Introduction to Nervous System
The organisation of the Nervous system
The nervous system is one of the smallest and yet the most complex of the 11 body systems.
Its subdivisions include the central nervous system and the peripheral nervous system.
Neurology deals with normal functioning and disorders of the nervous system.
A neurologist is a physician who diagnoses and treats disorders of the nervous system.
Functions of the Nervous System
Gathers information from both inside and outside the body – Sensory Function Transmits information to the processing areas of the brain and spine Processes the information in the brain and spine – Integration Function
Sends information to the muscles, glands, and organs so they can respond appropriately – Motor Function
DIVISIONS OF NERVOUS SYSTEM
- CENTRAL NERVOUS SYSTEM
- PERIPHERAL NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) consists :
BRAIN
The brain is the part of the CNS that is located in the skull and contains about 85 billion neurons.
SPINAL CORD
The spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. The spinal cord contains about 100 million neurons.
The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.
THE PERIPHERAL NERVOUS SYSTEM
Sensory (afferent) division –
Nerve fibres that carry information to the central nervous system
Motor (efferent) division –
Nerve fibres carry impulses away from the central nervous system.
This division is further subdivided into a
- Somatic nervous system
- Autonomic nervous system.
THE SOMATIC NERVOUS SYSTEM (SNS)[VOLUNTARY] :
conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the action of this part of the PNS is voluntary.
THE AUTONOMIC NERVOUS SYSTEM [INVOLUNTARY]:
conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary.
The ANS is comprised of two main branches, the sympathetic nervous system and the parasympathetic nervous system.
Structure of a Nerve
A nerve is a bundle of neuron fibres found outside the CNS.
Endoneurium.
Each fibre is surrounded by a delicate connective tissue sheath, an endoneurium.
Perimeurium.
Groups of fibres are bound by a coarser connective tissue wrapping, the perineurium, to form fibre bundles or fascicles.
Epineurium.
Finally, all the fascicles are bound together by a tough fibrous sheath, the epineurium, to form the cordlike nerve.
Mixed nerves.
Nerves carrying both sensory and motor fibres are called mixed nerves.
Sensory nerves.
Nerves that carry impulses toward the CNS only are called sensory, or afferent, nerves.
Motor nerves.
Those that carry only motor fibres are motor, or efferent, nerves.
Basic Cells of the Nervous System
Neuron
The basic functional cell of the nervous system Transmits impulses (up to 250 mph)
Parts of a Neuron
Dendrite – receive stimulus and carries it impulses toward the cell body
Cell Body with nucleus-nucleus & most of cytoplasm
Axon – fibre which carries impulses away from the cell body
Schwann Cells- cells that produce myelin or fat layer in the Peripheral Nervous System
Myelin sheath – dense lipid layer which insulates the axon – makes the axon look grey
A node of Ranvier – gaps or nodes in the myelin sheath. Impulses travel from dendrite to cell body to axon
Three Types of Neurons
Sensory neurons – bring messages to CNS
Motor neurons – carry messages from CNS
Interneurons – between sensory & motor neurons in the CNS
Impulses
A stimulus is a change in the environment with sufficient strength to initiate a response.
Excitability is the ability of a neuron to respond to the stimulus and convert it into a nerve impulse
All of Nothing Rule –
The stimulus is either strong enough to start an impulse or nothing happens
Impulses are always the same strength along a given neuron and they are self-propagating – once it starts it continues to the end of the neuron in only one direction- from dendrite to cell body to axon
The nerve impulse causes a movement of ions across the cell membrane of the nerve cell.
Synapse
Synapse – small gap or space between the axon of one neuron and the dendrite of another – the neurons do not actually tough at the synapse
It is the junction between neurons that uses neurotransmitters to start the impulse in the second neuron or an effector (muscle or gland) The synapse ensures oneway transmission of impulses
Neurotransmitters
These are the chemicals in the junction which allow impulses to be started in the second neuron
“Neurotransmitter is a type of chemical messenger that transmits signals across a chemical synapse, from one neuron to another.”
A neurotransmitter is the body’s chemical messenger. They are molecules that transmit signals from neurons to muscles, or between different neurons. The transmission of signals between two neurons occurs in the synaptic cleft.
The electrical signals that travel along the axon are briefly converted into chemical signals through neurotransmitters.
Types of Neurotransmitter
There are the following different types of neurotransmitters:
Excitatory neurotransmitters
These types of neurons increase the chances of the neuron firing an action potential. Epinephrine and norepinephrine are the two excitatory neurotransmitters.
Inhibitory neurotransmitters
These have inhibitory effects on the neurons and have fewer chances of the neuron firing an action potential. For eg., serotonin and gamma-aminobutyric acid (GABA).
Modulatory neurotransmitter
These can affect a large number of neurotransmitters at the same time. These can also influence the effect of other chemical messengers.
Reflex Activity
Reflex is an involuntary and sudden response to stimuli. It happens to be an integral component of the famed survival instinct.
Most of the common reflexes are a response to all the well-trained, accumulated knowledge of caution that we have internalized. It could be anything and ranges from the reflex action of abruptly withdrawing the hand as it comes in contact with an extremely cold or hot object. This action is termed as the reflex action. It has a subtle relation to instinct.
A point to be thought upon is that we all have our instincts differently depending on our past experiences and understanding. A reflex is a reaction triggered by this instinct. At times, we have no prior knowledge if the pan is hot or not. In other words, instinct has little to do with reflex.
References:
Principles of Anatomy and Physiology by Tortora Grabowski. Palmetto, GA, U.S.A.; fifteenth edition;pg no: 403-440
Ross and Wilson Anatomy And Physiology In Health And Illness;12TH EDITION;Pg no: 143-190
https://www.weber.edu/wsuimages/mollysmith/3510Presentations/Nerve.PPT
First Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise
First Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise
