5 The Nervous System

Overall organization

The nervous system detects changes inside and outside the body, processes information, and coordinates responses. It has two structural divisions: the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), which includes nerves and ganglia outside the CNS.

Neurons and supporting cells

Neurons are specialized cells that receive and transmit information. A typical neuron has a cell body, also called the soma, which contains the nucleus; dendrites, which receive many incoming signals; and an axon, which carries signals toward other cells. Axon endings communicate with neurons, muscles, or glands at junctions called synapses.

Glial cells support and protect nervous tissue and help it function. In the CNS, astrocytes help regulate the chemical environment around neurons, oligodendrocytes form myelin around CNS axons, microglia provide immune defense and remove cellular debris, and ependymal cells line fluid-filled spaces in the brain and spinal cord. In the PNS, Schwann cells form myelin around peripheral axons, while satellite cells support neuron cell bodies in ganglia. Myelin insulates axons and helps signals travel more quickly.

Pathways and functional divisions

Nervous-system pathways can be described by the direction their signals travel. Sensory (afferent) pathways carry information from receptors toward the CNS. Motor (efferent) pathways carry commands away from the CNS to effectors, such as muscles and glands.

The somatic nervous system carries sensory information from the body and controls skeletal muscle, usually under voluntary control. The autonomic nervous system regulates involuntary activity in smooth muscle, cardiac muscle, and glands; its major divisions are the sympathetic and parasympathetic systems. The enteric nervous system is a network in the digestive tract that helps regulate digestive activity and communicates with the autonomic system.

These divisions work together. For example, sensory input can reach the spinal cord and trigger a rapid motor response before the brain produces conscious awareness of the stimulus.

How neurons signal

A neuron's membrane maintains an electrical difference between the inside and outside of the cell. Incoming signals produce local changes in membrane voltage. If these changes bring a region of the neuron to threshold, it generates an action potential, a brief, all-or-none electrical signal. Sodium ions moving into the cell depolarize the membrane; potassium ions moving out help restore its resting state. An action potential travels along the axon without gradually fading.

In myelinated axons, the action potential is renewed at gaps in the myelin called nodes of Ranvier. This allows it to travel rapidly from node to node. At most synapses, an arriving action potential causes the neuron to release neurotransmitters. These chemicals cross the tiny synaptic gap and bind to receptors on a target cell, changing its activity. Signals from many synapses can combine to make a neuron more or less likely to fire.

The brain and spinal cord

The brain integrates sensory information and helps coordinate movement, thought, memory, emotion, and internal regulation. Its major regions have distinct contributions:

• The cerebrum has two hemispheres that support perception, voluntary movement, language, reasoning, and memory.

• The diencephalon includes the thalamus, which relays much sensory information, and the hypothalamus, which helps regulate internal conditions and endocrine activity.

• The brainstem consists of the midbrain, pons, and medulla. It connects the brain with the spinal cord and helps regulate essential functions such as breathing and heart activity.

• The cerebellum contributes to balance, posture, and the coordination and refinement of movement.

The spinal cord carries signals between the brain and much of the body and coordinates many reflexes. In a spinal-cord cross-section, central gray matter contains many neuron cell bodies and synapses, while surrounding white matter contains many axons traveling in bundles called tracts.

A reflex pathway can produce a quick response: sensory input enters the spinal cord, is processed there, and motor output travels to a muscle. Information may also be sent to the brain.

Protection of the CNS

The brain and spinal cord are protected by the skull and vertebral column, layers of tissue called meninges, and cerebrospinal fluid, which cushions the CNS.

Peripheral nerves and roots

A peripheral nerve is a bundle of axons wrapped in connective tissue. Some nerves carry sensory axons, some carry motor axons, and many carry both. Cranial nerves connect the brain with structures mainly in the head and neck; spinal nerves connect the spinal cord with the trunk and limbs.

Sensory neuron cell bodies are often grouped in ganglia outside the CNS. In a typical spinal nerve, sensory fibers enter the spinal cord through a dorsal root, and motor fibers leave through a ventral root. The roots join to form a mixed nerve, allowing information to travel in both directions between the CNS and the body.

How the system works together

Neurons transmit information through electrical signals along their axons and chemical signaling at most synapses. Glial cells support neurons, help maintain the nervous-tissue environment, and form myelin. The CNS processes and coordinates information, while peripheral nerves carry sensory input toward the CNS and motor output to the body. Together, these structures enable sensation, movement, reflexes, and regulation of internal functions.

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