Nervous System Functions and Components

Study Notes

The nervous system allows us to receive, integrate, and react to environmental stimuli to maintain homeostasis.

The central nervous system (CNS) includes the brain and spinal cord.
The peripheral nervous system (PNS) includes:
- Somatic sensory and motor neurons to skeletal muscles (voluntary)
- Autonomic sensory and motor neurons to smooth muscles and glands (involuntary)

Nervous tissue contains two types of cells:
- Neurons: transmit nerve impulses between parts of the nervous system
- Neuroglia (or glial cells): support, protect, and nourish neurons

Neurons are classified according to function:
- Sensory neurons: take impulses from a sensory receptor to the CNS (afferent)
- Interneurons: receive input from sensory neurons and other neurons, and communicate with motor neurons (association, connector)
- Motor neurons: take nerve impulses away from the CNS to an effector that carries out responses to environmental change (efferent, exit)
Neurons contain three basic parts:
- Cell body: contains nucleus and other organelles
- Dendrites: receive signals from sensory receptors or other neurons
- Axon: conducts nerve impulses away from the cell body
The axon may be covered by a protective myelin sheath, formed by Schwann cells containing myelin in plasma membranes.

The myelin sheath has gaps called nodes of Ranvier, which are areas with no myelin sheath.

There are over 100 substances known or suspected to be neurotransmitters, including:
- Acetylcholine (ACh)
- Norepinephrine (NE)
- Dopamine
- Serotonin
- Glutamate
- GABA (gamma aminobutyric acid)
Neurotransmitters transmit signals between nerves, and are used in:
- Nerve-muscle synapses
- Nerve-organ synapses
- Nerve-gland synapses
Acetylcholine and norepinephrine are active in both the CNS and PNS.

The CNS is made up of the spinal cord and the brain.
Both are wrapped in protective membranes called meninges, with spaces between them filled with cerebrospinal fluid.
The CNS is composed of two types of nervous tissue:
- Gray matter: short, non-myelinated fibers
- White matter: myelinated axons

The spinal cord extends from the base of the brain through the foramen magnum into the vertebral canal.
Components:
- Central canal: filled with cerebrospinal fluid
- Gray matter: "butterfly" shape
- White matter: contains interneurons grouped in tracts (ascending and descending) that cross over so that the left side of the brain controls the right side of the body.
Functions of the spinal cord:
- Provides a means of communication between the brain and peripheral nerves that leave the cord
- Is a center for reflex actions