Precious Chapter #12

Nervous SystemAs the most complex system, the nervous system serves as the body control

center and communications electrical-chemical wiring network. As a key homeostatic regulatory and coordinating system, it detects, interprets, and responds to changes in internal and external conditions. The nervous system integrates countless bits of information and generates appropriate reactions by sending electrochemical impulses through nerves to effectors organs such as muscles and glands. The brain and spinal cord are the central nervous system (CNS); the connecting nerve processes to effectors and receptors serve as the peripheral nervous system (PNS). Special sense receptors provide for taste, smell, sight, hearing, and balance. Nerves carry all messages exchanged between the CNS and the rest of the body.

Nervous Tissue- General characteristics and functions. It composed of neurons and associated supportive cells called neuroglial. Neurons - The functional units of the nervous system. Neurons are highly

specialized cells that generate and transmit electrical impulses (action potentials) permitting rapid communication between distant areas of the body. Neurons gather and process information from receptors in contact with the external environment, and generate appropriate response signals. Networks of neurons help integrate multiple sensory signals and are responsible for more complex behaviors such as abstract thinking, learning, memory, language, planning, etc. Neurons are permanent cells that do not regenerate when injured.

Neuroglial - Cells that provide metabolic support and immune protection for neurons. Neuroglial outnumber neurons by about 10:1 in the CNS. Neuroglial do not generate or conduct nerve impulses. However, unlike neurons, glial cells can regenerate if injured.

The central nervous system (CNS) - The brain and spinal cord. The brain is further divided into the cerebrum, diencephalon, basal ganglia, brainstem, and cerebellum.

The nervous system is divided into two main parts:

The Central Nervous System con’t,The peripheral nervous system (PNS) - All nervous

structures outside the CNS. Includes the cranial nerves which supply the head and neck, spinal nerves which supply the trunk and extremities, and the autonomic nervous system (ANS) which supplies the smooth muscle and glands of internal organs.

The Brain-

The brain has billions of neurons that receive, analyze, and store information about internal and external conditions. It is also the source of conscious and unconscious thoughts, moods, and emotions. Four major brain divisions govern its main functions: the cerebrum, the diencephalon, the cerebellum, and the brain stem.

The functional regions of the cerebrum.(figure 12.4)

Spinal Cord:The spinal cord is a continuation of the brain stem. It is

long, cylindrical, and passes through a tunnel in the vertebrae called the vertebral canal. The spinal cord has many spinal segments, which are spinal cord regions from which pairs (one per segment) of spinal nerves arise. Like the cerebrum and cerebellum, the spinal cord has gray and white matter, although here the white matter is on the outside. The spinal cord carries messages between the CNS and the rest of the body, and mediates numerous spinal reflexes such as the knee-jerk reflex.

Spinal cord:

Meninges membranes surrounding and supporting the spinal cord; from without inwards, the dura, arachnoid and pia mater. The subarachnoid space, between the arachnoid and pia mater contains CSF, a clear, slightly alkaline fluid. This space is widest in the lowest part of the vertebral canal and it is here, the lumbar cistern, that the CSF surrounds the cauda equina and filum terminale. CSF is formed by the choroid plexuses of the lateral, third and fourth ventricles of the brain.

The dura mater is strong, formed of elastic and fibrous tissue. It is the outermost covering of the spinal cord and provides the external layer of the dural sac, the tubular sheath that lies free in the vertebral canal. Superiorly it adheres to the margin of the foramen magnum where it is in continuity with the cranial dura mater; inferiorly it is anchored to the coccyx by the filum terminale. Within the vertebral canal, the dural sheath is separated from the periosteum of the vertebrae by the extradural or epidural space (described in more detail on page 148) but it extends along the dorsal and ventral nerve roots as far as the intervertebral foraminae to form dural root sleeves.

Meninges con’tThe arachnoid mater lines the dural sac, separated from it by a potential

space, the subdural space. The arachnoid mater is a thin avascular membrane that also ensheathes the spinal cord and the spinal nerve roots. It is connected to the underlying pia mater by delicate strands of connective tissue, the arachnoid trabeculae. Between the arachnoid and the pia mater lies the subarachnoid space, which contains the CSF.

The pia mater- is the innermost membrane covering the spinal cord and adheres closely to it. The pia mater ensheathes the spinal nerve roots and covers the spinal blood vessels.

Peripheral Nervous SystemThe peripheral nervous system (PNS) - All nervous

structures outside the CNS. Includes the cranial nerves which supply the head and neck, spinal nerves which supply the trunk and extremities, and the autonomic nervous system (ANS) which supplies the smooth muscle and glands of internal organs.

Autonomic Nervous System:The PNS has two parts: the autonomic nervous system

and the somatic nervous system. The autonomic system is divided into sympathetic branch and parasympathetic branch. The sympathetic nerves mobilize energy for the 'Fight or Flight' reaction during stress, causing increased blood pressure, breathing rate, and bloodflow to muscles. Conversely, the parasympathetic nerves have a calming effect; they slow the heartbeat and breathing rate, and promote digestion and elimination. This example of intimate interaction with the endocrine system is one of many that explain why the two systems are called the neuroendocrine system.

Somatic Nerves:The somatic nervous system, or voluntary nervous

system, enables humans to react consciously to environmental changes. It includes 31 pairs of spinal nerves and 12 pairs of cranial nerves. This system controls movements of skeletal (voluntary) muscles.