STRUCTURE AND FUNCTION OF THE NEURON
All neurons have the same structure but its important to note that there are some significant differences between different types of neurons in terms of the spatial arrangements of the dendrites and axon.
- Nucleus: contains the genetic code, protein synthesis
- Dendrites: communication with other neurons
- Axon: sends information to other neurons
Each neuron consists of many dendrites but only single axon, although the axon may be divided into several branches called collaterals.
Synapse
Synapse is the small gap between neurons in which neurotransmitters are released, permitting signaling between neurons. The two neurons forming the synapse are referred to as presynaptic and postsynaptic, reflecting the direction of information flow (from axon to dendrite). When a presynaptic neuron is active, an electrical current, action potential, is propagated down the length of the axon. When the action potential reaches the axon terminal, neurotransmitters are released into the synaptic cleft. Neurotransmitter bind to receptors on the dendrites or cell body of the postsynaptic neuron and create a synaptic potential. The synaptic potential is conducted passively (i.e. without creating an action potential).These passive currents form the basis of EEG. These different passive currents are summed together and if their summed activity exceeds a certain threshold, then an action potential (an active electrical current) will be triggered in this neuron.
Synapse is the small gap between neurons in which neurotransmitters are released, permitting signaling between neurons. The two neurons forming the synapse are referred to as presynaptic and postsynaptic, reflecting the direction of information flow (from axon to dendrite). When a presynaptic neuron is active, an electrical current, action potential, is propagated down the length of the axon. When the action potential reaches the axon terminal, neurotransmitters are released into the synaptic cleft. Neurotransmitter bind to receptors on the dendrites or cell body of the postsynaptic neuron and create a synaptic potential. The synaptic potential is conducted passively (i.e. without creating an action potential).These passive currents form the basis of EEG. These different passive currents are summed together and if their summed activity exceeds a certain threshold, then an action potential (an active electrical current) will be triggered in this neuron.
Electrical signaling and the action potential
Each neuron is surrounded by a cell membrane that acts as a barrier to the passage of certain chemicals. Within the membrane proteins molecules act as gatekeepers and allow in and out of particular chemicals, such as sodium (NA+) and potassium (K+) ions. The balance between these ions on the inside and outside of the membrane is such that there is normally a resting potential of -70mV across the membrane.
!Voltage-gated ion channels are only found in axons, which is why only the axon is capable of producing action potentials.
Each neuron is surrounded by a cell membrane that acts as a barrier to the passage of certain chemicals. Within the membrane proteins molecules act as gatekeepers and allow in and out of particular chemicals, such as sodium (NA+) and potassium (K+) ions. The balance between these ions on the inside and outside of the membrane is such that there is normally a resting potential of -70mV across the membrane.
!Voltage-gated ion channels are only found in axons, which is why only the axon is capable of producing action potentials.
- If a passive current of sufficient strength flows across the axon membrane,this begins to open the voltage-gated Na+ channels.
- When the channel is opened, then Na+ may enter the cell and the negative potential normally found on the inside is reduced (the cell is said to depolarize). At about −50 mV, the cell membrane becomes completely permeable and the charge on the inside of the cell momentarily reverses. This sudden depolarization and subsequent repolarization in electrical charge across the membrane is the action potential.
- The negative potential of the cell is restored via the outward flow of K+ through voltage-gated K+ channels and closing of the voltage-gated Na+ channels.
- There is a brief period in which hyperpolarization occurs (the inside is more negative than at rest). This makes it more difficult for the axon to depolarize straight away and prevents the action potential from traveling backwards.
- Myelin: a fatty substance that is deposited around the axon of some neurons that speeds conduction
Chemical signaling and the postsynaptic neuron
When the action potential reaches the axon terminal, the electrical signal intiaties events leading to the release of neurotransmitters into the synaptic cleft. Protein receptors in the membrane of the postsynaptic neurons bind to the neurotransmitters. Most of the receptors are transmitter-gated ion channels.
Glutamate and GABA the workhorse neurotransmitters of the brain,nearly every neuron produces one or other of these. Other common neurotransmitters are serotonin, dopamine, acetylcholine and noradrenaline.
Glutamate and GABA the workhorse neurotransmitters of the brain,nearly every neuron produces one or other of these. Other common neurotransmitters are serotonin, dopamine, acetylcholine and noradrenaline.
How do neurons code information?
The amplitude of an action potential does not vary but the number of action potential propagated per second varies along a continuum.
Spiking rate: rate of responding.
The amplitude of an action potential does not vary but the number of action potential propagated per second varies along a continuum.
Spiking rate: rate of responding.
The type of information that a neuron carries is related to the input it received and the output it sends to other neurons. The function of a region is determined by its inputs and outputs.
