Human Physiology

Lesson 40 : Organization of Nervous System

Organization Of Nervous System

The human nervous system is divided into two parts:

1. The central nervous system (CNS)
2. The peripheral nervous system (PNS).

The CNS includes the brain and the spinal cord and is the site of information processing and control. The CNS internally, both brain and spinal cord is bathed in a fluid called cerebrospinal fluid (CSF) which supplies nutrition to the neural tissues and act as a cushion to prevent injury during concussions.

The PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord). The nerve fibers of the PNS are two types:

1. Afferent fibers or sensory fibers: These fibers transit impulses from tissues or organs to the CNS.
2. Efferent fibers or motor fibers: These fibers transmit regulatory impulses from the CNS to the concerned peripheral tissues or organs.

Basic unit of nervous System:Neuron

The nervous system performs its functions by two types of cells, namely, neurons and glial cells. Neuron is the basic functional unit of nervous system. It composed of three parts, namely, cell body, dendrites and axon.

The neuronal cell body or soma consists of a nucleus, endoplasmic reticulum, golgi apparatus, mitochondria and cytoplasm contains certain granular bodies called Nissl’s granules. Short fibers which branched rapidly and projects out from the cell body containing Nissl’s granules are called dendrites (Fig 1). These dendrites received information (impulses) from other neurons and transmit impulses towards cell body.

The axon is a long fiber emerging from cell body at the axon hillock. The axon is the major route by which nerve cell sends signals (nerve impulses) to other neurons. The distal end of axon is branched and the branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters (Fig 2). The chemicals are released from the axon terminals into a junction called synapses during transmission of signals from neuron to other neurons or muscles. Synapses may be found on nerve cell bodies (axo-somatic), on dendrites (axo-dendritic), on axons (axo-axonal), on muscle (neuro-muscular).

There are two types of axons, namely, myelinated and non-myelinated. The myelinated nerve fibers (A) are enveloped with Schwann cells, which form a myelin sheath around the axon (Fig 3). The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibers are found in spinal and cranial nerves.

Unmyelinated nerve fiber (B) is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomic and the somatic nervous systems (Fig 4).

Based on the number of axon and dendrites, the neurons are divided into three types.

1. Multipolar neuron: it consist of one axon and two or more dendrites; found in the cerebral cortex.
2. Bipolar neuron: it consists of one axon and one dendrite; found in the retina of eye.
3. Unipolar neuron: the cell body has one axon only; found usually in the embryonic stage.

The following are the figures related to above discussion:

1. Typical Neuron
2. Typical Nerve Synapse
3. Myelinated Nerve Fiber
4. Unmyelinated Nerve fiber