Sensory and Motor Pathways

The sensory and motor pathways of the peripheral nervous system are separated by distinct characteristics:

• The sensory pathway is responsible for sensations and is composed of the dorsal column pathway, the spinothalamic pathway, and the spinocerebellar pathway.
• The motor pathway is composed of the pyramidal system and the extrapyramidal system.

Sensory Pathway

1. The posterior column pathway allows for the sensations of fine touch, vibration, two-point discrimination, and a sense of position and gets its name from that portion of the spinal cord the sensations travel along.
2. The spinothalamic pathway transmits information from the skin to the thalamus, including crude touch, pain, and temperature.
3. The spinocerebellar pathway transmits information to the cerebellum about muscle fiber length and tension and serves to communicate an unconscious proprioceptive sensation (position and movement).

Motor Pathway

PYRAMIDAL SYSTEM: the motor pathway known as the pyramidal system is responsible for voluntary muscle movement and actions such as swallowing and includes the

1. corticospinal (from the brain to the spinal cord) tract as well as the
2. corticobulbar (from the brain to cranial nerves) tracts.

This pathway is called the pyramidal system because the information travels to the paired white matter structures of the medulla known as the medullary pyramids.

The pyramidal corticospinal tract is responsible for motor movement and crosses over at the level of the medulla. This means the right side of the tract controls the right side of the body. Damage will result in a unilateral deficit below the area of injury only.

On the other hand, damage to the cortical area that begins the corticospinal tract will impact the signal before it crosses ("decussates") over, such that motor deficit will be seen on the opposite side of the body.

EXTRAPYRAMIDAL SYSTEM: The motor pathway known as the extrapyramidal system controls involuntary movements including posture and muscle tone and do not travel to the medullary pyramids. They do cross over to the opposite side, however (like the pyramidal tracts), at the midbrain or medulla.

The extrapyramidal pathway is specifically responsible for

• reflexes,
• locomotion,
• complex movements,
• and postural control.

Damage to the extrapyramidal system leads to different types of movement disorders (called "dyskinesia") from damage to the pyramidal system. Dyskinesia includes tremors, tics, rigidity, and other involuntary movements. (Damage to the pyramidal tract causes paralysis.) Examples of extrapyramidal diseases are Parkinson's and Huntington's chorea. These diseases can also have significant effects on the autonomic nervous system as they decrease the amount of neurotransmitters available throughout the body (i.e. low dopamine in Parkinson's leading to hypotension).

Disorders of Sensory and Motorway Pathways

Multiple Sclerosis: an autoimmune disease that attacks the myelin sheathing necessary for the fast conduction of sensory nerves, resulting in sensory loss and paresthesias. 

Muscular Dystrophy: autoimmune disease group any one of which attacks the muscle fibers or is congenital, resulting in weakness.

Alzheimer's Disease and Parkinson's Disease are deficits arising from damage in the central nervous system (brain).