Anatomy and Physiology Exam - Nervous System - Spinal Cord

Schwann Cells

• Glial cells in the Peripheral Nervous System (PNS) that wrap around nerve cells to produce the myelin sheath, which insulates axons and significantly increases the speed of electrical impulse conduction.

  • Separated by nodes of ranvier

Afferent

• Definition: Sensory pathways that carry nerve impulses, specifically sensory information including touch, pain, temperature, and vibration toward the Central Nervous System (CNS); from the body’s tissue and sensory organs to the spinal cord and onward to the brain

• Direction of Flow: Afferent signals arrive at the spinal cord for the peripheral nervous system (sensory input)

• Dorsal Root Ganglion: The cell bodies of these sensory neurons reside in a swelling outside the spinal cord known as DRG.

• Dorsal Root Entry: Sensory afferent fibers enter the back (dorsal side) of the spinal cord through the dorsal nerve root, now in the CNS.

Efferent

• Definition: Sensory pathways that carry motor commands away from the Central Nervous System (CNS) to the body’s muscles and glands (PNS).

• Direction of Flow: motor commands originate in the brain and descend down the spinal cord

• Exit Pathway: Originating in the gray matter of the anterior ventral horn of the spinal cord, efferent neurons emerge from these horns to form the ventral anterior root.

• The Mixed Nerve: Shortly after exiting, the ventral anterior root (motor) merges with the dorsal (sensory) root to form a mixed spinal nerve which travels out to the rest of the body.

• Function: Translates information from the CNS into physical actions split into two divisions; somatic and autonomic.

Sensory

Pathways that processes and relays incoming body sensations through the spinal cord to the brain via a structured entry and conduction system.

1. Entry via Dorsal Root

• Sensory receptors in the skin or muscles detect a stimulus.

• First-order neuron carries the electrical impulse from the receptor towards the spinal column.

• The cell bodies of these sensory neurons reside in a swelling outside the spinal cord known as Dorsal Root Ganglion.

• The central branch of the Afferent, sensory axon leaves the DRG and enters the back (dorsal side) of the spinal cord through the dorsal nerve root.

2. After entry

• Once in the spinal cord, the axons/paths split depending on the type of sensation.

  • Dorsal Column Pathway

  • Spinothalamic Pathway

  • Spinocerebellar Pathway

Motor

Pathways carry motor commands away from the Central Nervous System (CNS)/Brain to the body’s muscles and glands (PNS).

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• Upper motor neurons originate in the cerebral cortex or brainstem nuclei.

• Axons then bundle together and travel down the brainstem through white matter descending tracts (depends on the tract).

• The upper motor neurons travel down within these tracts along the spinal cord until they read the exact spinal segment responsible for the target muscle group.

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• The upper motor neurons then leave the white matter tract and enter the butter fly shaped gray matter of the anterior ventral horn of the spinal cord.

• The upper motor efferent neurons then synapse onto lower motor neurons whose cell bodies are in the ventral horn.

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• The lower motor neuron axon then exits the spinal cord through the ventral nerve root, they merge with the dorsal (sensory) root to form a mixed spinal nerve which travels out to the rest of the body/muscle fibers.

Somatic vs Visceral

Somatic and Visceral Sensory:

• Somatic Sensory: Carries sensory information from the skin, muscles, and joints toward the CNS.

• Visceral Sensory: Carries sensory information from internal organs (viscera) such as blood pressure or stomach fullness to the CNS.

• All sensory neurons, visceral or somatic, come into the spinal cord through the back, being houses in dorsal root ganglion and traveling in through the dorsal root

Somatic and Visceral Motor:

• Somatic Motor: Transmits motor commands from the CNS to the skeletal muscles, allowing for voluntary movement and reflexes.

  • One neuron chain

• Visceral Motor: Transmits motor commands from the CNS to smooth muscle, cardia muscle, and glands, regulating involuntary and unconscious internal functions such as heart rate and digestion.

  • Two neuron chain.

• Both somatic and visceral motor axons exit the spinal cord through the ventral root before splitting toward their targets. However somatic originates in ventral horn while visceral motor originates in lateral horn.

Somatic vs. Autonomic Effectors

• Represent the two distinct target categories controlled by the motor divisions of the motor system

• Somatic Effectors

  • Target Tissue - skeletal muscles only (voluntary, mostly conscious control)

  • Neural Pathway - a single lower motor neuron extends directly from the ventral horn of the spinal cord to the skeletal muscles.

• Autonomic Effectors

  • Target Tissue - Cardiac muscle, smooth muscle, and glandular tissue under involuntary control

  • Originates in lateral horns

  • Neural Pathway - Two-neuron chain (preganglionic and post ganglionic neurons synapsing at a ganglion).

