Spinal Cord Physiology

Spinal Cord: Principal Functions

The spinal cord has two principal functions in maintaining homeostasis: nerve impulse propagation and integration of information.

Spinal Cord: Nerve Impulse Propagation

The white matter tracts of the spinal cord carry nerve impulses between the brain and the rest of the body.

Spinal Cord: Information Integration

The gray matter of the spinal cord receives and integrates incoming sensory information and outgoing motor information.

Spinal Tracts: Nerve Impulse Propagation

One of the ways the spinal cord promotes homeostasis is through the movement of an action potential down a neuron's axon

Spinal Tracts: Sensory Direction

Sensory information travels upward through the spinal cord toward the brain.

Spinal Tracts: Motor Direction

Motor information travels downward from the brain through the spinal cord toward skeletal muscles and other effectors.

Spinal Tracts: Naming

A tract's name often indicates its location in the spinal cord and where the tract begins and ends; the location where a tract begins is named first, while the location where its axon terminals end is named last.

Sensory Tracts: Main Routes

Sensory nerve impulses travel toward the brain through two main routes on each side of the spinal cord: the spinothalamic tract and posterior funiculi.

Sensory Tracts: Spinothalamic Tract

The spinothalamic tract carries sensory information for pain, temperature, itch, and tickle.

Sensory Tracts: Posterior Funiculi

The posterior funiculi carry sensory information for touch, pressure, vibration, and conscious proprioception.

Sensory Systems

Sensory systems keep the CNS informed about changes in the external and internal environments.

Sensory Integration

Sensory integration is the processing of sensory information by interneurons in the spinal cord and brain.

Motor Response

Motor responses to integrated sensory information include muscle contractions and glandular secretions.

Cerebral Cortex: Voluntary Movement

The cerebral cortex plays a major role in controlling precise voluntary movements of skeletal muscles.

Brainstem and Other Regions: Automatic Movement

Other brain regions help integrate information needed to regulate automatic movements.

Motor Pathways: Main Types

Motor output to skeletal muscles travels through two types of descending pathways: direct pathways and indirect pathways.

Motor Pathways: Direct Motor Pathways

Direct motor pathways, also called pyramidal pathways, carry nerve impulses from the cerebral cortex that produce voluntary skeletal muscle movements via the lateral corticospinal, anterior corticospinal, and corticobulbar tracts.

Motor Pathways: Indirect Motor Pathways: Definition

Indirect motor pathways, also called extrapyramidal pathways, carry signals from the brainstem that regulate automatic movements and coordination via the rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts

Indirect Motor Pathways: Other Functions

Indirect pathways also help maintain skeletal muscle tone, sustain postural muscle contraction, and maintain equilibrium by regulating muscle tone in response to head movements.

Spinal Cord: Integration of Information.

One of the ways the spinal cord promotes homeostasis is by serving as an integrating center for some reflexes

Reflex

A fast, involuntary, unplanned sequence of actions that occurs in response to a specific stimulus, which could either be inborn or learned

Spinal Reflex

A reflex whose integration occurs in the gray matter of the spinal cord (e.g., patellar reflex)

Cranial Reflex

A reflex whose integration occurs in the brainstem (e.g., eye-tracking movements while reading)

Somatic Reflex

A reflex involving contraction of skeletal muscles.

Autonomic Reflex

An autonomic (visceral) reflex involving smooth muscle, cardiac muscle, or glands (e.g., Heart rate, digestion, urination, and defecation)

Reflex Arc

The specific pathway followed by nerve impulses that produce a reflex.

Reflex Arc: Five Components

The five functional components of a reflex arc are a sensory receptor, sensory neuron, integrating center, motor neuron, and effector.

Reflex Arc: Step 1

The sensory receptor detects a specific stimulus and produces a graded potential called a generator (receptor) potential that will trigger nerve impulses when it reaches the threshold level of depolarization

Reflex Arc: Step 2

Sensory neurons carry nerve impulses from the sensory receptor to the gray matter of the spinal cord or brainstem to allow conscious awareness that the reflex has occurred

Reflex Arc: Step 3

The integrating center consists of one or more regions of gray matter in the CNS where incoming information is processed and motor responses are determined.

Reflex Arc: Step 3: Monosynaptic Reflex Arc

A monosynaptic reflex arc has only one synapse in the CNS, directly connecting a sensory neuron with a motor neuron.

Reflex Arc: Step 3: Polysynaptic Reflex Arc

A polysynaptic reflex arc contains more than one CNS synapse and usually includes one or more interneurons.

