Nervous control

Question	Answer
Which functions is nervous control involved in?	Rapid movement, as well as collecting, processing, and responding to information from the environment. The brain is also an integrating centre for homeostasis, allowing it to be involved in longer term control too.
Name three ways in which the 'information' in the nervous system can be propagated.	Action potentials, neurotransmitters, and graded potentials.
Why are ion channels important in the nervous system?	Because they allow for depolarisation and the transmission of action potentials, be they instigated by voltage, or pressure/membrane distortion.
What are the two main divisions of the nervous system?	The central and peripheral nervous systems.
What are the two divisions of the central nervous system?	The brain and the spinal cord.
What are the two main divisions of the peripheral nervous system?	Motor neurons and sensory neurons.
What are motor neurons divided into?	Somatic and autonomic nervous systems.
What are the two divisions of the autonomic nervous system?	The sympathetic and parasympathetic divisions.
Generally, what can the sympathetic and parasympathetic divisions of the autonomic nervous system be said to be responsible for?	Fight or flight, and maintenance, respectively.
Describe somatic motor neurons.	Somatic motor neurons are monosynaptic, heavily myelinated cholinergic neurons; their long axons (alpha-motor neurons) extend from the CNS directly to the target tissue with no ganglia, and for fast propagation.
Describe autonomic motor pathways.	Autonomic motor pathways consist of two neurons: the first, pre-ganglionic, heavily myelinated cholinergic neuron which is short and extends from the CNS to a peripheral ganglion, and the second, post-ganglionic, unmyelinated, adrenergic, longer neuron which extends to an effector organ from the ganglion.
What are the cell bodies of sensory neurons found?	The dorsal root of the spinal cord.
Describe a somatic efferent pathway.	Sensory information arrives through dorsal root to dorsal horn of spinal cord → Information travels through an interneuron → An α-motoneuron is stimulated in the ventral horn and the action potential is propagated through the ventral root to the effector organ e.g. skeletal muscle.
What is the difference between a somatic efferent pathway and an autonomic efferent pathway?	The efferent neuron consists of two parts: the pre-ganglionic and the post-ganglionic.
What is significant about sympathetic chain ganglia in generating responses?	The peripheral sympathetic chain ganglia are connected, which allows for communication and the possibility for the generation of a systemic/widespread effect.
Describe sympathetic cAMP signal transduction.	Adrenaline/noradrenaline binds to α2 or β-adrenoceptor → binding causes conformational change which separates G protein complex (α separates from βγ) (α2-adrenoceptor linked with G-inhibitory protein complex; β-adrenoceptor linked with G-stimulatory complex) → The α subunit migrates through the membrane and binds to adenylate cyclase → Giα downregulates conversion of ATP to cAMP whereas Gsα upregulates it → cAMP phosphorylates protein kinase A →protein kinase A phosphorylates another protein e.g. myosin light chain phosphatase →cellular response e.g. dephosphorylation of myosin light chains and smooth muscle relaxation.
Describe sympathetic phospholipase C signal transduction.	Adrenaline/noradrenaline binds to α1-adrenoceptor → the Gs complex is separated into its α and βγ subunits → the α subunit migrates through the membrane and binds to amplifier enzyme phospholipase C, activating it → phospholipase C converts membrane proteins (phosphatidylinositol bisphosphate – PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3), secondary messengers → DAG activates protein kinase C which phosphorylates proteins to generate a response and propagate the signal → IP3 migrates from the membrane to the endoplasmic/sarcoplasmic reticulum and binds to a Ca2+ channel, opening it → in smooth muscle, the response is that Ca2+ binds to calmodulin which activates myosin light chain kinase, phosphorylating myosin light chains, and causing smooth muscle cell contraction.
Why are sympathetic cAMP and phospholipase C signal transduction mechanisms a good example of divergence?	Because the same neurotransmitter (adrenaline/noradrenaline) can initiate a different physiological response, depending on the receptor it binds to.
What type of membrane protein is the nicotinic ACh receptor?	An Na+ channel.
What type of membrane protein is the muscarinic ACh receptor?	A K+ channel.
What is a reflex?	An automated, patterned response to sensory stimuli.
Describe the typical steps of a reflex arc.	Stimulus → sensory receptor → sensory afferent neuron → CNS integration → efferent motor neuron → effector → response → feedback effect on stimulus.
Why would the assumption that a monosynaptic neuron reflex has minimal modulation be incorrect?	Because there are 2-3000 other synapses attached to the α-motoneuron which are stimulatory/inhibitory and dictate whether it fires – here is modulation.