Spinal Tracts - Key Terms

• Ascending and descending tracts head up or down

• Decussation - nerve fibers cross sides which explains why the left side of the brain controls and senses the right side of the body and vice versa.

• Contralateral means origin and destination are on opposite sides

  • Ex: injury on the left side of the spinal cord causes pain on right side of body

• Ipsilateral means origin and destination are on same side

  • Ex: a lesion on the right side of spinal cord causes right sided motor paralysis.

Ascending Tracts (sensory) - Dorsal Column Ascending Pathway

• A ipsilateral pathway that carries deep touch, visceral pain, vibration, and proprioception (knowing where your body is without looking) from the body up to the brain.

• Step-by-step Pathway:

1. First order Neuron - Sensory neuron detects a stimulus and enters spinal cord through dorsal root ganglion and ascends ipsilaterally to the medulla.

2. Second order Neuron - Once the first order neuron synapse onto the second order neurons in the medulla, they cross over (decussate) to the opposite side and ascend to the thalamus.

3. Third order Neuron - The third order neuron in the thalamus carries the signal to the cerebral cortex to process the information.

Ascending Tracts (sensory) - Spinothalamic Pathway

• A contralateral pathway that detects pain, pressure, temperature, light/crude touch, tickle, and itch

• Step by Step Pathway:

1. First order Neuron - Sensory receptors detect the sense, enters the spinal cord through the dorsal root ganglion. They synapse with second order neurons in the dorsal horn.

2. Second order Neuron - Second order neurons decussate to the opposite side of the spinal cord and ascend through the spinothalamic tract towards the thalamus.

3. Third order Neuron - Located in the thalamus, third order neurons carry information to the somatic sensory cortex.

Ascending Tracts (Sensory) - Spinocerebellar Pathway

• Detects unconscious proprioceptive signals

1. First order Neuron - Proprioceptive signals are detected and enters the spinal cord through the dorsal root ganglion.

2. Second order Neuron - they travel upward through the spinocerebellar tracts

3. Third order Neuron - Information reaches cerebellum to coordinate movement, posture and balance. However, does not reach conscious awareness.

Descending Tracts (Motor) - Corticospinal Tract

• Responsible for fine, precise, coordinated limb movements

• Two Neuron Pathway: Upper motor neurons originate in the cerebral cortex and descend to synapse with the lower motor neurons of the spinal cord that then directly innervate to skeletal muscles.

• 90% of fibers cross in the medulla (lateral) while 10% stay ipsilateral (anterior)

Descending Motor Tracts - Tectospinal Tract

• Coordinates reflex movements of the head and eye movements in response to sudden visual and auditory stimuli.

• Originates in the tectum (roof of brain), shortly after originating fibers cross over (decussate) to the opposite side of the brainstem and descend down the white matter (ventral) of the spinal cord but terminate in the neck region to control neck and shoulder muscles.

Descending Motor Tracts - Reticulospinal Tract

• Controls limb movements important to maintain posture

• Originates in the reticular formation and descends down the spinal cord to synapse on lower motor neurons in the spinal cord gray matter to regulate baseline posture, muscle tone, and influence limb extension/flexion.

Descending Motor Tracts - Vestibulospinal Tract

• Postural muscle activity in response to inner ear signals

• Originates in the vestibular nuclei of the brainstem that direct sensory information from the inner ear regarding the position and motion of your head.

• Unlike other tracts, it stays uncrossed (ipsilateral), sending signals down the same side of spinal cord to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head

Phrenic Nerve

• A vital motor nerve arising from the spinal cervical nerves C3 - C5

• This nerve travels down through the neck and chest, passing between the heart and lungs to reach the diaphragm

• You have two phrenic nerves, the right one controls the right side of the diaphragm, and the left one controls the left side of the diaphragm.

• It innervates the diaphragm, controlling the primary breathing movements (flatten and contract during inhalation and relax during exhalation).

Sciatic Nerve

• The largest and longest single nerve in the human body, originating in the lower back and running through the buttocks and the back of each leg into the feet.

• Function - it supplies sensation and motor control to the posterior thigh, lower leg, and foot.

• Sciatica - a symptom of sharp pain, burn, or tingling in the lower back that runs down the back of the leg caused by irritation, inflammation, or compression of the sciatic nerve.

Dorsal and Ventral Roots

• Dorsal Root

  • Composed of sensory nerve fibers entering the dorsal horn that carry sensory (afferent) information from the body (skin, organs, and muscles) into the spinal cord.

• Ventral Root

  • Composed of axons from motor and visceral efferent neurons that are in the grey matter of the spinal cord and carries them out of the front of the spinal cord to skeletal muscles and visceral organs.

