The spinal cord is a crucial part of the central nervous system that relays messages between the brain and the rest of the body. Even though the brain plays a pivotal role in controlling and regulating bodily functions, the spinal cord can also send and receive signals without needing input from the brain. This allows for basic reflexes and some motor functions to occur independently of the brain.
Anatomy of the Spinal Cord
The spinal cord runs from the base of the brain down through the spinal column. It is protected by the bony vertebrae that make up the spine. The spinal cord is composed of gray matter and white matter:
- Gray matter – contains neuron cell bodies
- White matter – contains axons with myelin sheaths
The gray matter is in an H-shape in the center of the cord and contains interneurons and motor neurons. The white matter surrounds the gray matter and consists of ascending and descending tracts that transmit signals up and down the spinal cord.
The spinal cord has 31 spinal nerve segments that branch off to connect to different parts of the body. Each segment corresponds with a pair of spinal nerves (one sensory; one motor) that relay signals between the spinal cord and the region of the body they innervate.
Neural Pathways in the Spinal Cord
There are specific neural pathways within the spinal cord that allow for communication without needing input from the brain:
- Reflex arc: This simplest pathway is a reflex arc, which allows for automatic, rapid responses to stimuli. It involves sensory neurons sending signals into the spinal cord gray matter where they synapse with interneurons that connect to motor neurons leading back out to the muscles. This creates a quick reaction, such as jerking your hand away from a hot stove.
- Ascending tracts: Sensory information travels up the spinal cord via ascending tracts in the white matter. Some ascend just a few segments to reflex centers while others go all the way to the brain.
- Descending tracts: Motor signals from the brain travel down the spinal cord via descending tracts in the white matter. Some stop at local reflex centers while others continue all the way to motor neurons.
- Propriospinal tracts: There are also intermediate neurons that create shortcuts to connect ascending and descending pathways within the spinal cord without involving the brain.
These pathways allow sensory information to enter the spinal cord and then be transmitted up to the brain or to nearby interneurons and motor neurons without needing input from the brain first. Likewise, motor signals from the brain can travel down the spinal cord and synapse with local motor neurons.
Spinal Cord Reflexes
Reflexes are automatic, subconscious reactions that occur through local pathways in the spinal cord. They provide rapid protective responses and do not require communicating with the brain first. Common spinal cord reflexes include:
- Stretch reflex: This reflex causes a muscle to contract when stretched in order to protect the muscle tissue. Muscle spindles detect the stretch and send signals to the spinal cord, which activates motor neurons to tell the muscle to contract.
- Withdrawal reflex: Also called a flexor withdrawal reflex, this causes a limb to jerk away from a painful stimulus. Pain receptors activate local interneurons in the spinal cord that stimulate motor neurons to pull the limb away.
- Crossed extensor reflex: When pain receptors are activated, this reflex causes the opposite limb to extend to support the body’s weight. Local interneurons coordinate the response.
- Autonomic reflexes: These reflexes control automatic functions like breathing, digestion, blood pressure, and pupil dilation through the autonomic nervous system. They do not require conscious input from the brain.
These reflexes provide rapid, protective reactions utilizing local circuits in the spinal cord itself. More complex motor functions and voluntary movements require communication with the brain.
Local Control of Motor Neurons
Local motor centers in the spinal cord can generate rhythmic patterns of motor neuron firing independently of the brain. This allows for basic motor functions like walking, swimming, and chewing to be carried out reflexively or through intrinsic spinal cord circuits.
Central pattern generators (CPGs) are neuronal networks within the spinal cord gray matter that can produce rhythmic firings of motor neurons without sensory feedback. CPGs coordinate automatic and rhythmic motor activities like locomotion, breathing, chewing, and swallowing.
For example, walking involves rhythmic alternating contractions in flexor and extensor muscles in the legs that are coordinated by CPGs. When the CPGs are activated, they stimulate motor neurons in a repeating oscillatory pattern to move the legs without the brain consciously controlling each muscle contraction.
