Neuro: Synapses and Receptors

Synapse

• The point of connection between a nerve cell and another cell.
• A specialized junction where a neuron communicates with a target cell.
• Target cells include nerves, muscles, and glands.

Electrical Synapses

• Relatively rare in vertebrates.
• Membranes of the two cells are in close contact.
• Enables a rapid and reliable transmission of nerve impulses.

Chemical Synapses

• More complex than electrical synapses.
• Presynaptic and postsynaptic cells are separated by a synaptic cleft.
• Transmission is accomplished by neurotransmitters released from synaptic vesicles.

Neurotransmitter Release

• The presynaptic nerve terminal contains vesicles filled with neurotransmitter molecules.
• When an action potential arrives, voltage-gated calcium channels open.
• Calcium influx triggers the release of neurotransmitters into the synaptic cleft.
• Diffuse across the cleft and bind to receptors on the postsynaptic membrane.

Neurotransmitter Receptor Interactions

• Neurotransmitter molecules have specific shapes that fit the shapes of their receptors.
• Binding of the neurotransmitter changes the receptor's shape, opening ion channels.
• Excitatory neurotransmitters cause depolarization of the postsynaptic membrane (e.g., sodium influx).
• Inhibitory neurotransmitters cause hyperpolarization (e.g., chloride influx).

Excitatory Postsynaptic Potential (EPSP)

• A local depolarization of the postsynaptic membrane.
• The amplitude is determined by the number of vesicles released.
• If sufficiently large, it initiates an action potential in the postsynaptic cell.

Inhibitory Postsynaptic Potential (IPSP)

• A local hyperpolarization of the postsynaptic membrane.
• Makes the postsynaptic membrane more negative.
• Decreases the likelihood of an action potential.

Details of a Synapse

• Synaptic vesicles, full of neurotransmitters, are positioned at the presynaptic terminal.

Action of neurotransmitters

• Neurotransmitters activate receptors, causing changes in the postsynaptic cell.
• Some neurotransmitters activate ion channels directly.
• Other neurotransmitters activate second messengers inside the postsynaptic cell.

Types of Neurotransmitters

• Acetylcholine: crucial at neuromuscular junctions and some synapses between neurons.
• Glutamate: the main excitatory neurotransmitter in the central nervous system (CNS).
• GABA: the main inhibitory neurotransmitter in the CNS.

Synaptic Transmission (steps)

• Step 1: Action potential arrives at the axon terminal.
• Step 2: Calcium channels open, triggering neurotransmitter release.
• Step 3: Neurotransmitters bind to receptors on the postsynaptic membrane.
• Step 4: Neurotransmitters diffuse away from the receptors and are removed from the synaptic cleft.

Neurotransmitter Action

• Neurotransmitters are synthesized from precursors.
• They are stored inside vesicles.
• They are released when they are triggered.
• They are inactivated by degradation or reuptake.

Classification of neurotransmitter

• By chemical structure.
• By function (excitatory or inhibitory).
• Acetylcholine
• Amino acids
• Monoamines
• Neuropeptides

Neural Circuits

• Neurons and circuit organization, including convergence and divergence.

Types of Neural Circuits

• Divergence (one neuron to many).
• Convergence (many neurons to one).
• Serial processing (step-by-step).
• Parallel processing (multiple pathways simultaneously).
• Reverberating (feedback loops).

Sensory Functions

• Levels of sensation and pathways, from body-to-brain, brain-to-body.
• Integration of sensation as a process involved in wakefulness & sleep.
• Sensory integration involves receiving, sorting, interpreting, and storing sensory information.

Sensory Receptors

• Structures and types of sensory receptors, including free nerve endings and encapsulated nerve endings.
• Types of stimulus they detect;
• Type of response to a stimulus;
• Location of receptors & origin of stimuli;

Tactile Receptors

• Touch—crude touch & discrimination.
• Types of rapidly adapting touch receptors, including Meissner corpuscles and hair root plexuses.
• Two types of slowly adapting touch receptors, including Type I cutaneous mechanoreceptors (Merkel discs) & Type II cutaneous mechanoreceptors (Ruffini corpuscles).
• Pressure.
• Vibration.

Pain

• Fast pain (acute) vs. slow pain (chronic).
• Sensory adaptation to pain.
• Causes of pain including, initial stimulus & referred pain.
• The modulation of pain involves the CNS as well as the PNS.

Sensation (as a process)

• The mechanisms for stimulus transduction into action potentials.

Sensory Modalities

• How one sensation can be distinguished from another. This reflects the types of stimuli detected by sensory receptors.

Proprioception

• Information about body position & movement.
• The importance of proprioceptors in maintaining posture, balance, & coordination.
• Types of proprioceptors, including muscle spindles & tendon organs.
• How proprioceptors detect changes in muscle length/tension.
• The adaptive nature of proprioceptive receptors & the importance of the brain's role in sensory integration.

Other Receptors

• The various types of sensory receptors that detect a variety of stimuli.

Receptor Potentials

• The way that stimuli are changed into electrical signals.

Sensory Pathways

• Somatic Sensory—The pathways for sensory information.
• The processes/pathways that involve first-, second-, and third-order neurons.
• Trigeminothalamic pathways.

Memory

• A process in which knowledge or skills acquired through instruction or experience are retained over time.
• Stages of memory.
• Types of memory (immediate, short-term, and long-term).
• Amnesia