Structure of Spinal Cord
The spinal cord is an elongated, cylindrical structure extending from the brain, just below the medulla oblongata. It is enclosed in the vertebral canal and surrounded by meninges and cerebrospinal fluid (CSF). The spinal cord extends from the first cervical vertebra to the lower border of the first lumbar vertebra, measuring about 45 cm in length and approximately as thick as a little finger.
Except for cranial nerves, the spinal cord serves as the main communication link between the brain and the rest of the body. Motor nerves from the brain descend through it to reach various organs, while sensory nerves carry information upward to the brain. Some actions, like reflexes, are controlled directly by the spinal cord without brain involvement.
Internal Structure
In cross-section, the spinal cord has gray matter in the center shaped like the letter “H,” surrounded by white matter. The gray matter contains:
- Sensory cells that receive impulses from the body’s periphery
- Lower motor neurons that transmit impulses to skeletal muscles
- Connector neurons that link sensory and motor neurons at the same or different levels
The gray matter’s posterior columns contain sensory neurons, while the anterior columns contain motor neurons. The white matter is arranged in three columns—anterior, posterior, and lateral—formed by ascending sensory tracts, descending motor tracts, and connector neuron fibers.
Sensory Nerve Tracts in the Spinal Cord
Two main sources of sensory input reach the brain via the spinal cord:
- From the Skin: Cutaneous receptors respond to pain, temperature, touch, and pressure. The impulses are transmitted to the opposite hemisphere of the cerebrum, where the sensation is perceived.
- From Muscles, Tendons, and Joints: Proprioceptors respond to stretch and help maintain balance, posture, and awareness of body position.
Motor Nerve Tracts in the Spinal Cord
Motor neurons carry impulses from the brain to muscles and glands. Their activation results in muscle contraction or glandular secretion. Voluntary muscle movements originate in the cerebrum, while involuntary coordination involves the brainstem, cerebellum, and midbrain.
Types of Motor Tracts
- Pyramidal (Corticospinal) Tracts: The main pathway for voluntary muscle control. These fibers pass through the internal capsule and decussate (cross over) in the medulla oblongata to form the lateral corticospinal tracts.
- Extrapyramidal Tracts: Involved in coordination and posture; connected with the basal nuclei and thalamus.
Upper and Lower Motor Neurons
The upper motor neuron originates in the cerebrum and sends fibers through the internal capsule to the spinal cord. The lower motor neuron lies in the anterior horn of the spinal cord and connects directly to skeletal muscles through motor end-plates, forming a motor unit. The neurotransmitter acetylcholine is released at these junctions to trigger muscle contraction.
Movements of Involuntary Muscles
Involuntary movements are controlled by upper motor neurons in the midbrain, brainstem, cerebellum, or spinal cord. These neurons help maintain posture, balance, and muscle tone through automatic reflexes.
Spinal Reflexes
A spinal reflex is an immediate, automatic response to a sensory stimulus, occurring without brain involvement. It involves three elements: sensory neurons, connector neurons, and lower motor neurons. Reflexes are protective and help the body respond quickly to changes in the environment.
Reflex Arc
The basic unit of a reflex is the reflex arc, which includes sensory input, processing within the spinal cord, and motor output. Reflexes can be classified as:
Monosynaptic Reflex
In this type, only two neurons are involved—one sensory (afferent) and one motor (efferent). The sensory neuron detects the stimulus and directly synapses with the motor neuron in the spinal cord. An example is the knee-jerk reflex, where tapping the tendon stretches muscle receptors, causing the leg to kick forward.
Polysynaptic Reflex
This reflex involves one or more interneurons between the sensory and motor neurons. The afferent neuron sends signals to interneurons in the gray matter, which then transmit them to multiple motor neurons. This allows for coordination of several muscles, often resulting in one group contracting while another relaxes (e.g., withdrawal reflex).
Plantar Reflex
The plantar reflex occurs when the sole of the foot is stroked with a blunt instrument:
- In healthy adults, it causes a downward movement (flexion) of the big toe.
- An upward movement (extension) of the big toe is known as the Babinski sign, named after Joseph Babinski. This sign can indicate damage to the brain or spinal cord in adults but is a normal primitive reflex in infants.
Clinical Importance
Testing spinal reflexes is an essential part of neurological examination. Abnormal reflex responses can help identify lesions, nerve damage, or spinal cord disorders.
Detailed Notes
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