List four ways of classifying neural reflexes.	By: effector division (i.e. somatic or autonomic), integration site (i.e. brain or spinal cord), the number of neurons in the pathway (i.e. monosynaptic or polysynaptic/autonomic), or origin (i.e. innate, such as pupillary light reflex, or learned, such as the Pavlovian reflex).
What are autonomic/visceral reflexes usually involved in regulating?	Internal organs, e.g. moving or limiting movement of content through hollow organs.
Where are autonomic/visceral reflexes integrated?	In the spinal cord and lower (subconscious brain). However, they can be modulated by higher (conscious levels).
Give a generalised sequence of events in an autonomic reflex.	Stimulus → sensory receptor in viscera → impulse in visceral (sensory) fibre →travels to dorsal root ganglion then to dorsal horn → integrated in the spinal cord, perhaps by the preganglionic neuron → impulse travels out of ventral horn along a preganglionic axon → it reaches an autonomic ganglion and synapses with a ganglionic neuron → these exist as chain ganglia so there can be divergence of response → impulse travels along the postganglionic axon to the visceral effector →a response occurs → there is feedback to the CNS as to the status of the stimulus.
In the pupillary light reflex, what division of autonomic control causes constriction of the pupil?	Parasympathetic activity causes pupillary constriction, meaning sympathetic activity causes pupillary relaxation.
Which areas of the brain are involved in the pupillary light reflex?	The thalamus and midbrain.
Outline the sequence of events in the pupillary light reflex (for both constriction and dilation).	Light hits the photoreceptors in the retina in one or both eyes → The pupillary reflex is activated and signals travel through the optic nerve to the thalamus, then to the midbrain → Signals then continue to travel through the efferent, parasympathetic neurons (if too much light, sympathetic if too little) running through cranial nerve III (the oculomotor nerve) to constrict the pupils in both eyes via circular pupillary muscles in the iris or dilate the pupils in both eyes via contraction of radial muscles lying perpendicular to the circular muscles → Less light hits the retina in both eyes (the consensual reflex) or more light hits it → Signals return to the thalamus that the light level is appropriate → Contraction of the pupillary muscles in the iris stops at the optimal diameter for the pupils and dilatory signals are inhibited, or contraction of the radial muscles stops at the optimal diameter for the pupils and constrictive signals are inhibited.
What is the pupillary light reflex?	An autonomic response to the intensity of light hitting the retina which protects it from overstimulation, or provides enough light to receive sufficient sensory input to function.
What is the baroreceptor reflex?	An autonomic response to changes in blood pressure which acts to maintain normal blood pressure (in health, 120/80 mmHg) in homeostasis.
Where are the baroreceptors predominately?	In the aortic arch and internal carotid arteries.
What are baroreceptors sensitive to?	Stretch.
When are baroreceptors active? Where to do they send impulses to?	The receptors are active during normal pressure and sending constant impulses to the medulla regarding the level of stretch, the heart rate, and the rate of change in pressure.
Name two sensory nerves that relay information between the aortic arch and internal carotid arteries and the medulla in the baroreceptor reflex.	The sinus nerve (carotid sinus) and depressor nerve (aortic arch).
Which branch of the autonomic nervous system acts to raise blood pressure if it has dropped?	The sympathetic branch acts to raise blood pressure, while the parasympathetic branch acts to lower it if it rises.
Outline the sequence of events in the baroreceptor reflex (for both lowered and raised blood pressure).	When arterial pressure changes, arterial walls are subjected to more or less stretch, and the sensory nerves coming from the carotid sinus (sinus nerve) and from the aortic arch (depressor nerve) become more or less active and send more or fewer impulses. Upon receiving more or fewer impulses from the baroreceptors, the cardiovascular centres respond by exciting sympathetic and inhibiting parasympathetic nerves (if BP has dropped) or exciting parasympathetic and inhibiting sympathetic nerves (if BP has risen). This either results in increased cardiac output, increased constriction of arterioles, and increased constriction of veins (if BP has dropped), or a decrease in these factors (if BP has risen).
What is the main integration centre of the autonomic nervous system?	The hypothalamus.
At the conscious level, which areas of the brain can have control over the autonomic nervous system?	The limbic lobe (involved in emotional responses), and the cerebral cortex (frontal lobe).
What are the effectors of somatic reflexes?	Skeletal muscles.
List three sensors involved in somatic reflexes.	Muscle spindles, Golgi tendon organs, and pain receptors.
Which types of receptors do proprioceptors include, and what do they sense?	Muscle spindles (sense stretch), Golgi tendon organs (sense force), and joint receptors (sense pressure, interpreted as position).
Describe the structure of a muscle spindle.	Muscle spindles are made up of small muscle fibres, the centre portions of which have no contractile proteins as there are nerve endings there.
