Section 2.2 Flashcards
What is the somatic nervous system, and what does it control?
The somatic nervous system comprises axons that innervate skeletal muscles, which are under voluntary control.
Where are the cell bodies of motor neurons for the somatic nervous system located, and how do they control muscle contraction?
With the exception of the head, the cell bodies of motor neurons are located in the ventral horn of the spinal cord.
Motor neurons of the somatic nervous system terminate directly on their effector or target muscles.
Upon stimulation, these motor neurons release acetylcholine, leading to muscle contraction.
How does muscle relaxation occur in the somatic nervous system, and is there inhibition of muscle contraction?
In the somatic nervous system, muscle relaxation occurs by decreasing the excitability of motor neurons.
Unlike the autonomic nervous system, there is no inhibition of skeletal muscle contraction; only excitation is involved in controlling muscle contraction and relaxation.
What role do motor neurons play in the nervous system, and what kinds of signals do they receive?
- Motor neurons serve as the convergence point for various sensory inputs from different parts of the brain, including the cortex, basal nuclei, cerebellum, and brainstem.
- They also receive sensory input directly from reflex pathways.
- Motor neurons accept a mixture of both excitatory (EPSPs - Excitatory Postsynaptic Potentials) and inhibitory (IPSPs - Inhibitory Postsynaptic Potentials) signals.
How do motor neurons decide whether to generate action potentials and contract muscles?
Motor neurons integrate the collection of excitatory and inhibitory signals they receive to determine whether or not to generate action potentials, ultimately leading to muscle contraction through lower motor neurons.
What are the potential consequences of damage to either upper or lower motor neurons?
Damage to either upper or lower motor neurons can result in severe and irreversible health problems such as paralysis or spasticity, impacting the ability to control and move muscles effectively.
What are upper motor neurons?
Upper motor neurons are motor neurons that originate either in the motor region of the cerebral cortex or in the brain stem and carry motor information down to the lower motor neurons.
The upper motor neuron descends in the spinal cord to the level of the appropriate spinal nerve root
What are lower motor neurons?
Lower motor neurons are motor neurons located in either the anterior grey column, anterior nerve roots or the cranial nerve nuclei of the brainstem and cranial nerves with motor function
What happens at the neuromuscular junction when an action potential reaches the motor neuron’s terminal button?
When an action potential reaches the terminal button of the motor neuron, voltage-gated Ca2+ channels open, allowing Ca2+ to enter the terminal button.
The increase in Ca2+ triggers the exocytotic release of vesicles containing acetylcholine (ACh) into the synaptic cleft.
What is the role of acetylcholine (ACh) in muscle contraction?
- Released ACh binds to nicotinic receptors on the motor end plate, opening cation channels, leading to a net influx of positive charge and depolarization. This is known as the end-plate potential.
- The end-plate potential depolarizes the motor end plate sufficiently, influencing the muscle membrane and causing voltage-gated Na+ channels to open.
- Once enough Na+ channels open, the muscle fiber reaches threshold, and a wave of excitation radiates from the motor end plate, resulting in muscle contraction.
How is the signal for muscle contraction terminated, and what enzyme is involved in this process?
Acetylcholine (ACh) needs to be removed from the synaptic cleft to prevent continuous muscle excitation.
An enzyme called acetylcholinesterase rapidly inactivates ACh within milliseconds, terminating the contraction signal.
What is the role of glutamate as a neurotransmitter, and where is it released from?
Glutamate is a neurotransmitter released by nociceptive afferent nerve fibers.
It plays a role in transmitting signals related to pain and sensory information.
What are the two types of glutamate receptors on dorsal horn neurons, and what actions are associated with each type?
Dorsal horn neurons have two types of glutamate receptors: AMPA and NMDA receptors.
The actions of glutamate on these receptors depend on which type is activated:
- Activation of AMPA receptors can lead to excitatory responses in dorsal horn neurons.
- Activation of NMDA receptors can also result in excitatory responses, but they are associated with more complex processes and may play a role in synaptic plasticity and pain processing.
What are the actions of AMPA receptors and NMDA receptors in dorsal horn neurons, and how do they affect signal transmission?
Activation of AMPA receptors leads to permeability changes, allowing sodium to enter the neuron, resulting in depolarization.
Once depolarization reaches a certain level, it dislodges the Mg2+ ion in the NMDA channel, activating NMDA receptors.
Activation of NMDA receptors allows calcium to enter the neuron, activating a second messenger pathway.
This activation makes the neuron more excitable than normal, which can explain why injured areas are more sensitive to stimuli that would not normally cause pain. For example, sunburned skin may be more sensitive to clothing rubbing against it.
Describe how the somatic nervous system is different from the autonomic nervous system.
The somatic nervous system controls voluntary muscle movements, with motor neurons located in the spinal cord and no synapses in ganglia. In contrast, the autonomic nervous system controls involuntary functions and consists of sympathetic and parasympathetic branches, with ganglia located outside the spinal cord.
