4 1 Neural Circuits The average human brain contains over 100 billion neurons connected via trillions of synapses They are connected to as many inputs from the body and outputs to the body How are these organized After all with only the principles of neurons and neural signaling that we have learned so far entire animals are controlled resulting in complex behavioral and physiological outputs At the very least the nervous system of an animal comprises two sets of neurons 1 Neurons that receive information from the environment acting as sensors these are afferent neurons 2 Neurons that send information out to the body causing a response these are efferent neurons Afferent neurons may synapse directly onto efferent neurons in the central nervous system CNS But typically the CNS contains interneurons neurons that connect sensory afferents to efferents In this way the CNS acts as an integrating center receiving all the sensory input from the periphery and computing a response which is sent back out to various target tissues in the periphery the effectors The complexity and size of the CNS varies widely across taxa The amount of branching in the nervous system determines how many synaptic connections each neuron can make For example if a cell has many dendrites or a large soma it can receive many inputs this is called convergence If a cell has an axon that splits into several projections all of which conduct action potentials simultaneously it can send input to many targets this is called divergence Convergence in the nervous system allows for spatial summation by the efferent neuron so the effector will respond to stimuli from a range of sources Divergence in the nervous system allows a single sensory input to affect multiple tissues and multiple physiological processes simultaneously We can map out pathways of afferent and efferent neurons in the body and think of them as forming a circuit a pathway along which electrical signals will flow There are several simple neural circuits that have been used as models for studying how neurons interact with each other Each of them is a reflex an unconscious behavioral output in response to sensory input requiring no conscious processing in the brain We will go through three such circuits 1 The patellar reflex in humans 2 The gill withdrawal reflex in sea slugs 3 The escape reflex in fish 1 The patellar reflex arc will integrate many of the principles we have covered so far in one simple circuit It will also introduce the chemical synapse between an efferent neuron that controls movement a motor neuron and a skeletal muscle the neuromuscular junction When you go to the doctor he or she will have you sit on the edge of the examination table with your lower legs hanging slack and your quadriceps thigh muscles relaxed A tendon on your knee patella will be tapped by a small hammer and your foot will reflexively kick forward How does this fast unconscious circuit or reflex arc work It starts with a sensory cell called a muscle spindle which is embedded in your quadriceps muscle 1 Muscle spindles detect stretch when the quadriceps muscle is stretched mechanically gated channels in the membrane of the muscle spindle open 2 This causes an action potential which travels through the axon of the muscle spindle to the spinal cord 3 In the spinal cord the small amino acid NT glutamate is released at an excitatory chemical synapse 4 Glutamate binds receptors on the efferent neuron causing an EPSP that depolarizes the neuron to threshold and initiates an action potential 5 This action potential travels through the axon of the efferent going back to the quadriceps muscle 6 At the neuromuscular junction between the efferent and the quadriceps muscle the small NT acetylcholine is released from the efferent neuron 7 Acetylcholine binds receptors on the skeletal muscle cells depolarizing them and causing them to contract and kick your foot forward You may notice that the axon of the sensory afferent branches in the spinal cord Skeletal muscle groups tend to be arranged in antagonistic pairs extensors and flexors where contraction of one opposes the other a phenomenon called reciprocity This allows a range of motion around a joint When the muscle spindle in the quadriceps detects stretch the quadriceps muscle will contract It is crucial to preventing damage that the opposing muscle the biceps muscle on the underside of the thigh does NOT contract at the same time Thus the same stimulus that excites the quadriceps inhibits the biceps muscle This is facilitated by an inhibitory interneuron in the spinal cord 1 Muscle spindles detect stretch when the quadriceps muscle is stretched mechanically gated channels in the membrane of the muscle spindle open 2 This causes an action potential which travels through the axon of the muscle spindle to the spinal cord 3 In the spinal cord the small amino acid NT glutamate is released at an excitatory chemical synapse 4 Glutamate binds receptors on the interneuron causing an EPSP that depolarizes the neuron to threshold and initiates an action potential 5 This action potential travels through the short axon causing the small amino acid NT glycine to be released on an efferent neuron 6 This inhibits the efferent neuron which innervates the biceps muscle causing an IPSP that prevents it from firing 7 Acetylcholine is not released onto the skeletal muscle cells of the biceps preventing them from depolarizing and causing the flexor muscle to contract 2 The sea slugs of genus Aplysia have become a common model system in neurobiology due to their small nervous systems and simple circuits controlling the gill withdrawal reflex The nervous system of Aplysia comprises about 20 000 neurons many of which are relatively large and thus easy to identify and probe with electrodes This allows us to inject current and record from the cells like in the voltage clamp technique When the skin of the animal s siphon is touched gently the sea slug defensively and reflexively withdraws its gill to protect it from damage Sensory afferents in the siphon synapse onto motor efferents in the gill When the siphon receives a mechanical stimulus the afferent fires and excites the efferent causing the gill to retract This reflex is plastic it can be modified by other neural inputs For example if the tail is painfully stimulated the synapses involved in the reflex circuit are strengthened Following painful stimulation of the tail even a weak stimulus to the
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