched Encapsulated endings Located in the deeper layers of the skin and in deeper internal tissues In joint capsules and help to signal the degree of joint rotation. Adapt very slowly Detect heavy and prolonged touch and pressure signals. 6. Pacinian corpuscles Adapt in a few hundredths of a second. Particularly important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues.

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PainPain Is a Protective Mechanism. Pain occurs

whenever any tissues are being damaged, and it causes the individual to react to remove the pain stimulus.

Even such simple activities as sitting for a long time on the ischia can cause tissue destruction because of lack of BF to the skin where it is compressed by the wt of the body.

When the skin becomes painful as a result of the ischemia, the person normally shifts wt subconsciously.

But a person who has lost the pain sense, as after spinal cord injury, fails to feel the pain and, therefore, fails to shift.

This soon results in total breakdown and desquamation of the skin (pressure sore) at the areas of pressure.

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Types of Pain

Pain has been classified into two

Fast Pain Slow Pain Fast pain is felt within about 0.1 second after a pain stimulus is

applied Slow pain begins only after 1 second or more and then increases

slowly over many seconds and sometimes even minutes. Fast pain is also described by many alternative names, such as sharp

pain, pricking pain, acute pain, and electric pain. Slow pain also goes by many names, such as slow burning pain,

aching pain, throbbing pain, nauseous pain, and chronic pain.

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Pain Receptors and Their Stimulation

The pain receptors in the body tissues are all free nerve endings. They are widespread in:

the superficial layers of the skin

certain internal tissues

the periosteum, the arterial walls

the joint surfaces the falx and tentorium in the cranial vault most other deep tissues are only sparsely supplied with nerve endings No nociceptors in the brain.

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Stimuli Excite Pain Receptors Three Types of Stimuli Excite Pain Receptors o Mechanicalo Thermal o Chemical.

o Fast pain is elicited by the mechanical and thermal types of stimuli o Slow pain can be elicited by all three types.o Some of the chemicals that excite pain receptors include:

bradykininserotoninhistaminepotassium ionsAcidsAcetylcholineproteolytic enzymes prostaglandins substance P

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Dual Pathways for Transmission of Pain Signals into the CNS

• Even though all pain receptors are free nerve endings, these endings use two separate pathways for transmitting pain signals into the CNS.

a fast-sharp pain pathway a slow-chronic pain pathway.

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Pain pathway……

The fast sharp pain signals are elicited by mechanical or thermal pain stimuli transmitted to the spinal cord by small type Aσ fibers at

velocities between 6 and 30 m/sec.

The slow-chronic type of pain is elicited mostly bypersisting mechanical or thermal stimuliis transmitted to the spinal cord by type C fibers at

velocities between 0.5 and 2 m/sec.

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Pain pathway……

Because of the double system of pain innervation, a sudden painful stimulus often gives a “double” pain sensation:

The sharp pain apprises the person rapidly of a damaginginfluence and, plays an important role in making the person reactimmediately to remove himself or herself from the stimulus.

The slow pain tends to become greater over time.

This sensation eventually produces the intolerable suffering oflong continued pain and makes the person keep trying to relievethe cause of the pain.

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Pain pathway……Dual Pain Pathways in the Cord and Brain Stem takes two

pathways: the neospinothalamic tract the paleospinothalamic tract.

Neospinothalamic tract conducts fast pain and terminate mainly in lamina I (lamina marginalis) of the dorsal horns and there excite second-order neurons of the neospinothalamic tract.

These give rise to long fibers that cross immediately to the opposite side of the cord through the anterior commissure and then turn upward, passing to the brain in the anterolateral columns.

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Pain pathway……Capability of the NS to Localize Fast Pain in the Body.

The fast-sharp type of pain can be localized much more exactly in

the different parts of the body than can slow chronic pain.

However, when only pain receptors are stimulated, without the

simultaneous stimulation of tactile receptors, even fast pain may

be poorly localized, often only within 10 cm or so of the stimulated

area.

Yet when tactile receptor that excite the dorsal column–medial

lemniscal system are simultaneously stimulated, the localization

can be nearly exact.

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Pain pathway……

Probable NT of the Type Aσ Fast Pain Fibers.

It is believed that glutamate is a neurotransmitter

substance secreted in the spinal cord at the type

Aσ pain nerve fiber endings.

This is one of the most widely used excitatory

transmitters in the CNS, usually having a duration

of action lasting for only a few milliseconds.

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Pain pathway…… Pathway for Transmitting Slow-Chronic Pain.

The paleospinothalamic pathway transmits pain mainly from the peripheral slow-chronic type C pain fibers, although it does transmit some signals from type Aσ fibers as well.

