Spinal cord: Anatomy and Function, Knee-Jerk Reflex Reaction

  • The spinal cord extends from the foramen magnum at the base of the skull to the second lumbar vertebra. Spinal nerves communicate between the spinal cord and the body. The inferior end of the spinal cord and the spinal nerves exiting there resemble a horse’s tail and are collectively called the cauda equina. A cross section reveals that the spinal cord consists of a superficial white matter portion and a deep gray matter portion.
Spinal cord

Cross Section (a) In each segment of the spinal cord, rootlets combine to form a dorsal root on the dorsal (posterior) side and a ventral root on the ventral (anterior) side. (b) Relationship of sensory and motor neurons. Image Source: openstax.org

  • The white matter consists of myelinated axons, and the gray matter is mainly a collection of neuron cell bodies. The white matter in each half of the spinal cord is organized into three columns, called the dorsal (posterior), ventral (anterior), and lateral columns. Each column contains ascending and descending tracts, or pathways. Ascending tracts consist of axons that conduct action potentials toward the brain, and descending tracts consist of axons that conduct action potentials away from the brain.
  • The gray matter of it is shaped like the letter H, with posterior horns and anterior horns. Small lateral horns exist in levels of the spinal cord associated with the autonomic nervous system. The central canal is a fluid-filled space in the center of it. Spinal nerves arise from numerous rootlets along the dorsal and ventral surfaces.
  • The ventral rootlets combine to form a ventral root on the ventral (anterior) side, and the dorsal rootlets combine to form a dorsal root on the dorsal (posterior) side of the spinal cord at each segment. The ventral and dorsal roots unite just lateral to it to form a spinal nerve. The dorsal root contains a ganglion, called the dorsal root ganglion. It contains the cell bodies of pseudo-unipolar sensory neurons. The axons of these neurons originate in the periphery of the body. 
  • They pass through spinal nerves and the dorsal roots to the posterior horn of the spinal cord gray matter. In the posterior horn, the axons either synapse with interneurons or pass into the white matter and ascend or descend. The anterior and lateral horns of the spinal cord gray matter contain the cell bodies of motor neurons, which regulate the activities of muscles and glands. 
  • Somatic motor neurons are in the anterior horn, and autonomic neurons are in the lateral horn. Axons from the motor neurons form the ventral roots and pass into the spinal nerves. Thus, the dorsal root contains sensory axons, and the ventral root contains motor axons. Each spinal nerve therefore has both sensory and motor axons.

Functions

  • It receives brain impulses that govern movement and autonomic processes. Sending information to the brain: The spinal cord nerves also send messages from the body to the brain, such as touch, pressure, and pain sensations. It may conduct motor reflexes independently of the brain. The patellar reflex, for example, causes a person’s knee to jerk involuntarily when touched in a certain area. It’s tasks include transmitting nerve impulses for movement, feeling, pressure, temperature, pain, and other activities.

Spinal Cord Reflexes

Knee-Jerk Reflex

  • The simplest reflex is the stretch reflex. A stretch reflex occurs when muscles contract in response to a stretching force applied to them. The knee-jerk reflex, or patellar reflex, is a classic example of the stretch reflex. The stimulus for this reflex is stretching of the quadriceps femoris muscle. When the patellar ligament is tapped, the quadriceps femoris muscle tendon and the muscles themselves are stretched. Sensory receptors within these muscles are also stretched, and the stretch reflex is activated. Consequently, contraction of the muscles extends the leg, producing the characteristic knee-jerk response. Descending neurons within the spinal cord synapse with the neurons of the stretch reflex and modulate their activity. This activity is important in maintaining posture and in coordinating muscular activity.
Spinal cord

Knee-Jerk Reflex:  The knee-jerk reflex is an example of a stretch reflex whereby the stretching of a muscle stimulates the same muscle to contract. Image Source: openstax.org

Withdrawal Reflex

  • The function of the withdrawal reflex, or flexor reflex, is to remove a limb or another body part from a painful stimulus. The sensory receptors are pain receptors, and stimulation of these receptors initiates the reflex. Following painful stimuli, sensory neurons conduct action potentials through the dorsal root to the spinal cord, where the sensory neurons synapse with interneurons, which in turn synapse with motor neurons. These neurons stimulate muscles, usually flexor muscles, that remove the limb from the source of the painful stimulus.
Spinal cord

Withdrawal Reflex: The withdrawal reflex moves a body part away from a painful stimulus

Further Readings