242
(^) Spinal ganglion Connecting (^)
(nerve cell bodies)^ Synapse (associative)^
neuron (^)
Cell body
Gray (^)
Sensory matter^
neuron (^)
Motor neuron^
(^) White (^)
matter (^)
(^) Synapse Spinal cord (^)
Axon
(^) Sensory nerve ending (^)
or receptor (tendon)^
(^) Motor nerve
®^
Lear ni ng^
ending or^
effector
Cen gage^
(in muscle) (^)
Start
©^
Figure 10- 8 The basic pathway of a reflex
arc, as illustrated- by the knee-jerk reflex.
causing the stimulation of stretch receptors within the
quadriceps femoris muscle. They send the impulse via
sensory neurons to the spinal cord for interpretation. The
impulse then travels to a motor neuron (response) back to
the muscles that contract and your leg extends.^
Reflexes also occur within our bodies to help main-tain
homeostasis. Heartbeat rate, digestion, and breath-ing rates
are controlled and maintained by reflexes concerned with
involuntary processes. Coughing (the choke reflex),
sneezing, swallowing, and vomiting are other examples of
automatic subconscious reactions to changes within or
outside our body.
Grouping of Neural Tissue
In the nervous system, a number of terms are used to de-
scribe nervous tissue organization. It is important to un-
derstand the meanings of these terms. The term white
matter refers to groups of myelinated axons (myelin has a
whitish color) from many neurons supported by neuro-glia.
White matter forms nerve tracts in the CNS. The gray areas
of the nervous system are called gray matter, consist-ing
of nerve cell bodies and dendrites. It also can consist of
bundles of unmyelinated axons and their neuroglia. The
gray matter on the surface of the brain is called the cortex.^
A nerve is a bundle of fibers located outside the CNS.
Most nerves are white matter. Nerve cell bodies that are
found outside the CNS are generally grouped together to
form ganglia (GANG-lee-ah). Because ganglia are made
up primarily of unmyelinated nerve cell bodies, they are
masses of gray matter.
Chapter 10
A tract is a bundle of fibers inside the CNS. Tracts
can run long distances up and down the spinal cord. Tracts
are also found in the brain and connect parts of the brain
with each other and with the spinal cord. As-cending tracts
conduct impulses up the cord and are concerned with
sensation. Descending tracts conduct impulses down the
cord and are concerned with mo-tor functions. Tracts are
made of myelinated fibers and therefore are classified as
white matter. Two other terms are of note: a nucleus is a
mass of nerve cell bodies and dendrites inside the CNS,
consisting of gray matter; horns are the areas of gray
matter in the spinal cord.
The Spinal Cord
The spinal cord begins as a continuation of the medulla ob-
longata of the brainstem. Its length is approximately 16 to
18 inches. Its diameter varies at different levels because it
is surrounded and protected by bone (the vertebrae) and by
disks of fibrocartilage (the intervertebral disks). It is made
up of a series of 31 segments, each giving rise to a pair of
spinal nerves. In addition to the above protection, the spinal
cord (as well as the brain) is further protected by the
meninges (men-IN-jeez), a series of connective tissue
membranes. Those associated specifically with the spinal
cord are called the spinal meninges (Figure 10-9).
The outermost spinal meninx is called the dura mater
(DOO-rah-MATE-ehr), which means tough mother. It
forms a tough outer tube of white fibrous con-nective
tissue. The middle spinal meninx is called the arachnoid
mater (ah-RACK-noyd MATE-ehr) or spider layer. It
forms a delicate connective membranous tube inside the
dura mater. The innermost spinal meninx is known as the
pia mater (PEE-ah MATE-ehr) or delicate mother. It is a
transparent fibrous membrane that forms a tube around and
adheres to the surface of the spinal cord (and brain). It
contains numerous blood vessels and nerves that nourish
the underlying cells.
Between the dura mater and the arachnoid is a space
called the subdural space, which contains serous fluid.
Between the arachnoid and the pia mater is the
subarachnoid space. It is here that the clear, watery ce-
rebrospinal fluid circulates. The meninges do not attach
directly to the vertebrae. They are separated by a space
called the epidural space. This space contains loose con-
nective tissue and some adipose tissue that acts as a pro-
tective cushion around the spinal cord.
The Anatomy and Functions of the Nose.
A major function of the spinal cord is to convey sensory
impulses from the periphery to the brain and to con-duct
motor impulses from the brain to the periphery.