In this pathway, the peripheral fibers terminate in the spinal cord almost entirely in laminae II and III of the dorsal horns, which together are called the substantia Gelatinosa (the location of the 1st synapse in pain pathway).

Most of the signals then pass thru one or more additional short fiber neurons within the dorsal horns themselves before entering mainly lamina V.

Here the last neurons in the series give rise to long axons that mostly join the fibers from the fast pain pathway, passing first thru the anterior commissure to the opposite side of the cord, then upward to the brain in the anterolateral pathway.

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Pain pathway……Neurotransmitter of Type C Nerve Endings- Substance P

Research experiments suggest that type C pain fiber terminals entering the SC secrete both glutamate transmitter and substance P transmitter.

The glutamate transmitter acts instantaneously and lasts for only a few milliseconds.

Substance P is released much more slowly, building up in concentration over a period of seconds or even minutes.

It has been suggested that the ‘’double” pain sensation one feels after a pinprick is due to the glutamate transmitter giving a faster pain sensation and the substance P transmitter giving a more lagging sensation.

It seems clear that glutamate is the neurotransmitter mostly transmitting fast pain into the CNS

Substance P is concerned with slow-chronic pain.

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Pain Suppression (Analgesia System) in the CNS

The degree to which a person

reacts to pain variestremendously.

This results partly from a

capability of the brain itself to

suppress input of pain signals

to the nervous system by

activating a pain control

system, called ananalgesia system.

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Pain Suppression….The analgesia system consists of three major components: 1. The periaqueductal gray and periventricular areas of the

mesencephalon send signals to

2. The raphe magnus nucleus, a thin midline nucleus located in the lower pons and upper medulla, and the nucleus reticularis paragigantocellularis, located laterally in the medulla.

From these nuclei, second-order signals are transmitted down the

dorsolateral columns in the spinal cord to

3. a pain inhibitory complex located in the dorsal horns of the SC

At this point, the analgesia signals can block the pain before it isrelayed to the brain.

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Pain Suppression….It has been observed that electrical stimulation either in the

periaqueductal grayarea or in the raphe magnus nucleus can suppressmany strong pain signals entering by way of the dorsalspinal roots.

Stimulation of areas at still higher levels of the brain thatexcite the periaqueductal gray area can also suppresspain. Some of these areas are:

1. the periventricular nuclei in the hypothalamus, lying adjacent to the third ventricle

2. to a lesser extent, the medial forebrain bundle

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Gate Control Theory Of Pain

• The gate control theory of pain argues that the sensory messages travel through the body’s pain highway i.e. from the stimulated nerves to the spinal cord.

• The messages undergo reprocessing here and then get transferred to thalamus, the brain’s depot for tactile information.

• The laminae II & III (substantia gelatinosa), the pain sensation is somewhat blocked so that the amount of pain a person feels is lessened.

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Pain neurtrasmitter substances

Several transmitter substances are involved in the analgesia system; especially involved are enkephalin and serotonin.

Many nerve fibers derived from the periventricular nuclei

and from the periaqueductal gray area secrete enkephalin at their endings.

The endings of many fibers in the raphe magnus nucleus release enkephalin when stimulated.

Fibers originating in this area send signals to the dorsal horns of

the spinal cord to secrete serotonin at their endings. serotonin →

local cord neurons to secrete enkephalin.

The enkephalin is believed to cause both presynaptic and

postsynaptic inhibition of incoming type C and type Aσ pain fibers where they synapse in the dorsal horns.

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Pain NT Substances

Brain’s Opiate SystemEndorphins andenkephalins are involved

About a dozen of opiate-like substances have nowbeen found at different points of the nervous system; allare breakdown products of three large proteinmolecules:

proopiomelanocortin,Proenkephalinprodynorphin.

Among the more important of these opiate-likesubstances are

ß-endorphin, met-enkephalin,leuenkephalin,dynorphin.

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Trigeminal Neuralgia

Trigeminal neuralgia causes facial pain.  Develops in mid to late life. Characterized by feeling like bursts of sharp, stabbing, electric-

shocks. Can last from a few seconds to a few minutes

Interferes with common daily activities such as eating and sleep.

Leads to irritability, severe anticipatory anxiety and depression, and life-threatening malnutrition.

Often called "tic douloureux" because of a characteristic muscle spasm that accompanies the pain.

In almost all cases (97%), pain will be restricted to one side of the face.

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What Is Tabes Dorsalis?• Tabes dorsalis is a slow degeneration of the covering of nerve

cells and nerve fibers that carry sensory information to the brain. The degenerating nerves are in the dorsal column of the spinal cord (the portion closest to the back of the body) and carry information that help maintain a person's sense of position. Tabes dorsalis results when a syphilis infection goes untreated.

