Neurology - Spinal Cord Physiology

Conscious proprioception

the awareness of the positions and movements of muscles, tendons and joints

Posterior column -What 2 tracts is it made up of? -What nerve impulses does it convey?

*gracile ("slender") fasciculus* *cuneate ("wedge shaped") fasciculus* Conveys nerve impulses for sensing touch, light pressure, vibration and conscious proprioception

Trigger zone -what is it? -where is it usually found? -what does movement of the area cause?

-area that causes an attack of neuralgia when stimulated -usually it's an area of skin supplied by the nerve -movement of the area increases pain (i.e. when an area of skin is stimulated that is innervated by the nerve in question, an attack of sx results)

Neuritis -what is it? -sx?

-inflammation of the nerve *-mainly the sheath and connective tissue are affected (usually the axon is not) * -constant dull pain -can also get numbness and tingling

Autonomic (visceral) reflex

-involve responses of smooth muscle, cardiac muscle, and glands. -generally not perceived -Ex; heart rate, digestion, urination, and defecation

Neuralgia -what is it? -associated pathology? -commonly affected nerves?

-nerve pain: recurrent attack of sudden excruciating pain along distribution of the nerve -no associated pathology -has a trigger zone -commonly affected are trigeminal and intercostal nerves

What is the stretch reflex good for in everyday life?

Helps to maintain posture. For example, if a standing person begins to lean forward, the gastrocs and other calf muscles are stretched. Consequently, the stretch reflexes are initiated in these muscles, which cause them to contract and reestablish the body's upright posture.

Propagation takes place in what matter? Integration takes place in what matter?

-More White -More Grey

2 main functions of a spinal cord:

1. Propagates nerve impulses (sensory/afferent & motor/efferent) 2. Integration (interneurons)

The 5 functional components of a reflex arc:

1. Sensory receptor 2. Sensory neuron 3. Integrating center 4. Motor neuron 5. Effector neuron

List the 4 Somatic Spinal Reflexes:

1. Stretch reflex 2. Tendon reflex 3. Flexor (Withdrawal) reflex 4. Crossed Extensor reflex

Indirect Pathway

Convey nerve impulses from the brain stem to cause automatic movements and help coordinate body movements with visual stimuli. They also maintain skeletal muscle tone, sustain contraction of postural muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head.

-More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. -Involves more than two types of neurons and more than one CNS synapse -Ex; hand burned by flame:

Polysynaptic

Name the direct pathways (3)

-lateral corticospinal tract -anterior corticospinal tract -corticobulbar tract

4 types of Reflex:

Spinal reflex Cranial reflex Somatic reflex Autonomic reflex

Name the indirect pathways (5)

- rubrospinal tract - tectospinal tract - vestibulospinal tract - lateral reticulospinal tract - medial reticulospinal tract

Crossed Extensor Reflex - key points to memorize

- when you step on a tack and withdraw or flex your leg, the other leg needs to extend so you don't fall over - works in coordination with *Flexor (withdrawal) Reflex* arc - *contralateral* - intersegmental reflex arc - reciprocal innervation

Example of inborn reflex: Example of learned reflex:

-Pulling hand away from heat -Slamming on brakes in an emergency while driving

Causes of neuralgia

-diabetes -post-herpes zoster (shingles)

Causes of Neuritis

-secondary to a pathology (leprosy, TB) -trauma to nerve -chronic exposure (to a toxin like lead, drugs or alcohol)

which has a trigger zone, neuritis or neuralgia?

neuralgia

what nerves are typical with neuralgia?

trigeminal & intercostal nerves

2 types of integration center

Monosynaptic Polysynaptic

a change in the internal or external environment:

Stimulus

Why are reflexes important?

-They protect us -Because reflexes are normally so predictable, they provide useful information about the health of the nervous system and can greatly aid diagnosis of disease

Flexor (Withdrawal) Reflex

-found in the* feet* -protective reflex moves the limb away from a possibly damaging stimulus -*intersegmental reflex arc * -*polysynaptic* -*ipsilateral* -*reciprocal innervation*

Somatic reflex

-results in contraction of skeletal muscle -Ex; conscious movement

Cranial reflex

-when integration happens in brain stem. -Ex; tracking movements of your eyes as you read

Spinal reflex

-when integration happens in grey matter of spinal cord. -Ex; patellar reflex (knee jerk)

Reciprocal innervation during the stretch reflex

Although the stretch reflex pathway itself is monosynaptic, a polysynaptic reflex arc to the antagonist muscles operates at the same time. It involves 3 neurons and 2 synapses. An axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center... the interneuron synapses with and inhibits the motor neuron that normally excites the antagonistic muscles. Thus, when the stretched muscle contracts during stretch reflex, antagonistic muscles that oppose the contraction relax.

