The Telencephalon: Cerebral Cortex and Basal Ganglia

The Cerebral Cortex

• The human cerebral cortex covers an area of about 2,200 to 2,400 cm2 due to extensive folding.
• The cerebral cortex is divided into distinct regions based on the underlying cytoarchitectonics by Brodmann.
• The lobes of the brain include the frontal, parietal, temporal, and occipital lobes.
• The frontal lobe is responsible for planning, cognitive control, and execution of movements.
• The parietal lobe receives sensory input about touch, pain, temperature, and limb position, and is involved in coding space and coordinating actions.
• The temporal lobe contains auditory, visual, and multi-modal processing areas.
• The occipital lobe processes visual information.
• The limbic lobe (not really a lobe) is involved in emotional processing, learning, and memory.

Ion Channels and Action Potentials

• Ion channels are formed by transmembrane proteins that create passageways through which ions can flow.
• Ion channels can be either passive (always open) or gated (open only in the presence of electrical, chemical, or physical stimuli).
• Passive current conduction is called electrotonic conduction or decremental conduction.
• A depolarizing current makes the inside of the cell more positive and therefore more likely to generate an action potential.
• Action potentials are an all-or-none phenomenon: The amplitude of the action potential does not depend on the size of the triggering depolarization, as long as that depolarization reaches threshold for initiating the action potential.
• Voltage-gated channels are of prime importance in generating an action potential because they open and close according to the membrane potential.
• Myelin allows for the rapid transmission of action potentials down an axon.
• Nodes of Ranvier are the spaces between sheaths of myelin where voltage-gated Na+ and K+ channels are located and action potentials occur.

Synaptic Transmission

• Neurotransmitters are synthesized by and localized within the presynaptic neuron and stored in the presynaptic terminal before release.
• Neurotransmitters are released by the presynaptic neuron when action potentials depolarize the terminal (mediated primarily by Ca2+).
• The postsynaptic neuron contains receptors specific for the neurotransmitter.
• Classes of neurotransmitters include amino acids, biogenic amines, and neuropeptides.
• Neurotransmitters must be removed from the receptor after binding, which can be accomplished by active reuptake back into the presynaptic terminal, enzymatic breakdown of the transmitter in the synaptic cleft, or diffusion of the neurotransmitter away from the region of the synapse.

The Structure of Neurons

• Neurons and glial cells make up the nervous system.
• Neurons are the cells that transmit information throughout the nervous system.
• Most neurons consist of a cell soma (body), axon, and dendrites.
• Neurons communicate with other neurons and cells at specialized structures called synapses, where chemical and electrical signals can be conveyed between neurons.

The Role of Glial Cells

• Astrocytes are a type of glial cell that helps form the blood-brain barrier.
• Astrocytes have an active role in modulating neural activity.
• Glial cells aid in the speed of information transfer by forming myelin around the axons of the neurons.
• An oligodendrocyte is a type of glial cell that forms myelin in the central nervous system.
• A Schwann cell is a type of glial cell that forms myelin in the peripheral nervous system.
• As part of the immune response of the nervous system, microglial cells are phagocytic cells that engulf damaged cells.

The Autonomic Nervous System

• The autonomic nervous system is involved in controlling the action of smooth muscles, the heart, and various glands.
• The sympathetic system uses the neurotransmitter norepinephrine and prepares the body for fight-or-flight responses by stimulating the adrenal glands.
• The parasympathetic system uses acetylcholine as a neurotransmitter and is responsible for decreasing heart rate and stimulating digestion.

Glial Cells

• Astrocytes are a type of glial cell that helps form the blood-brain barrier and has an active role in modulating neural activity.
• Glial cells aid in the speed of information transfer by forming myelin around the axons of neurons.
• Oligodendrocytes are a type of glial cell that forms myelin in the central nervous system.
• Schwann cells are a type of glial cell that forms myelin in the peripheral nervous system.
• Microglial cells are phagocytic cells that engulf damaged cells as part of the immune response of the nervous system.

Nervous System Structure

Central Nervous System

• The central nervous system consists of the brain and spinal cord.

Peripheral Nervous System

• The peripheral nervous system consists of all nerves and neurons outside of the central nervous system.

Autonomic Nervous System

• The autonomic nervous system is involved in controlling the action of smooth muscles, the heart, and various glands.
• The sympathetic system uses the neurotransmitter norepinephrine and prepares the body for fight-or-flight responses by stimulating the adrenal glands.
• The parasympathetic system uses acetylcholine as a neurotransmitter and is responsible for decreasing heart rate and stimulating digestion.

Brain Regions

Brainstem

• The brainstem houses fibers that pass from the cortex to the spinal cord and cerebellum, and sensory fibers that run from spinal levels to the thalamus and then to the cortex.
• The brainstem's neurons carry out many sensory and motor processes, including visuomotor, auditory, and vestibular functions as well as sensation and motor control of the face, mouth, throat, respiratory system, and heart.

Cerebrum

• The cerebrum is divided into two hemispheres, each containing a cerebral cortex, basal ganglia, limbic system, and diencephalon.

Diencephalon

• The thalamus is the relay station for almost all sensory information.
• The hypothalamus is important for the autonomic nervous system and endocrine system, controlling functions necessary for the maintenance of homeostasis, and is involved in control of the pituitary gland.

Cerebral Cortex

• The cerebral cortex is a continuous sheet of layered neurons in each hemisphere.
• The axons of cortical neurons and subcortical ganglia travel together in white matter tracts that interconnect neurons in different parts of the brain and spinal cord.

Neuronal Signaling

Neurons

• Neurons are the cells that transmit information throughout the nervous system.
• Most neurons consist of a cell soma (body), axon, and dendrites.
• Neurons communicate with other neurons and cells at specialized structures called synapses, where chemical and electrical signals can be conveyed between neurons.

Action Potential

• The resting membrane potential is the difference in voltage across the neuronal membrane during rest.
• The electrical gradient results from the asymmetrical distribution of ions across the membrane.
• The action potential is mediated by increased Na+ conductance, which leads to rapid depolarization of the membrane potential.

Synaptic Transmission

• The presynaptic cell is located before the synapse with respect to information flow; the postsynaptic cell is located after the synapse with respect to information flow.
• Nearly all neurons are both pre- and postsynaptic, since they both receive and transmit information.