Nervous System

• The human nervous system consists of: (a) Central nervous system (CNS) consisting of the brain and spinal cord.

  (b) Peripheral nervous system (PNS) consisting of nerves connecting the central nervous system and the rest of the body. The function of the PNS is to conduct sensory and motor signals between the CNS and the limbs and organs (receptors and effectors).

• A stimulus is a change in the environment that causes an organism to react. Stimuli are detected by sensory receptors.

• A response is a change in the body as a result of the stimulus. Effector cells are muscle cells or gland cells, which carry out the response to stimuli.

• Bodily functions are classified into voluntary actions and involuntary actions.

• Involuntary actions are actions that cannot be consciously controlled, such as heartbeat, peristalsis, vasoconstriction and reflex actions.

• Voluntary actions are actions that are consciously controlled.

Nervous tissue

• Nerve impulses are transmitted by nerves, which are bundles of neurones wrapped in connective tissue.

• A neurone is a nerve cell.

• There are three main types of neurones: (a) Sensory neurones – Respond to stimuli affecting cells of the sensory organ they are found in and relay signals to the CNS (b) Intermediate neurones (relay neurones) – Transmit nerve impulses from the sensory neurones to the motor neurones; found within the CNS (c) Motor neurones – Transmit nerve impulses from the CNS to the effector muscle cells or gland cells

• Neurones share common characteristics:

  (a) A relatively large cell body containing the nucleus and organelles.

  (b) Nerve fibres that increase the distance over which nerve impulses can be transmitted. There are two types of nerve fibres.

  • Axons are long, that conduct nerve impulses away from the cell body of the neurone.

  • Dendrons are branched projections that conduct nerve impulses towards the cell body.

  • At the terminal ends of axons and dendrons, the nerve fibre branches. These branches are known as dendrites. Where the axon is connected to muscles, these branches are also known as motor end plates.

Sensory Neurone

• The sensory neurone has a smooth and rounded cell body, a single long dendrite and a short axon. The dendron is structurally similar to an axon and is myelinated.

Motor Neurone

• The motor neurone consists of a cell body and a long thin axon covered by a myelin sheath.

• Around the cell body are branching dendrites that receive nerve impulses from other neurones and conduct them towards the cell body.

• The axon conducts signals away from the cell body towards the effector cells.

Synapses

• A synapse is a junction between two neurones or between a neurone and an effector.

• At a synapse, impulses from the axon of one neurone are transmitted to the dendrites of another neurone or to effector cells.

Reflex actions

• Reflex actions are involuntary responses to a specific stimulus. They cannot be consciously controlled.

• The pathway by which nerve impulses travel during reflex actions is called a reflex arc.

• It consists of:

  (a) Receptor

  (b) Sensory neurone

  (c) Intermediate neurone / relay neurone (located in CNS)

  (d) Motor neurone

  (e) Effector

• Receptors in the skin detect the stimulus.

• Nerve impulses are produced which are transmitted by the sensory neurone to the spinal cord.

• In the spinal cord, the nerve impulses are transmitted across a synapse to an intermediate neurone and then across another synapse to the motor neurone. Nerve impulses are also transmitted to the brain.

• Nerve impulses travel along the motor neurone to the motor end plate.

• The nerve impulses stimulate the motor end plate and cause the muscle to contract.