Reciprocal innervation

An arrangement in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists. It prevents conflict between opposing muscles and is vital in coordinating body movements.

How the tendon reflex works (FYI don't need to memorize this level of detail

As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold). Nerve impulses arise and propagate into the spinal cord along a sensory neuron. Within the spinal cord (integrating center) the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses. The muscle relaxes and relieves excess tension. Thus, as tension on the tendon organ increases, the frequency of inhibitory impulses increases, inhibition of the motor neurons to the muscle developing excess tension (effector) causes relaxation of the muscle. This protects the tendon and muscle from damage due to excessive tension. The sensory neuron from the tendon organ also synapses with an excitatory interneuron in the spinal cord. This excitatory interneuron synapses with motor neurons controlling antagonistic muscles, triggering contraction of antagnoist muscles (*reciprocal innervation*).

Stretch reflex

Causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via *monosynaptic reflex arc*. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee and ankle joints. Example = patellar reflex (knee jerk) -stimulated by *muscle spindles* -*ipsilateral* -reciprocal innervation; *relaxation* of antagonist -NOTE- the initial stretch causes a contraction in the stretched muscle. It helps avert injury by preventing overstretching of muscles. - It operates as a feedback mechanism to control muscle *length* by causing muscle contraction.

Direct Pathways

Convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles.

Spinothalamic tract -What nerve impulses does it convey?

Conveys nerve impulses for sensing pain, temperature, itch, tickle and deep pressure

an area of skin that provides sensory input to the CNS via the posterior roots of one pair of spinal nerves or via the trigeminal (V) nerve

Dermatomes

-starts with chicken pox -*Varicella Zoster Virus (Herpes)* is the virus that causes chicken pox in children -after recovery from chicken pox in childhood, the virus is not eliminated from the body but lies dormant within the *sensory ganglia* of cranial or spinal nerves and can become activated later in life -years later, when the immune system is depressed, the virus reactivates *-usually only 1 nerve root affected (one side of body)*

Herpes Zoster/ Post-Herpetic Neuralgia (Shingles)

Motor neuron

Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.

Muscle tone

In addition to large-diameter motor neurons that innervate typical skeletal muscle fibers, smaller-diameter motor neurons innervate smaller, specialized muscle fibers within the muscle spindles themselves. The brain regulates muscle spindle sensitivity through pathways to these smaller motor neurons. This regulation ensures proper muscle spindle signalling over a wide range of muscle lengths during both voluntary and reflex contractions. By adjusting how vigorously the muscle spindle responds to stretching, the brain sets an overall level of *muscle tone, which is the small degree of contraction present while the muscle is at rest.*

This condition affects intercostal nerves that travel between the internal and innermost intercostal muscles

Intercostal Neuralgia

What does the sensory nervous system do?

Keeps the CNS informed of changes in the external and internal environments. The sensory information is integrated (processed) by interneurons in the spinal cord and brain. Responses to the integrative decisions are brought about by motor activities (muscular contractions and glandular secretions).

-In the simplest type of reflex -The integrating center is a single synapse between a sensory neuron and a motor neuron. -Ex; patellar tendon (spinal) reflex:

Monosynaptic

What are the two categories of pathway via which motor output travels?

Motor output travels down the spinal cord in two types of descending pathways: Direct pathways Indirect pathways

Nerve impulses from sensory receptors.. -What do they do? -What are the two main routes called?

Nerve impulses from *sensory receptors* propagate *up the spinal cord* to the brain along two main routes on each side: 1. Spinothalamic tract 2. Posterior column

Reflex Arc

Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination. The pathway followed by nerve impulses that produce a reflex is a *reflex arc* (*reflex circuit*).

is the axon affected with neuritis?

Not usually mainly the sheath and connective tissue are affected

Integrating center

One or more regions of gray matter within the CNS act as an integrating center. In the simplest type of reflex, the integrating center has a single synapse (directly between a sensory and motor neuron = *monosynaptic reflex arc*). More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron (*polysnaptic reflex arc* involves more than one type of neuron and more than one CNS synapse)

