Hey, Guys! lets me tell you what you would be clear with by the end of this section. Here in this section, we would be clear with the Organization of the Nervous System and the role played by them. So, let’s start!
Organization of Nervous System
Basic functions of central nervous system
- To collect the information from the external as well as the internal environment through the sensory system.
- To store, compare and contrast the presented information with the past experience.
- To make a decision and pass it to the effector organ or gland through the motor system.
Difference between general senses and special senses
- Not all the sensory sensations are realized or reach the consciousness levels in the Central Nervous System, i.e. we are not aware of those sensations. e.g. Blood pressure, Peristaltic movements in GIT.
- Pieces of information reaching the cerebral cortex area are perceived at consciousness levels and we become aware of the sensation.
- Some of the motor responses are under our control while others are not. e.g. Lifting a cup of tea or shaking hand with your friend are under our control but secretion of saliva on seeing lemon or raw mango is not under our control :). We can have control in picking them but would be able to control our salivation.
- Motor responses under voluntary control are called as Somatic motor responses. e.g. writing, dancing, shaking hands, moving of limbs, etc.
- Motor responses which are not under voluntary control are called as Autonomic motor responses. e.g. heart beat, GI movements, etc.
Autonomous nervous system
Now having known about the different divisions of nervous system let us now move to the very important organ of Central Nervous System i.e. BRAIN which can be divided into different parts.
- Hindbrain consists of medulla, pons, and cerebellum.
- Midbrain along with medulla and pons is also called as brainstem.
- The forebrain is divided into diencephalon and telencephalon. Diencephalon is located in the inner part and is differentiated into the dorsal thalamus and ventral hypothalamus.
- The telencephalon of the forebrain is located in the outer area and is differentiated into cerebral hemispheres connected by the corpus callosum.
- These fundamental divisions of the brain can be discerned in the human embryo by the end of the 4th week when the Central Nervous System is a hollow tube. The nervous system is consists of two types of cells: – a) Neurons & b) Glial cells
Neurons are the true functional cells of central nervous system and conduct the information in the form of action potentials. They are concerned with the communication system. Recent shreds of evidence suggest that humans have 86 billion neurons of which 16 billion are in the cerebral cortex and 69 billion are in the cerebellum. Smaller mammals have smaller brains due to the fewer neurons and glial cells and not because that their neurons are smaller :).
- Cell body (perikaryon) of a neuron contains a nucleus, Golgi apparatus, ribosomes, mitochondria and Nissl bodies in the rough endoplasmic reticulum which is involved in protein synthesis.
- Dendrites are the highly branched extensions of the cell body up to 1mm in length. A neuron may have one or many dendrites, arranged in a typical pattern called dendritic tree. The majority of the synapses are formed on the dendrites.
- Axon usually arises from the cell body at the site termed as axon hillock. Axons have a diameter ranging from 0.2-20 μm in humans and length from few μm to over a meter. Maybe encapsulated in a myelin sheath and the terminals are swollen terminals/buttons termed as axon terminals.
Classification of Neurons
Nerve cells can be classified based on their structure, connections, and neurotransmitters they release.
Based on their shape and size of the cell body, its dendritic tree, axon length and the nature of connections it makes neurons are classified into unipolar, bipolar and multipolar neurons.
- Unipolar neurons have single neurite. e.g. sensory neurons
- Bipolar neurons have double neurites. e.g. retinal neurons
- Multipolar neurons have many neurites. e.g. major of the neurons in vertebrate nervous system.
- Pseudounipolar neurons start their life as bipolar neuron but later the two neurites get fused. e.g. neurons of dorsal root ganglion
Classification based on axon length
- Projection neurons have long axons extending into other regions of the nervous system.e.g. pyramidal and Purkinje cells.
- Interneurons have short axons and produce direct effects only in their immediate neighborhood. e.g. stellate cells
Classification based on connections
- Afferent / sensory neurons are the neurons which carry the sensory information from the receptors to the central nervous system.