• Treatment The treatment of choice for tabes dorsalis is antibiotics. The drug

that is normally recommended is penicillin, given intravenously.

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Motor Functions of the Spinal Cord -the Cord Reflexes

Sensory info is integrated at all levels of the NS → appropriate motor responses that begin in the SC with relatively simple

muscle reflexes → the brain stem with more complicated responses, and finally extend to the cerebrum, where the most complicated muscle skills are controlled.

Without the special neuronal circuits of the cord, even the most complex motor control systems in the brain could not cause any purposeful muscle movement.

To give an example, there is no neuronal circuit anywhere in the brain that causes the specific to-and-fro movement of the legs that is required in walking.

Instead, the circuits for these movements are in the cord, and the brain simply sends command signals to the spinal cord to set into motion the walking process.

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Motor Functions of the Spinal Cord cont’d… However, the role of the brain in the control of movement can not

be undermined. The brain gives directions that control the sequential cord

activities:to promote turning movements when they are requiredto lean the body forward during accelerationto change the movements from walking to jumping to monitor continuously and control equilibrium.

All this is done through “analytical” and “command” signals generated in the brain.

But it also requires the many neuronal circuits of the spinal cord that are the objects of the commands.

These circuits provide all but a small fraction of the direct control of the muscles.

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Organization of the Spinal Cord for Motor fxns

The cord gray matter

is the integrative area

for the cord reflexes.

Figure shows the

Typical organization of the cord gray

matter in a single

cord segment.

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Organization of the SC for Motor fxns…

Anterior Motor Neurons. Located in each segment of the anterior horns of the cord gray matter are several thousand neurons that are 50 to 100% larger than most of the others and are called anterior motor neurons.

They give rise to the nerve fibers that leave the cord by way of the anterior roots and directly innervate the skeletal muscle fibers.

The neurons are of two typesalpha motor neurons and gamma motor neurons.

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Organization of the SC for Motor fxns… Interneurons. Interneurons are present in all areas of the cord gray matter in the Dorsal horns Anterior horns Intermediate areas between them

Are small and highly excitable, often exhibiting spontaneous activity and capable of firing as rapidly as 1500/s

Have many interconnections with one another, and many of them also synapse directly with the anterior motor neurons

Interneurons and anterior motor neurons are responsible for most of the integrative functions of the SC

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Alpha Motor Neurons- give rise to large type A alpha (Aα) motor

nerve fibers, averaging 14 micrometers in diameter and branch several times after they enter the Muscle and are collectively called the motor unit.

The alpha motor neurons innervate extrafusal muscle fibers

Gamma Motor Neurons- The gamma motor neurons are located in the spinal cord anterior horns, transmit impulses through

much smaller type A gamma (Aγ) motor nerve fibers, which go to small, special skeletal muscle fibers called intrafusal fibers

See next fig.

Organization of the SC for Motor fxns…

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Organization of the SC for Motor fxns…

Fig

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Organization of the SC for Motor fxns…

Renshaw Cell Inhibitory System.

Renshaw cells are located in the anterior horns of the spinal cord,

in close association with the motor neurons.

These cells transmit inhibitory signals to the surrounding motor neurons, which is known as lateral inhibition.

It was found out that the Renshaw cells contain glycin and GABA receptors.

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Organization of the SC for Motor fxns…

Collateral inhibition effect is important for the

following major reason:

The motor system uses this lateral inhibition to focus, or sharpen, its signals in the same way that the sensory system uses the same principle—that is, to allow unabated transmission of the primary signal in the desired direction while suppressing the tendency for signals to spread laterally.

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Muscle Sensory Receptors

Proper control of muscle function requires not only excitation of the muscle anterior motor neurons but also continuous feedback of sensory information from each muscle to the SC, indicating the functional status of each muscle at each instant.

To provide this information, the muscles and their tendons aresupplied abundantly with two special types of sensory receptors: Muscle spindles found in the belly of the muscle and send

information to the NS about muscle length or rate of change of length, and

Golgi tendon organs which are located in the muscle tendons and transmit information about tendon tension or rate of change of tension.

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Muscle Sensory Receptors…......

The signals from these two receptors are either entirely or almost entirely for the purpose of intrinsic muscle control. Operate almost completely at a subconscious level.

Transmit tremendous amounts of information to:the spinal cord the cerebellum even to the cerebral cortex, helping each of these

portions of the NS function to control muscle contraction.

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Receptor Function of the Muscle Spindle Structure and Motor Innervation of the Muscle Spindle.

Skeletal muscle contains two types of muscle fibers:Small intrafusal skeletal muscle fibers Large extrafusal skeletal muscle fibers.

The area midway between two ends of intrafusal fibers has few or no actin and myosin filaments.

Hence does not contract

Instead, it functions as a sensory receptor, as described later. These portions are excited by small gamma motor nerve fibers also called gamma efferent fibers

The large alpha efferent fibers (type A alpha nerve fibers) innervate the extrafusal skeletal muscle.

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Receptor Function of the Muscle…..

Sensory Innervation of the Muscle Spindle. The receptor portion of the muscle spindle is its central portion.They are stimulated by stretching of this mid portion of the spindleand are excited in two ways:1. Lengthening the whole muscle stretches excites the mid

portion of the spindle 2. Even if the length of the entire muscle does not change,

contraction of the end portions of the spindle’s intrafusal fibers stretches the mid portion of the spindle and therefore excites the receptor.

Two types of sensory endings are found in this central receptor area of the muscle spindle.

the primary ending and the secondary ending. Primary Ending. transmits sensory signals to the spinal cord Secondary Ending. Innervates receptor region on one or both

sides of the primary ending.

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Receptor Function of the Muscle…..

Division of the Intrafusal Fibers There are also two types of muscle spindle intrafusal fibers: 1. Nuclear bag muscle fibers (one to three in each spindle), in

which several muscle fiber nuclei are congregated in expanded “bags” in the central portion of the receptor area,

2. Nuclear chain fibers (three to nine), which are about half as large in diameter and half as long as the nuclear bag fibers and have nuclei aligned in a chain throughout the receptor area

(See figure on the next slide)The primary sensory nerve ending is excited by both the nuclear

bag intrafusal fibers and the nuclear chain fibers.

Conversely, the secondary ending is usually excited only by nuclear chain fibers.

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Response of Both the 1º and the 2º Endings to the Length of the Receptor

Static Response Vs Dynamic Response When the receptor portion of the muscle spindle is stretchedslowly, the No of impulses transmitted from both the 1º and the 2º endings ↑almost directly in proportion to the degree of stretchingand the endings continue to transmit these impulses for severalminutes.

This effect is called the static response of the spindle receptor,meaning simply that both the 1º and 2º endings continue totransmit their signals for so long.

When the length of the spindle receptor ↑ suddenly, the 1º ending(not the 2º ending) is stimulated, this excess stimulus of the 1º

ending is called the dynamic response

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Muscle Stretch Reflex• The simplest

manifestation of muscle spindle function is the muscle stretch reflex (monosynaptic)

• Whenever a muscle is stretched suddenly, excitation of the spindles causes reflex contraction of the large skeletal muscle fibers of the stretched muscle and also of closely allied synergistic muscles.

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Neuronal Circuitry of the Stretch Reflex.

Figure on the previous slide demonstrates the basic circuit of the muscle spindle stretch reflex.

Figure shows a type Ia proprioceptor nerve fiberoriginating in a muscle spindleand entering a dorsal root of the spinal cord.

A branch of this fiber then goes directly to the AH of the cord gray matter and synapses with these neurons

AH neurons send motor nerve fibers back to the same muscle from which the muscle spindle fiber originated.

Thus, this is a monosynaptic pathway that allows a reflex signal to return with the shortest possible time delay back to the muscle after excitation of the spindle.

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Flexor Reflex (Withdrawal Reflex)A sensory stimulus from a limb

→cause the flexor muscles of

the limb to contract

→withdrawing the limb from the stimulating object.

This is called the flexor reflex.

Most powerfully by stimulation of pain endings, such as by

a pinprick, heat, or a wound, for which reason it is also called a nociceptive reflex, or simply a pain reflex.

Fig

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• Neuronal Mechanism of the Flexor Reflex.Figure on slide 75 shows the neuronal pathways for the

flexor reflex. A painful stimulus is applied to the hand → excitation of the

flexor muscles of the upper arm → withdrawing the hand from the painful stimulus.

The pathways for eliciting the flexor reflex pass to the spinal cord interneuron pool of neurons then to the motor neurons.

Most of the signals of the reflex traverse many more neurons and involve the following basic types of circuits:

Diverging circuits to spread the reflex to the necessary muscles for withdrawal

Circuits to inhibit the antagonist muscles, called reciprocal inhibition circuits

Circuits to cause after discharge lasting many fractions of a second after the stimulus is over

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Knee Jerk Reflex

• Clinically, a method used to determine the sensitivity of the stretch reflexes

The knee jerk can be elicited by simply striking the patellar tendon with a reflex hammer; this stretches the quadriceps muscle and excites a dynamic stretch reflex that causes the lower leg to “jerk” forward.

A sudden stretch of muscle spindles is all that is

required to elicit a dynamic stretch reflex.

The muscle jerks are used by neurologists to assess

the degree of facilitation of spinal cord centers.

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