In addition to CPGs, the spinal cord can modify commands from the brain using local reflexive circuits. For example, signals from the brain initiate walking, but spinal cord reflexes help adjust muscle forces and joint positions to maintain balance and react to the environment.
Communication Between Spinal Cord and Brain
While the spinal cord can control some functions on its own, communication with the brain is essential for voluntary and skilled movements. Ways the spinal cord and brain communicate include:
- Ascending pathways: Sensory information is constantly traveling up axons in ascending spinal tracts to reach sensory processing centers in the brain. The brain then perceives sensations and can respond.
- Descending pathways: Motor signals travel down axons in descending spinal tracts to activate local motor neurons in the spinal cord or CPGs. This allows the brain to voluntarily control movement and coordinate muscle contractions.
- Brain modulation: The brain sends signals down to the spinal cord to modulate reflexes and CPGs based on variables like different environments, desired actions, and available feedback. This allows the brain to adapt motor outputs.
- Cerebellum: The cerebellum integrates sensory input and motor output between the brain and spinal cord to coordinate skilled voluntary movements and make postural adjustments.
Two-way communication between the brain and spinal cord allows sensory information to reach the brain for perception and decision-making while the brain can control voluntary skilled movements by sending signals to initiate and modulate motor neuron activity.
Injuries to Spinal Cord Communication Pathways
Damage to the spinal cord can disrupt communication between the spinal cord and brain. Common injuries include:
- Complete spinal cord injury: This severs all neural connections between the spinal cord below the injury and brain. Reflexes may remain intact below the injury but no voluntary control.
- Incomplete spinal cord injury: Some pathways remain intact allowing for some communication between the spinal cord and brain. Some function may be preserved.
- Brown-Sequard syndrome: A hemisection injury affects one side of the spinal cord. Ipsilateral motor loss and proprioceptive loss occurs with contralateral pain/temperature sensation loss.
- Anterior cord syndrome: Motor connections are disrupted while some sensory pathways remain intact. This results in motor deficits with some preserved sensation.
- Posterior cord syndrome: Damage to the posterior spinal cord affects ascending sensory pathways resulting in altered sensations.
Depending on the location and severity of spinal cord damage, different types of communication deficits can result between the spinal cord and brain.
Treating Spinal Cord Injuries
Treatment options for spinal cord injuries focus on limiting further damage and trying to restore function. Interventions may include:
- Surgery to stabilize vertebrae or remove bone fragments/disc material
- Methylprednisolone within 8 hours to reduce inflammation
- Rehabilitation with physical therapy to regain strength and function
- Assistive devices like braces, walkers, wheelchairs to maximize independence
- Electrical stimulation to activate muscles below injury
- Experimental treatments like stem cell transplants to try to repair damaged pathways
The extent of recovery depends on the severity and location of original injury. Research continues into new treatments to restore communication between the spinal cord and brain after injury.
Key Takeaways
In review, key points about spinal cord communication include:
- The spinal cord can transmit signals without input from the brain through local reflex arcs, ascending/descending pathways, and propriospinal neurons.
- Reflexes mediated through the spinal cord provide rapid, automatic protective responses.
- Central pattern generators in the spinal cord can coordinate rhythmic motor patterns.
- Two-way communication with the brain allows skilled voluntary movements and modulation of spinal cord circuits.
- Injuries can damage spinal cord pathways and disrupt critical communication with the brain.
The spinal cord plays an integral role in allowing basic functions to occur independently of the brain through local circuits and reflexes. However, intentional movement and modulation of motor responses relies on integration with the brain through ascending and descending spinal tracts.
Conclusion
In conclusion, the spinal cord is capable of sending and receiving signals without input from the brain through local circuits and reflex arcs. This allows for rapid protective reflexes and basic motor functions like walking to occur automatically. However, intentional skilled movements require two-way communication with the brain through ascending sensory and descending motor pathways. Spinal cord injuries can severely impair this critical communication and result in deficits in both reflexive responses and voluntary control. Understanding spinal cord anatomy and communication pathways provides insight into how to improve function after injury.