- Efferent/ motor neurons are the neurons which carry the command from the central nervous system to the effector organ (muscle or gland)
As illustrated in the picture sensation of temperature is carried by the afferent neurons to the CNS and the response i.e. to withdraw the finger from the hot object is carried by the efferent neurons.
Classification based on the neurotransmitters they secrete
- GABAergic neurons release γ-aminobutyrate.e.g. cerebellar Purkinje cells, interneurons of the cerebral cortex, interneuron of the cerebellar cortex, afferents of the caudate nucleus, interneurons of the hippocampus, afferents of globus pallidus and substantia nigra pars reticulata.
- Glutamatergic neurons release glutamate. e.g. Primary afferents of cranial and spinal nerves, neurons of the visual system, thalamocortical neurons, cerebral cortical pyramidal cells, hippocampal pyramidal cells, propriospinal neurons, cerebellar granule cells.
- Dopaminergic neurons release dopamine. eg. neurons in the zona compacta of the substantia nigra.
- Microglia are the immune system cells derived from bone marrow monocytes which have migrated to the nervous system during development. There are the macrophages of the nervous system which continuously scrutinizes the environment for any damage. It phagocytoses the debris generated due to apoptosis in the CNS and is responsible to neuroinflammation.
- Astrocytes are the irregularly shaped cells having long processes resembling dendrites. They lack Nissl bodies, unlike neurons. They have a wide variety of functions such as:- they maintain the local potassium levels by removing them from the ECF and dumping into the capillaries, terminate the actions of small transmitters molecules by removing them from the synaptic cleft, regulate blood-brain barrier, detoxification of ammonia and free radicals, as radial glial cells they guide the neurons to their proper destination in the developing brain.
- Oligodendrocytes in CNS and Schwann cells in PNS myelinate the axons of the neuron and nerves respectively.
- Ependymal cells are the lining cells of the spinal cord and the vestibular system of the brain. They are mainly involved in the formation and circulation of CSF and they make up the blood-CSF barrier.
- Satellite glial cells are present in the sensory, sympathetic and parasympathetic ganglia. They sensitive to injury and inflammation and are the major contributor to the chronic pain elicited during these conditions.
Common terms used in Neuroanatomy and Neurophysiology
- Gray matter-Collection of cell bodies within the Central Nervous System
- White matter-Collection of axons within the Central Nervous System
- Bundle of axons within the Central Nervous System, moving up and down, have common origin and termination – Tracts
- Bundle of axons moving up to upper levels of Central Nervous System – Ascending tracts
- Bundle of axons moving downward to lower levels of Central Nervous System –Descending tracts
- White matter connecting the Central Nervous System anteroposteriorly–Association fibers
- White matter connecting the left and right side of the brain – Commissure
Want to remember the gray matter and white matter in a simpler way! Well, let us imagine small gray color rats sitting together with long white tails :). The group of rats is our gray matter and the bundle of white tails is our white matter. Don’t you think we can remember it better now? Cell bodies of the neurons put together to form the gray matter and being myelinated with white sheath bundle of axons form the white matter.
- Cortex – Gray matter present on the surface of the Central Nervous System. e.g. cell bodies collected on the cerebral hemisphere are called cerebral cortex, cell bodies of the cerebellum are called as the cerebellar cortex.
- Nucleus– Gray matter embedded within the Central Nervous System and surrounded by white matter. e.g. nucleus of VII cranial nerve
- Reticular formation– when the sensory and motor fibers cross the gray matter gets fragmented and gets scattered and a network of the white and gray matter is formed forming reticular formation.
- The nerve is the collection of axons outside the Central Nervous System. e.g. parasympathetic nerve
The difference between tracts and nerve is that tracts are inside Central Nervous System and nerves are outside the Central Nervous System. Tracts are myelinated by the oligodendrocytes and nerves are myelinated by Schwann cells.
With this, I hope we have learned the basic terms used in neuroanatomy and neurophysiology. You can reach me for further clarifications. All the best!