Page 438 - ACCCN's Critical Care Nursing

Neurological Assessment and Monitoring 415

Brain
CENTRAL Higher-order functions
NERVOUS such as memory, learning
and intelligence
Cranial nerves SYSTEM
Information
Spinal cord processing
Spinal nerves Sensory Motor
information commands
with with
afferent efferent
division division
includes
PERIPHERAL
NERVOUS
SYSTEM
Somatic Autonomic
nervous nervous system
system

Parasympathetic Sympathetic
division division

Special sensory Somatic sensory
receptors receptors monitor
provide sensations skeletal muscles, Smooth
of smell, taste joints, skin surface; Skeletal muscle
vision, balance and provide position sense muscle
hearing and touch, pressure,
pain and temperature Cardiac
sensations muscle
Glands
Visceral sensory receptors monitor
internal organs, including those of
cardiovascular, respiratory, digestive,
urinary and reproductive systems
RECEPTORS EFFECTORS
FIGURE 16.1 The functional divisions of the nervous system. 1

receive, process and transmit information. Most synaptic neurotransmitter synthesised in the cell body, along with
contacts between neurons are either axodendritic (excit- enzymes and lysosomes. The movement of materials
atory) or axosomatic (inhibitory). A neuron’s dendritic between the cell body and synaptic knobs is called axo-
tree is connected to many neighbouring neurons and plasmic transport. Some materials travel slowly, at rates
receives positive or negative charges from other neurons. of a few millimetres per day. This transport mechanism
The input is then passed to the soma (cell body). The is known as the ‘slow stream.’ Vesicles containing neu-
primary role of the soma and the enclosed nucleus is to rotransmitter move much more rapidly, travelling in the
perform the continuous maintenance required to keep ‘fast stream’ at 5–10 mm per hour which increases syn-
the neuron functional. Most neurons lack centrioles, aptic activity. Axoplasmic transport occurs in both direc-
important organelles involved in the organisation of the tions. The flow of materials from the cell body to the
cytoskeleton and the movement of chromosomes during synaptic knob is anterograde flow. At the same time,
mitosis. As a result, typical CNS neurons cannot divide other substances are being transported towards the cell
and cannot be replaced if lost to injury or disease. The body in retrograde flow (’retro’ meaning backward). If
fuel source for the neuron is glucose; insulin is not debris or unusual chemicals appear in the synaptic knob,
required for cellular uptake in the CNS. retrograde flow soon delivers them to the cell body. The
arriving materials may then alter the activity of the cell
A myelin sheath, consisting of a lipid-protein casing,
covers the neuron and provides protection to the axon by turning appropriate genes on or off. Retrograde flow
and speeds the transmission of impulses along nerve cells is the means of transport for viruses, pathogenic bacteria,
from node to node. (see Figure 16.2b). Myelin is not a heavy metals and toxins to the CNS, with resulting
3
continuous layer but has gaps called nodes of Ranvier disease such as tetanus, viral encephalitis and lead intoxi-
(see Figure 16.2a). cation. Defective anterograde transport seems to be
involved in certain neuropathies, including critical illness
Each synaptic knob contains mitochondria, portions of neuropathies. 4
the endoplasmic reticulum, and thousands of vesicles
filled with neurotransmitter molecules. Breakdown Synapses
products of neurotransmitter released at the synapse are The human brain contains at least 100 billion neurons,
reabsorbed and reassembled at the synaptic knob. The each with the ability to influence many other cells.
synaptic knob also receives a continuous supply of Although there are many kinds of synapses within the