FUNCTIONAL ORGANIZATION OF NERVOUS TISSUE
What is Nervous System?
It is a major regulatory system that detects sensation and controls movements,
physiologic processes, and intellectual functions.
It consists of the brain, spinal cord, nerves and sensory receptors.
How is the Nervous System divided:
In human, there is only one nervous system with two subdivisions which are called;
a) The central nervous system (CNS)
b) The peripheral nervous system (PNS).
CNS consists of brain and spinal cord. The brain is located within the skull and the
spinal cord is located within the vertebrae canal, formed by the vertebra..
-It is the major site for processing information, initiating responses, and integrating
mental processes.
PNS consist of nerve, ganglia, sensory receptors and plexuses.
-Nerves are bundles of axons and their sheaths that extend from the CNS to peripheral
structures, such as muscles, and glands, and from sensory organs to the CNS.
-Forty-three pairs of nerves originate from CNS to form the PNS.
-Of these, 12 pairs known as Cranial Nerves originate from the brain.
-Other 31 pairs known as Spinal Nerves originate from the spinal cord.
-Also located outside the CNS, are (a) A collection of Neuron Cell Bodies called
Ganglion, and (b) An extensive network of axons and in some case are called Plexus.
The PNS has two subcategories; namely the Afferent and the Efferent divisions.
The Afferent or Sensory division transmits electrical signals known as Action Potentials
from sensory receptors to the CNS.
-The cell bodies of sensory neurons are located in ganglia near the spinal cord or near the
origin of certain Cranial Nerves.
The Efferent or Motor division transmits electrical signals from CNS to effector organs,
such as muscle and glands.
-Efferent division has two subdivisions namely: (a) Somatic Nervous System, and (b)
Autonomic Nervous System (ANS).
Somatic Nervous System: It transmits action potentials from the CNS to skeletal
muscles.
-Its cell bodies are located within CNS and their axons extend through nerves to form
synapses with skeletal muscle cells.
-Skeletal muscles are voluntarily controlled through this system.
Autonomic Nervous System (ANS): It transmits electrical signals from CNS to smooth
muscle, cardiac muscle and certain glands.
-ANS has two sets of neurons that exits in a series between the CNS and effector organs.
-Cell bodies of the first neurons are within the CNS and the send their axons to
autonomic ganglia where neuron cell bodies of the second neurons are located.
, -Smooth or Cardiac muscles are controlled involuntarily.
-ANS is subdivided into the Sympathetic and the Parasympathetic divisions and Enteric
Nervous System.
The sympathetic division is responsible for body’s physical activity.
The Parasympathetic division regulates resting or vegetative functions such as digesting
food or emptying urinary bladder.
The Enteric Nervous System consists of interjoining of cells (Plexuses) within the wall of
digestive tract.
Functions of Nervous System:
1. Control of Muscles and Gland: The nervous system controls the major
movements of the body through the control of skeletal muscle.
-Some smooth muscle in the walls of blood vessels contracts only when stimulated by the
nervous system or hormones.
2. Homeostasis: The regulatory and coordinating activities of the nervous system
are important for maintaining homeostasis. Cells in our body function
dependently of each other to maintain homeostasis.
-For example, heart cells must contract at a rate that ensures adequate blood delivery,
skeletal muscles of respiration must contract at a rate that ensures oxygenation of
blood, and the kidney cells must regulate blood volume and remove waste products.
3. Mental Activity: The brain is the center of mental activities, including
consciousness, thinking, memory and emotions.
4. Sensory Input: Sensory receptors are located in the skin, muscles, joints, internal
organs, and specialized sensory organs such as touch, temperature, taste, smell,
sound, blood pressure, pH of body fluids and body position.
5. Integration: The brain and spinal cord are the major organs for processing
sensory input and initiating responses. The input may produce an immediate
response, may be stored as memory or may be ignored.
Cells of the Nervous System:
The nervous system is made up of nerve cells called Neurons and nonneural cells.
-Nerve cells (neurons) receive stimuli and transmit electrical signals.
-Nonneural cells are called Neuroglia or Glial cells and their functions include
support and protect neurons and perform other functions.
Neurons or Nerve Cells:
-They receive stimuli and transmit electrical signals to other nerve cells or to effector
organs.
-Nerve cell is made up of Cell Body also known as Soma, Dendrites and Axons also
called nerve fibers.
-Neurons are specialized for communication.
Neuron Cell Body:
-Soma contains a single large and centrally located nucleus with a distinct nucleolus.
-Such organelles like ER, Mitochondria and Golgi Apparatuses surround the nucleus.
-In the cytoplasm of the cell body are lipid droplets and melanin pigments,
neurofilaments and microtubules.
