3.2.1 Cell types

The Types of Cells Found Within the Nervous System and their Anatomical and Functional Localization in Layers in Different Parts of the Cortex

The nervous system is composed of many different types of cells, each of which plays a specific role in the functioning of the nervous system. In the cortex, the outer layer of the brain, different types of cells are organized into distinct layers that serve specific functions. Here are some of the most important cell types found in the nervous system and their localization in the cortex:

Type of cell:	Description:
Neurons	These are the primary cells responsible for transmitting electrical signals within the nervous system. They are located throughout the cortex but are most abundant in layer IV, the innermost layer of the cortex.
Glial cells	These cells play a supportive role in the nervous system, providing structural support and insulation for neurons. They are most abundant in the outer layers of the cortex.
Microglia	These are a type of glial cell that play a role in the immune system of the central nervous system (CNS). They are located throughout the cortex but are most densely concentrated in the deeper layers of the cortex.
Astrocytes	These are another type of glial cell that play a role in maintaining the structural integrity of the CNS and in regulating the chemical environment around neurons. They are located throughout the cortex but are most densely concentrated in the outer layers of the cortex.
Oligodendrocytes	These are glial cells that produce myelin, a fatty substance that insulates the axons of neurons and helps to increase the speed of electrical signals transmitted by neurons. They are located primarily in the deeper layers of the cortex.

The functional specialization of different regions of the cortex is reflected in the arrangement of these cell types and the distribution of neural connections within the cortex. For example, the primary sensory cortex, which is responsible for processing sensory information, has a different arrangement of cell types and neural connections than the motor cortex, which is responsible for controlling movement.

Neurons

Neurons are the primary cells responsible for transmitting electrical signals within the nervous system. They are specialized for this function and have distinct structural features, such as dendrites, which receive signals from other neurons, and an axon, which transmits signals to other neurons.

In the cortex, neurons are organized into different layers, each with a specific function. The different layers of the cortex are designated by Roman numerals, with layer I being the outermost layer and layer VI being the innermost layer. Here are some of the key functional localizations of neurons within the cortex:

1. Layer IV: This is the innermost layer of the cortex and is often referred to as the “input layer.” It is composed of large, bushy neurons that receive incoming signals from other areas of the brain and from sensory organs.
2. Layer V: This layer contains the largest neurons in the cortex and is responsible for transmitting signals to other areas of the brain. This layer is sometimes referred to as the “output layer.”
3. Layer VI: This innermost layer of the cortex receives signals from the thalamus, a structure that serves as a relay station for sensory information. Neurons in this layer are responsible for sending signals to the brainstem and spinal cord.

The different regions of the cortex, such as the sensory cortex, the motor cortex, and the association cortex, each have their own distinct arrangements of neurons and neural connections, which are optimized for their specific functions. The distribution of neurons within the cortex reflects the functional specialization of different regions of the brain and helps to ensure that signals are transmitted efficiently and accurately from one area of the brain to another.

Neurons are composed of a soma or cell body, which contains a nucleus and intracellular organelles such as the Golgi apparatus and mitochondria. The dendrites receive signals from other neurons and transmit them to the cell body. The axon arises from the axon hillock, which is located at the base of the cell body. At the axon hillock, an action potential is initiated and travels along the axon towards the axon terminal. Schwann cells play a role in insulating the axon by producing a myelin sheath that increases the speed of action potential transmission. The axon terminals make synaptic connections with other neurons by releasing neurotransmitters into the synaptic cleft, which then bind to receptors on the postsynaptic neuron to transmit the signal (Amunts, 2007).

Glial cells

The glial cells are made up of 6 types of cells:

• Astrocytes
• Oligodendrocytes
• Microglial
• Ependymal cells
• Schwann cells
• Satellite cells

Astrocytes:

Oligodendrocytes:

Microglial:

Ependymal cells:

Scwann cells:

Satellite cells:

Cell type:	Function:
Astrocytes	Provide structural and metabolic support. Participate in repair.
Oligodendrocytes	Produce myelin and provide electrical insulation.
Microglia	Participate in defence and immune responses.
Ependymal	Assist in the production and movement of CSF.
Schwann	Produce myelin and provide electrical insulation.
Satellite	Provide structural and metabolic support for cell bodies of neurons.

