Divisions of the Nervous System and the Brain

Contrast the locations and functions of the central and peripheral nervous systems

[Turn to figure 16.8 Page 286 before continuing] The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops. The Central Nervous System (CNS) includes the brain and spinal cord which lie at the center of the body. The CNS integrates (sums up) the information it recieves from all over the body allowing us to make decisions. The Peripheral Nervous System (PNS) is composed of nerves (bundles of neurons, see Fig. 16.9 Page 287), lies to either side of the body and connects the CNS to other parts of the body.

The Peripheral Nervous System (PNS)contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body. The axons and dendrites are surrounded by a white myelin sheath. Cell bodies are in the central nervous system (CNS) or ganglia. Ganglia are collections of nerve cell bodies within the PNS. Cranial nerves in the PNS take impulses to and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. They enter the cord through the dorsal root and leave through the ventral root.

Two main components of the PNS:

1. sensory (afferent) pathways that provide input from the body into the CNS.

2. motor (efferent) pathways that carry signals from CNS to muscles and glands (effectors).

Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the conscious level contributes to perception of our external environment. Sensory input from the PNS is processed by the CNS and responses are sent by the PNS from the CNS to the organs of the body.

There are two major subdivisions of the PNS motor and sensory pathways: the somatic and the autonomic. The somatic nervous system includes all nerves that serve the musculoskeletal system. The autonomic nervous system includes the motor neurons that control the internal organs automatically and usually without need for concious intervention.

Differentiate between the functions of the sympathetic and parasympathetic divisions of the autonomic nervous system

The peripheral nervous system consists of all body nerves. Motor neuron pathways are of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands). The Somatic Nervous System (SNS) includes all nerves controlling the muscular system and external sensory receptors. External sense organs (including skin) are receptors. Muscle fibers and gland cells are effectors. The Autonomic Nervous System is that part of PNS consisting of motor neurons that control internal organs The autonomic system controls muscles in the heart, the smooth muscle in internal organs such as the intestine, bladder, and uterus. It has two subsystems. The Sympathetic Nervous System is involved in the fight or flight response. The Parasympathetic Nervous System is involved in relaxation. Each of these subsystems operates in the reverse of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis. For example: when you are scared the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect. Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect to a secondary motor neuron which in turn innervates the target organ.

Neurotransmitter substance released on stimulation of the parasympathetic nervous system is Acetylcholine.

Neurotransmitter substance released on stimulation of the sympathetic nervous system is Adrenaline.


Parasympathetic Effect

Sympathetic Effect










PUPIL SIZE (diameter)




Increased to digestive organs, etc. but decreased to skeletal muscle.

Decreased to digestive organs, but increased to skeletal muscle

Identify the source gland for adrenalin and explain its role in the "fight or flight" response

The adrenal gland which is situated atop your left kidney (see fig. 16.12 Page 290) consists of an inner portion called the medulla and an outer portion called the cortex. When stimulated by the sympathetic nervous system the adrenal medulla releases adrenalin (also called epinephrine) and noradrenaline (also called norepinepherine). Together they cause:

• Increased heart rate - blood circulates faster

• Constriction of arteries going to digestive system kidney, skin - diverts blood to where it is needed immediately, i.e., skeletal muscles

• Dilation of arteries going to skeletal muscle - carries more blood to skeletal muscles, ready for action

• Dilates bronchioles - better gaseous exhange in the lungs

• Slows gut movement - blood diverted from intestine

• Hair stands on end - makes a mammal appear bigger and more frightening

• increase sweat secretion - cool body if activity speeds up

• Dilates pupil -better peripheral vision

• Contracts bladder and anal sphincter - this is no time to go to the washroom!

• Relaxation of bladder - elimination experienced in cases of severe fright, possibly to reduce weight

Identify and give functions for each of the following: - medulla oblongata - cerebrum - thalamus - cerebellum - hypothalamus - corpus callosum

The brain is composed of three parts: the cerebrum (seat of consciousness), the cerebellum, and the medulla oblongata (these latter two are "part of the unconscious brain").

The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping. The hypothalamus regulates homeostasis (constancy of the internal environment). It has regulatory areas for thirst, hunger, sleep, body temperature, water balance, and blood pressure, and links the Nervous System to the Endocrine System (via the pituatury gland). The midbrain and pons are also part of the unconscious brain.

The thalamus serves as a central relay point for incoming nervous messages. It recieves all sensory impulses (except those associated with the sense of smell) and channels them to the appropriate regions of the cerebrum. Sometimes called the gatekeeper to the cerebrum because it only alerts the cerebrum to info that it feels requires immediate attention. We are not aware of most of the sensory impulses recieved by the CNS.

The cerebellum is the second largest part of the brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates balance.

The cerebrum is the only area responsible consciousness. The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory data and motor functions. The cerebrum governs intelligence and reasoning, learning and memory. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.

Important: Use Fig. 16.13 on Page 291 to label the above diagram using the bold print words in the above section of text. This is the diagram that examiners (and I) will use to ask you to identify structures of the human brain.

Explain how the hypothalamus and pituitary gland interact as the neuroendocrine control centre

The hypothalmus controls the pituitary gland and thus body homeostasis. It has centres for hunger, satiety, sleep, thirst, body temperature, and blood pressure. The pituitary gland is divided into two portions, the anterior pituitary and the posterior pituitary. Neurosecretory cells in the hypothalmus respond to neurotransmitter substances and produce two hormones (ADH and Oxytocin) that are stored in and released from the posterior pituitary. The hypothalmus controls the anterior pituitary by producing specific releasing hormones that cause the anterior pituitary to release one of the several hormones produced in the anterior pituitary. The hypothalmus can also secrete specific release inhibiting hormones that stop or prevent the anterior pituitary from releasing one of the hormones produced there. The hypothalmic-releasing/release-inhibiting hormones travel from the hypothalmus to the pituitary by way of a portal system. [Hormones produced in the anterior pituitary , the master gland, include Growth hormone, Prolactin, and Melanocyte-stimulating hormone.