Module 6: Nervous System
6.1 – Objectives
By the end of this section, you should be able to:
• Draw and label a diagram of the human brain, showing all the major regions, important gyri
and sulci, and the major functional areas of movements, sensory, vision, hearing,
speech, and so on.
• Name two main types of brain cells.
• Draw and label a chemical synapse.
• Describe the events underlying synaptic transmission.
• Name the four classes of neurotransmitters.
• Name the main excitatory and inhibitory neurotransmitters in the brain.
• Describe the ionic mechanisms and the changes in membrane potential associated with an
excitatory postsynaptic potential (EPSP) and an inhibitory postsynaptic potential (IPSP).
• Define spatial and temporal summation.
• Draw and explain the arrangement of the motor system.
• Define the motor cortex.
• Draw a simple diagram of the corticospinal tract.
Draw a simplified diagram of a muscle spindle.
• Draw a diagram of the reflex arc for the stretch reflex (for example, knee jerk reflex), and
describe the sequence of events in this reflex.
• Describe alpha-gamma coactivation.
• Name three specific functions of the cerebellum.
• Name seven behaviors influenced by the limbic system.
• Name seven major functions of the hypothalamus.
• List the two divisions of the Autonomic Nervous System (ANS).
• Describe the pathways of the Parasympathetic NS (PSYN) and Sympathetic NS (SYN).
List the functions of the PNS and SNS. 6.2 – Introduction
The nervous system consists of the central nervous system (CNS) and peripheral nervous
The CNS is made up of the brain and spinal cord, while the nerves outside the CNS that
go to muscles and organs like the heart are considered part of the PNS.
We can therefore divide the PNS into somatomotor (going to skeletal muscles) and
autonomic (going to other organs) nervous systems.
In this section, we will look at some of the other types of cells in the CNS and some of
the sensory systems of the body, and then we will examine how the brain controls
muscles for movement.
6.4 - Basic Structure of the Brain
There are two cerebral hemispheres—a left and
a right hemisphere.
o The left hemisphere sends signals to
activate muscles on the right side of the
o Similarly, sensory information from the
right side of the body travels to the left
hemisphere (and vice versa).
The brain stem, which controls some of the
most basic functions of the body like heart rate
and respiration, is made up of the midbrain,
pons, and medulla oblongata.
o The medulla is continuous with
the spinal cord. At the back, or
o Just above the brain stem is the
cerebellum, which is mainly responsible for coordinated movement.
o Not shown at right, the diencephalon consists of the thalamus and
As you can see on the diagram at right, there are many bumps (called gyri) and dips
(called sulci) on the surface of the brain.
These folds are most prominent in humans and increase
the surface area of the brain.
The locations of the sulci and gyri are quite consistent
between individuals (with only minor differences in size
and shape) and are so prominent that they have specific
Each cerebral hemisphere can be divided up into four
lobes based on these "landmarks." Within each lobe are
regions that have very specific functions. 6.6– Functional Structure of the Brain
a) Frontal Lobe
The Primary Motor Cortex processes input from skeletal muscles throughout the
body, while the Motor Association Area (Premotor Cortex) and the Prefrontal Cortex
integrate movement information with other sensory inputs to generate perception
(interpretation) of stimuli.
b) Temporal Lobe
The temporal lobe contains the Primary Auditory Cortex and Auditory Association
Areas, which receive and process signals from the auditory nerve and integrate them
with other sensory inputs. Other portions of the Temporal Lobe are involved with
smell (olfaction) and in mediating short-term memory storage and recall.
c) Parietal Lobe
Contains the Primary Somatosensory Cortex, which receives input from the major
sense organs (the skin, musculoskeletal system, and taste buds). The Association
Areas of the lobe integrate sensory information with other association areas of the
cortex to form meaningful perceptions.
d) Occipital Lobe
Is the area of the cerebral cortex responsible for vision. It contains the Primary Visual
Cortex, which receives input directly from the optic nerve, as well as Visual
Association Areas that further process visual information and integrates it with other
Processes sensory information and coordinates the execution of movement in the
body. As the structure with the largest number of neurons in the brain, the
cerebellum receives input from somatic receptors, receptors for equilibrium, and
valance and motor neurons from the cortex.
a) Corpus Callosum:
Serves as a pathway between the two cerebral hemispheres. This allows the brain to
integrate sensory and motor information from both sides of the body, and to
coordinate whole-body movement and function.
