Nervous System Overview

Organ Systems That Coordinate

The body uses organ systems to coordinate activities and maintain homeostasis. The nervous and endocrine systems are the primary coordinators.

• Nervous system: Responses are fast, brief, and can be highly specific in their target.

• Endocrine system: Responses are slower, last longer, and often have more general effects.

Nervous System Functions

The nervous system has three main functions essential for maintaining homeostasis and responding to environmental changes.

• Monitoring: Monitors the body's internal and external environments.

• Integration: Integrates sensory information.

• Coordination: Coordinates voluntary and involuntary responses.

Structural and Functional Organization

Central and Peripheral Nervous Systems

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• CNS: Consists of the brain and spinal cord; responsible for processing and integrating information.

• PNS: Consists of all neural tissue outside the CNS; connects the CNS to limbs and organs.

PNS Functional Divisions

The PNS is further divided based on function:

• Afferent division (sensory): Brings information to the CNS from receptors in body tissues and organs.

• Efferent division (motor): Carries information away from the CNS to effectors (muscles and glands).

Nervous Tissue

Cell Types in Neural Tissue

Nervous tissue is composed of two main cell types: neurons and neuroglia.

• Neurons: Basic functional units of the nervous system; communicate with one another and with other cells.

• Neuroglia: Support and regulate the environment around neurons; much more numerous than neurons.

Anatomy of a Representative Neuron

Neurons have a specialized structure for communication:

• Cell body (soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits electrical impulses away from the cell body.

• Axon terminals: Release neurotransmitters to communicate with other cells.

Additional info: The axon hillock is the site where action potentials are initiated.

Neuron Regeneration

Most neurons lack centrioles, so typical CNS neurons cannot divide and cannot be replaced by mitosis if destroyed by injury or disease.

• Implication: Damage to CNS neurons is often permanent.

Functional Classification of Neurons

Types of Neurons

Neurons are classified based on their function:

• Sensory neurons (afferent): Receive information from sensory receptors and relay it to the CNS.

• Motor neurons (efferent): Carry instructions away from the CNS to effectors (muscles and glands).

• Interneurons (association neurons): Located entirely within the CNS; interconnect other neurons and play a role in higher functions such as memory, planning, and learning.

Neuroglia

Types of Neuroglial Cells

Neuroglia make up about half of all neural tissue and are essential for supporting neurons.

• In the CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells

• In the PNS: Satellite cells, Schwann cells

Additional info: Oligodendrocytes form myelin sheaths in the CNS, while Schwann cells perform this function in the PNS.

Membrane Potential and Action Potentials

Resting Membrane Potential

All living cells have a polarized plasma membrane. The resting membrane potential of neurons is typically -70 mV (negative inside relative to outside).

• Measured in millivolts (mV)

• Maintained by ion gradients across the membrane

Action Potentials

An action potential is a propagated change in membrane potential of excitable cells, such as neurons.

• Voltage-gated channels: Open or close in response to changes in membrane potential.

• Threshold: Level of depolarization required to initiate an action potential (typically around -60 mV).

Characteristics of an Action Potential

• All-or-none principle: Every stimulus that brings the membrane to threshold will result in an identical action potential.

• Propagation: Action potential travels down the length of the neuron.

Generation of an Action Potential

• Begins at the initial segment of the axon (axon hillock).

• Membrane depolarizes to threshold (-60 mV).

• Voltage-gated Na+ channels open, membrane rapidly depolarizes up to +30 mV.

• Na+ channels close, K+ channels open, repolarizing the membrane.

• Channels close, membrane returns to resting potential.

Equation:

Propagation of Action Potentials

• Continuous propagation: Occurs in unmyelinated fibers; relatively slow (about 1 m/s).

• Saltatory propagation: Occurs in myelinated axons; much faster (18–140 m/s). Depolarization occurs only at Nodes of Ranvier.

Synapses and Neurotransmitters

Structure of a Synapse

A synapse is the site where a neuron communicates with another cell.

• Presynaptic neuron: Neuron sending information; axon terminal contains vesicles with neurotransmitters.