Reflex Arc: Step 4

The motor neuron carries nerve impulses from the integrating center out of the CNS to the responding part of the body (effector)

Reflex Arc: Step 5

The effector is the muscle or gland that responds to the motor nerve impulse.

Reflex Arc: Step 5: Somatic Reflex

If the effector is skeletal muscle, the response is a somatic reflex.

Reflex Arc: Step 5: Autonomic Reflex

If the effector is smooth muscle, cardiac muscle, or a gland, the response is an autonomic reflex.

Reflex Arc: Sequence

Sensory receptor → sensory neuron → integrating center → motor neuron → effector.

Reflexes: Health

Reflexes provide information about the health of the nervous system because they are normally so predictable that any damage or disease anywhere is shown.

Stretch Reflex

A reflex that causes a skeletal muscle to contract in response to stretching of that muscle via a monosynaptic reflex arc, meaning it involves one synapse between a sensory neuron and a motor neuron.

Stretch Reflex: Step 1

Slight stretching of a muscle stimulates sensory receptors called muscle spindles that detect changes in muscle length.

Stretch Reflex: Step 2

The stretched muscle spindle generates nerve impulses that travel along a somatic sensory neuron through the posterior root and into the spinal cord.

Stretch Reflex: Step 3

In the spinal cord (integrating center), the sensory neuron forms an excitatory synapse directly with a motor neuron in the anterior gray horn.

Stretch Reflex: Step 4

If the excitation reaches threshold, the motor neuron generates nerve impulses that travel through the anterior root and peripheral nerve to the stretched muscle.

Stretch Reflex: Step 5

Acetylcholine release triggers muscle action potentials, causing the stretched muscle to contract, relieving the stretching and preventing excessive stretching that could cause injury.

Stretch Reflex: Overall Sequence

Muscle stretch → muscle spindle → sensory neuron → spinal cord → motor neuron → acetylcholine release → skeletal muscle contraction.

Stretch Reflex: Ipsilateral Reflex

A reflex in which sensory nerve impulses enter the spinal cord on the same side that motor impulses leave it; all monosynaptic reflexes are ipsilateral.

Stretch Reflex: Large vs Small Motor Neurons

Large-diameter motor neurons activate typical skeletal muscle fibers, while smaller motor neurons regulate specialized muscle fibers within muscle spindles.

Muscle Spindle Motor Neurons

Smaller motor neurons regulate the sensitivity of muscle spindles to stretching over a wide range of muscle lengths during voluntary and reflex contractions

Muscle Tone

A small degree of skeletal muscle contraction is normally present at rest, as the brain adjusts muscle spindle sensitivity to maintain an appropriate muscle tone.

Muscle Tone: Injury Prevention

Stretch reflexes help prevent muscle injury by limiting excessive stretching.

Reciprocal Innervation

A neural process where the activation of an agonist muscle is coordinated with the simultaneous inhibition and relaxation of its antagonistic pair

Reciprocal Innervation: Stretch Reflex

The muscle spindle sensory neuron sends an axon collateral to an inhibitory interneuron, which inhibits the motor neuron supplying the antagonistic muscle that prevents it from contracting while the stretched muscles contracts

Reciprocal Innervation: Purpose

Reciprocal innervation prevents opposing muscles from contracting against each other and helps coordinate movement.

Stretch Reflex: Polysynaptic Component

A polysynaptic reflex involving an inhibitory interneuron occurs simultaneously with the monosynaptic stretch reflex, containing three neurons (a sensory neuron, an inhibitory interneuron, and a motor neuron) and two synapses

Stretch Reflex: Brain Input

Axon collaterals from the muscle spindle sensory neuron relay the message to the brain through ascending pathways to provide conscious awareness of the stretch or contraction and coordinate muscular movement

Stretch Reflex: Posture

The stretch reflex helps maintain posture by causing stretched postural muscles to contract, maintaining an upright posture.

Stretch Reflex

Stretch reflex operates as a feedback mechanism to control muscle length by causing muscle contraction

Tendon Reflex

A negative feedback mechanism that controls increasing muscle tension by causing muscle relaxation before tension becomes so great it may damage the muscle.

Tendon Organs

The sensory receptors for the tendon reflex lie at the junction where skeletal muscles meet tendons and detect changes in muscle tension caused by passive stretch or muscular contraction.

Tendon Reflex: Step 1

When tension is applied to the tendon, a tendon organ is stimulated and depolarized to threshold.