What part of muscle spindle innervation allows for greater control over movement?	α-γ co-activation
Describe α-γ co-activation.	The muscle spindles may send efferent impulses via sensory nerves to the spinal cord in accordance with skeletal muscle movement, which stimulates γ-neurons, resulting in the contraction of the intrafusal muscle fibres of the spindle. This contraction lengthens the non-contractile spindle, resulting in γ-neuron activity. This is again integrated in the spinal cord, and impulses may now be sent along α-motoneurons, which cause skeletal muscle contraction. An example of this may be in being handed a heavy tray.
Muscle spindles are involved in which reflex?	The stretch reflex!
What is a myotactic unit?	A myotactic unit consists of all pathways controlling a joint (all nerves, receptors, and muscles).
What is the purpose of Renshaw cells?	To reduce/dampen reflexes in order to prevent them from becoming too aggressive.
What do Renshaw cells do with motor neuron input?	Renshaw cells are interneurons that synapse with recurrent branches (those that synapse in the CNS) of α-motoneurons. They receive excitatory inputs, and make inhibitory synapses upon the same motor neurons which can limit motor neuron firing to dampen/reduce reflexes and prevent them becoming too aggressive.
What reciprocal inhibition is involved in Renshaw cell activity?	Synergist muscles may also be excited by spindle afferents making excitatory monosynaptic connections, and antagonist muscles are inhibited with inhibitory synapses on motor neurons in reciprocal inhibition.
Describe the structure of a Golgi tendon organ.	Golgi tendon organs consist of sensory nerve endings interwoven among collagen fibres, and link the muscle and the tendon.
What is the function of the Golgi tendon reflex?	To prevent tendons from becoming overstretched.
Describe the Golgi tendon reflex.	When the tendon is stretched by muscle contraction, the 1b sensory nerves are compressed by the collagen in the Golgi tendon organ and afferent neurons integrate via an inhibitory interneuron and inhibit further motor neuron excitation, inhibiting contraction and preventing damage.
What reciprocal activity is involved in the Golgi tendon reflex?	Like Renshaw cells in the muscle spindle reflex, the Golgi tendon organ also makes an excitatory synapse on another interneuron to make an inhibitory synapse on synergist muscle motor neurons. Further, it makes an excitatory synapse on an interneuron which makes an excitatory synapse on motor neurons to antagonist muscles, causing them to contract so that the original muscle is not only relaxed but stretched out.
What are pain receptors also known as?	Nociceptors.
Describe the sequence of events in the flexor/pain reflex.	A painful stimulus activates nociceptors → the primary sensory neuron enters the spinal cord and diverges → one collateral activates ascending pathways for sensation (pain) and postural adjustment → this results in a withdrawal reflex pulling the limb away from the painful stimulus → the other activates a crossed extensor reflex which supports the body as the weight shifts away from the painful stimulus (e.g. stand on a pin, and the opposite leg’s extensors (posterior quadriceps) contract).
In reflexive movement, there is predominately spinal integration, and some input to the brain. How do postural reflexes differ?	Integration occurs in the cerebellum to maintain balance, and there is input to the cortex, which is where movement can be consciously integrated.
What is feedforward stimulation in postural reflexes?	In postural reflexes, there is feedforward stimulation, typically in expectation of a threat. As the brain initiates movement, there is feedforward activity which adjusts the posture before or as the body moves. There is also feedback after unanticipated postural disturbance, so that posture can be appropriately adjusted once more.
What is the difference between rhythmic movements and somatic reflexes? What is similar?	While both use somatic muscles as effectors, rhythmic movements are learned, while somatic reflexes are innate.
Where are rhythmic movements integrated?	In the cortex, at the conscious level.
After initiation, what happens to the control of rhythmic movements?	It becomes subconscious, and there are ‘central pattern generators’ in the spine which maintain motion. These combine both voluntary and reflexive movements. The corticospinal tract consists of the cortex (e.g. primary motor cortex), the medulla oblongata, and the spinal cord, so that movements may be initiated and then maintained.
Describe the sequence of events in the control of voluntary movement.	Sensory input from the sensory and motor cortices is received by the prefrontal cortex and motor association areas → these impulses are transmitted through the basal nuclei and the thalamus and back to the prefrontal cortex and motor association areas (‘planning and decision making’) →impulses travel back to the motor cortex and then to the cerebellum for coordination and timing → impulses travel to the brain stem for execution via the spinal cord and somatic motor nerves – influence on posture, balance and gait → sensory receptors transmit impulses to the spinal cord, cerebellum and sensory cortex as continuous feedback.

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	Created by sophietevans over 10 years ago