Thalamus: Receives sensory input as it travels from the spinal cord and integrates
sensory information before sending it to the cortex.
Hypothalamus: Controls a variety of endocrine functions (body temp, thirst, food
intake, etc.), mainly through directing the release of hormones.
c) Pituitary Gland
Primarily regulates other endocrine organs and is regulated by the hypothalamus.
The anterior pituitary gland is derived from epithelial tissue of the pharynx.
o Anterior pituitary hormones include LH, FSH, ACTH, TSH, GH, and prolactin.
The posterior pituitary derives from neural tissue of the hypothalamus.
o The posterior pituitary releases the hormones vasopressin and oxytocin. d) Midbrain
Bridges the lower brainstem with the diencephalon above.
Its primary function is in controlling eye movements, and it also exerts control over
auditory and visual motor reflexes.
Acts as a relay station for transferring information between the cerebellum and the
Along with the centers in the medulla, the pons also coordinate and controls
Is the portion of the brainstem that has primary control over involuntary functions
such as breathing, blood pressure and swallowing
It is also here that fibers from the corticospinal tract, which originate in the motor
cortex, cross over to the opposite side of the spinal cord to innervate muscles on the
opposite side of the body.
a) Optic Nerves
From each eye meet at the Optic Chiasma where they cross over and continue on as
optic tracts to the lateral geniculate bodies of the thalamus.
From there, axons extend to their respective hemisphere on the primary visual area
of the occipital lobe.
b) Brain Stem
Is an extension of the spinal cord.
Consists of 3 regions: the midbrain, pons, then medulla.
Is a center for many involuntary functions (i.e. breathing).
Incorporates 9 cranial nerves.
a) Primary Motor Cortex
At the posterior end of the frontal lobe.
The Primary Motor Cortices process information relating to skeletal muscle
movement. When electrically stimulated, this region will cause a specific muscle to
b) Primary Somatosensory Cortex
At the anterior end of the parietal lobe.
The Primary Somatosensory Cortex receives sensory information from the opposite
side of the body. The sensations of pain, temperature, touch, and vibration are
c) Language and Mathematical Area
Most often located in the left hemisphere (even for left-handed people).
This area serves as a general interpretive center, enabling a person to understand
visual and auditory information and in turn to generate written and spoken
responses. 6.7 – Neurons and Glial Cells
The brain is made up of neurons and glial cells.
o Neurons are the information transmitting and processing cells of the body.
o Glial cells make up 90% of the brain and only provide the environment for
neurons to function properly.
6.8 – Neurons
There are 3 types of neurons:
o 1) Bipolar Neurons: (2 processes extending from the cell body). They are a form
of neurons found in the eye.
o 2) Unipolar Neurons: (1 process extending from the cell body). They are located
in the peripheral nerves outside the CNS and are generally sensory in nature,
transmitting signals to and from the spinal cord.
o 3) Multipolar Neurons: Contain many branching dendrites and one axon. These
types are the most commonly seen in the CNS.
6.9 – Glial Cells
Glial cells are the support cells of the brain, as they maintain the delicate internal
environment of the CNS.
There are 5x as many compared to nerve cells.
Beside support, they also regulate the nutrients and specific interstitial environment of
Some types: Astrocytes, Microglia, and Oiligodedrocytes.
6.10 – The Language of the Nervous System and Neural Coding
Information travels down an axon in the form of action potentials
These action potentials are the language of the nervous system.
Example, if you hold a heavy object, there will be more action potentials compared to
holding a lighter object. This is called neural coding. The weight of the object is “coded”
into action potentials.
6.11 – Synaptic Transmission: The Chemical Synapse
Nerve cells communicate with on other with chemical synapse.
A presynaptic nerve will release a neurotransmitter that will affect a postsynaptic nerve.
The structure and process is very similar to a neuromuscular junction, but it is a bit
6.12 – Structure of a Chemical Synapse
1. Axon Terminal (of the presynaptic cell) containing:
a. Voltage-Gated Calcium Ion Channels (Ca ) ++
b. Synaptic Vesicles (contains neurotransmitter)
2. Synaptic Cleft
3. Postsynaptic cell containing:
a. Chemical Receptors
b. Chemically Gated Ion Channels (ligand gated ion channels) Theses open
when a chemical attaches to them, in this case, the neurotransmitter. 6.13 – Sequence of Events at a Chemical Synapse
1. Presynaptic Neurons: Synthesizes neurotransmitters that are stored in the synaptic
2. An action potential in the presynaptic neuron depolarizes the membrane and activates
Voltage-Gated Ca Channels. The calcium ions flow into the axon terminal (white balls).