Cerebral Cortex Cells

The cerebral cortex contains billions of neurons which are as follows:

• Pyramidal cells
• Fusiform cells
• Stellate cells

The other cells present in the cortex are horizontal cells such as the cells of Martinetti and Cajal-Retzius.

Pyramidal cells:

Fusiform cells:

These cells are present in the deepest layer of the cortices. They project their dendrites toward the cortical surface, however, the axon has all the possibility to become association, projection oriented, and commissural. They usually project toward the thalamus.

Stellate cells:

These cells resemble a star because their projections are planar. They are all abundant within the cortex. The dendrites have spines usually known as spiny or spiny cells. They are present in the internal pyramidal layer in order to release the glutamate (excitatory neurotransmitter) that functions as the excitatory interneurons. Aspin cells release GABA and act as inhibitory interneurons.

Horizontal cells:

Cajal-Retzius can only be found in the superficial part of the cortex. They are found in an adult brain and are considered super rare. They consist of one axon and a dendrite. They synapse within the molecular layer.

Cerebral Cortex Layers

Neuroscientists discovered neurons consist of a laminar alignment. This discovery indicates that neurons have laminar layers. These layers are parallel to the brain surface and are differentiated by size and shape.

The cerebral cortex, which is the outer layer of the brain, is composed of six distinct layers, each with its own unique set of neurons and neural connections. The six layers of the cortex are designated by Roman numerals, with layer I being the outermost layer and layer VI being the innermost layer. Here is a brief description of each layer:

The six layers of the cerebral cortex are as follows:

• Molecular layer (layer I)
• External granular (layer II)
• External pyramidal (layer III)
• Internal granular (layer IV)
• Internal pyramidal (layer V)
• Multiform layer (layer VI)

Molecular layer (plexiform):

The outermost layer, also known as the molecular layer, is thin and contains few neurons. It is composed mainly of support cells called astrocytes and is important for the maintenance of the structural integrity of the cortex.

External granular layer:

This layer is comprised of stellate cells, and small neurons and is involved in processing sensory information. Stellate cells are responsible for giving a granular appearance to layers. Other structures which are cellular can be seen in the form of pyramidal cells. These cells send dendrites to the molecular layer, then axons go deeper to the synapsing cortex. These axons are long enough to produce association fibres that travel toward the white matter in order to reach the CNS.

External pyramidal layer:

This layer consists of pyramidal cells containing medium-sized neurons and is involved in processing sensory information and in transmitting signals to other areas of the brain. Superficial cells are smaller in this layer. The apical dendrites of pyramidal cells extend in a superficial manner to reach the molecular layer, then the basal processes go toward the subcortical white matter to join in order to project back to the cortex.

Internal granular layer:

The innermost layer, also known as the internal granular layer, contains large neurons that receive incoming signals from other areas of the brain and from sensory organs. This layer receives the stimuli that arrive from the periphery even known as the ‘input cortical station’. This layer consists of stellate cells and small pyramidal cells, the axon of the stellate cell stays in the cortex and local synapses. In pyramidal cells, the axons have synapsed deep within the cortex, or in some other cases they might leave the cortex and join the fibres of white matter.

Internal pyramidal layer:

This layer contains the largest neurons in the cortex and is responsible for transmitting signals to other areas of the brain. It consists of large pyramidal cells. It is considered the main source of corticofugal fibres. It sends signals to fibres that can mediate motor activity within the motor cortex (Wilder, 1959).

Multiform layer:

This innermost layer receives signals from the thalamus, a structure that serves as a relay station for sensory information, and sends signals to the brainstem and spinal cord. The deepest layer of cortex overlies between the white matter subcortical. It possesses fusiform cells that contain fewer dominant interneurons and pyramidal cells. The axon of the Fusiform cell and pyramidal cell distribute the corticothalamic projection fibres and corticocortical commissural fibres that end in the thalamus (Amunts, 2007).

References:

(1) Amunts, K., Schleicher, A. and Zilles, K., 2007. Cytoarchitecture of the cerebral cortex—more than localization. Neuroimage, 37(4), pp.1061-1065.

(2) Wilder, J. (1959). The Anatomy of the Nervous System, Its Development and Function. American Journal of Psychotherapy, 13(4), pp.940–940. doi:10.1176/appi.psychotherapy.1959.13.4.940.