Tendon reflex

Operates as a feedback mechanism to control muscle *tension* by causing muscle relaxation before muscle force becomes so great that tendons might be torn. It is less sensitive than the stretch reflex but it can override the stretch reflex when tension is great (e.g. dropping a heavy weight). It is *ipsilateral*. The sensory receptors are the Golge tendon organs, which lie within a tendon near its junction with a muscle. In contrast to the muscle spindles which are sensitive to changes in muscle length, GTOs detect and respond to changes in muscle tension that are caused by passive stretch or muscle contraction. -Relaxation of muscle when too much *tension* -*inhibitory interneuron* that synapses with a motor neuron -*ipsilateral* -*polysynaptic* -stimulates *Golgi tendon organs (GTO)* -reciprocal innervation; *contraction of antagonist*

How the stretch reflex works (FYI don't need to memorize this level of detail

Slight stretch of muscle stimulates sensory receptors in the muscle called *muscle spindles* that monitor changes in the length of the muscle). In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve into the spinal cord. In the integrating center of the spinal cord, the sensory neuron makes an excitatory synapse with and thereby activates a motor neuron in the anterior gray horn. If the excitiation is strong enough, one or more nerve impulses arise in the motor neuron and propagate along its root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form NMJs with skeletal muscle fibers of the stretched muscle. Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector) and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves stretching.

Flexor (Withdrawal) Reflex - how it works (FIY)

Stepping on a tack, for example, stimulates dendrites (sensory receptor) of a pain-sensitive neuron. This sensory neuron then generates nerve impulses, which propagate into the spinal cord. Within the spinal cord (integrating center) the sensory neuron activates interneurons that extend to several spinal cord segments. The interneurons activate motor neurons in several spinal cord segments. As a result, the motor neurons generate nerve impulses, which propagate towards the axon terminals. Acetylcholine released by the motor neurons cause the flexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg. This reflex is protective because contraction of flexor muscles moves a limb away from the source of a possibly damaging stimulus. It is ipsilateral and polysynaptic. A feature of polysynaptic reflex arc is that several muscle groups are stimulated (needed for moving an entire limb). Because nerve impulses from one sensory neuron ascend and descend the spinal cord and activate interneurons in several segments of the spinal cord, this type of reflex is called an *intersegmental reflex arc*.

Which part of the brain plays a big role in controlling precise voluntary muscular movements?

The cerebral cortex (the outer part of the brain). (other parts of the brain provide integration for regulation of automatic movements).

Sensory receptor

The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a specific *stimulus* (i.e. a change in the internal or external environment) by producing a graded potential called a generator (or receptor) potential. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron.

Names of spinal tracts Sensory vs motor

The name of a tract in spinal cord indicates its position in the white matter and where it begins and ends e.g. anterior corticospinal tract is located in the *anterior* white column, it begins in the *cerebral cortex* (superficial gray matter of the cerebrum of the brain) and ends in the *spinal cord*. It conveys impulses from the brain towards the spinal cord, it is a *motor* or *descending* tract.

Sensory neuron

The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem. From here, relay neurons send nerve impulses to the area of the brain that allows conscious awareness that the reflex has occurred.

Crossed extensor reflex

The pain impulses from, for example, stepping on a tack, also initiate a *crossed extensor reflex* to help maintain balance when one foot is hurt. Stepping on a tack stimulates the sensory receptor of pain-sensitive neuron in the right foot. This sensory neuron then generates nerve impulses, which propagate into the spinal cord. Within the spinal cord (integrating center) the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord. The interneurons excite motor neurons on several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals. Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. It is a contralateral reflex arc... sensory impulses enter one side of the spinal cord and motor impulses exit on the opposite side.

Effector

The part of the body that responds to the motor nerve impulse, such as a muscle or gland. Its action is called a *reflex*. If the effector is skeletal muscle, the reflex is a *somatic reflex*. If the effector is smooth muscle, cardiac muscle or gland, the reflex is an *autonomic reflex*.

White matter function in spinal cord Gray matter function in spinal cord

White matter tracts in the spinal cord are highways for nerve impulse propagation (sensory input travels along these tracts towards the brain and motor output travels from the brain along these tracts towards skeletal muscles and other effector tissues). The gray matter of the spinal cord receives and integrates incoming and outgoing information.

In a stretch reflex, does the brain receive a signal as well?

Yes, to know what's going on. Thats why you feel it but after. Axon collaterals of the muscle spindle sensory neuron also relay nerve impulses to the brain over specific ascending pathways.

Reflex

a fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus. Can be inborn or learned

Intersegmental reflex arc

nerve impulses from one sensory neuron that ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord. Allows a single sensory neuron to activate several motor neurons, thereby stimulating more than one effector (as contrasted to the monosynaptic stretch reflex which involves muscles receiving nerve impulses from one spinal cord segment only).

Is the *reciprocal innervation* that occurs during a *stretch reflex* monosynaptic or polysynaptic?

polysynaptic