THE GROSS ORGANIZATION OF THE BRAIN
Gray matter, white matter and cerebrospinal fluid
Neurons are organized within the brain to form white matter and gray matter. Gray matter consists of neuronal cell bodies. White matter consists of axons and support cells (Glia)
There are 3 different kinds of white matter tract:
- Association tracts: cortical within hemisphere
- Commisure: cortical between hemisphere → corpus callosum
- Projection tract: cortical to subcortical
The brain consists of four ventricles filled with CSF:
- The lateral ventricles found in each hemisphere
- Third ventricle lies centrally around the subcortical structure
- Fourth ventricle lies in the brainstem (hindbrain)
Cerebrospinal fluid (CSF) carries waste metabolites, transfers some messenger signals and provides a protective cushion for the brain.
The cerebral cortex
The cerebral cortex (3mm thick)consists of two folded sheets of gray matter organized into two hemispheres. Having a folded structure permits a high surface area to volume ration and thereby permits efficient packaging. The raised surfaces of the cortex are termed gyri/gyrus, the dips or folds are called sulci/sulcus.
The cerebral cortex (3mm thick)consists of two folded sheets of gray matter organized into two hemispheres. Having a folded structure permits a high surface area to volume ration and thereby permits efficient packaging. The raised surfaces of the cortex are termed gyri/gyrus, the dips or folds are called sulci/sulcus.
Most of the cortex contains 6 main cortical layers, termed the neocortex. Other cortical regions are mesocortex and the allocortex.
The lateral surface of the cortex of each hemisphere is divided into 4 lobes: the frontal, parietal, temporal and occipital lobes. The dividing line between the lobes is sometimes prominent, as is the case between frontal and temporal lobes (sylvian fissure) but in other cases the boundary cannot readily be observed (eg between occipital and temporal lobes)
The lateral surface of the cortex of each hemisphere is divided into 4 lobes: the frontal, parietal, temporal and occipital lobes. The dividing line between the lobes is sometimes prominent, as is the case between frontal and temporal lobes (sylvian fissure) but in other cases the boundary cannot readily be observed (eg between occipital and temporal lobes)
There are four different ways in which regions of cerebral cortex maybe divided:
- Regions divided by the pattern of gyri and sulci.
- Regions divided by cytoarchitecture: Brodmann's areas.
- Regions divided by functions → primary visual cortex and the primary motor cortex
- Regions divided by connectivity
THE SUBCORTEX
The basal ganglia
The basal ganglia are large rounded masses that lie in each hemisphere. They surround and overhand the thalamus in the center of the brain. They are involved in regulated motor activity and the programming and termination of action → Disorders: hypokinetic hyperkinetic (e.g. Parkinson's and Huntington disease). The basal ganglia are also implicated in the learning of the rewards, skills and habits.
Main structure of basal ganglia
The basal ganglia are large rounded masses that lie in each hemisphere. They surround and overhand the thalamus in the center of the brain. They are involved in regulated motor activity and the programming and termination of action → Disorders: hypokinetic hyperkinetic (e.g. Parkinson's and Huntington disease). The basal ganglia are also implicated in the learning of the rewards, skills and habits.
Main structure of basal ganglia
- Caudate nucleus
- Putamen
- Globus pallidus
The limbic system
The limbic system is important for relating to the organism to its environment based on current needs and the present situation, and based on previous experience. It is involved in the detection and expression of emotional responses.
- Amygdala has been implicated in the detection of fearful or threatening stimuli
- Cingulate gyrus have been implicated in the detection of emotional and cognitive conflicts
- Hippocampus is particularly important for learning and memory
- Mamillary bodies are two small round protrusions that have traditionally been implicated in memory
- Olfactory bulbs lie on the under-surface of the frontal lobes. Their connections to the limbic system underscore the importance of smell for detecting environmentally salient stimuli(e.g. food) and its influence on mood and memory.
The diencephalon
The two main structures that make up the diencephalon are the thalamus and the hypothalamus. The thalamus is the main sensory relay for all senses (except smell) between the sense organs (eyes, ears etc) and the cortex. It also contains projections to almost all parts of the cortex and the basal ganglia. At the posterior end of the thalamus lie the lateral geniculate nucleus and the medial geniculate nucleus. There are the main sensory relays to the primary visual and primary auditory cortices.
The hypothalamus lies beneath the thalamus and consists of a variety of nuclei that are specialized for different functions primarily concerned with the body. Body temperature, hunger, thirst, sexual activity and regulation of endocrine functions. Tumors in this region can lead to eating and drinking disorders, precocious puberty, dwarfism and gigantism.
The midbrain and hindbrain
Midbrain
Midbrain
- Superior colliculi → integrate information from several senses (vision, hearing and touch)
- Inferior colliculi → specialized for auditory processing
- Substantia nigra → cell loss in this region is associated with the symptoms of Parkinson's disease
Hindbrain
- Cerebellum → important for dexterity and smooth execution of movement
- Unilateral lesion: poor coordination on the same side of the body as the lesion
- Bilateral lesions: wide staggering gait, dysarthria (slurred speech), nystagmus (eyes moving in a to-and-fro motion) - Pons → Link between cerebellum and the cerebrum. Receives information from visual areas to control eye and body movements.
- Medulla oblongata → regulates vital functions such as breathing swallowing, heart rate and the wake-sleep cycle.