2 Nervous System Physiology

Neurons and their signals

Neurons are organized to receive, integrate, and transmit signals. The cell body contains the nucleus, dendrites receive many incoming signals, and the axon carries signals away from the cell body. Axon terminals communicate with other neurons, muscles, or glands. Glial cells support the neural environment, and some form myelin, an insulating covering that speeds conduction along axons.

Membrane potentials and action potentials

Neurons maintain a voltage difference across their membranes because ions such as sodium $(\mathrm{Na}^{+})$ and potassium $(\mathrm{K}^{+})$ are unevenly distributed inside and outside the cell. At rest, the inside is usually negative relative to the outside. Selective ion channels allow ions to cross the membrane, while the sodium–potassium pump helps maintain the ion gradients that support this electrical state.

Incoming signals usually produce graded potentials, local voltage changes whose size depends on the input. If depolarization at the axon's initial segment reaches threshold, voltage-gated channels generate an action potential. Sodium entry rapidly depolarizes the membrane, and potassium exit then repolarizes it.

An action potential is all-or-none: a stronger stimulus does not make an individual action potential larger, but it can increase how frequently the neuron fires. Refractory periods briefly limit repeat firing and help action potentials travel in one direction. In myelinated axons, the signal is regenerated at gaps in the myelin, enabling rapid, saltatory conduction.

Synaptic communication

At a chemical synapse, an arriving action potential opens voltage-gated calcium channels in the axon terminal. Calcium entry triggers the release of neurotransmitter, which crosses the synaptic cleft and binds to receptors on the target cell. Depending on the receptor and the ions or intracellular pathways involved, the signal may excite or inhibit the target. Neurotransmitter is cleared or broken down so that the signal does not continue indefinitely.

The resulting postsynaptic potentials are graded. An excitatory postsynaptic potential (EPSP) moves the membrane closer to threshold, while an inhibitory postsynaptic potential (IPSP) moves it farther away or otherwise reduces the chance of firing. A neuron combines inputs from many synapses: spatial summation combines signals arriving at different locations, and temporal summation combines signals arriving close together in time. If the net effect reaches threshold, the neuron fires.

At electrical synapses, ions pass directly between neighboring cells through connecting channels, allowing very rapid transmission.

Sensory transduction and processing

Sensory transduction is the conversion of physical or chemical stimuli into electrical signals by sensory receptors. Receptors may detect the external environment, internal body conditions, or the position and movement of body parts. Different receptor types respond to different kinds of stimuli:

• Mechanoreceptors respond to pressure or movement.

• Thermoreceptors respond to temperature.

• Chemoreceptors respond to chemicals.

• Photoreceptors respond to light.

• Nociceptors respond to potentially damaging stimuli.

A stimulus changes a receptor's membrane potential. This change may trigger action potentials in a sensory neuron or alter neurotransmitter release from a specialized receptor cell. Stimulus intensity is commonly represented by the size of the receptor potential and the frequency of action potentials. The active receptors and neural pathways help encode stimulus location and type.

Afferent pathways carry sensory signals into the central nervous system (CNS), where spinal cord and brain circuits process them. Many sensory pathways relay through the thalamus before reaching relevant areas of the cerebral cortex. The brain's processing gives rise to conscious perception.

Reflexes and sensory responses

Touching a hot surface activates temperature- and pain-sensitive receptors. Sensory signals enter the spinal cord, where circuits can initiate a rapid withdrawal reflex. Other signals ascend to the brain, where the stimulus is consciously perceived and integrated with context to guide further action.

This example shows that a rapid reflex response can be organized in the spinal cord without waiting for conscious processing, while information can also travel to the brain.

Motor pathways and movement

Efferent pathways carry motor commands from the CNS to effectors. In the somatic motor system, a lower motor neuron carries a signal from the brainstem or spinal cord to skeletal muscle. At the neuromuscular junction, acetylcholine activates receptors on the muscle and can initiate contraction.

Voluntary movement depends on coordinated activity across several brain regions. The cerebral cortex helps plan and initiate actions, while descending upper motor neurons influence lower motor neurons and spinal circuits. The basal ganglia contribute to selecting and regulating movements, and the cerebellum helps coordinate timing, precision, and balance. Sensory feedback, including information about muscle stretch and joint position, allows ongoing movement to be adjusted.

Autonomic regulation

The autonomic nervous system (ANS) regulates involuntary activity in cardiac muscle, smooth muscle, and glands. Its sensory inputs monitor internal conditions, and its outputs adjust organ activity to help maintain homeostasis. Many autonomic pathways use a two-neuron chain: a preganglionic neuron from the CNS synapses in a ganglion with a postganglionic neuron, which acts on the target tissue.

The sympathetic division generally prepares the body for increased demand. The parasympathetic division generally supports maintenance functions such as digestion and recovery. Their effects depend on the organ and receptor, and are not always simple opposites.

In both divisions, preganglionic neurons release acetylcholine onto nicotinic receptors in autonomic ganglia. Most parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors, while most sympathetic postganglionic neurons release norepinephrine. An important exception is that sympathetic neurons to most sweat glands release acetylcholine.

Homeostasis and integrated responses

Autonomic reflexes help stabilize internal conditions. For example, sensors detect a change in blood pressure and signal the brainstem; autonomic output then adjusts heart activity and blood-vessel tone.

The enteric nervous system is an extensive network in the digestive tract that can coordinate local digestive activity. It is also influenced by sympathetic and parasympathetic pathways.

Together, sensory receptors transduce stimuli into neural activity, which the CNS integrates to shape perception and action. Motor pathways control skeletal muscle and coordinate movement, while autonomic circuits adjust organs and glands to support homeostasis.