Tendon Reflex: Step 2

Nerves impulses propagate along the sensory neuron into the spinal cord

Tendon Reflex: Step 3

In the spinal cord, the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron

Tendon Reflex: Step 4

The inhibitory interneuron releases an inhibitory neurotransmitter that hyperpolarizes the motor neuron, causing it to generate fewer nerve impulses.

Tendon Reflex: Step 5

Reduced motor neuron activity causes the muscle to relax, relieving the excessive tension on the tendon.

Tendon Reflex: Inhibitory Pathway

Tendon organ → sensory neuron → inhibitory interneuron → motor neuron inhibited → muscle relaxes.

Tendon Reflex: Protection

As tension on a tendon organ increases, the frequency of inhibitory nerve impulses increases, inhibiting the motor neurons controlling the muscle and causing the muscle to relax, protecting the tendon and muscle from damage caused by excessive tension.

Flexor Reflex: Definition

The flexor reflex, or withdrawal reflex, is a polysynaptic reflex that causes a limb to quickly move away from a painful or potentially damaging stimulus.

Flexor Reflex: Step 1

Stepping on a tack stimulates the dendrites of a pain-sensitive sensory neuron.

Flexor Reflex: Step 2

The sensory neuron generates nerve impulses that travel into the spinal cord.

Flexor Reflex: Step 3

In the spinal cord, the sensory neuron activates interneurons that extend across several spinal cord segments.

Flexor Reflex: Step 4

The interneurons activate motor neurons in several spinal cord segments, causing them to generate nerve impulses that propagate toward the axon terminals

Flexor Reflex: Step 5

Motor neurons release acetylcholine at neuromuscular junctions, causing the flexor muscles in the thigh to contract, withdrawing the leg away from the source of the stimulus

Flexor Reflex: Ipsilateral Nature

The flexor reflex is ipsilateral because sensory input and motor output occur on the same side of the spinal cord.

Flexor Reflex: Polysynaptic Nature

The flexor reflex is polysynaptic because it involves multiple interneurons and more than one synapse in the CNS.

Intersegmental Reflex Arc

A reflex pathway in which a sensory neuron activates multiple interneurons across several spinal cord segments.

Intersegmental Reflex Arc: Motor Response

Because several motor neurons are activated at the same time, multiple muscle groups can contract simultaneously.

Flexor vs. Stretch Reflex: Main Difference

The flexor reflex uses multiple spinal cord segments and several motor neurons, while the stretch reflex uses a single spinal cord segment and a monosynaptic pathway.

Crossed Extensor Reflex

A polysynaptic reflex where a painful stimulus on one side of the body causes the opposite limb to straighten and support body weight, helping maintain balance.

Crossed Extensor Reflex: Step 1

Stepping on a tack stimulates a pain-sensitive sensory receptor in the right foot.

Crossed Extensor Reflex: Step 2

The sensory neuron generates nerve impulses that travel into the spinal cord.

Crossed Extensor Reflex: Step 3

In the spinal cord, the sensory neuron activates several interneurons that cross to the opposite side and synapse with motor neurons in several spinal cord segments.

Crossed Extensor Reflex: Step 4

Interneurons excite motor neurons that control extensor muscles in the opposite limb, which generate more nerve impulses that propagate toward the axon terminals

Crossed Extensor Reflex: Step 5

Motor neurons release acetylcholine at neuromuscular junctions, causing extensor muscles in the opposite limb to contract and support the body's weight

Crossed Extensor Reflex: Balance

The crossed extensor reflex helps maintain balance by shifting body weight onto the opposite limb when the stimulated limb is withdrawn.

Crossed Extensor Reflex: Contralateral

A crossed extensor reflex is contralateral because sensory impulses enter one side of the spinal cord while motor impulses exit the opposite side.

Contralateral Reflex

A reflex in which sensory input enters one side of the spinal cord and motor output leaves the opposite side, coordinating extension of the opposite limb with flexion (withdrawal) of the stimulated limb.

Crossed Extensor Reflex: Polysynaptic Nature

The crossed extensor reflex is polysynaptic because sensory neurons communicate with several interneurons and motor neurons through multiple synapses.

Crossed Extensor Reflex: Intersegmental Nature

The crossed extensor reflex involves interneurons and motor neurons across several spinal cord segments.

Reciprocal Innervation

A reflex arrangement in which one set of muscles contracts while the opposing set relaxes.

Reciprocal Innervation: Flexor Reflex

During the flexor reflex, flexor muscles of the stimulated limb contract while its antagonistic extensor muscles relax.

Reciprocal Innervation: Purpose

Reciprocal innervation prevents opposing muscles from contracting against each other and allows efficient, coordinated movement.