13 nervous tissue

Our nervous system allows us to sense and respond to our external environment and also allows us to control the internal activities of our bodies to maintain homeostasis.

There is only one nervous system. However, anatomists and physiologists have found it useful to partition it into various divisions. Structurally, it can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

The peripheral nervous system (PNS) includes the cranial nerves that arise from the brain and the spinal nerves that arise from the spinal cord. Also included are ganglia, which are clusters of nerve cell bodies located outside the CNS.

Together the CNS and PNS collect information with the help of receptors in the PNS, process and evaluate sensory input so the CNS can determine the appropriate response, and then respond by sending impulses via the PNS to effectors (muscles and glands).

Sympathetic division, which typically speeds up body activities

Parasympathetic division, which typically slows down body activities

IF YOU HAD PAINFUL BLOATING OF YOUR STOMACH, WHAT FUNCTIONAL DIVISION OF YOUR NERVOUS SYSTEM WOULD BE RESPONSIBLE FOR SENDING IMPULSES TO YOUR CENTRAL NERVOUS SYSTEM (CNS)?

A VISCERAL SENSORY

B AUTONOMIC MOTOR

C SOMATIC SENSORY

D SOMATIC MOTOR

E SYMPATHETIC DIVISION

There are two distinct cell types that form nervous tissue:

Neurons, which are excitable Glial cells, which are supportive

Nervous tissue cells of the CNS

Neurons are the basic structural unit of the nervous system and conduct impulses from on part of the body to another. They have a high metabolic rate, they have extreme longevity, and most cease mitosis after fetal development.

Network of neurons

Read clinical view about the possibility of new neurons in adults.

Dendrites conduct impulses towards the cell body while axons conduct impulses away from the cell body.

Neurons are classified into three types: Sensory (afferent) neurons, motor (efferent) neurons, and interneurons (association neurons). It is estimated that 99% of all neurons are interneurons.

Glial cells (neuroglial cells) are capable of mitosis, do support, nourish, and protect neurons, and collectively account for one-half the volume of the nervous system. There are four types found in the CNS (shown above) and two types found in the PNS.

Glial cells of the CNS

The most abundant type of glial cells are the astrocytes, so named because of the cytoplasmic projections that give the cells a star-like shape.

Astrocytes envelop the brain capillaries and reduce capillary permeability. This helps create the blood-brain barrier.

Ependymal cells line the internal cavities (ventricles) of the brain and the central canal of the spinal cord (CNS). These cells, and nearby capillaries, form a network called the choroid plexus, which produces cerebrospinal fluid (CSF).

Choroid plexi are formed by the ependymal cells that line the ventricles and by nearby blood vessels. CSF is essentially a filtrate of plasma.

In lateral ventricles of brain

Microglial cells are wandering phagocytic cells that ingest and remove debris throughout the CNS.

Oligodendrocytes are cells that ensheathe portions of axons in the CNS by repeatedly wrapping around an axon to create a myelin sheath.

Oligodendrocytes creating a myelin sheath.

Satellite cells are flattened cells that arrange around neuronal cell bodies in ganglia of the PNS and help regulate the movement of nutrients and waste products

Note numerous dark satellite cells arranged around pink neuronal cell bodies in a ganglion of the PNS. They regulate the movement of nutrients and wastes.

Satellite cells

Cell bodies of a ganglion

Schwann cells (neurolemmocytes) are responsible for forming the myelin sheath around axons in the PNS.

Schwann cells (neurolemmocytes) in action in the PNS

Schwann cells (neurolemmocytes) finishing up the job of myelination in the PNS.

Read about tumors of the CNS in the clinical view in the text.

ACCORDING TO THE CLINICAL VIEW ON TUMORS OF THE CNS, WHICH OF THE FOLLOWING IS MOST LIKELY TO DEVELOP INTO A NEOPLASM (TUMOR) IN THE CNS?

A INTERNEURONS

B AXONS

C MITOCHONDRIA IN NERVE CELL BODIES

D DENDRITES

E ASTROCYTES

A nerve is a cable-like bundle of parallel axons. While a single axon tends to be microscopic, nerves are larger and tend to be visible to the naked eye.

Sensory nerves convey sensory information to the central nervous system and all axons are conducting impulses in the same direction.

Motor nerves convey motor impulses from the central nervous system to the muscles and glands and all the axons are conducting impulses in the same direction.

Mixed nerves carry both types of information and some axons are transmitting impulses in one direction, while other axons are transmitting impulses in the opposite direction.

Read about nerve regeneration and spinal cord injuries in the clinical view in your text.

Read about nervous system disorders in the clinical view in the text.

Nervous tissue development begins in the embryo during the third week of development. A neural groove appears and by the end of the third week the sides of this groove come together and fuse along the midline to form a neural tube.

The neural tube has an opening near the future head and an opening near the future buttocks.

If the openings in the neural tube do not close, the developing human will have a neural tube defect.

Read about neural tube defects in the clinical view in the text.

The cranial part of the neural tube expands to form the brain.

The caudal part of the neural tube expands to form the spinal cord.

ACCORDING TO THE CLINICAL VIEW ON NEURAL TUBE DEFECTS (NTDs), WHICH OF THE FOLLOWING IS CORRECT?

A ANENCEPHALY CAN BE DETECTED WITH ULTRASOUND

B SPINA BIFIDA OFTEN CAUSES PARALYSIS

C SPINA BIFIDA OCCULTA MAY BE ASYMPTOMATIC

D VITAMIN B12 CAN REDUCE INCIDENCE OF NTDs

E ALL OF THE ABOVE

EVERYTHING PAST THIS POINT IS EXTRA OR FOR USE IN EXAMS

Neuroglial cells creating a myelin sheath

Glial cell wrapped around neuron

Figure 14.co

Neurology is the study of the nervous system

A stimulus, if sufficiently strong, will generate a nerve impulse (action potential)

Figure 14.01

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of the cranial nerves arising from the brain and the spinal nerves that arise from the spinal cord.

The autonomic motor component has two subdivisions

Structural division of the nervous system.

Functional division of the nervous system

The nervous system conveys information rapidly and enables us to respond to stimuli.

The nervous system enables us to orient our bodies to changes in the internal and external environment.

The nervous system allows coordination of body activities and assimilation of experiences for memory, learning, and intelligence.

The nervous system provides for instinctual behavior, such as rooting and sucking to facilitate breast feeding.

The nervous system is composed of conductive neurons and supportive neuroglia (glial) cells

Neuroglial cells wrap a myelin sheath around the axons and dendrites of some nerve cells. This sheath is interrupted by Nodes of Ranvier.

Nerve impulse rapidly jumping from node to node.

Neurons can be sensory (afferent), motor (efferent), or interneurons (found only in CNS) which connect the sensory and motor neurons

A nerve is a collection of nerve fibers (neurons) outside the CNS

Figure 14.04

Figure 14.09

Figure 14.03b