The Sensory Nervous System - Essay Example

Perception of stimulus- Role of Central and Peripheral nervous systems The increased complexity of the human body, brought about by evolution, alsoincreased the need for co-ordination and control of the various parts of the body. This is brought about by the nervous system. There are two distinct sections of the human nervous system, the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system comprises of the brain and the spinal cord, whereas the peripheral nervous system includes the spinal nerves and cranial nerves.

The PNS essentially acts as a channel to relay sensory impulses to the CNS from the various surfaces of the human body in the case of somatic or voluntary nervous system. The basic unit of the entire nervous system is the neuron. The neurons themselves have three parts 1) the dendrite-which receive message impulses from other cells and transmits it to the cell body 2) the cell body 3) the axon-which transmits messages away from the cell body. There are three types of neurons namely sensory or afferent neurons, motor or efferent neurons and intermediate neurons.

The sensory neurons are responsible for the transmission of messages from the various sensory receptors or organs in the human body, which receive the external stimulus, to the central nervous system. The intermediate or relay neurons are present only in the CNS. The motor neurons transmit the message impulses from the CNS to the corresponding effector organ which might be a muscle or a gland which responds to the received impulse. The plasma membrane of the neuron has an unequal distribution of electrical charges between its inner and the outer surfaces, i.e., positive on the inner membrane and negative on the outer membrane.

When a message impulse is to be transmitted the polarities reverse temporarily resulting in what is known as an action potential. This action potential which begins in one spot spreads to the adjacent membranes thereby transmitting the message across a neuron. While the action potential allows transmission within a neuron neurotransmitters allow the transmission of messages between different neurons through the synaptic cleft. The synapse is the space between two nerve cells. (Farabee, 1992).

Depending on the type of external stimuli, various parts of the brain are targeted through various pathways through the spinal cord. For example the somatosensory system has three modalities. One particular modality being the discriminatory touch enables the human body to perceive shapes and sizes by touch alone. In this system sensory inputs are provided through the axons of the sensory neurons. These neurons have their cell bodies placed in a clump outside the spinal cord called the dorsal root ganglion, one ganglion for every spinal nerve.

The sensory neurons, unlike other neurons do not transmit the impulses through their cell body. They have the ability to pass the signals from the distal axon process to the proximal axon process thus by-passing the cell body entirely. These axons which initially carry the impulses from the receptor organs are called the primary afferents as they bring these impulses to the spinal cord. The cross section of the spinal cord shows a wide anterior and narrow posterior fissure. The outer portion of the spinal cord is composed of white matter that contains ascending and descending nerve tracts.

This white matter is divided into dorsal, ventral and lateral white columns. The primary afferents descend into the dorsal white matter of the spinal cord and directs the spinal cord toward the brain. The axons finally synapse at the medulla oblongata. The neurons in the medulla oblongata are called the secondary afferents. The secondary afferents then form a new tract called the medial lemniscus or the midline ribbon through the brainstem to the thalamus. The thalamus is the central relay point for messages that come to the brain.

From the thalamus the final synapse is made to the neuron in the cerebral cortex which is the ultimate target. ( Molavi, 1997) The cerebral cortex is a thin layer of grey matter which covers the two hemispheres of the cerebrum, the largest part of the human brain. The cortex is divided into four lobes, Occipital, Frontal, Temporal and Parietal. Each lobe is responsible for control of various functions. The occipital lobe controls visual perception, the temporal lobe is responsible for auditory signals and language processing, the parietal lobe takes care of the sensory perceptions like touch, taste, pain, cold, heat etc.

and the frontal lobe is in charge of speech, muscular activity and thought process. Thus the peripheral nervous system and the central nervous system together act as coordinating centers of the human body. For the human body to work in a controlled coordinated manner, all of the above mentioned units of the central nervous system should work harmoniously. Chemical imbalances, either due to natural causes like ageing or drug induced, lead to various neurological illnesses like Parkinson’s disease, depression, drug induced euphoria etc.

( Molavi, 1997). CitiationsFarabee, M.J (1992). The Nervous System. Retrieved March 24 2009, from http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookNERV.htmlMolavi, Diana Weedman (1997). Baic SomatoSensory Pathway. Neuroscience Tutorial, 1997, Retrieved March 24 2009, from http://thalamus.wustl.edu/course/bassens.html