3. Calcium causes the synaptic vesicles to fuse to the wall of the synaptic terminal, causing
exocytosis and the release of the neurotransmitter.
4. Neurotransmitter diffuses across the cleft and acts on the chemical receptors found on
the postsynaptic cell membrane.
5. Receptors cause the opening of the chemically gated ion channels.
6. The postsynaptic membrane potential changes, causing a depolarization or
hyperpolarization depending on the type of neurotransmitter.
NOTE: A depolarization increases the probability of an action potential on the postsynaptic
neuron, while a hyperpolarization decreases the likelihood.
6.14 – Neurotransmitters
Are chemicals released by neurons at their axon terminals.
They are synthesized within the axons and are stored in the synaptic vesicles to be
released in response to an action potential.
After being released, the neurotransmitter diffuses across the synaptic cleft and
produces a response in the postsynaptic neuron.
Depending on the type neurotransmitter, this response may be excitatory (turns ON a
neuron), leading to a depolarization. If the depolarization is strong enough, it MAY fire
an action potential. On the other hand, the neurotransmitter could produce an
inhibitory (turns OFF a neuron) response, leading to a hyperpolarization of the
postsynaptic membrane and making it harder to generate an action potential.
There are 4 Classes of Neurotransmitters: Acetylcholine (Ach), Biogenic Amines, Amino
Examples (most common):
o Excitatory Glutamate
o Inhibitory Gama-amino butyric acid (GABA)
At the NMJ, a SINGLE action potential in the motor neuron produced a SINGLE action
potential in the muscle cell, causing the muscle to contract.
However, a single action potential on a presynaptic neuron will NOT produce an action
potential on a postsynaptic neuron.
6.17 - Ionic Basis of Postsynaptic Potentials ESPSs and IPSPs
An excitatory neurotransmitter will cause the opening of
the chemically gated channels. These gates are selective
for ONLY Positive Ions and will+allow the influx of
predominately sodium ions (Na ) into the cell.
This will cause a local depolarization of the membrane
called an Excitatory Postsynaptic Potential (EPSP).
The EPSP is a very local event that diminishes with time
and distance from the point of origin and, as a result, is
also called a graded potential. The influx of sodium will depolarize the region of the dendrite but it will NOT fire an
This is because there are no voltage-gated channels on the dendrites or cell body of the
Voltage-gated channels are necessary for the production of an action potential, and the
action potential beings at the axon hillock where there is the highest concentration of
Thus, in order to generate the action potential, the EPSP must depolarize the axon
6.19 – EPSPs
EPSP gets smaller with the distance it has to travel.
Therefore, in order to cause a sufficient depolarization to open the voltage-gated
sodium channels located at the axon hillock, the positive current of the EPSP must be
strong enough to spread all the way from the synapse where it originated to the axon
hillock. Now we can have an action potential.
6.20 – Spatial and Temporal Summation of Synaptic Potentials
The strength of an EPSP can be increased in 2
ways: (1) Spatial Summation of EPSPs and (2)
Temporal Summation of EPSPs
o 1) Spatial summation of EPSPs is the
additive effect produced by MANY
EPSPs that have been generated at
MANY different synapses on the same
postsynaptic neuron at the same time.
o 2) Temporal summation of EPSPs is the
additive effect produced by many EPSPs
that have been generated at the SAME
synapse by a series of high-frequency
action potentials on the presynaptic
Note: EPSP and an Action potential are both DIFFERENT. The EPSP, which occurs ONLY on the
dendrites and cell body, will decrease with time and distance from its point of origin. However,
the action potential is all-or-nothing and is usually only found on the axon. Also, EPSPs can be
added one on top of the other, while action potential cannot.
6.23 – Inhibitory Postsynaptic Potentials IPSPs
There are also inhibitory neurotransmitters whose
effects are to shutoff nerve cells. The
neurotransmitters in this situation create a
hyperpolarization called an Inhibitory Postsynaptic
Potential (IPSP). 6.24 – IPSPs
Inhibitory neurotransmitters produce a hyperpolarization by opening different
chemically gated channels.
These channels, depending on the type of neurotransmitter, will either let chloride ions
(Cl) into the cell (adding negative charge) or let positive potassium (K ) out (removing
The overall effect is the sam