• Synaptic cleft: Gap between neurons where neurotransmitters diffuse.

• Postsynaptic neuron: Neuron receiving information; has receptors for neurotransmitters.

Important Neurotransmitters

• Norepinephrine (NE): Common neurotransmitter in the brain and autonomic nervous system (ANS); released by adrenergic synapses.

• Dopamine, GABA, serotonin: Function as CNS neurotransmitters.

• Nitric oxide (NO), carbon monoxide (CO): Gases that act as neurotransmitters.

Protection and Support Structures

Meninges

The meninges are three layers of specialized membranes that provide stability, protection, and facilitate cerebrospinal fluid (CSF) flow around the brain and spinal cord.

• Dura mater

• Arachnoid

• Pia mater

Additional info: Cranial meninges are continuous with spinal meninges and nerves.

Spinal Cord

Structure and Segments

The spinal cord is the major neural pathway between the brain and the PNS, integrating information and controlling spinal reflexes.

• Central canal: Narrow internal passageway filled with CSF.

• Segments: 31 segments, each giving rise to a pair of spinal nerves.

Additional info: The cauda equina is a bundle of spinal nerves below the end of the spinal cord.

Brain Structure and Function

Major Regions of the Brain

The brain contains almost 97% of the body's nervous tissue and is divided into several major regions:

• Cerebrum: Conscious thought, sensations, intellectual functions, memory storage and processing.

• Diencephalon: Thalamus, hypothalamus, epithalamus; processing centers, relay station, autonomic functions, hormones.

• Brain stem: Midbrain, pons, medulla oblongata; processing centers, relay station, autonomic functions.

• Cerebellum: Motor activities, coordination.

Motor, Sensory, and Association Areas of the Cerebral Cortex

The cerebral cortex contains distinct areas for motor, sensory, and association functions.

• Precentral gyrus (frontal lobe): Primary motor cortex; sends motor information for voluntary movement.

• Postcentral gyrus (parietal lobe): Primary somatosensory cortex; receives sensory information (touch, pressure, pain, temperature).

• Visual cortex (occipital lobe): Receives visual information.

• Gustatory cortex (frontal lobe): Receives taste information.

• Auditory cortex: Receives hearing information.

• Olfactory cortex (temporal lobe): Receives smell information.

Cranial and Spinal Nerves

Cranial Nerves

There are twelve pairs of cranial nerves, identified by Roman numerals I–XII, and classified as primarily sensory, primarily motor, or mixed.

Spinal Nerves

Spinal nerves are mixed nerves grouped according to the region of the vertebral column.

Nerve Plexuses

Nerve plexuses are networks of nerve trunks with neuronal fibers from multiple spinal nerves.

• Cervical plexus: Innervates muscles of the neck and diaphragm.

• Brachial plexus: Innervates pectoral girdles and upper limbs.

• Lumbar and sacral plexuses (lumbosacral plexus): Innervate pelvic girdle and lower limbs.

Dermatome

A dermatome is a specific region of the body surface monitored by a pair of spinal nerves. Clinically important for determining location of damage or infection of a spinal nerve.

Reflexes and the Autonomic Nervous System

Reflexes

A reflex is a rapid, automatic response to a specific stimulus, helping preserve homeostasis with rapid adjustments.

Events in a Reflex Arc

• Receptor detects stimulus.

• Sensory neuron transmits signal to CNS.

• Integration center processes information.

• Motor neuron transmits response to effector.

• Effector carries out response.

Somatic vs. Autonomic Nervous System

The somatic nervous system (SNS) controls voluntary movements, while the autonomic nervous system (ANS) regulates involuntary functions.

Divisions of the ANS

Neurotransmitters at Specific Synapses

• Preganglionic fibers: Are cholinergic, releasing acetylcholine (ACh).

• Postganglionic sympathetic fibers: Most are adrenergic, releasing norepinephrine (NE).

Parasympathetic Division Function

The parasympathetic division is known as the "rest-and-digest" division.

• Constricts pupils

• Increases digestive secretions and tract activity

• Stimulates urination and defecation

• Constricts bronchioles

• Decreases heart rate