Dorsal Root Ganglion

• A distinct ovoid swelling or bulge along the dorsal root of the spinal cord that houses the cell bodies of primary sensory neurons bringing info into the spinal cord.

Branches of the different divisions that mixed sensory and motor signals go to the different regions of the body

• Dorsal Ramus (Posterior Branch) - Smaller branch heading toward the back that provides somatic motor, visceral motor, and sensory innervation to the deep muscles and skin of the back.

• Ventral Ramus (Anterior Branch) - Much larger branch that curves around the lateral and anterior body wall that provides somatic motor, visceral motor, and sensory innervation to the anterior and lateral trunk as well as the limbs.

• Meningeal Rami - Tiny branches that immediately re-enter the vertebral canal after the spinal nerve forms to supply the “axial structures",” meninges, vertebrae, and spinal blood vessels.

Epidural Space

• The area located between the inner wall of the bony vertebral canal and the dura matter (the tough collagenous membrane surrounding the spinal cord) that is filled with fat and blood vessels.

• Often the site where anesthetics like an epidural during childbirth are injected.

CSF (Cerebrospinal Fluid)

• A clear, colorless, water-like fluid circulating through the brains ventricles and the subarachnoid space around the spinal cord.

• It cushions the CNS, provides chemical stability, and floats the brain to reduce its effective weight.

First, Second, and Third Order Neurons

• First-order neuron: Detects the sensory stimulus and delivers it from the receptor (typically sitting in the dorsal root ganglion) and projects it into the spinal cord or brainstem to synapse with the second neuron.

• Second-order neuron: Receives the signal from the first order neuron where its axon typically crosses/decussates before ascending up to the thalamus.

• Third-order neuron: Carries the signal from the thalamus up to the primary sensory area of the cerebral cortex for conscious perception.

Upper and Lower Motor Neurons

• Upper motor neuron: Originates in the cerebral cortex or brainstem and descends through the brainstem and spinal cord (via like the corticospinal tract) to synapse on lower motor neurons.

• Lower motor neuron: Its cell body is in the cranial nerve nuclei of the brainstem or ventral horn of the spinal cord and its axon directly innervates skeletal muscle fibers (the final common pathway).

Layers surrounding a Spinal Nerve

The Meningeal Coverings:

• where the spinal roots originate inside the vertebral canal, they are enclosed by the three meninges

1. Pia Matter - The innermost, delicate layer that tightly adheres directly to the spinal cord and its roots

2. Arachnoid Matter - The wispy, web-like middle layer seperated from the Pia matter by the subarachnoid space (which contains css)

3. Dura Matter - The outermost, tough, fibrous membrane layer

The Peripheral connective tissue layers:

• Where the dorsal and ventral spinal roots unite to form a true spinal nerve outside the spinal cord, the meninges transition into three distinct layers of peripheral connective tissue

1. Epineurium - The outermost dense connective tissue sheath that encloses the entire spinal nerve

2. Perineurium - A smooth, intermediate layer of specialized connective tissue that wraps around each individual bundle (fascicle) of nerve fibers.

3. Endoneurium - The innermost delicate connective tissue layer that surrounds individual nerve axons and their myelin sheaths

Reflexes

• A quick, involuntary, movement/reaction of glands or muscle to sensory stimulation that occur without our intent or even our awareness.

• Every classic reflex travels along a neural pathway called a reflex arc that consist of 5 steps:

1. Receptor - a specialized sensory nerve ending or receptor that detects a specific stimulus and converts it to an electrical signal

2. Sensory (Afferent) Neuron - An electrical conduction that carries the nerve from the receptor into the dorsal horn of spinal cord (CNS - brain or spinal cord)

3. Integration Center:

• Monosynaptic - A direct, single synapse between the sensory neuron and a motor neuron (no interneurons).

• Polysynaptic - One or more interneurons (relay neurons) process the signal and coordinate complex responses.

4. Motor (Efferent) Neurons - A nerve cell that transmits the outgoing command signal away from the CNS out toward the periphery

5. Effector - The target muscle, cardiac/smooth muscle, or gland that executes the final action

• Ex: Flexor Withdrawal Reflex - A painful or dangerous stimulus triggers sensory pain fibers, activating flexor motor neurons to pull a limb away while simultaneously inhibiting extensor muscles. However, crossed extensor reflex maintain balance by extending the other leg (contralateral) to bare the weight of being on the one leg.

• Ex: Patellar Reflex - Doctor taps the patellar tendon which stretches the quadriceps tendon, sensory neurons carry this stretch signal to the spinal cord which synapses onto a motor neuron that causes the quadriceps to contract and leg to kick out while reciprocal inhibition (prevents muscles from working against each other) relaxes the hamstring.